(CARBONYL COMPOUNDS) (CARBONYL COMPOUNDS)
Al
Aldedehyhydedess anandd keketotonneses cocontntaainin ththe e sasameme fufuncnctitiononaall grgrououpp,, ththee cacarbrbononylyl grgrououpp (>
(>C=C=O)O).. IInn aaldldehehydydeses ththee cacarbrbononylyl grgrououpp is is atattatachcheded eieiththerer to to twtwoo hyhydrdrogogenen at
atomomss (a(ass in in foformrmalaldedehyhydede)) or or to to ononee hyhydrdrogogenen atatomom anand d onone ae alklkylyl grgrououp;p; wh
whililee inin keketotoneness ththe e cacarbrbononylyl grgrououpp is is alalwawaysys atattatachcheded to to twtwo o alalkykyll grgrououpsps..
Li
Likeke ththe care carbobon-n-cacarbrbonon dodoubublele bobondnd of alof alkekenenes,s, tthhe e cacarbrboonn-o-oxxygygeenn ddoouubblele oof f tthhee cacarbrboonnyyll ggrorouupp is is cocompmpoosseedd oof f oonnee aandnd o
onnee bboonndd..
IInn tthhee ccaarrbboonnyyll ggrroouupp,, ccaarrbboonn aattoomm iis s iin n aa ssttaattee ooff sspp22 hybridization. Thehybridization. The C
C--OO bboonndd iiss pprroodduucceedd bbyy oovveerrllaapp ooff aa sspp22 ororbbititalal of cof cararbobonn wiwithth a p-a p-ororbbititaall of of o
oxyxyggenen. O. On tn thhee ooththeerr hhaanndd, t, the he CC-O-O bboonndd is is ffoormrmeedd bby ty thhee sisiddeewwaays ys ooveverrlalapp o
off pp-o-orbrbititaalsls ooff ccaarbrboonn aanndd ooxyxyggeenn. T. Thhee rreemamainininingg twtwoo sspp22 ororbitbitalsals of carbof carbonon ffoorrmm bboonndd wwiitth th thhe se s--oorrbbiittaall ooff hhyyddrrooggeenn oorr sspp22 ororbibitatalsls of of cacarbrbonon of of ththee alalkykyll
AL
ALDE
DEHY
HYDE
DE AN
AND K
D KET
ETON
ONES
ES
St
gr
grououp. p. NoNow w sisincnce e ththe e ththrereee bobondnds os of f ththe e cacarbrbononylyl cacarbrbonon ututililizizee spsp22 orbitals,orbitals, th
theyey liliee in in ononee pplalanene anand d arare e 12120000 apaparartt (s(simimililararitityy wiwithth C=C=C)C).).) Ho
Howewevever, ir, it ist is imimpoportrtanantt to nto nootete ththat tat thehe cacarbrbonon oxoxygygenen dodoubublele bobondnd isis dif
differferenentt frofromm carcarbobon-n-cacarborbon dn dououblblee bobondnd. D. Dueue to to gregreateaterr eleelectctronronegegatiativivityty of of oxy
oxygengen atoatom, thm, thee ππ-electro-electron cloud is attan cloud is attachedched towardtowards oxygen. Cons oxygen. Consequensequentlytly ox
oxygygen en atattataininss aa papartrtiaiall nenegagatitiveve chcharargege anand d cacarbrbonon a a papartrtiaiall poposisititiveve chcharargege..
Th
Thisis popolalarr nanatuturere of of ththee cacarbrbononylyl grgrououpp cacaususee inintetermrmololececulular ar atattrtracactitionon inin al
aldedehyhydedess or or keketotoneness anand d hehencncee acaccocoununtsts fofor r ththeieirr hihighgherer boboililiningg popoinintsts ththanan th
thatat ofof hyhydrdrococararbobonsns anandd etethehers rs ofof cocompmpararabablele momolelecuculalarr weweigightht.. MoMorereovoverer,, pola
polarr natnature ure of of the the carbcarbonyl onyl grougroup p also also expexplainlainss the the dipodipolele momemomentnt inin al
aldedehyhydedess anandd keketotonenes. s. HoHoweweveverr ththee hihighgh vavalulueses of of didipopolele momomementntss (2
(2.3.3-2-2.8.8 D)D) ofof alaldedehhydydeses anandd keketotonenes s cacannnnotot bebe acaccocoununtetedd fofor r ononlyly byby in
induductctivivee efeffefectct;; ththisis cacan bn bee acaccocoununtetedd foforr if if cacarbrbononylyl grgrououpp is is aa reressononanancece hyb
gr
grououp. p. NoNow w sisincnce e ththe e ththrereee bobondnds os of f ththe e cacarbrbononylyl cacarbrbonon ututililizizee spsp22 orbitals,orbitals, th
theyey liliee in in ononee pplalanene anand d arare e 12120000 apaparartt (s(simimililararitityy wiwithth C=C=C)C).).) Ho
Howewevever, ir, it ist is imimpoportrtanantt to nto nootete ththat tat thehe cacarbrbonon oxoxygygenen dodoubublele bobondnd isis dif
differferenentt frofromm carcarbobon-n-cacarborbon dn dououblblee bobondnd. D. Dueue to to gregreateaterr eleelectctronronegegatiativivityty of of oxy
oxygengen atoatom, thm, thee ππ-electro-electron cloud is attan cloud is attachedched towardtowards oxygen. Cons oxygen. Consequensequentlytly ox
oxygygen en atattataininss aa papartrtiaiall nenegagatitiveve chcharargege anand d cacarbrbonon a a papartrtiaiall poposisititiveve chcharargege..
Th
Thisis popolalarr nanatuturere of of ththee cacarbrbononylyl grgrououpp cacaususee inintetermrmololececulular ar atattrtracactitionon inin al
aldedehyhydedess or or keketotoneness anand d hehencncee acaccocoununtsts fofor r ththeieirr hihighgherer boboililiningg popoinintsts ththanan th
thatat ofof hyhydrdrococararbobonsns anandd etethehers rs ofof cocompmpararabablele momolelecuculalarr weweigightht.. MoMorereovoverer,, pola
polarr natnature ure of of the the carbcarbonyl onyl grougroup p also also expexplainlainss the the dipodipolele momemomentnt inin al
aldedehyhydedess anandd keketotonenes. s. HoHoweweveverr ththee hihighgh vavalulueses of of didipopolele momomementntss (2
(2.3.3-2-2.8.8 D)D) ofof alaldedehhydydeses anandd keketotonenes s cacannnnotot bebe acaccocoununtetedd fofor r ononlyly byby in
induductctivivee efeffefectct;; ththisis cacan bn bee acaccocoununtetedd foforr if if cacarbrbononylyl grgrououpp is is aa reressononanancece hyb
Re
Resosonanancncee in in cacarbrbononylyl grgrououpp alalsoso exexplplaiainn ththe se shohortrterer C=C=OO bobond nd (a(and nd hehencncee greater
greater bond bond energyenergy) than ) than the C=the C=CC Ot
Otheherr imimpoportrtanantt didiffffererenencece inin cacarbrbonon-o-oxyxygegenn anand d cacarbrbonon-c-cararbobonn dodoubublele bo
bondndss lilies es in in ththee fafactct ththatat ththe e cacarbrbononylyl grgrououpp unundedergrgoeoess nunuclcleoeophphililicic adaddidititionon re
reacactitionon whwhililee ololefefininicic unundedergrgoeoess elelecectrtropophihililicc adaddidititionon rereacactitiononss Ald
Aldehehydydeses shoshoww andand
Al
Aldedehyhydedess anandd keketotonneses araree fufuncnctitiononalal isisoomemersrs ofof oxoxirirananeses (cy
(cycliclicc ethethersers),), ununsasaturturateatedd alalcohcoholols s anandd ununsatsaturauratedted etethehers.rs.
Ketones show
Ketones show ,, Examples, of functional Examples, of functional is
isomomererisism arem are gigivevenn ababovovee inin alaldedehyhydedes.s. Isomerism:
Isomerism: chainchain funcfunctiontionalal isomisomeriseris ..
Ch
Chainain isoisomemers:rs: Fun
Functioctionalnal IsomIsomers:ers:
chain
chain fufunctnctionionalal anan mesmesmermerismism..
Chain isomers: Chain isomers:
Metamers: Metamers:
Primary alcohols form aldehydes while secondary alcohols form ketones.
Controlled oxidation can be carried out by using CrO3-pyridine
Controlled oxidation of alcohols can also be done by pyridinium dichromate (PDC) or pyridinium chlorochromate PCC which is a mixture of pyridine CrO3 and HCl in 1:1:1 ration. This reagent also does not attack double bonds.
Preparation 1. By oxidation of alcohols.
(Colli reagent).
alcohols can be oxidized to ketones by aluminum ter-butoxide in presence of acetone.
Secondary alcohols are oxidized to ketones and acetone is reduced to isopropanol (secondary alcohol).
Unsaturated secondary alcohols are oxidized to unsaturated ketones.
Calcium formate, on
pyrolysis, give formaldehyde calcium formate with calcium salt of any other fatty acid gives aldehydes; calcium salts of fatty acids other than calcium formate yield ketones.
Oppenauer oxidatio . Secondary
Calcium salts of dibasic acids, on heating gives cyclic ketones
Instead of using calcium salt of an acid, vapours of acid or mixture of acids can be passed over heated MnO at 3000C.
It is believed that here carboxylic are first converted into manganese salts which decomposes to form aldehyde or ketone.
Al
Aldedehhydydeses araree foformrmeded whwhenen ththe te triripplele bbonondd pr
presesenentt on on ththee tetermrmininalal cacarbrbonon atatoom,m, hohowewevever, r, keketotoneness aarere foformrmeded whwhenen ththee tr
tripiplele bobondnd is preis presesentnt on noon non-n-tetermrmininalal cacarbrbonon..
Ho
Howewevever,r, rerememembmber er ththatat vivinynyll boboraraneness foformrmeded frfrom om tetermrmininalal alalkykyneness (u(usesedd foforr pr
prepeparariningg)) ststililll hahaveve onone e hyhydrdrogogenen atatomom ththatat rereacact t wiwithth frfreseshh momolelecuculele of of di
diboborarancncee to to loloww yiyieleldd ofof alaldedehyhydede. T. Thuhus, s, it it isis adadvivisasablblee toto ususe se steteriricacallllyy hi
hindnderereded alalkykyll boboraranene ininststeaead d ofof didiboboraranene,, esespepecicialallyly duduriringng prprepepararatatioionn of of ald
aldehehydeydes. s. OnOnee sucsuchh stestericricalallyly hihindndereeredd alkalkylyl boboranranee is is didisiasiamyl myl boboranrane.e.
Gem
Gem -D-Dihihalalidideses hahavivingng twtwoo hahalologegenn at
atomomss on on ththee tetermrmininalal cacarbrbonon atatomom gigiveve alaldedehyhydedes, s, whwhililee gem gem –dihalides, having –dihalides, having tw
twoo hahalologegenn atatomomss on on nonon-n-cacarbrbonon gigiveve keketotonenes.s.
(ii) Hydr
(ii) Hydroboroboratioatio of alkynof alkynes.es.
5.
Th
Thisis memeththodod is is nonot t uusesedd mumuchch bebecacaususee aaldldehehydydeses arare e afaffefectcteedd byby aalklkalalii anandd dih
dihalalideidess are uare ususualallyly preprepapared red frofrom thm thee cacarborbonylnyl cocompompounundsds ththememselselvesves.. Hy
Hydrdrogogenen cycyananididee gigivevess alaldedehyhydedes, s, whwhililee alalkykyll cy
cyananidideses gigiveve keketotonenes.s. Fr
Fromom acacidid chchloloriridedes, s, keketotoneness cacan n bebestst bebe prprepeparareded byby ususiningg wweaeakekerr org
organanomeometatallillicc reareagegent, ent, e.g.g. li. lithithiumum didialkalkycycupruprateate oror didialkalkylyl cacadmdmiumium..
Ac
Acidid chchloloriridedess cacan n bebe re
reduducecedd inintoto aaldldehehydydeses wiwitthh hyhydrdrogogenen inin boboililiningg xyxylelene ne ususiningg papalllladadiuiumm as aas a cat
catalyalyst-st-supsuppoportertedd onon babariuriumm susulphlphatate. e. ThThisis rereacactiotionn is is cacallelledd an
andd ususeded fofor prr prepeparariningg bubut nott not ketketoneones.s.
6.
6. FroFro GriGrigngnardard rereagagenents.ts.
7.
7. ByBy ththe e rereduductctioio of of acacidid chchloloriridede anand d esestetersrs
Rosenmund Rosenmund reducation
Ba
Barriuiumm susulplphhaatete aacctsts as as aa ppooiisosonn ffoorr PdPd ccaattaalylysstt aanndd pprerevevennttss rereeedduucacattioionn RC
RCHOHO to to RCRCHH22OH. QuOH. Quininololininee anandd susulplphhurur araree bebetttterer popoisisononiningg agagenentsts fofor Pdr Pd ca
catatalylystst.. FoFormrmalaldedehyhydede cacannnnotot be pbe prerepapareredd memeththodod sisincnce foe formrmylyl chchloloriridede isis un
unststabablele atat roroomom tetempmpererataturure.e. Aci
Acidd chchlorlorideides as are re rereadadilyily redreduceucedd toto aldaldehehydydeses by by litlithiuhium tm tri-ri-teterr butoxyalu
butoxyaluminumminum hydridehydride, , LiAl(OCMLiAl(OCMee33))33H orH or trtri-i-n-n-bubutytyl til tinn hyhydrdridide,e, SnSn(C(C44HH99))33H.H.
Es
Estetersrs cacan n be be rereduducecedd eaeasisilyly toto alaldedehyhydedess by by sosodidium um alalumumininiuiumm hyhydrdrididee NaAlH
NaAlH44 or di-isobor di-isobutyutyl alumul alumuniumnium hydrhydrideide (DIB(DIBAL-HAL-H), Al[(), Al[(CHCH33 ))22 CHCH22CHCH22]]22H.H.
(( ).). NNititririleless wwhehen ren reduducecedd bbyy memeanans of s of stannou
stannouss chloridechloride and hand hydrochloydrochloric acid ric acid in absoin absolute etlute ether foher followed llowed by hydby hydrolysisrolysis yield
yield aldehydaldehydes. Thes. This reactis reaction is ion is knownknown
8. F
8. Fromrom ninitritrilesles Stephen reductionStephen reduction
Step
Nitriles can also be reduced selectively by di-isobutyl aluminum hydride to imines which upon hydrolysis gives aldehydes.
Ketones cannot be prepared by this method. (i) Oxo process:
The net reaction appears to be an addition of formaldehyde through anti-Markownikoff rule; this reaction in known as
and applied for the preparations of aldehydes only. (ii) Wacker process:
Ketones (but not aldehydes) are prepared by the ketonic hydrolysis of acetoacetic ester or its alkyl derivatives by heating with dil. aq. acid or dil. alcoholic solution of alkali.
9. From alkenes
hydroformylation or carbonylation
e.g. benzaldehyde can be prepared by
(i) Boiling benzyl chloride with a solution of cupric or head nitrate
(ii) Oxidizing toluene with chromium trioxide in presence of acetic anhydride to trap benzaldehyde acetate and thus avoid its oxidation to benzoic acid
(iii) Treating toluene with chloride in carbon tetrachloride and decomposing the complex precipitated with water
11. Fo aromatic aldehyde an ketones. Aromati aldehydes¸
(Laboratory method).
(Laboratory method)
Remember that side chains bigger that CH3 are oxidized by Etard reaction at the end carbon atom. e.g.
Partial oxidation of toluene can also be brought about by (a) manganese
dioxide and 65% H2SO4 at 310K (vapour phase oxidation) or catalytic oxidation with air diluted with nitrogen at 770 K in presence of oxides of Mn, Mo or Zr.
These methods constitute industrial methods for the preparation of benzaldehyde.
(iv) Treating benzene (aromatic compound) with mixture of carbon monoxide and dry HCl gas under pressure and in the presence of anhydrous AlCl3
Since formyl chloride (HCOCl) is unstable, formyl group (-CHO) can be introduced in the form CO+HCl or HCN+HCl.
(v) Treating benzene or an aromatic compound having activating group (like –OH, -OC2H5etc.) with a mixture of hydrogen cyanide and hydrogen chloride in the presence of anhydrous AlCl3 or ZnCl2
(vi) this reaction involves the conversion of aromatic compounds to aldehydes in the presence of a 20 amine and formic acid.
(Gattermann aldehyde synthesis)
are prepared by Friedel –Craft acylation
(c) For the preparation of the ketones of the type Ar. CO. Ar’ if one of the aryl contains deactivating group, it should be present in the acid chloride moiety. For example,
The alternate reactants i.e. nitrobenzene and benzoyl chloride cannot be used because strongly deactivating nitro group prevents the acylation reaction.
Aromatic ketones (as well as aliphatic ketones) can be prepared by treating the acid chloride with dimethyl cadmium.
The use of dimethyl cadmium is preferred over the use of Grignard reagents because the product (ketones) does not the react with dimethyl cadmium
The extreme ease with which B-keto acids undergo decarboxylation is applied for the preparation of ketones (aliphatic as well as aromatic).
Lower aldehydes and ketones are soluble in water due to hydrogen bonding between negative oxygen of carbonyl group and positive hydrogen of water. Higher members (having more than five carbon atoms) are practically insoluble in water, but soluble in organic solvents like alcohol and ether.
Ketones fro B-ket acids
2. Aldehydes and ketones have higher boiling points as compared to
corresponding alkenes. This is due to between the two carbonyl groups which are stronger forces than the van der Waals forces
existing in alkanes.
Further, aldehydes and ketones cannot form intermolecular hydrogen bonds with each other which are stronger forces than the dipole-dipole attraction
hence they have lower boiling points than the corresponding alcohols which can easily form hydrogen bonds. Thus boiling points of aldehydes and ketones are higher than hydrocarbons but lower than alcohols of comparable masses.
3. Aldehydes and ketones have larger dipole moments than alkyl halides and ethers confirming that a dipolar structure, C+-O- contributes to the structure of aldehydes and ketones.
All these are examples of nucleophilic (nucleus loving) additions i.e. addition of nucleophiles (electron rich species) on electron deficits atoms.
Since the mobile electrons of carbons-oxygen double bond are strongly pulled towards oxygen, carbonyl carbon is electron-deficient and carbonyl
oxygen is electron-rich. The electron deficient (acidic) carbonyl carbon is most susceptible to attack by electron rich nucleophilic reagents, that is, by bases. Hence
Note that in the transition state, oxygen has started acquiring negative charge which it will have bear in the product. Actually,
(i.e. its ability to carry a negative charge)
The polarity of the carbonyl group is not the cause of reactivity; t is simply another manifestation of the electronegativity of oxygen.
4. Nucleophilic addition. Addition of HCN, NaHS Grignard reagent etc.
th typica reaction of aldehyde an ketone is nucleophilic addition
(Not the presence of charge on O which it can easily carried)
it is tendency of oxygen to
acquir electrons whic is responsible
The reactivity of the carbonyl group towards the nucleophilic addition of the reactions depends upon the
and also the site where nucleophile attacks. Thus, substituent or factor in the carbonyl compound that increases the positive charge on the carbonyl carbon (i.e. electronegative group) will increase its reactivity towards addition reactions and vice versa. Hence the introduction of alkyl group or any other electron donating group on the carbonyl carbon decreases its reactivity; thus formaldehyde (having no
group) is more reactive than other aldehydes (having one alkyl group) which in turn are more reactive than ketones (having two alkyl group), i.e.
Similarly, among substituted aldehydes, having –I group,
magnitud of the positive charg on th
carbonyl carbon atom (electrophilic characte of carbonyl carbon) on crowding around the carbonyl carbon atom (steric effect),
Among ketones the reactivity decreases with the increase in + I effect of the alkyl group and also with increase in bulkiness of the alkyl group on carbonyl carbon.
Since in
Nucleophilic additions to aldehydes and ketones are catalyzed by acids (sometimes, by Lewis acids). In presence of acid, carbonyl oxygen gets protonated. This prior protonation increase the electrophilic character of the
carbonyl, carbon and thus lowers the Eact for nucleophilic attack, since it permits oxygen to accept π electrons without having a negative charge.
aromatic aldehyde th positive charge on the carbonyl carbon atom can delocalize over benzene nucleus, these are les reactive tha aliphati aldehydes.
(Undergoes nucleophilic attack more readily)
Aldehydes and ketones react with water in presence of acid or base to form hydrate.
Like the general nucleophilic additions, hydrate formation follows the following order.
Aldehydes react with alcohols in presence of dry HCl gas to acetals, e.g.
Since the reaction is reversible, therefore excess of alcohol is used to shift the equilibrium towards acetals formation. Acetals are readily cleaved by acids and are stable towards base.
r ge -diol formation)
Ketones however, do not react with monohydric alcohols. Of course, ketones and aldehydes react with dihydric alcohols to form cyclic ketals and cyclic acetals respectively.
Acetals (cyclic acetals) and ketals (cyclic ketals) are used protecting the carbonyl groups. Since aldehydes are more reactive then ketones, alcohols react preferentially with aldehydes leaving ketones group free.
HCN is a weak acid thus a poor source of CN-(the nucleophile). However addition of a base that generates CN- from HCN furnishes ample supply of CN-. Thus NaCN in presence of H
2SO4 generally used as a source of CN- .
All aldehydes, but only lower ketones (acetone, butasnone, 3- pentanone and pinacolone) form cyanohydrins. Higher ketones do to form cyanohydrins because of steric interference.
Cyanohydrins are good synthetic reagents as they can be converted into α-hydroxyl acids, α-amino acids and α,β-unsaturated
carboxylic acid.
(Strecker synthetic)
The bisulphite addition compounds decompose on heating with dil. acids or bases, to regenerate the carbonyl compound. Hence, this reaction is used for the purification and separation of carbonyl compounds.
Recall that formaldehyde reacts Grignard reagents (or alkyllithiums) to give primary alcohols, aldehydes other than HCHO give secondary alcohols and ketones give tertiary alcohols
Reaction of aldehydes and ketones with α-bromoesters in the presence of metallic zinc and
ether to give β-hydroxy ester is known as The β-hydroxy
esters are easily dehydrated to unsaturated esters having stable conjugated system.
Higher aliphati ketone and aromatic ketone do not reac with NaHS
(v)
(vi Addition of organozin compounds. (Reformatsky reaction).
An organozinc compound is first formed which then adds on the carbonyl group in a manner analogous to that of a Grignard reagent.
Since organozinc reagents are less reactive than Grignard reagents, they do not react further with the ester group.
An ylide is a neutral molecule having a negative carbon adjacent to a positive hetero atom (e.g. P or S), each atom has an octet of electrons and directly bonded to each other. Aldehydes and ketones react with phosphorus ylides to yield alkenes and triphenylphosphine oxide. The reaction, known as Witting reaction, has proved to be a valuable method for synthesizing alkenes.
Thus, the net result of the reaction is the replacement of carbonyl oxygen, =O, by the group =CRR’. The reaction is carried out under mild conditions and in presence of solvents like tetrahydrofuran (THF) and dimethyl sulfoxide
(DMSO).
Writing reaction has a great advantage over most other alkene syntheses in that
no ambiguity exits as to the location of the double bond in the product. (vii) Addition of ylide (Wittig reaction).
The ylide acting as a nucleophile attacks the carbonyl carbon of the aldehyde or ketones to form an unstable intermediate betaine followed by
oxaphosphetane which then spontaneously loses triphenyl phosphine oxide to form an alkene. The driving force for the Wittig reaction is the formation of very strong P-O bond.
(viii) Cannizaro reaction: Aldehydes which do not have any α-hydrogen atom,
when treated with a concentrated solution of NaOH or KOH, undergoes a simultaneous oxidation and reduction (disproportionation) forming a salt of
carboxylic acid and alcohol e.g.
Since acetaldehyde (CH3CHO) has hydrogen atoms, it does not undergo Cannizzaro reaction; while trichloroacetaldehyde, CCl3CHO having no
α-hydrogen atom, undergoes Cannizzaro reaction.
Mechanism.
[2 methypropnal, (CH3)2CH. CHO]
α Cannizzaro reaction. This exceptional
behavior is probly due to + I effect to the two alkyl groups.
The reaction is believed to follow the following three steps:
First step: The first step is the reversible addition of hydroxide ion (nucleophile) to carbonyl group to form ion I.
Second step: The hydroxyalkoxide ion I now transfers its hydride ion directly to another aldehyde molecule, the latter is thus reduced to alkoxide ion and the former (ion I) oxidized to an acid.
Third step: The acid and alkoxide ion so obtained exchange their protons to give the more stable pair: acid anion and alcohol.
Remember that isobutyraldehyde although
contains -hydroge atom, i undergoes
Note that one molecule of the aldehyde as a hydride donor and the other acts as a hydride acceptor. In other words, Cannizzaro reaction is an example of self- oxidation and reduction.
When the reaction is carried out in D2O instead of H2O, no C –D bond is
formed indicating that the hydrogen comes forms the aldehyde and not form the solvent.
Cannizzaro reaction, between two different aldehydes each having α-hydrogen atoms.
When one of the aldehydes is formaldehyde, it always undergoes oxidation
(rather than other aldehyde) since formaldehyde is more nucleophilic than other aldehydes.
Here half-part of the molecule is oxidized and other half part is reduced.
Crossed Cannizzaro reaction:
Aldehydes having α-hydrogen atom can also be made to undergo Cannizzaro type of reaction, if reaction is carried out in presence of aluminum ethoxide. But in such case, acid and alcohol react together to form ester as the final product. The reaction is now known is
The reaction takes place in presence of alkoxide and forms alkynol ( and alkynediol with CH≡CH); this reaction is
known as
Tischenko reaction
(ix) Addition of termina alkynes.
Ammonia and some ammonia derivatives like hydroxylamine (NH2 hydrazine (H2N – NH2), phenylhydrazine (H2N – NHC6H5) and semicarbazide (H2N –NHCONH2) react with aldehydes and
ketones in weakly acidic medium.
These derivatives are crystalline solids and used for the identification of carbonyl compounds
5. Replacement of carbonyl oxygen. OH),
The oximes can be hydrolysed back to the parent aldehydes and ketones on treatment with acids, further, oximes have sharp and specific melting points so oxime formation is used for the separation and identification of aldehydes and ketones.
Oximes from all aldehydes and mixed ketones (not simple ketones) can exist in two geometrical isomeric forms For example,
Ketoximes when treated acid catalysts like conc. H2SO4, PCl5, H3PO4, SOCl2 or C6H5SO2Cl, undergo rearrangement to form substituted amides. This reaction is known as
In case, ketoxime has two different alkyl or aryl groups’ difference amides are formed from different isomeric oximes. For example.
It is the anti-alkyl group that migrates.
Cyclohexanone oxime when treated with such reagent undergoes Beckmann rearrangement to form caprolactam, a reagent used for synthesizing nylon.
(ii) .
(iii)
Simple hydrazones have low melting points hence occasionally used to identify carbonyl compounds (difference from 2, 4-dinitrophenylhydrazones). However, they form the basis for the Wolf-Kishner reduction.
Reaction with hydrazine, H N.NH (hydrazone formation)
(iv) In addition to ammonia derivatives, thioalochols and PCl5 also reacts, in which carbonyl oxygen is replaced by two atoms or groups.
Marcaptals of ketones especially actone, are used for preparing sulphonals, used sedatives.
α In addition to nucleophilic
addition reactions carbonyl compound exhibit the unusual acidity of α-hydrogen atoms. Actually, in the nucleophilic additions carbonyl group acts as a functional group, while in the acidity of α-hydrogen atoms, it acts as a substituent and exerts on the adjacent (alpha) carbon atoms.
(vi) Reactio with phosphorus pentachloride
The unusual acidity of the α-hydrogen is due strong electron-withdrawing nature of carbonyl group which in turn makes αcarbon also electron
-withdrawing. Hence, in presence of base, it easily loses hydrogen as proton and itself converted into which is stabilized by resonance.
Note that the two important properties of a carbonyl group viz., susceptibility to nucleophilic attack and acidity of α-hydrogen is due to the to
accommodate the negative charge.
Important reactions of carbonyl compounds due to acidic hydrogen, i.e. due to enols and enolate are discussed here under.
Aldehydes and ketones containing at least one α -hydrogen atom (i.e., a -hydrogen atom attached to the α-carbon atom with
respect to the functional group aldehyde or ketone) when treated with dilute base like adding on the aldehydic group. Aldol condensation may take place
carbanion
ability of oxygen
between (a) same or different aldehydes (b) an aldehyde and a ketone and (c) same or different ketones.
Aldol when heated loses a molecule of water to from unsaturated compound.
Since formaldehyde, trichloroactcetalehyde (Cl3C.CHO) and benzaldehyde (C6H5CHO) do not have any α-hydrogen atom, they do not undergo Aldol condensation in presence of dilute base.
Although here, all possible products are obtained, yet by using different catalysts, one product may be
made to predominate. In presence of base, α-hydrogen atom of lower aldehyde
is more acidic and so migrates, while in presence of an acid, α-hydrogen atom
the higher aldehyde is more acidic.
(a) Condensation between tw same aldehyde molecules
It is the α-hydrogen atom of the ketone which is involved in Aldol condensation.
Although formaldehyde does not have any α-hydrogen atom, it undergoes Aldol condensation on treatment with a
Aldol condensation of acetone molecules produce different product under different condition.
(i) Two molecules of acetone condense together in the presence of barium hydroxide to form diacetone alcohol.
(c) Condensation between aldehyde and ketone.
(d) Condensation between formaldehyde molecules.
strong base.
Diaetone alcohol, on heating loss a molecule of water of form mesitly oxide.
(ii) In the presence of dry hydrogen chloride gas, actone molecules condense to form a mixture of mesityl oxide and phorone.
Note that here the understand compound is isolated and not the Aldol or Ketol. (iii) Acetone forms mesitylene (1,3,5-trimethlbenzene) on distillation with concentrated sulphuric acid
Again here, it is the unsatured compound that is isolated and not the Aldol or Ketol.
Let us take the example of the condensation of two acetaldehyde molecules. The reaction takes place in the three steps as follows:
First step: The baser (OH- ion) removes a hydrogen ion from α-carbon atom of one of the aldehyde molecule to form resonance stabilized carbanion, I.
Second step: The carbanion I (enolate ), being a nucleophile, adds to the carbonyl carbon atom of the second molecule of acetaldehyde to form the anion of Aldol.
Third step: The Aldol anion now takes a proton form the solvent (water) forming Aldol.
Note that the catalyst (OH-) is regenerated in the step. Mechanism of base catalysed aldol condensation.
of the aldehyde molecules undergoes enolisation which then attacks the protonated carbonyl group another aldehyde molecule.
Β-Hydroxyaldehyde or ketone so, formed undergoes dehydration easily forming
a double bond at β-carbon atom leading to α,β-unsaturated aldehyde or ketone
which is quite stable due to conjugation.
If the double bond is in conjugation with the aromatic ring, the product becomes so stable that unsaturated aldehyde or ketone is isolated as the final product instead of β-hydroxycarbonly compound. For example:
A dialdehyde, a ketoaldehyde or a diketone undergoes Aldol condensation to form 5-or- 6 membered cyclic compounds.
In the above ketoladehyde, although three different enolates are possible, it is the enolate from the ketone side of the molecule that add to the aldehyde. This is because of greater reactivity of aldehydes towards nucleophilic addition than the ketone due to electronic as well as steric factors.
Claisen –Schmidt reaction Perkin reaction, knoevenagel reaction (all discussed futher in aromatic aldehydes), halogenations and haloform reaction.
α Aldehydes and ketone having α-hydrogen atom, treated
with Cl2 or Br2 in solvents like water, chloroform, acetic acid or ether lead to α
-mono di-or tri-halogenated product.
Intramolecular aldol condensation (Cyclization via aldol condensation).
Other reactions releated to aldol condensation.
The excess of alkali decomposes the trihalogen compound to give haloform.
Acetaldehyde and methyl ketones (CH3.CO.R) react rapidly with halogens (Cl2, Br2 or I2) in the presence of alkali to form haloform. This reaction is usually known as since haloform (CHX3) is the main product. It involves the formation of carbanion.
Diethyl ketone (C2H5CO.C2H5) has no –COCH3 group, hence it does not undergo haloform reaction. Holoform reaction is used as a diagnostic test for detecting the presence of –COCH3 group in a compound.
Thus the haloform reaction may also be used for distinguishing the methyl ketones from other ketones since the former forms haloform while the latter does not form any haloform, e.g.
It is important to note that ethyl alcohol (CH3CH2OH) and secondary alcohols having one of the alkyl groups as methyl, although does not contain a carbonyl group, also respond haloform reaction. It is due to the fact that such alcohols are first oxidised by halogen to acetaldehyde (CH3CHO) or methyl ketone respectively, which in turn gives the haloform reaction because of the presence of –CO. CH3 grouping.
Thus in short haloform reaction is given by all compounds containing either of the following groupings.
Remember that hypohalite does not attack carbon-carbon bond present in the molecule. For example,
The reaction consists mainly of two important : (A) halogenations of –COCH3 grouping to form –COCX3 and (B) elimination of CX3 part a :C-X
3 anion.
(A) Halogenations of –COCH3 grouping. This part of the reaction involves following steps:
(a) The base :B- takes up the hydrogen atom form the carbonyl compound (recall that hydrogen atoms are acidic in nature.)
(b) Electrophilic attack by the halogen at the negatively charged carbon of carbanion.
(c) Repetition of the above two steps till all the three hydrogen atoms –COCH3 are replaced by halogen atoms. Note that the removal of hydrogen from -COCH2 X is easier than from –COCH3 because the presence of halogen atom increase the acidity of the hydrogen atoms. Similarly, removal of hydrogen from –COCHX2 is easier that from –COCH2X.
(B) Elimination of –CX3 part. This part the of reaction again involved the following three steps:
(a) Nucleophilic attack of –OH on trihaloacetone.
(b) Loss of :C-X
3 to form haloform anion and acetic acid.
(d) Proton exchange to form a more stable pair of acetate ion (CH3COO-) and haloform (HCX3).
Aldehydes are easily oxidized to the corresponding acid and thus act as strong reducing agents.
Aldehyde can also be oxidised by much milder oxidising agents like
(ammonical silver nitrate), [blue colored alkaline
9. Oxidation.
Tollen’s
solution of cupric ion (Fehling solution 1) complexed with sodium potassium tartrate (Fehling soluation 2)] and (alkaline solution of cupric ion complexed with citrate ions). Thus these regents are reduced by aldehydes.
On the other hand, ketones are not oxidized by milder oxidizing agents and thus they do not reduce Tollen’s reagent Fehling and Benedict solution ( However, stronger oxidizing agents like acid dichormate alk. KMnO4 and hot conc. HNO3 oxidise ketones to carboxyclic acids.
Benedict solution
Benzadehyde (aromatic aldehydes) although reduces Tollen’s regent, it does not reduce Fehling and Benedict solutions.
In case ketones is unsymmetrical, cleavage takes place in such a way that carbonyl group is retained by smaller alkyl group (Popoff’s rule). For example.
Aldehydes and ketones with a methyl or methylene group adjacent to the carbonyl group are oxidized by SeO2 to give dicarbonyl compounds. For example,
Hypohalites (-OX where X=Cl, Br or I) oxidize CH
3CHO and methyl ketone to acid salt along with formation of haloform (haloform reaction).
Ketones are also oxidized by Caro’s acid (H2SO5) or perbenzoic acid (C6H5CO3H) or peracetic acid (CH3CO3H) to form esters.
The reaction is called In case of aliphatic ketones oxygen is inserted between carbonyl carbon and the alkyl group. However, in case of aromatic ketones both products are formed.
Aldehydes and ketones are reduced to the primary and secondary alcohols respectively by catalytic hydrogenation (H 2 in presence of Ni or Pt), nascent hydrogen (sodium amalgam and acid or sodium and alcohol) lithium aluminium hydride (LiAlH4) sodium borohydride or aluminium isopropoxide (Me2CHO)3Al in iso-propanol. Reducation by means of aluminum
Baeyer Villiger oxidation
isopropoxide is known as MPV reduction does not reduce –NO2, -CH=CH2, -C≡C-, etc.
Both these regents reduce aldehydes and ketones to 10 and 20 alcohols respectively. Neither of the two reagents reduce the C=C bond.. However the two regents differ in the following respect:
(i) LiAlH4 also reduces ester and acid chloride to alcohols, while NaBH4 does not affect these groups.
(ii) The hydride ion in LiAlH4 is very basic and thus it reacts violently with water, hence it is used in dry solvents like dry ether and THF. Moreover, the product exists as alkoxide ion, so it converted into alcohol by using aqueous HCl or aq. NH4Cl solution.
Meerwein-Ponndorf-Verley (MPV) reeducation;
reduces the carbonyl group as well as C=C bond, but not esters.
It is important to that the reveres of MPV reduction (i.e. oxidation of secondary alcohols to ketnoes) in presence of aluminium ter-butoxide is known as
Aldehydes and ketones are reduced to the corresponding alkanes by means of amalgamated zinc and hydrochloric acid or alkaline hydrazine solution (b) Catalytic hydrogenation (c) MPV reduction. Oppenauer oxidation. (d) (Clemmensen reduction) (Wolf-Kishner reduction or Hung Milnon reaction).
The same conversion can be made by heating aldehydes and ketones with red phosphorus and hydroiodic acid
Two molecules of ketones undergo reduction in prances of Mg/Hg to form which is converted into
when treated with mineral acids.
Conversion of pinacol to pinacolone is known as
(e)
(f) Bimolecular reduction or Pinacol reduction.
pinacolone
pinacol-pinacolone rearrangement.
Lower aldehydes undergo polymersation to form different products under different conditions.
Aldehydes restore the pink colors of Schiff’s reagent (Schiff’s reagent is a dilute solution of rosaniline hydrochloride in water whose red colour has been discharged by passing sulphur dioxide).
Benzaldehyde reacts with ammonia to form hydrobenzamide. Aldehydes other than HCHO give aldehyde ammonia, while HCHO forms urotropine.
Mechanism:
11. Polymerisation
12. Shift’s test.
Ketone do not restore Schiff’s reagent colour.
Special reactions of Aromatic Aldehydes and Ketones (i) Reaction with ammonia.
Benaldehyde reacts with primary aliphatic or aromatic amines to form
In a crossed Cannizzaro reaction, if one of the aldehydes is formaldehyde, it is always oxidized (and not reduced) to formic acid.
Benzaldehyde when heated with aqueous ethanolic NaCN (or KCN) undergoes self-condensation to form
(ii) Reaction with amines. Shift’s base.
(iii) Crossed Cannizzaro reaction.
(iv Benzoin condensation
Condensation between an aldehyde or ketones with compounds containing active methylene group in the presence of ammonia (its derivative (amines, pyridine, piperidine etc.) to form unsaturated compound is known as
Condensation of an aromatic aldehyde with acid anhydride in presence of sodium salt of the acid from which anhydride is derived to form,
α, β-unsaturated acid in known as
Note that in the second example it is α-carbon atom of the propionic anhydride that reacts with the aldehydic group.
This is an example of crossed Aldol condensation in which aromatic aldehydes or ketones (v) Knoevenagal reaction.
kenoevengel reaction.
(vi) Perkin reaction
Perkin reaction.
with or without α-hydrogen atom react with aldehydes, ketones or ester having
α-hydrogen atoms in the presence of dilute alkali to form α, β-unsaturated
carbonyl compounds.
The reaction may also take place between two ester molecules, at least one of which has α-hydrogen atom
is a relatively harmless but powerful lachrymator or tear gas and is used by police to disperse mobs.
Like aliphatic aldehydes and ketones, aromatic aldehydes and ketones react with PCl5 to give dichloro derivative. For example,
Aromatic aldehydes and ketones undergo electrophilic substitution reactions, like nitration, sulphonation and halogenations, in the m-position . However, these reactions are slow because of the deactivating influence of the carbonyl group on the benzene ring, moreover, certain side reaction like oxidation, etc. make the yield poor.
It can be prepared by general methods. It can be manufactured by (i) the controlled oxidation of methane or natural gas,
Phenacyl chloride
(ix) Reaction with phosphorus pentchloride.
(x) Reactions of benzene nucleus.
Individual members of Aldehydes and Ketones 1. Formaldehyde, Methanal HCHO.
(ii) passing water gas (CO+H2) at low pressure through an electric discharge, and (iii) oxidation of methanol with air over a heated catalyst (Cu or Ag).
It is a colorless, pungent smelling gas extremely soluble in water.
Since it is a solution under the name of
or in the form of solid polymers, (polymer) and (trimer) which on heating liberate HCHO. Chemically, it gives most of the chemical properties of aldehydes discussed earlier like reaction with HCN, NaHSO3, Grignard reagent, NH2OH, H2N.NH2 , oxidation, reduction, Cannizzaro reaction and polymerization. On account of presence of hydrogen in place of alkyl group, formaldehyde is more reactive than other aldehydes and reacts in different manner with some regents.
Hexamethylene tetramine is used as a urnary antiseptic under the trade name of urotropine.
2. Condensation with phenol: formation of bakelite
Properties.
gas, it is marketed as 40% aqueous
formalin paraformaldehyde
metaformaldehyde
3. Polymerisation: When an aqueous solution of formaldehyde is evaporated to dryness, paraformaldehyde is formed. When gaseous formaldehyde is allowed to stand at room temperature, metaformaldehyde (trioxame) is produced.
Both of them are white solids and regenerate HCHO on heating.
As mentioned above, formaldehyde is used as its 40% aqueous solution under the name of formaline. It is used as a preservative for biological and anatomical specimens, it is used in the preparation of urotropine, in the preparation of bakelite, a synthetic plastic, in silvering of mirror.
It can be manufacture in the following ways :
1. By hydration of acetylene.
2. By the catalytic dehydrogenation of ethanol in presence of heated copper (3000C).
Uses:
3. From ethylence
Acetaldehyde is a colourless liquid with strong pungent and irritating odour. In water it is hydrated to the extent of 58% forming ethylidene hydroxide. The aqueous solution has an agreeable smell.
Chemically, it gives most of the properties of aldehydes. It does not undergo Cannizzaro reaction.
Polymerisation:
Both these polymers give acetaldehyde when distilled with dil. H 2SO4.
Acetaldehyde is used
(i) as an antiseptic inhalant in nose troubles.,
(ii) in the preparations of chemicals like acetic acid, ethyl alcohol, etc.
(Wacker process).
Properties:
(iii) in the preparation of acetaldehyde ammonia, a rubber accelerator. (iv) in the preparation of paraldehyde, a hypnotic and sporofic.
(v) in the preparation of metaldehyde, used as solid fuel in sprit lamp, and (vi) in the preparation of days and drugs.
It is manufactured (i) by the oxidation of isopropyl alcohol with oxygen at 5000C, (ii) by the catalytic dehydrogenation of isopropyl alcohol, and (iii) By Wacker process (from propene).
Acetone (Ketone, in general) condenses with chloroform or bormoform in presence of alkali to form addition product.
Acetone condenses with ammonia to form diacetone amine.
Acetone is reduced by magnesium-amalgam and water to give pinacol
1. It is used for storing acetylene. 2. It is very frequently used a solvent.
3. It is used in the preparation of chloroform, iodoform (antiseptic), chloratone (hypnotic and sedative), etc.
(bimolecular reduction).
(Oil of bitter almonds), C6H5CHO. It occurs as glucoside in bitter almonds and hence it is commonly named as oil of bitter almonds. Amygdalin on hydrolysis with dilute acids or the enzyme emulsion gives benzaldehyde, glucose and hydrogen cyanide.
Commercially, it is prepared from toluene in the following way.
C6H5COCH3. Commercially it is prepared by the oxidation of ethylbenzene with air in the presence of V2O5 or oxides of Mn, Zn, etc. at about 5000C.
It is used in perfumery and in medicine as hypnotic (sleep producing drug) under the name of . Two molecules of acetophenone condense in presence of aluminum ter-butoxide to form
4. Benzaldehyde (amygdalin)
5. Acetophenone, Acetylbenzene, Methylphenyl keton
hypnone
On oxidation with perbenzoic acid, it forms phenyl acetate
Quiones are unsaturated cyclic diketones. Two quinones of benzene are possible (m-benzoquione is not possible as it is not possible to construct such formula by maintain tetravelency of carbon).
Note that quinones are non-aromatic conjugated cyclic diketones , Since they are highly conjugated they
Benzoquinone, being the most important is commonly known as quinone. It is prepared by the oxidation of hydroquione or aniline.
(Baeyer-villiger oxidation).
6. Quinones.
α β UNSATURATEDCARBONYL COMPOUNDS
As the name represent these compounds contain unsaturation between and carbon atoms with respect to carbonyl group i.e. –C=C-C=O-. Such molecules are quite stable due to the presence conjugated system of double bond. Such molecules give properties of the double bond carbonyl group and some additional properties due to the interaction of the two groups. Due to electron withdrawing nature of the >C=O group, the reactivity of C=C towards electrophilic reagents decreases as compared to an isolated double bond, On the other hand, C=C group undergo nucleophilic addition reactions which are uncommon for simple alkenes.
