CHEMISTRY FORM 6 SEM 3 Chapter 6.pdf

6.1

Aldehyde and Ketone – Both aldehyde and ketone are carbonyl

compound (organic with C=O in it). Both has the same molecular

formula as C

H

O

6.1.1

Nomenclature of aldehyde

functioning group of aldehyde are

and end with –al

butanal 3-methylpentanal 4-chloro-3-methylbutanal

6.1.2

Ketone : functioning group of ketone are

and end with –

one

propanone butanone Pentan-2-one

hexan-3-one 2,4-dimethyl

Explanation : Butane is a non-polar molecule, where molecules were

held by temporary dipole-induced dipole. Propanal and propanone are

polar molecule, which has a permanent dipole-permanent dipole

attraction forces. However, the dipole moment of ketone is greater

than aldehyde, so ketone usually have higher b.p than aldehyde.

Propan-1-ol has high boiling point due to strong hydrogen bond

between them, however, ethanoic acid has greater hydrogen bond than

propan-1-ol, since they form dimer between them

Molecule Butane (C₄H₁₀) Propanal (C₂H₅COH) Propanone; CH₃COCH₃ Propan-1-ol C₃H₇OH Ethanoic acid CH₃COOH RMM 58 58 58 60 60 Boiling point (o_C) 1 48 56 97 118

δ+ δ-

δ+

δ-

Solubility : Lower aliphatic

aldehydes such as methanal &

ethanal

are soluble in water because they are able to form

hydrogen bond with water (as shown in diagram). Higher

member of aliphatic carbonyl compounds are insoluble as there

are presence of hydrophobic alkyl group

δ-

Isomerism of aldehyde and ketones : Given the molecular formula of

organic compound are C

H

O, out line all possible isomers of the

organic compound

6.2

Chemical preparation of aldehyde and ketone

6.2.1

Aldehyde

1. Oxidation of primary alcohol

Controlled oxidation of alcohol by using acidified potassium

dichromate (VI)

Prevention : Having excess alcohol over oxidant & distilled off

aldehyde.

+ H

O

propan-1-ol

propanal

2. Using catalytic oxidation (Cu at 400

_{C) on a 1}

_alcohol

propan-1-ol

propanal

This process is also known as dehydrogenation as hydrogen is

produced.

6.2.2

Ketone

1.

Oxidation of secondary alcohol

Oxidation of alcohol by using acidified potassium dichromate (VI)

Unlike oxidation on 1

alcohol, it does not need prevention as ketone

formed will not further oxidise to other substances.

+ H

O

Propan-2-ol

propanone

2.

Using catalytic oxidation (Cu at 400

_{C) on a 2}

_alcohol

propan-2-ol

propanone

This process is also known as dehydrogenation as hydrogen is

produced.

6.3

Chemical reaction of aldehyde and ketone

6.3.1

Reduction of aldehyde and ketone.

Reagent : LiAlH

(lithium aluminium hydride) in dry ether

propanal

propan-1-ol

From reaction above, we can tell that 1

alcohol is formed back using

reduction. Hence, we can summarised the reaction as

6.3.2

Oxidation of aldehyde

Aldehyde can be further oxidised to form carboxylic acid.

Reagent : KMnO

/ H

(acidified potassium manganate (VII)

Propanal

propanoic acid

6.3.3 Addition reaction of aldehyde and ketone

Reagent : HCN (hydrogen cyanide) + little KCN

The nitrile compound formed is then further hydrolysed under acidic

condition, to form 2-hydroxybutanoic acid according to the equation

The nitrile compound formed can also be reduced to form amine, where

Reagent : R-MgBr (Grignard reagent)

When aldehyde react with Grignard reagent (Chap 4.4.2), a secondary

alcohol is formed

KETONE

Reagent : HCN (hydrogen cyanide) + little KCN

The nitrile compound formed is then further hydrolysed under acidic

condition, to form butanoic acid according to the equation

Reduction

Reagent : R-MgBr (Grignard reagent)

When ketone react with Grignard reagent (Chap 4.4.2), a tertiary alcohol is

6.3.4

Condensation reaction of aldehyde / ketone

For aldehyde :

Reagent : (NO

)

C

H

NHNH

(2,4-dinitrophenylhydrazine)

An orange precipitate surfaced when 2,4-dinitrophenylhydrazine is

added to aldehyde or ketone

For ketone

Reagent : (NO

)

C

H

NHNH

(2,4-dinitrophenylhydrazine)

Similar to aldehyde, an orange precipitate will surface after the

reaction

Test Aldehyde Ketone

Triiodof orm

• Reagent : (I₂ + NaOH) Iodine in sodium hydroxide Only work for ethanal _{as it has methyl-carbonyl} group

• _{Observation : a yellow crystal of} triiodomethane is observed

Equation :

CH₃CH=O + 3 I_{2 + NaOH } CHI₃ + HCOO–_Na+ _{+ 3 HI}

• Reagent : (I₂ + NaOH) Iodine in sodium hydroxide Only work for those which _{has methyl-carbonyl} group

• _{Observation : a yellow crystal of} triiodomethane is observed

Equation : CH₃COCH₃ + 3 I₂ + NaOH  CHI3 + CH3COO–Na+ + 3 HIHI

Test Aldehyde Ketone

Fehling’s solution

• _{Reagent : Fehling solution [solution of complex copper} (II) ion]

• _{Positive Test : only works for aldehyde}

• _{Observation : blue solution turns to red precipitate of} Cu₂O Equation : red ppt. No reaction occur for ketone Tollen’s reagent

• _{Reagent : Tollen solution [solution of complex Ag(NH}

3)2]+] • _{Positive test : only work for aldehyde}

• _{Observation : colourless solution turn grey solid (silver} mirror) Equation : silver mirror No reaction occur for ketone

propanone  2-bromopropane

propanal  2-hydroxybutanoic acid

2-butenal  butanoic acid

2. Outline a chemical test to distinguish between a) propanal and propanone

b) ethanal and propanal

c) pentan-2-one and pentan-3-one

Reagent : Fehling / Tollen reagent (for aldehyde) ; Iodine in NaOH (for ketone)

Observation : Red brick precipitate formed when added propanal while no changes for propanone

Equation : CH₃CH₂CHO + 2 Cu2+ _{+ 5 OH-  CH}

3CH2COO- + Cu2O + 3 H2O

Reagent : Iodine in NaOH

Observation : Yellow precipitate formed when added to ethanal but no changes for propanal

Equation : CH₃CHO + 3 I_{2 + OH-  HCOO}- _{+ CHI}

3 + 3 HI

Reagent : Iodine in NaOH

Observation : Yellow precipitate formed when added to pentan-2-one but no changes for pentan-3-one

Equation : CH₃CH₂CH₂COCH₃ + 3 I_{2 + OH- }

CH₃CH₂CH₂COO- _{+ CHI}

6.5

Natural Compound with Carbonyl Group – Carbohydrates

6.5.1

Simplest form of carbohydrates that cannot be hydrolysed to simple sugar
Examples : glucose and fructose

Glucose Fructose

Also known as aldose (functioning group of CO–H)

Also known as ketose (functioning group CO–CH₃)

Open ring close ring open ring close ring

Adding glucose to Fehling solution will turn the blue solution into a red precipitate

(positive test : aldehyde)

Adding fructose to Fehling will show no changes to Fehling solution

6.5.2 Disaccharide

Disaccharides are 2 monosaccharide joined together by glycosidic link.
The process of joining 2 monosaccharides are condensation process as water

molecule is given off as side product. Molecular formula of disaccharide is C₁₂H₂₂O₁₁.Example

3 most common disaccharides

=> sucrose (sugar cane) => maltose (barley) => lactose (milk)
Disaccharide can be break-up and formed back 2 monosaccharides by

hydrolysis of water

6.5.3

Polysaccharides

Polysaccharide ~ polymer containing long chains of

monosaccharide units. Example : starch and cellulose. They have

the empirical formula C

H

O

.

All saccharides are bond using glycosidic ring and they can be

hydrolysed by heating with diluted acid where

(C

H

O

)

+ n H

O

n C

H

O

.

Cellulose are mainly found in cell wall of plants. Cotton is almost

pure cellulose. It can be use to manufacture synthetic fiber

known as rayon.





→



A , B , C C , E C , D , E C C , D E

Element Carbon, C Hydrogen, H Oxygen, O

Mass 66.7 11.1 22.2

Mol = mass / mol 5.51 mol 11.1 mol 1.39 mol Ratio 5.51 / 1.39 = 4 11.1 / 1.39 = 8 1.39 / 1.39 = 1

Orange precipitate is observed

Carbonyl compound

LiAlH₄ in dry ether

Alcohol / hydroxyl group

Cl₂ under UV

Iodine in NaOH

C gives yellow precipitate / crystal while F will not C₆H₅COCH₃ + I_{2 + OH-  C6}H₅COO- + CHI₃ + HI

mass of CN needed = 0.03 x 60 = 1.8 g [1]

H < D < G

 chlorine on the aryl ring is very inert /strong C-Cl bond / overlap between unhybridise Cl with C in benzene ring1]

 chlorine on C=O is reactive because of highly δ+ carbon atom bonded to electronegative O and Cl/ due to inductive effect [1]

CH₃CHO + HCN  →KCN CH₃CH(OH)CN

CH₃CH(OH)CN + 2 H₂O + H+ _{ CH} 3CH(OH)COOH + NH4+ hydrolysis CH₃CHO → CH₃CH(OH)COOH 44 90 or 4.40 g → 9.00 g % yield = 5.40 ×100 / 9.00 = 60%

E will give yellow precipitate when react with alkaline iodine solution Product formed are CH₃CH₂CH₂COO– _{+ CHI}

3

D reduced to form pentan-3-ol [1] while E is reduced to pentan-2-ol, which is optical active / chiral carbon atom