CHEM1102 Lecture Notes 11

Chemistry 1103

Charlie Bond

MCS Rm 4.16/4.27

[email protected]

What is Organic Chemistry? Organic Reactions I II Alkanes (Ch 21) Conformational Analysis (Ch 21) Stereochemistry I II III (Ch 22) S Alkyl Halides I II (Ch 24)II

Alcohols - Nomenclature

• IUPAC names

– the parent chain is the longest chain that contains the -OH group

– number the parent chain in the direction that gives the -OH group the lower number

– change the suffix -ee to -olol

• Common names

– name the alkyl group bonded to oxygen followed by the word alcoholalcohol

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Alcohols - Nomenclature

Ethanol

(Ethyl alcohol) (Propyl alcohol)1­Propanol (Isopropyl alcohol)2­Propanol

1­Butanol (Butyl alcohol) OH OH OH OH 2­Butanol

(sec­Butyl alcohol) 2­Methyl­1­propanol(Isobutyl alcohol)

2­Methyl­2­propanol (tert­Butyl alcohol) OH Cyclohexanol (Cyclohexyl alcohol) OH OH OH

Alcohols - Nomenclature

• Compounds containing

– two -OH groups are named as diols,

– three -OH groups are named as triols, etc.

CH₃CHCH₂ HO OH CH₂CH₂ OH OH CH₂CHCH₂ OH HO HO 1,2­Ethanediol

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Polyols and sugars

Larger alcohol-containing molecules are of massive importance in biochemistry as they are used by cells as fuel, starting materials for making essential molecules (e.g. ribose in DNA), and to store information.

Ignose and Godnose

Albert Szent-Gyorgyi isolated ascorbic acid and wanted to call the compound Ignose in his publication (as it resembles the sugars glucose and fructose). The journal editor refused and so he tried to call it Godnose instead.

Eventually they settled on hexuronic acid, until Charles King discovered that it was the same compound as vitamin C

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Physical Properties

• Alcohols are polar compounds

– both the C-O and O-H bonds are polar covalent δ -δ+ δ+ O H H H C H

Hydrogen Bonding

– Association of ethanol molecules in the liquid state (only two of the three possible hydrogen bonds to the upper oxygen are shown here).

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Boiling Points

– alcohols have higher boiling points and are more soluble in water than hydrocarbons

CH₃CH₂CH₂OH CH₃CH₂CH₂CH₃ CH₃OH CH₃CH₃ CH₃CH₂OH CH₃CH₂CH₃ CH₃CH₂CH₂CH₂CH₂OH HOCH₂CH₂CH₂CH₂OH CH₃CH₂CH₂CH₂CH₂CH₃ Structural Formula Name Molecular Weight (g/mol) Boiling Point (°C) Solubilityin Water methanol 32 65 infinite ethane 30 ­89 insoluble ethanol 46 78 infinite propane _{44 ­42 insoluble} 1­propanol 60 97 infinite butane 58 0 insoluble 1­pentanol _{88 138} 2.3 g/100 g 1,4­butanediol ₉₀ 230 infinite hexane 86 69 insoluble

Acidity of Alcohols

• Most alcohols are about the same or

slightly weaker acids than water

– aqueous solutions of alcohols have the same pH as that of pure water

CH₃O H _{O H} H [CH₃O- ][H₃O+] [CH₃OH] CH₃O H O H H + K_a = + + = 3.2 x 10­16 pK_a = 15.5

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Acidity of Alcohols

– pK_a values for several low-molecular-weight alcohols

(CH₃)₃COH (CH₃)₂CHOH CH₃CH₂OH H₂O CH₃OH CH₃COOH HCl Compound _pKa ­7 15.5 15.7 15.9 17 18 4.8 hydrogen chloride acetic acid methanol water ethanol 2­propanol 2­methyl­2­propanol Structural  Formula Stronger      acid Weaker     acid *Also given for comparison are pK_a values for  water,  acetic acid, and hydrogen chloride.

Acidity of Phenols

• Phenols are significantly more acidic than

alcohols

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Acidity of Phenols

– the greater acidity of phenols compared with alcohols is the result of the greater stability of the phenoxide ion relative to an alkoxide ion

These three Kekulé  structures are equivalent  These three contributing structures delocalize the negative charge onto  the carbons of the ring O: :O: O: O: O: : : : : : : : : :

Rule of Thumb for OH pKa

• Compound

pKa

• Alkylsulfonates

0

• Carboxylic acids

5

• Phenols

10

• Alcohols

15

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Basicity of Alcohols

• In the presence of strong acids, the oxygen atom of an alcohol behaves as a weak base

– proton transfer from the strong acid forms an oxonium ion

– thus, alcohols can function as both weak acids and weak bases CH₃CH₂-O-H H O H H O H H H2SO4 CH₃CH₂-O H H CH₃CH₂-O H H _HH O H Ethyloxonium ion (pK_a ­2.4) •• Hydronium ion (pK_a ­1.7) Ethanol + + ++ ₊ •• + + +

Reaction with Active Metals

• Alcohols react with Li, Na, K, and other active metals to liberate hydrogen gas and form metal alkoxides

– Na is oxidized to Na+ and H+ is reduced to H

2

– alkoxides are somewhat stronger bases that OH

-– alkoxides can be used as nucleophiles in nucleophilic substitution reactions

– they can also be used as bases in _β -elimination

reactions

2CH₃CH₂OH + 2Na 2CH₃CH₂O-Na+ + H₂

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Conversion of ROH to RX

• Conversion of an alcohol to an alkyl halide

involves substitution of halogen for -OH at

a saturated carbon

– the most common reagents for this purpose are the halogen acids, HX, and thionyl

chloride, SOCl₂

Conversion of ROH to RX

with HCl, HBr, and HI

– low-molecular-weight 1° and 2° alcohols are unreactive under these conditions

CH₃COH CH₃ CH₃ HCl CH₃CCl CH₃ CH₃ H₂O 2­Chloro­2­ methylpropane 2­Methyl­2­ propanol 25°C + +

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Conversion of ROH to RX

– water-insoluble 3° alcohols react by bubbling gaseous HCl through a solution of the alcohol dissolved in

diethyl ether or THF

– 1° and 2° alcohols require concentrated HBr and HI to form alkyl bromides and iodides

+ HCl + H₂O OH CH₃ 0°C ether Cl CH₃ 1­Chloro­1­methyl cyclohexane 1­Methyl­ cyclohexanol OH + HBr + H2O 1­Butanol 1­Bromobutane (Butyl bromide) Br

Reaction of a 3° ROH with HX

• 3° Alcohols react with HX by an S_N1 mechanism

– Step 1:_{Step 1:} a rapid, reversible acid-base reaction transfers a proton to the OH group

– this proton-transfer converts the leaving group from

OH-, a poor leaving group, to H

2O, a better leaving group CH₃-C CH₃ CH₃ O H O H H H CH₃-C CH3 CH₃ O H H O H H + 2­Methyl­2­propanol (tert­Butyl alcohol) An oxonium ion + rapid and reversible •• + + • • •• •• ••

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Reaction of a 3° ROH with HX

– Step 2:Step 2: loss of H₂O from the oxonium ion gives a 3° carbocation intermediate

– Step 3:_{Step 3:} reaction with halide ion completes the reaction CH₃-C CH₃ CH₃ O H H CH₃-C+ CH₃ CH3 O H H slow, rate determining An oxonium ion + + A 3° carbocation intermediate S_N1 CH₃-C+ CH3 CH₃ Cl _{CH3-C Cl} CH3 CH₃ fast + 2­Chloro­2­methylpropane (tert­Butyl chloride)

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Reaction of a 1° ROH with HX

• 1° alcohols react by an S_N2 mechanism

– Step 1:_{Step 1:} proton transfer to OH converts the leaving

group from OH-, a poor leaving group, to H

2O a better

leaving group

– Step 2:Step 2: nucleophilic displacement of H₂O by Br

-Br - OH H slow, rate  determining + + S_N2 OH H Br H H H O O H H rapid and reversible An oxonium ion + + OH H + OH

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Reaction of ROH with HX

• Same as for alkyl halides: Reactions are

governed by a combination of electronic

and steric effects

Increasing rate of carbocation formation 3° alcohol       2° alcohol      1° alcohol        S_N1 S_N2 Increasing rate of displacement of H₂O governed by steric factors governed by electronic factors never react by S_N2 never react by S_N1

From MasteringChem

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Dehydration of Alcohols

• An alcohol can be converted to an alkene

by elimination of H and OH from adjacent

carbons (a

β

-elimination)

– 1° alcohols must be heated at high

temperature in the presence of an acid catalyst, such as H₂SO₄ or H₃PO₄

– 2° alcohols undergo dehydration at somewhat lower temperatures

– 3° alcohols often require temperatures only at or slightly above room temperature

Dehydration of Alcohols

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Dehydration of Alcohols

• When isomeric alkenes are obtained, the

more stable alkene (the one with the

greater number of substituents on the

double bond) generally predominates

(Zaitsev’s rule

Zaitsev’s rule

)

CH₃CH₂CHCH₃ OH _{85% H} 3PO4 CH₃CH=CHCH₃ CH₃CH₂CH=CH₂ 1­Butene   (20%) 2­Butene   (80%) 2­Butanol + heat

Dehydration of a 2° Alcohol

• A three-step mechanism

– Step 1:Step 1: proton transfer from H₃O+ to the -OH group converts OH-, a poor leaving group, into H₂O, a better leaving group

CH₃CHCH₂CH₃ HO O H H H O H H CH₃CHCH₂CH₃ O H H + An oxonium ion rapid and reversible + + +

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Dehydration of a 2° Alcohol

– Step 2:Step 2: loss of H₂O gives a carbocation intermediate

– Step 3:_{Step 3:} proton transfer from an adjacent carbon to H₂O gives the alkene and

regenerates the acid catalyst

H O H CH₃CHCH₂CH₃ CH₃CHCH₂CH_{3 H2O} + slow, rate determining + + A 2° carbocation intermediate CH₃-CH-CH-CH₃ H O H H CH₃-CH=CH-CH₃ O H H H rapid + + + + E1

Dehydration of a 1° Alcohol

• A two-step mechanism

– _{Step 1:Step 1: proton transfer gives an oxonium ion}

– Step 2:_{Step 2:} proton transfer to solvent and loss of H₂O gives the alkene and regenerates the acid catalyst CH₃CH₂-O-H H O H H O H H CH₃CH₂ O H H rapid and reversible + + H O H H C H CH₂ OH H H H O H H + + H C C H H H + O H H + slow, rate determining E2

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Hydration-Dehydration

• Acid-catalyzed hydration of an alkene and dehydration of an alcohol are competing processes

– large amounts of water favor alcohol formation

– scarcity of water or experimental conditions where water is removed favor alkene formation

– Le Chatelier’s Principle An alkene An alcohol C C H OH H₂O   acid catalyst + C C