Lecture 18- Aldehydes And Ketones

General

Organic Chemistry Two credits Second Semester 2009

King Saud bin Abdulaziz University for Health Science

Reference Book: Organic Chemistry: A Brief Course, by Robert C. Atkins and Francis A. Carey Third Edition

Instructor: Rabih O. Al-Kaysi, PhD.

Lecture 18

Chapter 11

Aldehydes and Ketones

Nucleophilic Addition to the Carbonyl Group

Nomenclature

IUPAC Nomenclature of Aldehydes O

O H

O

H

O

HCCHCH

Base the name on the chain that contains the carbonyl group and replace the -e ending of the hydrocarbon by -al.

IUPAC Nomenclature of Aldehydes O

O H

H

4,4-dimethylpentanal O

5-hexenal O

HCCHCH

2-phenylpropanedial (keep the -e ending before -dial)

IUPAC Nomenclature of Aldehydes

O when named as a substituent formyl group

C

H

when named as a suffix carbaldehyde or carboxaldehyde

Substitutive IUPAC Nomenclature of Ketones O

O

CH3CH2CCH2CH2CH3

CH3CHCH2CCH3 CH3

H3C

Base the name on the chain that contains the carbonyl O group and replace -e by -one. Number the chain in the direction that gives the lowest number to the carbonyl carbon.

Substitutive IUPAC Nomenclature of Ketones O

O

CH3CH2CCH2CH2CH3

CH3CHCH2CCH3 CH3

3-hexanone

4-methyl-2-pentanone H3C

O

4-methylcyclohexanone

Functional Class IUPAC Nomenclature of Ketones O

O

CH3CH2CCH2CH2CH3

O H2C

CHC CH

CH2

CH2CCH2CH3

List the groups attached to the carbonyl separately in alphabetical order, and add the word ketone.

Functional Class IUPAC Nomenclature of Ketones O

O

CH3CH2CCH2CH2CH3

ethyl propyl ketone

CH2CCH2CH3

benzyl ethyl ketone

O H2C

CHC CH

CH2

divinyl ketone

Structure and Bonding: The Carbonyl Group

Structure of Formaldehyde

planar bond angles: close to 120° C=O bond distance: 122 pm

The Carbonyl Group

very polar double bond O 1-butene

propanal

dipole moment = 0.3D

dipole moment = 2.5D

Resonance Description of Carbonyl Group

•• •

– •• • •

C

C +

O•

• O•

nucleophiles attack carbon; electrophiles attack oxygen

Bonding in Formaldehyde

Carbon and oxygen are sp2 hybridized

Bonding in Formaldehyde

The half-filled p orbitals on carbon and oxygen overlap to form a π bond

Physical Properties

Aldehydes and ketones have higher boiling than alkenes, but lower boiling points than alcohols. boiling point –6°C O

OH

49°C

97°C

More polar than alkenes, but cannot form intermolecular hydrogen bonds to other carbonyl groups

Sources of Aldehydes and Ketones

Many aldehydes and ketones occur naturally

O

2-heptanone (component of alarm pheromone of bees)

Many aldehydes and ketones occur naturally

O H trans-2-hexenal (alarm pheromone of myrmicine ant)

Many aldehydes and ketones occur naturally

O H citral (from lemon grass oil)

Synthesis of Aldehydes and Ketones

A number of reactions already studied provide efficient synthetic routes to aldehydes and ketones.

from alkenes ozonolysis from alkynes hydration (via enol) from arenes Friedel-Crafts acylation from alcohols oxidation

What about..?

aldehydes from carboxylic acids

R 1. LiAlH4 2. H2O

O

O

C

C

R

OH

H PDC, CH2Cl2

RCH2OH

Example

benzaldehyde from benzoic acid O

O

COH

CH

1. LiAlH4 2. H2O (81%)

CH2OH

PDC CH2Cl2 (83%)

What about..?

ketones from aldehydes

R 1. R'MgX 2. H3O+

O

O

C

C

R

H OH RCHR'

R' PDC, CH2Cl2

Example

3-heptanone from propanal O C

CH3CH2

O CH3CH2C(CH2)3 CH3

H

(57%)

1. CH3(CH2)3MgX 2. H3O

+

OH CH3CH2CH(CH2)3 CH3

H2CrO4

Reactions of Aldehydes and Ketones: A Review and a Preview

Reactions of Aldehydes and Ketones Already covered in earlier chapters: reduction of C=O to CH2 Clemmensen reduction Wolff-Kishner reduction reduction of C=O to CHOH addition of Grignard and organolithium reagents

Principles of Nucleophilic Addition to Carbonyl Groups: Hydration of Aldehydes and Ketones

Hydration of Aldehydes and Ketones

C

O •• ••

H2O

••

HO ••

C

••

O ••

H

Substituent Effects on Hydration Equilibria OH

O + H2O

C R

R

R'

C OH

compared to H electronic: reactants

alkyl groups stabilize

steric: product

alkyl groups crowd

R'

Equilibrium Constants and Relative Rates of Hydration C=O rate

hydrate

K

%

Relative

CH2=O

CH2(OH)2

2300

>99.9

2200

CH3CH=O

CH3CH(OH)2

1.0

50

1.0

(CH3)3CCH=O

(CH3)3CCH(OH)2 0.2

17

0.09

(CH3)2C=O

(CH3)2C(OH)2

0.14

0.0018

0.0014

When does equilibrium favor hydrate?

when carbonyl group is destabilized alkyl groups stabilize C=O electron-withdrawing groups destabilize C=O

Substituent Effects on Hydration Equilibria

OH

O + H2O

C R

R

R

C OH

R = CH3: K = 0.000025 R = CF3: K = 22,000

R

Mechanism of Hydration (base)

Step 1: H

–

•O• • • ••

+

C

O •• ••

••

HO ••

C

•• •– O• ••

Mechanism of Hydration (base)

Step 2: ••

HO ••

C

H

H ••

HO ••

C

••

OH ••

–• • + • O• ••

•• •– O• ••

H

O •• ••

Mechanism of Hydration (acid) Step 1:

H C

C

O •• ••

+ OH ••

+

H

O •• + H H

+

•O• • •

H

Mechanism of Hydration (acid) Step 2:

H •O• • •

H

+

C

+ OH ••

H •O •

+

H

C

••

OH ••

Mechanism of Hydration (acid)

Step 3: H

••

+O

H

•O •

H

C

••

O

H

••

H + ••

H

H ••

OH ••

•O •

+

H

C

••

OH ••

Cyanohydrin Formation

Cyanohydrin Formation

C

O •• ••

+ HCN

• •N

C

C

••

O ••

H

Cyanohydrin Formation

• •N

– C ••

C

O •• ••

Cyanohydrin Formation H • •N

C

C

•• •– O• ••

H

O •• + H H

• •N

C

C

••

O ••

H

• O •• •

H

Example

Cl Cl

O

Cl

NaCN, water Cl CH then H2SO4

OH CHCN

2,4-Dichlorobenzaldehyde cyanohydrin (100%)

Example

O CH3CCH3

NaCN, water then H2SO4

OH CH3CCH3 CN (77-78%)

Acetone cyanohydrin is used in the synthesis of methacrylonitrile