Chapter 8

ESTERSNorfazrin Mohd Hanif

Faculty of Applied Science

UiTM Negeri Sembilan

STRUCTURE OF ESTERS

Carboxylic acid derivatives in which the hydroxy group (-OH) is replaced by an alkoxy group (-OR).

General formula:

R C

O

O R' or RCOOR' or RCO2R'

Esters have pleasant odors such as smell like fruits (apples or bananas) and flower, especially those with low molecular weight.

NOMENCLATURE

Acid part Alcohol

1. Name the alcohol part 1st. R as an alkyl group.2. Name the acid part by changing the ‘-ic acid’

to ‘-ate’.

NOMENCLATURE

According to the IUPAC system the alcohol part of theester (R΄) is named first.

ethyl

This is followed by the name of the acid where the–ic ending of the acid has been changed to –ate.

ethanoate

ethyl ethanoate

NOMENCLATURE

PREPARATION

• FROM CARBOXYLIC ACIDS

• FROM ACYL CHLORIDES

• FROM ACID ANHYDRIDES

• Esterification:

- The reaction between an alcohol and a carboxylic acid to form an ester and H2O.

FROM CARBOXYLIC ACIDS

R C

O

O H O R'H

H+

CH3CH2-O-H CH3 C

O

O H

CH3-O-H C

O

OHH

+

H+

R C

O

O R'

C

O

OCH3

CH3 C

O

OCH2CH3

H2O

H2O

H2O

carboxylic acid alcohol ester

EXAMPLES

ethanol ethanoic acid ethyl ethanoate

methanol benzoic acid methyl benzoate

H+ = catalyst

FROM ACYL CLORIDES

Acyl chloride react with an alcohol and phenols to produce an ester.

Does not require catalyst even through alcohol is a weak nucleophilebecause acyl chloride is highly reactive.

The reaction is irreversible unlike the reaction of carboxylic acid with alcohol.

Pyridine is added to prevent side reaction between HCl and ROH.

R C

O

Cl R-OH R C

O

OR HClpyridine

H3C C

O

Cl CH3 C

O

OCH2CH3O HCH3CH2 HClpyridine

Example:

FROM ACID ANHYDRIDES

R C

O

O C

O

R R' OH R C

O

OR' R C

O

OH

EXAMPLE

H3C C

O

O C

O

CH3 CH3CH2 OH H3C C

O

OCH2CH3 H3C C

O

OH

H3C C

O

O C

O

CH3

OH

H3C C

O

O H3C C

O

OH

ester carboxylic acid

ethanoic anhydride ethanol ethyl ethanoate ethanoic acid

phenol phenyl ethanoate ethanoic acidethanoic anhydride

Acid anhydrides react with alcohol to produce esters and carboxylic acids.

Does not required catalyst, but still requires heating.

The reaction is slower than the reaction with acyl chlorides

REACTIONS

• HYDROLYSIS

• AMMONOLYSIS

• REACTION WITH GRIGNARD REAGENT

• TRANSESTERIFICATION

HYDROLYSIS

Ester molecules undergoes hydrolysis to form carboxylic acids or carboxylate salts and alcohols.

Hydrolysis occurs in either acidic or basic condition.

The reaction with pure water is so slow.

The reaction is catalyzed by dilute acid such as HCl or H2SO4 and heated under reflux.

R C

O

O R H OH R C

O

OH ROHester

heat

carboxylic acid alcohol

R C

O

O R H OH

R C

O

OH ROH

R C

O

O ROH

H+

OH

carboxylate salts

carboxylic acid

AMMONOLYSIS

Esters reacts with ammonia, primary and secondary amines to produce amides and alcohols.

R C

O

O Rester

R C

O

NH2 R OH

R C

O

O Rester

N R C

O

NH R OHH

H

R

R

R C

O

O Rester

N R C

O

N R OHH

R

R

R

R

primary amide

secondary amide

tertiary amide

EXAMPLE

H3C C

O

O CH2CH3 N H3C C

O

NH2 CH3CH2 OH

H

H

ammonia

H

N

H

HH

0-5OC

primary amine

secondary amine

REACTION WITH GRIGNARD REAGENT

Esters undergo nucleophilic acyl substitution then nucleophilic addition with two equivalents moles of Grignard reagent, RMgX to yield two different alcohols which one of them is tertiary.

R C

O

O Rester

2RMgX R C

R

R

OH ROHH3O+

ether

EXAMPLE

CH3CH2 C

O

O CH3

ester

2CH3MgCl CH3CH2 C

CH3

CH3

OH CH3OHH3O+

ether

tertiary alcohol

Grignard reagent

MECHANISM

R C

O

OR R' MgX R C

O MgX

OR

R'

R C

O

R'

The initial product is unstable and it loses a magnesium alkoxide to form a ketone.

This ketone is more reactive towards Grignard reagent and will react with a second molecule of the Grignard reagent.

R' MgX R C

O MgX

R'

R'

R C

O

R' R C

OH

R'

R'

H3O+

The reaction is followed by hydrolysis to give tertiary alcohol with two alkyl group (R’) coming from Grignard reagent.

TRANSESTERIFICATION

Substitution of one alkoxy group for another in an ester.

Alcoholysis of ester to prepare more complex esters from simple esters.

Can take place under either acidic (in HCl or H2SO4) or basic conditions.

R C

O

O R'ester

R'-OHH+

heat

EXAMPLE

R'' OH R C

O

O R''esterlarge excess alcohol

CH3 OH

CH3CH2CH2 C

O

OCH3 CH3CH2CHCH3

OH

CH3CH2CH2 C

O

OC

CH3

H

CH2CH3

H+

heat

Thank you…