Ester

Learning Outcome:• Draw the structural formula of the ester that could be produced by

the condensation reaction between an alcohol and a carboxylic acid, given their structural formulae, and write an equation for the reaction.

• Give systematic name for the ester formed

• Identify, draw structures and name the alcohol and acid (or carboxylate salt) that could be obtained by hydrolysis of an ester, given its structural formula.

• Explain the use of heating under reflux and the presence of a trace of concentrated sulfuric acid in the laboratory production of ester.

• Explain the steps involved in isolating pure ester.

Ester• General formula of esters is CnH2n+1COOCmH2m+1.

(R-COO-R’)

• Ester is obtained by combining the general formulae of carboxylic acids and alcohols.

• The functional group of an ester is carboxylate group –COO-

General Esterification

• RCOOH + R’OH ↔ RCOOR’ + H2O • Ester is prepared through esterification,

using concentrated sulfuric acid under reflux method.

Nomenclature of Esters• Name the alcohol part (replace –anol with –yl) followed by carboxylic acid part (replace –ic with

–ate). Number(s) in the parent alcohol & carboxylic acid is retained.

Example :HCOOH + CH3OH HCOOCH3 + H2OMethanoic methanol Methyl methanoate acid

Types of esters :

• 2-methyl-1-prophyl ethanoate• 2-propyl butanoate• 2-propyl-2-ethylbutanoate

IsomerSame molecular formula but different structural formula

Classification Carboxylic acidCH2CH3-COOH

EsterCH3-COO-CH3

Functional group

Physical properties

Chemical properties

Very polar carboxyl group polar ester group

Strong H-bonds between moleculesNon-valotile liquid at r.t.pHigher MP/BPVery soluble in water

Weak DDI between moleculesGas at r.t.pLower MP/BPInsoluble in water

Acidic, neutralisation react with alcohols condense to ester

Fairly unreactiveHydrolysed to acid and alcohol

Physical Properties

• State at room temperature pressure (r.t.p)

Ester are volaitle liquid at r.t.p compare to its isomeric acid (carboxylic acid) with same molecular mass

Why?

Ester has a polar ester groupMolecules held together loosely by weak DDIHE at r.t.p easily break the weak DDI

Carboxylic acid has a very polar carboxyl groupHeld together by stronger H-bondHE at r.t.p cannot break the strong H-bond

Physical Properties• O δ- Melting and Boiling Point

- C δ+ bp = 126oC range = 120-125oC O δ- Moderate lower than ROH and RCOOH higher than alkane and alkene- Polar ester functional group, ester

molecules can form intermolecular dipole-dipole interactions.

DDI

Solubility in water

• Solubility in water is low.• small molecular mass ester group are soluble in water.• Has a polar ester group, can form hydrogen bond with

very polar water molecules, but not extensive (blocked by the non-polar parts)

• Solubility decreases when carbon chain length increase.

Hydrogen bond

Blocked by non-polar HC chain

• Odour Sweet and fruity smell

Example : Allyl hexanoate (pineapple)

Benzyl acetate( pear, strawberry, jasmine)

Steric Factors

• Larger alkyl group (non polar hydrocarbon) hamper the reactivity of the functional group, making it less reactive.

• Less steric factors more reactive• This apply to all organic compounds with functional

groups.

Less steric

More reactive

Preparation of esters

Preparation of esters

• An ester is prepared by reacting, under reflux, an alcohol with a carboxylic acid in the presence of concentrated sulfuric acid as a catalyst.

• The ester forming (process) reactions are called esterifications.

• Water is produced in this (type) reaction – condensation

H+ R’OH + HO – C – R ↔ R’O – C – R + H2O

|| Reflux || O O

Alcohol Carboxylic Ester Water acid

Acid loses a OH group and alcohol loses a proton. A molecule of water is eliminated.

This reaction is reversible, attain equilibrium.

Concentrated H2SO4 used only a trace as catalyst. H2SO4 is a dehydrating agent. It removes water causing the equilibrium to move to the right and thus increase the yield of ester.

• Complete the following reactions:

• Draw the alcohol and carboxylic acid used to synthesizes the ester below:

Esterification 1.Reflux

heat mixture in a vessel with a water-cooled condenser to prevent loss of volatile material

2. Separation-separating funnel to separate organic layer from aqueous layer

3. Purificationdistillation to obtain pure ester

1. Reflux• Refluxing is a process of repeating vapourisation

and condensation of organic substances to prepare organic compounds by heating the mixture in reflux setup.

• Purpose – increase the rate of reaction (by heating)

- to prevent the loss of volatile organic compounds (by condenser).

Species present after reflux- Ester, H2O, ROH, RCOOH and H2SO4.

Reflux method

2. Separation• Transfer the cooled reaction mixture into a

separating funnel.• Purpose

– To separate 2 immiscible layers• Add some distilled water to remove water

soluble compound- ROH, RCOOH and H2SO4.• Stopper, shake, release the gases formed• Place it back to retort stand, remove the stopper

and allow it to separate into 2 layers.

• Remove the aqueous layer.• Add some Na2CO3 (aq) to remove acids

such as H2SO4 and RCOOH by converting them into more water soluble form.

• Transfer the clean ester to conical flask.• Add anhydrous calcium chloride to absorb

excess water.• Equation?

3. Purification• Purpose

– Distill it to obtain pure ester.• Record the boiling range, odour and

appearance.

Question• The following structures of two synthetic

flavour compounds. For each compound:

a) Draw the structures of the carboxylic acid and alcohol from which they could be prepared.

O CH2CH3

|| | CH3 – C – O – CH2CHCH2CH3

• O ||CH3CHCH2 – C – O – CH2CH2CH2CH3

| CH3

Carboxylic acid Alcohol

b) Write an equation for the esterification reaction.

O CH2CH3

|| |CH3 – C – O – CH2CHCH2CH3

• O ||CH3CHCH2 – C – O – CH2CH2CH2CH3

| CH3

Hydrolysis of Esters

Hydrolysis of esters• Esters undergo hydrolysis when refluxed with

aqueous acid or base.• Hydrolysis is a chemical reaction where the

splitting of a molecule is accompanied by the addition of water molecule.

• In the hydrolysis of ester the –H appears on the alcohol and the –OH on carboxylic acid.

• Ester broken down to its carboxylic acid and alcohol.

Hydrolysis of Ester• Acid catalysed hydrolysis of ester

RCOOR’ + H2O ↔ RCOOH + R’OH Reflux• O ||

CH3CH2 – O – C – CH3 + H2O ↔ O || CH3CH2OH + CH3-C - OH

• Alkaline hydrolysis of ester (similar to saponification)

RCOOR’ + OH- RCOO- + R’OH Carboxylate salt alcohol RCOO- (aq) + H+ (aq) RCOOH (aq/l/s)

• This method is preferred because the acid is removed from the equilibrium as its salt.

• This drives the equilibrium to the right and so alkaline hydrolysis is essentially irreversible, giving a higher yield of products.

O O || + NaOH || + HOR’ R – C – OR’ R – C – O-Na+

• To obtain the acid from the salt, hydrogen ions are required, in the form of dilute mineral acid

• O O || + HCl (aq) || + NaCl (aq)

R-C – O-Na+ R- C –OH

• http://www.youtube.com/watch?v=fZ8FlZ82HKA