Esters

What are esters?

☺Esters are chemical organic compounds, which are formed when an alcohol reacts with a carboxylic acid.

☺Esters are made by reacting an alcohol with an organic acid.

☺Esters encompass a large family of organic compounds with broad applications in medicine, biology, chemistry and industry.

Structure of esters

Preparation of estersEsters occur naturally, but can be made in the laboratory by reacting an alcohol

with an organic acid. A little sulfuric acid is needed as a catalyst. This is the general word equation for the reaction:

alcohol + organic acid → ester + water

For example:

methanol + butanoic acid → methyl butanoate + water

The diagram shows how this happens, and where the water comes from:

NamingNamed after alcohol & carboxylic acid

fromwhich they are derived.

Names of esters end in –oate.

CH3C

CH2 O

O

CH3

ethyl ethanoate

This part from the acid and is named after it

This part comes from the alcohol & is named after it

Esterification is the reaction of a carboxylic acid and alcohol in the presence of an acid catalyst to produce an ester.

The main chain of an ester comes from the carboxylic acid, while the alkyl group in an ester comes from the alcohol.

Esterification

Ester hydrolysis

Breakdown of an ester by water.Process sped up by catalysisCan use an acid to catalyse (H2SO4)Alkali catalysts (e.g. sodium hydroxide)

can also be used but instead of producing carboxylic acid a carboxylate salt is formed.

Alkaline hydrolysis goes to completion & hence is usually preferred.

Carboxylic acids can be esterified using diazomethane:

RCO2H + CH2N2 → RCO2CH3 + N2

Using this diazomethane, mixtures of carboxylic acids can be converted to their methyl esters in near quantitative yields, e.g., for analysis by gas chromatography. The method is useful in specialized organic synthetic operations but is considered too expensive for large scale applications.

Alcoholysis of acyl chlorides and acid anhydrides

• Alcohols react with acyl chlorides and acid anhydrides to give esters:

RCOCl + R'OH → RCO2R' + HCl

(RCO)2O + R'OH → RCO2R' + RCO2H

• The reactions are irreversible simplifying work-up( refers to the series of manipulations required to isolate and purify the product of a chemical reaction). Since acyl chlorides and acid anhydrides also react with water, anhydrous conditions are preferred. This method is employed only for laboratory-scale procedures, as it is expensive.

Transesterification • Involves changing one ester into another

one.It is widely practiced: RCO2R' + CH3OH → RCO2CH3 + R'OH

Like the hydrolysation, transesterification is catalysed by acids and bases. • The reaction is widely used for degrading

triglycerides, e.g. in the production of fatty acid esters and alcohols. Poly(ethylene terephthalate) is produced by the transesterification of dimethyl terephthalate and ethylene glycol:

(C6H4)(CO2CH3)2 + 2 C2H4(OH)2 → 1/n {(C6H4)(CO2)2(C2H4)}n + 2 CH3OH

Carbonylation Alkenes undergo "hydroesterification" in the

presence of metal carbonyl catalysts. Esters of propionic acid are produced commercially by this method:

C2H4 + ROH + CO → C2H5CO2R

The carbonylation of methanol yields methyl formate, which is the main commercial source of formic acid. The reaction is catalyzed by sodium methoxide:

CH3OH + CO → CH3O2CH

Addition of carboxylic acids to alkenes

• In the presence of palladium-based catalysts, ethylene, acetic acid, and oxygen react to give vinyl acetate:

C2H4 + CH3CO2H + 1/2 O2 → C2H3O2CCH3 + H2O

SaponificationOne of the organic chemical reactions known to ancient man was the preparation of soaps through a reaction called saponification. Natural soaps are sodium or potassium salts of fatty acids, originally made by boiling lard or other animal fat together with lye or potash (potassium hydroxide). Hydrolysis of the fats and oils occurs, yielding glycerol and crude soap.

Bar SoapsSoap is made by combining lye (sodium hydroxide is the chemical name) with a liquid and fats and oils. Once these ingredients are mixed, a chemical reaction starts to take place. The result of this reaction is soap and if you've made it right, there isn't any lye left. Just good clean glycerin rich soap.

Liquid Soaps

The major difference between bar soaps and liquid soaps is the alkali used to saponify the oils. All soap, whether hard or liquid, starts with a simple chemical reaction between oils and an alkali. With bars soaps, it's sodium hydroxide. With liquid soaps, it's potassium hydroxide.

Odors and smells

One of the most important physical property of esters is their variety of smells. They occur naturally in plants and animals. Small esters, in combination with other volatile compounds, produce the pleasant aroma of fruits. In general, a symphony of chemicals is responsible for specific fruity fragrances; however, very often one single compound plays a leading role. For example, an artificial pineapple flavor contains more than twenty ingredients but ethyl butyrate is the major component that accounts for the pineapple-like aroma and flavor. It is amazing that so many fragrances and flavors can be prepared by simply changing the number of carbons and hydrogens.

Name Flavor or Fragrance

Propyl acetate

Pears

Octyl acetate

Oranges

Isoamyl acetate

Banana

Ethyl Butyrate

Pineapple

Butyl acetate

Apple

Methyl trans-cinnamate

Strawberry

PerfumesPerfumes have a pleasant smell and they stimulate receptors in the nose. Some perfumes are obtained from natural sources, such as lavender oil or sandalwood oil. Others are made synthetically.

Commercial perfumes need particular properties to be successful. The table summarizes these properties and why they are needed.

Property Why the property is important?

non-toxic does not poison the wearer

does not irritate the skin prevents the wearer from suffering rashes

evaporates easily - very volatile perfume molecules reach the nose easily

insoluble in water it is not washed off easily

does not react with water avoids the perfume reacting with perspiration

Aspirin and Salicylic Acid

Aspirin-is used to relieve pain and reduce inflammation-is an ester of salicylic acid and acetic acid

Oil of wintergreen-is used to soothe sore muscles-is an ester of salicylic acid and methanol

Done by:

Bleona Çoba&Anxhela Blloshmi