Naming esters Basically, an organic group is substituted in for the H in a carboxylic acids The first part of the name comes from the alcohol. Second part then comes from the acid e.g. methyl ethanoate CH3COOCH3 Key point about esters: Made from carboxylic acids or acyl chlorides. They are unreactive compared with acids and acyl chlorides Esters are structural isomers of carboxylic acids ETHYL METHANOATE

Structural isomerism about the functional group Classification Carboxylic acid Ester Functional Group R-COOH R-COOR’ Name Propanoic acid Methylethanoate Physical properties O-H bond gives rise No hydrogen bonding to hydrogen bonding; insoluble in water get higher b.p and greater solubility in water Chemical properties acidic fairly unreactive reacts with alcohols hydrolysed to acids

Key reactions: Esterification The reagents are an alcohol + strong acid catalyst (e.g conc. H2SO4 ) Reflux conditions are required to produce an ester Example prep: Preparation of ethyl ethanoate CH3CH2OH(l) + CH3COOH(l) CH3COOC2H5(l) + H2O(l) ethanol ethanoic acid ethyl ethanoate water N.B: Conc.H2SO4 is a dehydrating agent. It removes water causing the equilibrium to move to the right so increasing the ester yield

Key reactions: Esterification 2 The reagents are an alcohol + acyl chloride Dry conditions (diethyl ether) are required to prevent the acyl chloride hydrolysing. The ester is prepared at room temperature as the acyl chloride is so reactive. Example prep: Preparation of methyl ethanoate CH3OH(l) + CH3COCl(l) CH3COOCH3(l) + HCl(g) methanol ethanoyl methyl hydrochloric chloride ethanoate acid N.B: Acid chlorides are very reactive and MUST be kept dry as they react with any water present.

Key reactions: Esterification 3 The reagents are an alcohol + acid anhydride Room temperature can be used for this reaction but it is slow. Usually, the mixture will be warmed slightly to make the reaction more efficient. Example prep: Methyl ethanoate CH3OH(l) + (CH3CO)2O(l) CH3COOCH3(l) + CH3COOH(l) methanol ethanoic methyl ethanoic anhydride ethanoate acid N.B: Acid anhydrides are not as reactive as acyl chlorides so this esterification reaction is slower than that of the acid chloride + alcohol. BUT, the reaction is safer as it is less exothermic and acid anhydrides are less toxic. Ethanoic anhydride

Reversing esterification: Hydrolysis of esters ESTER + WATER CARBOXYLIC ACID + ALCOHOL HCOOH + C2H5OH METHANOIC ETHANOL ACID ETHYL METHANOATE The products of hydrolysis depend on the conditions used... acidic CH3COOCH3 + H2O CH3COOH + CH3OH alkaline CH3COOCH3 + NaOH CH3COO- Na+ + CH3OH N.B: The carboxylic acid can be made by treating the salt with HCl, forming the carboxylic acid and NaCl Water soluble ionic salt

Uses of esters Despite being fairly chemically unreactive, esters are useful as ... • flavourings apple 2-methylbutanoate pear 3-methylbutylethanoate banana 1-methylbutylethanoate pineapple butylbutanoate rum 2-methylpropylpropanoate • solvents nail varnish remover - ethyl ethanoate • plasticisers in polymer preparation (PVC vs uPVC)

Polyesters Condensation reaction between monomers forming a ester group link. Terylene was the first man-made polyester.

Triglycerides: Naturally occuring esters Triglycerides are the most common component of edible fats and oils and are esters of glycerol (propane-1,2,3-triol) CH2OH CHOH

Fats Ester group Fatty acid part (carboxylic acid) Alcohol group

Saturated vs unsaturated fats Triglyceride - unsaturated

Saturated vs unsaturated Saturated fats contain no carbon-to-carbon double bonds. The straight fatty acids chains pack closely together. Unsaturated fats contain at least one carbon-to-carbon double bond. This bends the fatty acid chains and they cannot pack closely together. Saturated fats are less healthy than unsaturated fats because the dense packing makes them harder for the body to digest.

Keypoints - Fats and oils are ESTERS of glycerol and fatty acids. - Unsaturated compounds are identified by the presence of at least 1 C=C double bond. - Monounsaturated means there is 1 C=C double bond. Polyunsaturated means LOTS of C=C double bonds. Omega 3 fats are named as such because the first C=C double bond is on the 3rd carbon from the CH3 end of the fatty acid. Omega 6 = 6th carbon.

Saponification – alkaline hydrolysis of an ester The production of soaps requires the alkaline hydrolysis of triglycerides (triglycerol esters) Example preparation of a soap:

Saponification CH2OH CHOH Soaps are sodium or potassium salts of fatty acids. They are made by boiling fats or oils with sodium or potassium hydroxide. The products are the salt of the fatty acid and glycerol. Potassium soaps tend to be liquids whereas sodium soaps tend to be solids. Soaps are inefficient in acid conditions as the carboxylic acid forms. This is less soluble in water than the salt. The equilibrium is well to the right. CH2OH CHOH

Properties of soap Soaps are also inefficient in hard water because of the calcium and magnesium ions it contains. Calcium and magnesium salts of fatty acids are insoluble in water (scum).

Transesterification Transesterification is the process of exchanging an alkyl group (R’’ in the example below) on the ester with that on an alcohol (R’ group on the alcohol below). Transesterification most famous application is probably in the production of Bio-diesel, but it can also be used to make margarine. Please see the following website for more information on esters: http://www.chemguide.co.uk/organicprops/esters/background.html

Transesterification Biodiesel, an alternative fuel produced straight from vegetable oil, animal oil/fats, tallow and waste cooking oil. The greatest source of suitable oil comes from crops such as rapeseed, palm or soybean. In the UK it is rapeseed that offers the greatest potential for biodiesel production. There are three basic routes to biodiesel production from oils and fats: 1. Base catalysed transesterification of the oil. 2. Direct acid catalysed transesterification of the oil. 3. Conversion of the oil to its fatty acids and then to biodiesel. Almost all biodiesel is produced using base catalysed transesterification. This is the most economical process as it requires low temperatures and pressures and produces a 98% yield. Please see the following website for more information: http://www.esru.strath.ac.uk/EandE/Web_sites/02-03/biofuels/what_biodiesel.htm

Transesterification Production of margarine: The process of transesterification can be used to introduce unsaturated alkyl chains into the fatty acid part of the ester. In this case the R’ group of the alcohol would be mono or polyunsaturated. For more information see: http://www.chemguide.co.uk/organicprops/esters/background.html#top http://www.margarine.org.uk/whatisspread-use.html