CARBOXYLIC ACIDS AND THEIR DERIVATIVES

Alkanoic (Carboxylic) Acids General Formula: R-COOH Naming: Suffix -anoic acid CH3COOH ethanoic acid CH2ClCH2COOH 3-chloropropanoic acid

General Properties Organic acids contain a carbonyl group (C=O) and a hydroxyl group (-OH) together on the end carbon. Low mass carboxylic acids are oily liquids with a strong smell and are miscible with water. Higher mass carboxylic acids are waxy solids with no smell and are not miscible with water. Carboxylic acids have significantly higher boiling points than their corresponding alcohols or alkanes.

Preparation can be made by the oxidation of primary alcohols (those alcohols with the -OH group on the end carbon). An oxidising agent such as acidified potassium dichromate or acidified potassium permanganate may be used. can also be prepared using certain bacteria. Vinegar is made by fermenting sugar (from barley, apple juice or whey) to make ethanol, then oxidising the ethanol to ethanoic (acetic) acid, using a special bacteria.

Reactions Organic acids are weak acids. They react with bases, carbonates and reactive metals like inorganic acids, only more slowly. The H from the -OH group is lost forming RCOO - and H+. The name -oic acid changes to -anoate ion. e.g. the ion formed from methanoic acid (HCOOH) is the methanoate ion (HCOO -). e.g. CH3COOH(aq) + NaOH(aq) CH3COONa(aq) + H2O(l) carboxylic acids also react with alcohols to form esters and water: HCOOH + HOCH2CH3 CH3CH2OCHO + H2O methanoic acid ethanol ethyl methanoate water The reaction requires an acid catalyst. Usually a drop of concentrated sulfuric acid is used (it also acts as a dehydrating agent, uses up the water & speeds up the reaction). Derivatives of Carboxylic Acids include acid (acyl) chlorides, esters, amides and amino acids.

Esters Esters contain the functional group COO. R and R’ are alkyl chains. E.g. CH3COOCH2CH2CH3 Naming: Name the part of the molecule that comes from the alcohol first, like a side chain (e.g. ethyl). Then name the part that comes from the acid, like an acid ion (e.g. methanoate) CH3CH2CH2COOCH2CH3 ethyl butanoate

General Esters have lower melting and boiling points than the carboxylic acid and alcohol from which they were formed. Esters are not very soluble in water except for methyl methanoate. Low mass esters are volatile with distinctive, often fruity odours. Used as solvents, perfumes and in flavourings. The scents of many flowers are due to esters. Higher mass esters are waxy solids with little odour. Complex esters are contained in fats and oils.

Preparation of esters Esters are produced by esterification reactions. These are condensation reactions involving an alcohol and a carboxylic acid (with conc, H2SO4 as catalyst) or an alcohol and an acid chloride. carboxylic acid and alcohol – heated under reflux with conc. Sulfuric acid as catalyst. Excess acid is then neutralised with sodium carbonate, and anhydrous magnesium sulfate is added to remove excess water. Because the esters are more volatile than the rest of the mixture they can be removed by fractional distillation. acid chloride and alcohol – acid chloride dropped into pure alcohol in a fume cupboard. Reaction is fast and yield is high. No heat or catalyst required. Most efficient way to prepare esters.

Hydrolysis of esters This is the reverse of esterification – the ester is split into two smaller molecules. Under acidic conditions the product is an alcohol and a carboxylic acid. Under basic (alkaline) conditions the products are an alcohol and a carboxylate ion/salt. More product is favoured under alkaline conditions since the alkali shifts the equilibrium in favour of the more unreactive carboxylate salt. The alkaline hydrolysis of esters is known as saponification. Soap making is the saponification of the triester of glycerol.

The saponification of triester of glycerol The carboxylate salt, sodium stearate is what we know as soap. Other long chain carboxylate salts of sodium are also used as soap.

Acid (Acyl) Chlorides Derived from carboxylic acids with a chlorine atom replacing the hydroxyl group (-OH) of the carboxylic acid COOH group. COOH COCl the IUPAC name for acid chloride is alkanoyl chloride. Acid chloride functional group naming follows the usual rules: e.g. CH3COCl is ethanoyl chloride, CH3CH2COCl is propanoyl chloride. Acid chloride functional group

Properties Acid chlorides have low melting and boiling points as there is no hydrogen bonding between molecules. They exist as pungent smelling, fuming liquids.

Reactions They are highly reactive because the carbon of the acid chloride group is attached to two very electronegative atoms, Cl and O. This means there is a relatively large + charge on the carbon atom, and it is therefore open to attack by nucleophiles. The reactions that acid chlorides undergo are nucleophilic substitutions. With water: acid chlorides react violently with water to form a carboxylic acid and hydrogen chloride. The hydrogen chloride dissolves in moist air to form droplets of hydrochloric acid. This is the fumes that are given off by acid chlorides.

Reactions (Continued) With ammonia: acid chlorides react readily with ammonia to form amides and ammonium chloride: Ethanoyl 2NH3 ethanamide ammonium chloride ammonia chloride with aminoalkanes: acid chlorides react with primary aminoalkanes forming secondary amides and hydrogen chloride: with alcohols: acid chlorides react readily with alcohols to form an ester and hydrogen chloride. This is often a preferred way to prepare an ester as it needs no catalyst.

Reaction Summary for Ethanoyl Chloride Ethanoic acid PCl5 or SOCl2 Ethanoyl chloride Ethyl ethanoate Ethanamide N-ethyl ethanamide