Advanced Higher Chemistry Unit 3 Carboxylic acids

Carboxylic Acids General Formula – RCOOH Systematic and trivial names are common acetic acid ethanoic acid formic acid methanoic acid TrivialSystematic CH 3 COOH HCOOH

The carboxyl group (-COOH) is a polar group containing an acidic –OH group. Hydrogen bonding is possible within carboxylic molecules and also with other molecules containing other relevant functional groups. Hydrogen bonding results in Higher boiling points methanoic acid (RFM=46) – b.pt. 101°C ethanol (RFM=46) – b.pt. 79°C Increased solubility in water

Hydrogen bonding is stronger in carboxylic acid than in alcohols due to the electron withdrawing effect of the carbonyl group. The shape of carboxylic acid molecules allow the formation of dimers (as above) held together by 2 hydrogen bonds. Dimers exist in pure carboxylic acids in both the solid and liquid states and even in the gaseous state at temperatures just above the boiling point.

As with other homologous series, solubility decreases as chain length increases. Carboxylic acids, alcohols and water all contain the hydroxyl group. The OH bond is polar and can be broken heterolytically to produce H + ions. Molecules containing a OH group are thus potentially acidic. Alcohols are much less likely to dissociate to produce hydrogen ions than water (K a for ethanol is , K a for water is 1.8 x ). Carboxylic acids dissociate to a small extent so are weak acids (K a for ethanoic acid is 1.7 x ). carboxylate ion

Carboxylic acids are acidic but alcohols are not due to the carboxylate ion being stabilised by the delocalisation of electrons over the whole carboxylate group. This is a more realistic representation of the ion. Stabilisation of the ion means that it is less likely to combine with a hydrogen ion. The dissociation equilibrium lies further to the right making the acid stronger.

Bonding in Carboxylic Acids Bonding in the –OH group is sigma bonding. Bonding in the C=O is a sigma and pi bond (see aldehydes and ketones).

Synthesis of Carboxylic Acids Oxidation of primary alcohol Oxidising agents –Tollens’ Reagent acidified potassium dichromate acidified potassium dichromate acidified potassium permanganate acidified potassium permanganate hot copper(II) oxide hot copper(II) oxide Hydrolysis of a nitrile (see Halogenalkanes – synthesis of nitriles)

Hydrolysis of esters or amides

Reactions of Carboxylic Acids Neutralisation and salt formation e.g. with alkalis: CH 3 COOH + NaOH  NaCH 3 COO + H 2 O with metal carbonates 2CH 3 COOH + Na 2 CO 3  2NaCH 3 COO + CO 2 + H 2 O with some metals 2CH 3 COOH + Mg  Mg(CH 3 COO) 2 + H 2

They undergo condensation reactions with alcohols to form esters (see Reactions of Alcohols). They can be reacted with ammonia or amines (which are both basic) to form the corresponding ammonium salt which is then heated to drive off water and produce amides.

This type of reaction is used on the production of nylon, which is a condensation polymer. During polymerisation the amino group, -NH 2, on one monomer molecule is condensed with a carboxylic group on another monomer molecule to form a secondary amide. For this reason, nylons can be called polyamides.

Reduction using lithium aluminium hydride (reaction is similar to reduction of aldehydes and ketones) to produce primary alcohols in high yield.

Exercise Now attempt the exercise on page 44 of your Unit 3(b) notes.