Carboxylic Acids And Derivatives

These have the ending-oic acid but no number is necessary for the acid group as it must always be at the end of the chain. The numbering always starts from the carboxylic acid end. If there are carboxylic acid groups on both ends of the chain then it is called a - dioic acid. Eg ethandioic acid

The carboxylic acid are only weak acids in water and only slightly dissociate, but they are strong enough to displace carbon dioxide from carbonates.

The smaller carboxylic (up to C4) acids dissolve in water in all proportions but after this the solubility rapidly reduces. They dissolve because they can hydrogen bond to the water molecules

The carboxylic acid salts are stabilised by delocalisation, which makes the dissociation more likely. The delocalised ion has equal C-O bond lengths. If delocalisation did not occur, the C=O bond would be shorter than the C-O bond. The pi charge cloud has delocalised and spread out. The delocalisation makes the ion more stable and therefore more likely to form.

increasing chain length pushes electron density on to the COO- ion, making it more negative and less stable. This make the acid less strong. Propanoic acid is less acidic than ethanoic acid. Alkyl groups electron releasing. Electronegative chlorine atoms withdraw electron density from the COO- ion, making it less negative and more stable. This make the acid more strong. Chlorine electron withdrawing. Chloroethanoic acid is more acidic than ethanoic acid

Carboxylic acids can form salts with metals, alkalis and carbonates. acid + metal (Na) —> salt + hydrogen 2CH3CO2H + 2Na —> 2CH3CO2-Na+ + H2 acid + alkali (NaOH) —> salt + water CH3CO2H + NaOH —> CH3CO2 -Na+ + H2O acid + carbonate (Na2CO3) —> salt + water + CO2 2CH3CO2H + Na2CO3 —> 2CH3CO2-Na+ + H2O + CO2

The effervescence caused by production of CO2 with carboxylic acids with solid Na2CO3 or aqueous NaHCO3 can be used as a functional group test for carboxylic acids

Carboxylic acids cannot be oxidised by using oxidising agents but methanoic acid is an exception as its structure as effectively an aldehyde group. It forms carbonoic acid (H2CO3) which can decompose to give CO2

Carboxylic acids react with alcohols, in the presence of a strong acid catalyst, to form esters and water. Carboxylic acids + alchohol <—> ester + water

Esters have two parts to their names, eg methyl propanoate CH3CH2COOCH3 The bit ending in–anoate comes from the carboxylic acid and includes the C in the C=O bond. The bit ending in–yl comes from the alcohol that has formed it and is next to the single bonded oxygen.

The reaction is reversible. The reaction is quite slow and needs heating under reflux, (often for several hours or days). Low yields (50% ish) are achieved. An acid catalyst (H2SO4) is needed.

Esters are sweet smelling compounds that can be used in perfumes and flavourings. Non-toxic, soluble in solvent. Volatile turns into gas easily and does not react with water. Esters can be used as solvents for polar organic substances. Ethyl ethanoate is used as a solvent in glues and printing inks. Esters can be used as plasticisers for polymers

Although polar, they do not form hydrogen bonds (reason: there is no hydrogen bonded to a highly electronegative atom). They have a lower b.p. than the hydrogen-bonded carboxylic acids they came from. They are also almost insoluble in water. Often pure polymers have limited flexibility because the polymer chains cannot move over each other. Incorporating some plasticiser into the polymer allows the chains to move more easily and the polymer can become more flexible.

Esters can be hydrolysed and split up by either heating with acid or with sodium hydroxide.

With acid reagents: dilute acid (HCl) conditions: heat under reflux This reaction is the reverse reaction of ester formation. When an ester is hydrolysed a carboxylic acid and an alcohol are formed. Ethyl propanoate CH3CH2CO2CH2CH3 + H2O <—> CH3CH2CO2H + CH3CH2OH This reaction is reversible and does not give a good yield of the products.

With sodium hydroxide reagents: dilute sodium hydroxide conditions: heat under reflux This reaction goes to completion. Using excess sodium hydroxide will ensure the ester is completely hydrolysed. The carboxylic acid salt product is the anion of the carboxylic acid. The anion is resistant to attack by weak nucleophiles such as alcohols, so the reaction is not reversible. CH3CH2CO2CH3 + NaOH —> CH3CH2CO2-Na+ + CH3OH methyl propanoate sodium propanoate methano

Both formation of esters and hydrolysis of esters can be done by heating under reflux.

The liquid ethyl benzoate can be hydrolysed by sodium hydroxide by heating under reflux for 30 minutes After the reaction is complete allow the mixture to cool to room temperature and add hydrochloric acid. A precipitate of benzoic acid will be produced that can be filtered off using suction filtration. C6H5COONa + HCl —> C6H5COOH + NaCl Sodium benzoate is soluble in water because it is ionic. Benzoic acid, however, is insoluble. This is because even though the polar COOH group can form hydrogen bonds, the benzene ring is non-polar. In organic compounds there are often polar parts and non-polar parts. The solubility in water of a compound will controlled by whether the polar or non polar part is of greater importance.

Fats and oils are esters of glycerol and long chain carboxylic acids (fatty acids) Vegetable oils and animal fats can be hydrolysed to give soap, glycerol and long chain carboxylic (fatty) acids. Glycerol (propane-1,2,3-triol) forms hydrogen bonds very easily and is readily soluble in water. It is used in cosmetics, food and in glues. Soap Long chain carboxylic (fatty) acids, produced by the hydrolysis of fats, act as soaps. The polar CO2 - end is hydrophilic and mixes with water. The long non-polar hydrocarbon chain is hydrophobic and mixes with grease. So this allows the grease and water to mix and be washed away.

Biodiesel is a mixture of methyl esters of long chain carboxylic acids. Vegetable oils can be converted into biodiesel by reaction with methanol in the presence of a (strong alkali) catalyst. It can be argued that the biodiesel produced from this method is classed as carbon–neutral as any carbon dioxide given off when the biofuel is burnt would have been extracted from the air by photosynthesis when the plant grew. This does not take into account any energy needed to irrigate plants, extract the oil, heat the reaction with methanol mixture or process the fuel. If the energy for these processes comes from fossil fuels then the biofuel produced is not carbon neutral. It also does not take into account the effect on land available for food production.

Acyl chlorides are much more reactive than carboxylic acids The Cl and –OCOCH3 groups are classed as good leaving groups (to do with less effective delocalisation.) This makes acyl chlorides and acid anhydrides much more reactive than carboxylic acids and esters

Acid anhydrides have a similar reactivity to acyl chlorides and therefore bring about the same changes in functional groups. The main difference is the by-products. Acyl chlorides mostly give off HCl. Acid anhydrides give off RCOOH