Chapter 25 – Summary
25.1 Carboxylic acids: Structure and Naming
A carboxylic acid (RCOOH) contains the functional group COOH, called the carboxyl group, which has an OH group attached to a carbonyl carbon atom. There are many familiar carboxylic acids. The R group may be a hydrogen atom (as in formic acid, HCOOH), an alkyl group (as in acetic acid, CH2COOH), or an aryl group (as in benzoic acid, C6H5COOH). Simple carboxylic acids are best known by common names based on Latin and Greek words that describe their source (e.g., formic acid, Latin formica, meaning “ant”). Greek letters, not numbers, designate the position of substituted acids in the common naming convention. IUPAC names are derived from the LCC of the parent hydrocarbon with the -e ending of the parent alkane replaced by the suffix -oic and the word acid.
25.2 Physical Properties of Carboxylic Acids
Many carboxylic acids are colourless liquids. Carboxylic acids are highly polar molecules and readily engage in hydrogen bonding, so they have relatively high boiling points compared to other substances of comparable molar mass. Boiling points increase with molar mass. Carboxylic acids having one to four carbon atoms are completely miscible with water. Solubility decreases with molar mass. Carboxylic acids have strong, often disagreeable, odours.
25.3 Formation and Reactions of Carboxylic Acids.
A carboxylic acid is formed by the oxidation of an aldehyde with the same number of carbon atoms. Because aldehydes are formed from primary alcohols, these alcohols are also a starting material for carboxylic acids. Carboxylic acids are considered to be some of the most acidic organic compounds however are still considered weak acids. This means they do not dissociate 100% in water. They partially dissociate to form carboxylate salts and hydronium ions. They react with bases to form salts and with carbonates and bicarbonates to form carbon dioxide gas and the salt of the acid. Carboxylic acids also undergo a reaction known as esterification which allows them to react with alcohols to form esters and water.
25.4 Ionization and Neutralization of Carboxylic Acids
Soluble carboxylic acids are weak acids in aqueous solutions. They will partially dissociate to form carboxylate anions and hydronium cations. Since they are moderately acidic, they will turn litmus paper from blue to red. Carboxylic acids neutralize bases to form carboxylate acid salts. When reacting with solutions of carbonate and bicarbonate they also form carbon dioxide gas.
25.5 Esters: Structure, Properties and Naming
Esters contain a carbonyl group with a second oxygen atom bonded to the carbon atom in the carbonyl group by a single bond. Esters have the general formula RCOOR′, where R may be a hydrogen atom, an alkyl group, or an aryl group, and R′ may be an alkyl group or an aryl group but not a hydrogen atom. (If it were hydrogen atom, the compound would be a carboxylic acid). The names for esters include prefixes that denote the lengths of the carbon chains in the molecules and are derived following nomenclature rules similar to those for inorganic acids and salts. Esters are produced by the reaction of acids with alcohols.
25.6 Reactions of Esters
Esters are produced by the reaction of acids with alcohols.
Esters are pleasant-smelling compounds that are responsible for the fragrances of flowers and fruits. Ester molecules are polar but have no hydrogen atom attached directly to an oxygen atom. They are therefore incapable of engaging in intermolecular hydrogen bonding with one another and thus have considerably lower boiling points than their isomeric carboxylic acids counterparts. Because ester molecules can engage in hydrogen bonding with water molecules, however, esters of low molar mass are somewhat soluble in water. Esters are common solvents.
Esters can be synthesized by esterification, in which a carboxylic acid and an alcohol are combined under acidic conditions.
Esters are neutral compounds that undergo hydrolysis, a reaction with water. Hydrolysis is the most important reaction of esters. Under acidic conditions, hydrolysis is essentially the reverse of esterification and gives a carboxylic acid and alcohol. When carried out under basic conditions, the process is called saponification and gives a carboxylate salt and an alcohol. Inorganic acids also react with alcohols to form esters. Some of the most important esters in biochemistry are those formed from phosphoric acid.
