NOMENCLATURE

COMMON NAMES

Carboxylic acids are found in most plants and animals and they can be easily isolated. When fruits ferment and becomes sour it produces acetic acid. That is how we get vinegar, a compound used since the early days of human civilisation.

Tutorial 12.1

Suggest a procedure you would use to isolate acids from the leaves of plants.   Answer

Formic acid is the cause of the stinging irritation when ants bit you. Formica is Latin for ant and that is how it got its name in 1670.

By and large most of the important natural carboxylic acids are still better known by their popular name, especially in the commercial sector. So you may want to understand what your non-chemist bosses are saying.

IUPAC NOMENCLATURE

So far we have dealt with alkyl halides, alkyl alcohols, and alkyl amines. These are alkanes with a substituent attached to the carbon in the chain. The IUPAC nomenclature is rather straight forward. Pick the longest alkyl chain bearing the substituent and add the name of the substituent to the suffix.

For carboxylic acid the carbon of the substituent − the carboxy group, −COOH − is also consider as part of the alkyl group. When we pick the longest chain it would have an additional carbon from the carboxy group. Let us take the simple compound CH₃(CH₂)₄COOH. The alkyl group is a pentyl (5 carbons), but we will name the compound hexanoic. That is, a derivative of a six-member alkane. Once you get used to this discrepancy the system it works fine.

So in the IUPAC system, the carbon in the carboxy group takes the number 1 position and instead of carboxylic acid we use -oic acid.

Because the carbon of the carboxy group is considered part of the alkane chain, it assumed the position 1. So the C-1 for the common name becomes C-2 in the IUPAC system.

Tutorial 12.2

What is the IUPAC name for the compound CH₃CH₂−CH(CH₃)−CH₂COOH?   Answer

To avoid any misunderstanding we agreed to use numericals for the IUPAC system and Greek numericals (α, β, γ, δ, etc) for the common names. So 3-methylpentanoic acid is the same as β−methylvaleric acid.

PHYSICAL PROPERTIES

Formic acid has an even higher boiling point than water. This is not surprising when we inspect the interaction between the molecules.

The movement of electrons along a π−bond is easier than along a σ−bond. So the hydrogen bonding (coulombic attraction) between two acid molecules is greater as compared with two water molecules.

Needless to say, if the carboxylic acid is mixed with water, strong hydrogen bonding is formed between the acid and water molecules. This causes the carboxy group to be soluble in water. For smaller carboxylic acid such attraction is sufficient to "pulled" the hydrocarbon portion of the molecule to go along with water. However as the hydrocarbon portion increases in size the resistance increases. So after hexanoic acid, carboxylic acid can be said to be insoluble in water.

SYNTHESIS of RCOOH

• From Grignard Reagents
• By oxidation of alcohols and aldehydes

CHEMICAL PROPERTIES

We started by stating that the carboxy group can be look upon as a substituent in an alkane. Then look can be deceiving. The carboxy group will not show nucleophilic substitution reactions or elimination reactions like the halides, alcohols or amines.

As a matter of fact the chemical properties of carboxylic acid is about reactions upon the carboxy group.

ACIDITY

In aqueous solution carboxylic acid dissociates to give the carboxylate ion and a proton. The driving force is the resonance stabilisation of the carboxylate ion formed. The electrons in the p−orbitals can move freely throughout the plane.

Tutorial 12.3

How do we know that there is resonance in the carboxylate ion?     Answer

Carboxylic acids are weak acids. For example the degree of dissociation of a 0.1M aqueous solution of acetic acid is only about 1.3%, whereas at such concentration the hydrochloric acid is almost totally dissociated.

Tutorial 12.4

The acidity constant (K_a) for acetic acid is 1.8x10⁻⁵, while that for trichloroacetic acid (Cl₃CCOOH) is 0.23. Explain why trichloroacetic acid is a stronger acid?     Answer

Soap is prepared by reacting a sufficiently large carboxylic acid with sodium hydroxide.

To an ESTER

The carboxylic salt can be used as a nucleophile to replace the bromide in an alkyl bromide; in a polar solvent like dimethylformamide (DMF) and at temperature of about 100�C.

Like all SN reactions there is a possibility of an elimination reaction by the nucleophile. So this reaction is good with primary and secondary bromides, but not for tertiary bromides. For tertiary bromide the alkene will be the major product.

With a diazomethane

There is another unique reaction worth mentioning. This is the reaction of the acid with the reactive diazomethane (CH₂N₂).

DMF

The product is the ester; methyl carboxylate.

NUCLEOPHILIC SUBSTITUTION

One prominent reaction of carboxylic acid is the substitution of the hydroxyl group with other nucleophiles like Cl‾, ‾OR, and ‾NH₂. The carbon of the carboxy group is slightly electron deficient, because of the more electronegative oxygen. So nucleophiles are attracted to it. However the hydroxyl group is not a good leaving group. It will not be easily detached from the carbon.

Carboxylic acid can easily be converted to acyl halide by thionyl chloride, phosphorus pentachloride or phosphorus tribromide. This is because the initial reaction is with the proton to convert the hydroxyl group to an easier leaving group. This is the same methodology used to replace the hydroxyl group in the alcohol with a halide group.

Note that the reactions must be free of water or alcohol. The halide in the acyl halide is easily replaced by hydroxyl or alkoxyl groups.

This reaction requires the help of an acid. Without any added acid, like sulphuric acid, the proton of the carboxy group will provide the proton. But being a weak acid it is not such a good catalyst.

slow

There are now three oxygens that can be protonated; two from the hydroxyl and one from the alkoxyl. If the proton is attached to a hydroxyl, then we have the following series of reactions.

slow

On the other hand if the alkoxyl group is protonated than we will get the original carboxylic acid. So it is not possible to obtain a high yield of the ester. However this is favoured for industrial preparation as the process is comparative clean, and does not involve expensive reagents. The production of ester can be driven by the use of a large excess of carboxylic acid and the continuous removal of the water formed. Such technology is readily available.

It is important to note that this is a series of fast reactions, except for the attack of the alcohol on the carbonyl group and the leaving of the substituent from the carbonyl group.

To an AMIDE

This reaction is even more difficult than the esterification. When ammonia is mixed with the acid it will undergo an acid-base reaction to form a salt; and not a nucleophilic substitution reaction.

The salt is isolated and heated at a high temperature of about 190�C to give the amide and water.

190�C