Key Information & Summary
- Carboxylic acids are organic compounds having hydroxyl group attached to the acyl group.
- Carboxylic acid derivatives contain functional groups that undergo nucleophilic acyl substitutions.
- Carboxylic acids, ester, amide, thioester, acyl phosphate, acyl chloride and acid anhydride are named as longest alkane name replacing its ending “e” with “oic acids”, “ate”, “amide”, “thiooate”, “yl phosphate”, “yl chloride” and “oic anhydride”, respectively.
- Carboxylic acids are synthesized from alcohols and aldehydes. Ester, thioester, acyl phosphate and acyl chloride may be synthesized by the reaction of carboxylic acid with alcohol, thiol, ATP and PCl5.
- Carboxylic acids have high boiling points and are soluble in water due to H-bonding.
- Carboxylic acid derivatives undergo various types of reactions including acyl group substitution, reduction, reactions with organometallic compounds etc.
Carboxylic acids find applications in pharmaceuticals, food, detergent and cosmetic industries.
INTRODUCTION TO CARBOXYLIC ACIDS AND DERIVATIVES
Carboxylic acid is an organic compound having hydroxyl group attached to the acyl group. The hydroxyl group of carboxylic acid may be substituted with other functional groups resulting in the formation of carboxylic acid derivatives. In other words, the functional groups that undergo nucleophilic acyl substitutions are called carboxylic acid derivatives. Carboxylic acid, carboxylate, ester, amide, thioester, acyl phosphate, acyl chloride and acid anhydride are the carboxylic acid derivatives as shown below. Acyl group (RCO-) is common in all these derivatives.
NOMENCLATURE OF CARBOXYLIC ACID
According to IUPAC system, carboxylic acids are named by dropping the word “e” from longest alkane and adding the word “oic acid”. For example, CH3CH2CH2COOH having four carbon atoms would be named as butanoic acid. In case of presence of double bond in the carbon chain, “anoic acid” is changed to “enoic acid” and location of double bond is represented by a number. For example, CH3CH=CHCOOH is named as 2-butenoic acid.
For esters, “oic acid” is replaced by “ate” e.g. CH3CH2CH2COOC2H5 is ethyl butanoate. Amides are named by replacing “oic acid” with “amide” e.g. CH3CONH2 is named as ethanamide. For thioesters, alkane name is followed by thiooate, e.g. CH3COSCH3 is named as methyl ethanethiooate. For acyl phosphates, acyl group name is written followed by “phosphate e.g. benzoyl phosphate. However, if an alkyl group is attached to O of phosphate, this akyl group is named after acyl group followed by “phosphate” e.g. acetyl adenosyl phosphate.
For acyl cholrides, “e” in the alkane is replaced with “yl chloride” e.g. CH3CH2COCl will be named as propanoyl chloride. In case of acid anhydrides, “acid” is replaced by anhydride e.g. (CH3CH2CH2CO)2O is named as butanoic anyhydride.
FORMATION OF CARBOXYLIC ACID DERIVATIVES
Carboxylic acids: Carboxylic acids are generally synthesized from alcohols and aldehydes as shown below.
Oxidative cleavage of alkenes also results in the formation of carboxylic acid.
Esters: Fischer reaction is the simplest method of formation of esters. In this reaction an acid reacts with alcohol to form ester. For example, ethanoic acid reacts with ethanol to form ethylethanoate.
Amides: Amides are produced by the reaction of ammonia or amines with acid chlorides. For instance, reaction of ammonia with propanoyl chloride will result in the formation of propanamide.
Thioester: Carboxylic acid reacts with a thiol group (e.g. R-SH) to form thioester with the release of water molecule.
Acyl phosphate: Acyl phosphate may be synthesized by the reaction of carboxylic acid with ATP which results in the formation of acyl phosphate with the release of ADP.
Acyl chloride: Carboxylic acids react with phosphorous pentachloride (PCl5) to form acyl chloride as shown below in a generalized reaction.
Acid anhydride: Acid anhydride is formed by the reaction of salt of carboxylic acid like sodium acetate with an acyl halide, for instance acetyl chloride as shown in the reaction below.
PHYSICAL PROPETIES OF CARBOXYLIC ACID DERIVATIVES
Carboxylic acids have high boiling points due to hydrogen bonds between the molecules. Carboxylic acid derivatives also form hydrogen bonds with water molecules which makes them soluble in water. Carboxylic acids with less than ten number of carbon atoms are liquids while carboxylic acids with more than 10 carbon atoms are wax like solids. They are colourless and have a particular odour. Bacteria on skin also produce carboxylic acids that produces odour. However, the odour of carboxylic acids diminishes with increasing number of carbon atoms.
REACTIONS OF CARBOXYLIC ACID DERIVATIVES
Carboxylic acid derivatives undergo various types of reactions including acyl group substitution, reduction, reactions with organometallic reagents etc.
Acyl group substitution: Nucleophiles attack the carboxylic acid derivatives from one side of the carbonyl carbon thereby converting it to tetrahedral geometry. The X group serves as the leaving group and when it leaves, the carbon reforms double bond with oxygen. At the end of reaction, nucleophile becomes the new X group.
The order of reactivity of various carboxylic acid derivatives is as follows: acyl phosphate>thioester>ester=carboxylic acid>amide>carboxylate (acyl phosphate is the most reactive and carboxylate is the least reactive). This difference in reactivity is due to the basicity of leaving group as weaker bases are better leaving groups.
Reduction: Carboxylic acid derivatives are reduced to either aldehydes or alcohols. Reduction reactions include catalytic dehydrogenation, complex metal hydride reactions, diborane reduction etc. For example, acyl halides are reduced to aldehyde in the presence of palladium as catalyst under mild conditions as shown in the reaction below.
Reactions with organometallic reagents: Carboxylic acid derivatives including esters, anhydrides and acyl chlorides react with organometallic compounds to form alcohol products. For example, esters react with Grignard reagents to form tertiary alcohols.
APPLICATIONS OF CARBOXYLIC ACID DERIVATIVES
Carboxylic acids are present in the body, for instance, prostaglandin (PGE2) induces labour in pregnant women and raises blood pressure. Carboxylic acid derivatives are also present naturally in citric fruits, fats and some dairy products. Various industrially manufactured food items like preservatives (benzoic acid), soft drinks, vinegar (acetic acid) also contain carboxylic acid derivatives. They are also used in many pharmaceutical products like aspirin (acetyl salicyclic acid), fungicides (e.g. caprylic acid), laxatives (e.g. hydrobutanedioic acid) etc. They are also present in soaps, detergents, cosmetics (e.g. oleic acid), paints (e.g. linoleic acid) and lubricants (sorbic acid).