Carboxylic Acid
Carboxylic Acids, Esters, Fats and Oils
6.1 CARBOXYLIC ACIDS
6.1.1 Structure and Nomenclature of Carboxylic Acids
Structure of Carboxylic Acids
Carboxylic acids are organic compounds that contain at least one carboxyl group in their structure. A
carboxyl group is a functional group consisting of a carbonyl and a hydroxyl written COOH or —CO2 H.
Classification
A. Monocarboxylic acids The general formula for saturated monocarboxylic acids can be written as:
where R is either H or an alkyl group for aliphatic Carboxylic acids. When R is phenyl
(aryl) group, the structure represents aromatic carboxylic acids.
Example The structure of the first three saturated monocarboxylic acids are written as follows:
B. Di- and tricarboxylic acids Carboxylic acids containing two and three carboxyl groups in their
structure are called dicarboxylic and tricarboxylic acids respectively.
Example The structure of the first three saturated dicarboxylic acids are written as follows:
Example Citric acid is a typical tricarboxylic acid, having the following structure:
Nomenclature of Carboxylic Acids
A. Common names for a Straight chain monocarboxylic acids
B. Branched chain and substituted carboxylic acids - the position of the side chain or substituents is
indicated by Greek letters, α, β, γ, δ... for designating the 1st, 2nd, 3rd,… position of carbon
atoms as shown below:
Dicarboxylic acids also possess common names which are based on their sources.
In IUPAC system, dicarboxylic acids are named as alkane dioic acids
Aromatic carboxylic acids Compounds which have a carboxyl group directly attached to an aromatic
ring are classified as aromatic carboxylic acids. The simplest aromatic carboxylic is benzoic acid
IUPAC names of carboxylic acid
Branched chain and substituted monocarboxylic acids -In IUPAC system, the positions of the substitutes
are indicated by Arabic numerals as 1, 2, 3. The numbering of the chain starts from the carboxyl carbon
and it is assigned C-1 position. C-2 position in the IUPAC system corresponds to the α-position in the
common naming system .
Aromatic carboxylic acids according to IUPAC system, aromatic carboxylic acids are named as
benzene carboxylic acids.
6.1.2 Physical Properties of Carboxylic Acids
1. State The lower aliphatic acids are liquids, whereas the higher members are solids.
Benzoic acid and most of its derivatives are also colourless solids.
2. Odour The odours of the lower aliphatic acids progress from irritating odour of methanoic acid
and ethanoic acids to the unpleasant odour of the butanoic, pentanoic and hexanoic acids. The
higher acids have little odour because of their low volatility.
3. Boiling Point Carboxylic acids have higher boiling points than alcohols of the similar size.
For example, ethanoic acid (CH3 COOH) boils at 118°C while propan-1-ol (CH3 CH2 CH2 OH)
boils at 97.2°C.
The higher boiling points of the carboxylic acids are also caused by hydrogen bonding between two
molecules of acid to produce a dimer.
4. Solubility Carboxylic acids up to four carbon atoms mix well with water in any proportion. The
solubility in water decreases with the increasing molecular mass and higher acids are almost
insoluble. The carboxylic acids dissolve in water due to formation of hydrogen bonding with
water molecules.
Among the aromatic acids, benzoic acid is sparingly soluble in water at room temperature.
However, all carboxylic acids are soluble in organic solvents like alcohol, ether, benzene etc.
6.1.3 Chemical Properties of Carboxylic Acids
Some of the common reactions of carboxylic acids are
I) Reaction as an acid Carboxylic acids are weak acids that ionize partially and O-H bond cleavage.
a. Reaction with metals: Carboxylic acids react with active metals such as Na, K, Mg, Ca etc. to
form salts and hydrogen gas.
The salts of carboxylic acids are named by writing the name of the metal first, followed by the name of
the acid replacing the ending -ic acid by -ate.
b. Reaction with Bases: Carboxylic acids react with strong bases like NaOH or KOH to form the
corresponding salts and water.
ii) Formation of Esters: In this reaction, carboxylic acids are heated with alcohols in the presence of
concentrated sulphuric acid. The reaction is called esterification.
6.1.4 Preparation of Carboxylic Acids
One of the important methods for preparation of carboxylic acids is oxidation.
i) Oxidation of Primary Alcohols: The primary alcohols are oxidized to the carboxylic acids by their
reaction with potassium permanganate.
ii) Oxidation of Alkyl benzenes: Aromatic compounds containing alkyl group as substituent undergo
oxidation to form aromatic acids. The reaction involves oxidation with potassium permanganate. The
alkyl group is oxidized to carboxyl group irrespective of its size.
iii) Preparation of acetic acid (Ethanoic acid): Acetic acid is one of the important carboxylic acids
which is used as food preservative. It can be prepared in laboratory by the oxidation of ethanol with
potassium permanganate. It can also be obtained by passing the vapours of ethanol through copper oxide.
6.1.5 Fatty Acids
They are a carboxylic acid with a long hydrocarbon chains and found in all cells. The hydrocarbon chains
of animal fatty acids are more saturated than those of vegetable origin.
With only a few exceptions, the fatty acids are all straight-chain compounds. Most fatty acids contain an
even number of carbon atoms. Fatty acids that do not contain carbon-carbon double bonds are termed as
saturated fatty acids, and those that contain one or more double bonds are called unsaturated fatty acids.
Table 6.6 Examples of naturally occurring saturated fatty acids
6.1.6 Uses of Carboxylic Acids
Acetic acid is used as a solvent and as a starting material in the preparation of acetates, acetic anhydride,
etc. It is also used to prepare the vinyl acetate polymer which is used in paints and adhesives. Vinegar
contains about 8-10 % acetic acid which is used in many food items. Aspirin, the common painkiller, is
prepared by the reaction of salicylic acid (2-hydroxylbenzoic acid) with acetic anhydride.
One of the most important industrial applications of long chain carboxylic acid is for making soaps,
detergents, and shampoos. They are also important in the manufacture of greases, crayons, plastics and as
raw materials for the production of synthetic odors and flavors.
6. 2 ESTERS
Many esters are pleasant smelling substances and are responsible for the flavor of fruits for example,
apples, pears, banana, pineapple, strawberry, etc. Oils, fats and waxes of plants or animal origin are all
esters. Many esters are found in flowers.
6.2 .2 Structure and Nomenclature of Esters
They are derivatives of carboxylic acids in which the hydroxyl group of carboxylic acid has been
replaced by an alkoxy group.
Esters can be represented by the general formula where R = hydrogen, alkyl or an aryl group and R′ =
alkyl or an aryl group.
Esters are named by the common system as well as by IUPAC system. In both the cases, the name
consists of two parts. The first part is named on the basis of the portion coming from alcohol and the
second part of the name is based on the portion from acid.
While writing the name of an ester, first the name of alkyl group is written first followed by the name of
the acid by replacing -ic acid with -ate.
6.2.3 Physical Properties of Esters
i) Odour esters have pleasant odour. The odour of many fruits and flowers result from mixtures of
carboxylic esters, and many of them are used in perfumes and food flavoring.
ii) Boiling points The boiling points of esters increase with increasing molecular mass. Branched-chain
esters have lower boiling points than their straight-chain isomers.
Esters have lower boiling points than carboxylic acids and alcohols of comparable molecular mass.
This is because ester molecules cannot form hydrogen bonds with each other.
iii) solubility Esters of low molecular mass are fairly soluble in water. Since esters can form hydrogen
bonding with water, the solubility of esters in water decreases with increasing molecular mass. All esters
are soluble in organic solvents.
6.2.4 Chemical Properties of esters
i) Hydrolysis The general reaction for acid-catalyzed hydrolysis of esters can be written as:
Esters also undergo base-catalyzed hydrolysis to give salts of carboxylic acids/soap and alcohols. Base-
catalyzed ester hydrolysis is called saponification. The general reaction for base-catalyzed hydrolysis of
esters:
ii) Reduction Esters are reduced to primary alcohols by special reducing agents like lithium aluminum
hydride, LiAlH4 . The general reaction for reduction of esters is given by:
6.2.5 Preparation of Esters
Esters can be synthesized by heating a mixture of a carboxylic acid and an alcohol in the presence of an
acid catalyst. This reaction is called esterification and is a common method for the preparation of esters.
In this condensation reaction, the hydroxyl group (–OH) from the acid and a hydrogen atom (–H) from
the alcohol are eliminated in the form of water, as indicated by the dotted rectangle in the above reaction.
6.2.6 Uses Esters
They have numerous uses as solvents, medicines, clothing (e.g. polyesters), fragrances in perfumes, and
plasticizers (e.g. octyl phthalate).
Table 6.8 Some common fruits and the esters responsible for their flavour
6. 3 FATS AND OILS
6.3.1 Source and Structure of Fats and Oils
Fats and oils belong to a class of biomolecules called lipids. They are triesters of glycerol which are
collectivity known as triglycerides or triacylglycerols. If the substance is solid or semisolid at ordinary
temperature, it is termed as a fat and if it is fluid, it is called an oil.
Structure of Fats and Oils
Fats and oils are represented by the following general structural formula:
Where R1 , R2 and R3 may be the same or different hydrocarbon groups.
Fats are esters of glycerol and mostly saturated fatty acids and oils are liquid esters primarily derived
from unsaturated fatty acids and glycerol. The structures of some common triglycerides are shown
below:
Variation in the structure of fats and oils occur in the fatty acid portion of the triglyceride (or
triacylglycerol).
6.3.2 Physical Properties of Fats and Oils
Fats and oils are greasy to the touch, and have lubricating properties; they are not readily volatile; and
may be burned without leaving any residue or ash. Fats like butter, lard and tallow are solids at room
temperature. On the other hand, oils are mainly obtained from plants, e.g., corn, peanut, cotton seed, olive
and soyabean oils which are liquids.
All oils and fats are colourless, odourless and neutral substances in pure state. They are lighter than water
and immiscible with it. They are soluble in organic solvents e.g. benzene, ether and chloroform etc.
6.3.3 Hardening of Oils
Oils can be converted to fats by addition of hydrogen (hydrogenation) in presence of nickel catalyst. This
process of converting oils to hard fats is known as hardening of oils. This reaction is used in the
preparation of margarine.
6.3.4 Rancidity
Fats and oils are quite reactive substances. When stored for any considerable length of time, especially
when the temperature is high and the air has free access to them, they deteriorate and they develop an
unpleasant odour which is rancidity.
It is caused mainly due to the hydrolysis of ester linkage and oxidation across the double bonds. Some
spoil very much more rapidly than others. The rancidity of a given fat is not necessarily the result of long
storage under unfavorable conditions. The fat may have been spoiled and rancid from the moment of its
production. In other words, to obtain a sound and sweet fat, the raw material must be sound. The fat thus
obtained must be stored under favorable conditions and its consumption should not be delayed.
