Carboxylic Acids: Preparation - Nomenclature - Properties

Carboxylic Acids

The functional group of all carboxylic acids is carboxyl group (-COOH) This group consists of hydroxyl and carbonyl groups.

The structural formula of carboxylic acids is:

represents alkyl group or hydrogen atom. Their general formula is (CnH2nO2).

Nomenclature of Carboxylic Acids

Carboxylic acids are named according to IUPAC system as shown in the fol- lowing steps:

1.      The longest carbon chain that includes carboxyl group is chosen and numbered. Carboxyl group is given number 1.

2.     Branched groups are written in alphabetical order and with their numbers mentioned.

3.     The word “acid” is added.

4.     The name of the alkane which corresponds to the longest carbon chain is written and suffix (-oic) is added in the end.

Preparation of Carboxylic Acids

Carboxylic acids are prepared through many methods. As we have mentioned before, they are prepared by oxidation of primary (1°) alcohols or aldehydes. We will explain preparation of carboxylic acids using Grignard reagent. In this reaction, firstly, alkyl halide is used then CO2. As shown below, in order to prepare wanted carboxylic acid, hydrolysis (reaction with water) in acidic medium is realized.

The carboxylic acid prepared through this method has one extra carbon than the alkyl group of Grignard compound.

Physical Properties of Carboxylic Acids

The carboxylic acids with low molecular weights (lower than C₁₀) are liquids with pungent and unpleasant odors. As their molecular weights increase, their boiling points also go up and the boiling points are higher than those of alcohols with the same number of carbons. The reason is that carbonyl and hydroxyl group in acid molecule ionize at a high rate and hydrolysis occurs (more than alcohols) between acid molecules.

Carboxylic acids with low molecular weights are highly soluble in water. As their molecular weights increase, their solubilities decrease.

Chemical Properties of Carboxylic Acids

A) Acidity of Carboxylic Acids:

Carboxylic acids assume acid property by carboxyl group’s donation of proton. Therefore, they react with bases readily and form water and salt.

B) Reactions of carboxylic acid with carbonates and bicarbonates

When carboxylic acids react with carbonates and bicarbonates, CO2 gas is formed. This reaction is one the important reactions to differentiate carbonyl group in carboxylic acids.

C) Reaction of carboxylic acid with alcohols (esterification reaction)

Carboxylic acids react with alcohols along with HCl or H2SO4 catalysts and form esters. This reaction is reversible as seen below and called as esterification.

D) Reduction of Carboxylic Acids

Carboxylic acids are reduced by reducing agents such as lithium aluminum hydride (LiAlH4) and form primary alcohols. But they cannot be reduced with H2/Ni.

 Example 1

Prepare propanoic acid starting from ethanal.

Solution:

The equations of the reactions are as follows:

Example 2

Prepare propanoic acid starting from ethyl chloride.

Solution:

In this method, a carboxylic acid is obtained with one more extra carbon than the number of carbons of the starting alkyl.

 (Vitamin C ), C which is known as ascorbic acid is a carboxylic acid which is found in citrus fruits most. It has an important place in our lives. Vitamin C helps improving defense of body by strengthening immune system. It is given as pills to patients.

Vitamin C is found in many fruits.

See also

1.      Alcohols: nomenclature – synthesis - Properties

2.     Alkyl halides: nomenclature - synthesis - Properties

References

1.      K. J. Denniston c J. J.Topping c and R. L.Caretc “General Organic and Biochemistry”c Mc-Graw- Hillc New York   (2004).

2.     K.W. Whittenc R.E. Davis  and L. M. Peckc “General Chemistry” 7th ed. Holt Rinehart and Winstonc New York (2010).

3.     Clayden, J.; Greeves, N. and Warren, S. (2012) Organic Chemistry. Oxford University Press. pp. 1–15. ISBN 0-19-927029-5.

4.     Streitwieser, Andrew; Heathcock, Clayton H.; Kosower, Edward M. (2017). Introduction to Organic Chemistry. New Delhipages=3–4: Medtech (Scientific International, reprint of revised 4th edition, Macmillan, 1998). ISBN 978-93-85998-89-8.