Describe properties of carboxylic acids.

1. Physical state: the first three aliphatic acids are colourless liquids with pungent smell. The next six are oily liquids with an odour of rancid butter while the higher members are colourless, odourless waxy solids. Benzoic acid is referred to as crystalline solid.

2. Solubility the first four aliphatic members are soluble in water due to intermolecular hydrogen bonding with water molecules.

With increasing size of alkyl groups, the non-polar part of the molecule predominates thereby reducing the solubility in water. The higher members are almost insoluble in water.

3. Boiling points: carboxylic acids have quite high boiling points due to presence of intermolecular hydrogen bonding which results in the formation of dimeric structures. 

Due to dimeric structure, the effective molecular mass of the acid becomes double the actual mass. Hence, carboxylic acids have higher boiling points than alcohols of comparable molecular masses. Moreover, O-H bond in carboxylic acids is more polar than O-H bond in alcohols. This is due to electron withdrawing effect of carbonyl group on O-H. Hence, H-bonds in carboxylic acids are comparatively stronger than those of that in alcohols.

4. Melting points: in first ten members of homologous series, the alternation effect is observed. The alternation effect implies that the melting point of an acid with even number of carbon atom is higher than the acid with odd number of carbon atoms above and below it. However, no such effect is observed in homologues with more than ten carbons. The alternation effect can be explained on the basis of the fact that the carboxylic acids with even number of carbon atoms, the terminal methyl group and carboxyl group of the opposite sides of zig-zag carbon chain. Hence, they fit better in the crystal lattice and it results in stronger intermolecular forces. On the other hand, acids with odd number of C atoms have carboxyl and terminal methyl number of C atoms has carboxyl and terminal methyl groups on the same side of zig-zag carbon chain. Therefore, such molecules being relatively unsymmetrical, fit poorly in the crystal lattice. This causes weaker intermolecular forces and accounts for the relatively lower melting points.
268_carboxylic acid.png 
Even number of C-atoms, fit better, in crystal lattice, have higher m.pts (Terminal groups are on opposite side)
2096_carboxylic acid1.png 
Odd numbers of C-atoms, fit properly, in crystal lattice, have lower m.pts. (Terminal groups are on same side).    

The melting and boiling points of aromatic acids are generally higher than those of aliphatic acids of similar molecular masses. This is presumably due to the fact that planar benzene ring in these acids can pack closely in the crystal lattice than zig-zag aliphatic acids.

   Related Questions in Chemistry

  • Q : Adiabatic compression A lean natural

    A lean natural gas is available at 18oC and 65 bars and must be compressed for economical pipeline transportation. The gas is first adiabatically compressed to 200 bars and then isobarically (i.e. at constant pressure) cooled to 25°C. The gas, which is

  • Q : Alkaline medium The amount of KMnO 4

    The amount of KMnO4 required to prepare 100 ml of 0.1N solution in alkaline medium is: (a) 1.58 gm (b) 3.16 gm (c) 0.52 gm (d) 0.31 gmAnswer: (a) In alkaline medium KMnO4 act as ox

  • Q : Dissolving Group IV Carbonate Explain

    Explain how dissolving the Group IV carbonate precipitate with 6M CH3COOH, followed by the addition of extra acetic acid.

  • Q : Unit of mole fraction Provide solution

    Provide solution of this question. Unit of mole fraction is: (a) Moles/litre (b) Moles/litre2 (c) Moles-litre (d) Dimensionless

  • Q : Explain the catalyst definition and

    Catalyst is a substance which accelerates the rate of a chemical reaction without undergoing any change in its chemical composition or mass during the reaction. The phenomenon of increasing the rate of a reaction with the help of a catalyst is known as catalysis.

  • Q : Describe Point Groups. For any

    For any symmetric object there is a set of symmetry operations that, together, constitute a mathematical group, called a point group.It is clear from the examples that most molecules have several elements of symmetry. The H2O

  • Q : What is synthetic rubber and how it

    To meet human needs, scientists have started preparing synthetic rubbers. Besides having similar properties as natural rubbers they are tougher, more flexible and more durable than natural rubber. They are capable of getting stretched to twice its length. Though, it reverts to its original shape

  • Q : Problem related to molarity Provide

    Provide solution of this question. Increasing the temperature of an aqueous solution will cause: (a) Decrease in molality (b) Decrease in molarity (c) Decrease in mole fraction (d) Decrease in % w/w

  • Q : How reactive is Trimethylindium towards

    Illustrate the reason, how reactive is Trimethylindium towards oxygen and water?

  • Q : Explain reactions of carbonyl oxygen

    In these reaction oxygen atom of carbonyl group is replaced by either one divalent group or two monovalent groups. Reaction by ammonia derivatives: aldehydes and ketones react with a number of ammonia derivatives such as hydroxylaminem hydrazine, semicarbazide etc. in weak acidic medium.

©TutorsGlobe All rights reserved 2022-2023.