--%>

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 : Concentration of Sodium chloride

    Provide solution of this question. If 25 ml of 0.25 M NaCl solution is diluted with water to a volume of 500ml the new concentration of the solution is : (a) 0.167 M (b) 0.0125 M (c) 0.833 M (d) 0.0167 M

  • Q : 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

  • Q : How reactive is Trimethylindium towards

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

  • Q : Molecular energies and speeds The

    The average translational kinetic energies and speeds of the molecules of a gas can be calculated.The result that the kinetic energy of 1 mol of the molecules of a gas is equal to 3/2 RT can be used to obtain numerical values for the

  • Q : Simulate the column in HYSYS The

    The objective of this work is to separate a binary mixture and to cool down the bottom product for storage. (Check table below to see which mixture you are asked to study). 100 kmol of feed containing 10 mol percent of the lighter component enters a continuous distillation column at the m

  • Q : Pressure and power for adiabatic

    a) Air flowing at 1 m3/s enters an adiabatic compressor at 20°C and 1 bar. It exits at 200°C. The isentropic efficiency of the compressor is 80%. Calculate the exit pressure and the power required. b) Steam enter

  • Q : Define alum Illustrate alum?

    Illustrate alum?

  • Q : Problem on vapour pressure Choose the

    Choose the right answer from following. If P and P are the vapour pressure of a solvent and its solution respectively N1 and N2 and are the mole fractions of the solvent and solute respectively, then correct relation is: (a) P= PoN1 (b) P= Po N2 (c)P0= N2 (d)

  • Q : Colligative property problem Which is

    Which is not a colligative property: (a) Refractive index (b) Lowering of vapour pressure (c) Depression of freezing point (d) Elevation of boiling point    

  • Q : Precipitation problem On passing H 2 S 

    On passing H2S  gas through a solution of Cu+ and Zn+2 ions, CuS is precipitated first because: (i) Solubility product of CuS is equal to the ionic product of ZnS (ii) Solubility product of CuS is equal to the solubility product o