Answer to Question #166402 in Organic Chemistry for Hafiz Faizal

Question

Give a short explanation for each of the following observations:

(i) Alcohols with three or fewer carbon atoms are soluble in water. Those with four carbon atoms are sparingly soluble and those with five of more carbon atoms are insoluble in water

. (ii) Propanol (Mw = 60) has a higher boiling point than butane (Mw = 58).

(iii) The decreasing order of acidity of alcohol is 1° > 2° > 3°

(iv) 2-Chloroethanol is more acidic than ethanol. 

Answer 1

(i) Carbon atoms in alcohol are hydrophobic and do not interact well with water. But there is a hydroxyl group, which interacts well with water and, with a short carbon chain length, can compensate for the hydrophobic interaction.

(ii) The longer the carbon chain, the stronger the intermolecular interaction, the higher the boiling point.

(iii) Methyl groups have a positive inductive effect, this effect destabilizes the alcoholate ion and reduces the positive charge on the hydrogen atom. The positive inductive effect grows with an increase in the number of methyl groups; the hydroxyl group of a tertiary alcohol experiences the strongest effect of this effect.

(iv) The halogen atom, located at the α-carbon atom of alcohol, has a pronounced negative inductive effect effect. The latter, by the polarization of the Ha1-C, C-C, C-0 bonds, leads to polarization of the hydroxyl group and an increase in the effective positive charge on the hydrogen atom of the hydroxyl group. The polarization of the hydroxyl group facilitates easier dissociation with the formation of an alcoholate anion and a proton.