Question

What is the major driving force for the reaction below? In which of the following reactions does a reversal of polarity at a carbon atom take place?

Answer 1

The reaction of a carboxylic acid with thionyl chloride yields the formation of the acid chloride with the liberation of sulfur dioxide gas and hydrochloric acid gas. SOCI2 + SO2(g) + HCl(g) POH CI The chloride ion is a better leaving group than hydroxide ion, therefore, the chloride ion in acid chloride tends to leave. Thus, the acid chloride is not thermodynamically stable than its corresponding carboxylic acid. This reaction is not an exothermic reaction since no amount of heat is liberated during the reaction. The nucleophilicity of an ion is inversely proportional to its electronegativity. Higher the electronegativity of an ion, poor will be the nucleophile and vice-versa. Since oxygen is more electronegative than chlorine, therefore, hydroxide ion is a better nucleophile than chloride ion. In the above reaction, sulfur dioxide and hydrochloric acid are liberated as gases in the surrounding irreversibly. Hence, the major driving force for the given reaction is "the byproducts SO2 and HCI bubbled out of solution irreversibly". The Grignard reagent can be synthesized by treating an organic halide with magnesium in the presence of an ether. The reaction involved in the formation of the Grignard reagent is shown as follows:

Ether R-X + Mg 8- 8+ R—MgX → Organic halide Magnesium Grignard reagent The organic halide must be aprotic in nature, otherwise the protic halide yield in the liberation of hydrogen gas and inhibits the formation of the Grignard reagent. Since among the given options, compound D is a halide containing hydroxyl group. therefore, it will resist the formation of the Grignard reagent. OH Mg _Br Br Mg + H2(g) Hence, the correct option is ‘D'. When carbon is bonded with a more electronegative atom, then it will carry a partial positive charge. But if it is bonded with a more electropositive atom, then it will attain a partial negative charge. In the following reaction, the carbon attains a partial positive charge in the reactant side as it is bonded with a more electronegative bromine atom and it attains a partial negative charge since it is bonded with lithium atom (more electropositive atom than carbon). Hence, the reversal of polarity on the carbon atom takes place in this reaction.

å S- Li(s) gt While this does not happen in the rest of the reactions as the carbon attains a partial positive charge in both reactant and product side.