Which of the following alcohol is most reactive in the reaction where O-H bond breaks?

Explanation

Option A : Primary alcohols are not very reactive when the O-H bond breaks because the carbon atom to which the hydroxyl group is attached is not electron deficient. Option B : Tertiary alcohols are not very reactive when the O-H bond breaks because the carbon atom to which the hydroxyl group is attached is not electron deficient. In tertiary alcohol, the carbon atom to which the hydroxyl group (-OH) is attached is bonded to three other alkyl groups (-R). This carbon atom has a partial positive charge due to the electronegativity difference between the carbon and oxygen atoms, but it is not highly electron deficient. As a result, tertiary alcohols are not very reactive when the O-H bond breaks, and they do not undergo reactions such as nucleophilic substitution or elimination very easily. Option: C The bond between oxygen and hydrogen in methanol is not a hydrogen bond as alluded to in a previous answer, it's a polar covalent bond. Secondly, when methanol ionises it forms methoxide and a proton, not a methyl cation and a hydroxide ion. Now that's out of the way, methanol becomes more reactive when a proton is removed because doing so makes the remaining molecule (methoxide) very unstable, and so it wants to grab a proton (or other electrophile), from anywhere. This is reflected in the high pKa of methanol of 16. When the 'acidic' proton is removed from methanol, a negative charge sits solely on the oxygen atom. There's no resonance stability to spread the charge over several atoms, like an alkanoic acid for example. Option D : Secondary alcohols are not very reactive when the O-H bond breaks because the carbon atom to which the hydroxyl group is attached is only moderately electron deficient. In secondary alcohol, the carbon atom to which the hydroxyl group (-OH) is attached is bonded to two other alkyl groups (-R) and one other carbon atom (-R'). This carbon atom has a partial positive charge due to the electronegativity difference between the carbon and oxygen atoms, but it is not highly electron deficient. As a result, secondary alcohols are not very reactive when the O-H bond breaks, and they do not undergo reactions such as nucleophilic substitution or elimination very easily.