What is the main reason for the decreased reactivity of tertiary alcohols in dehydration reactions compared to primary alcohols?

The decreased reactivity of tertiary alcohols in dehydration reactions is primarily due to steric hindrance around the carbon atom to which the hydroxyl (-OH) group is attached. In tertiary alcohols, the carbon atom bonded to the hydroxyl group is also attached to three other carbon atoms. This creates a crowded environment, making it physically difficult for the reacting species, such as acids or leaving groups, to access the carbon atom during the elimination reaction leading to dehydration.

This steric bulk can impede the formation of a carbocation intermediate, which is a key step in the dehydration process. While tertiary alcohols can form stable carbocations, if the surrounding groups are too bulky, they can hinder the appropriate alignment and interaction needed for the elimination reaction to proceed efficiently. In contrast, primary alcohols, which have less steric hindrance, are more reactive because the simpler structure allows easier access for the necessary reagents and a more favorable transition state during the reaction.

Thus, the correct understanding of decreased reactivity in tertiary alcohols comes down to the significant impact of steric hindrance, which limits the efficiency of dehydration reactions compared to their primary counterparts.