Why are tertiary alcohols very inactive in acid catalyzed hydro-alkoxy additions?

Question

Ryan Andrews · Accepted Answer

Because they are bulky and sterically-hindered.

I've drawn here the mechanism for an acid-catalyzed hydro-alkoxy addition using just ethanol and strong acid:

You can plainly see that the alcohol has to end up acting as the nucleophile, but tert-butanol...

...is too bulky and the reaction would take a very long time to proceed. The alcohol would have to move around slowly until it finds the right orientation with the cationic intermediate for the backside attack. By the time it finds the right orientation so that orbital blockage is not an interference, the good leaving group #OH_2^(+)# would have fallen off (no joke)---the water substituent on the protonated alcohol is not nearly as good of a base as a tertiary alkyl group would be (pKa of alkyl >> pKa of water).

tert-butanol is better off doing acid-base reactions at a high temperature, such as the first step in an elimination reaction.

Isaac Aronson · Answer

undefined Tertiary alcohols are very inactive in acid-catalyzed hydro-alkoxy additions because the formation of the carbocation intermediate, which is necessary for the reaction to proceed, is hindered by steric effects. Tertiary alcohols have three alkyl groups attached to the carbon bearing the hydroxyl group, leading to significant steric hindrance. As a result, the formation of the carbocation intermediate is energetically unfavorable, making the reaction slow and less likely to occur.