Primary alcohols and secondary alcohols can be oxidized with chromic acid, but tertiary alcohols cannot. (i) How do the structural differences between the alcohols account for the observed reactions?

Question

Primary alcohols and secondary alcohols can be oxidized with chromic acid, but tertiary alcohols cannot. (i) How do the structural differences between the alcohols account for the observed reactions?

Answer 1

This one is best shown with a picture! See below from:

https://chem.libretexts.org/Courses/SUNY_Potsdam/Book%3A_Organic_Chemistry_II_%28Walker%29/19%3A_Oxidation_and_Reduction/19.06%3A_Oxidation_of_alcohols_and_aldehydes

(It won't let me paste it in, but you can go check it out for yourself)

In the oxidation of an alcohol, the alcohol is converted to a ketone (or aldehyde in the case of a primary alcohol). For this to occur, there must be at least one hydrogen available on the carbon attached to the oxygen in the alcohol to serve as an electron donor when the double bond is formed (see step 3 of the picture above). Tertiary alcohols do not have this H available, because by definition they have three non-hydrogen groups attached to that carbon. Therefore, the double bond can't form and, since the chromic acid-alcohol complex you see in step 3 is not very stable (if it were things would stop there), everything would just break apart into its original components. Effectively, step 1 might happen from time to time if you put chromic acid and a tertiary alcohol in a container together, but the reaction couldn't proceed and they would eventually break back apart due to instability.

TLDR: Tertiary alcohols do not the required spare hydrogen to lose during the ketone/aldehyde formation step of the oxidation reaction.

Hope that helps!