Question

What is the major difference between a cyclic hemiacetal and a cyclic acetal?

Answer 1

It is apparent that ketal and ketone share the same first three letters. That makes it easier to remember that:

• acetals form from aldehydes
• ketals form from ketones

That implies that:

• cyclic hemiacetals form from cyclization of aldehydes
• cyclic hemiketals form from cyclization of ketones

Let's take a look at how a cyclic hemiacetal forms.

I drew the less obvious arrows, but I'll leave the proton transfers for you to practice. The formation of a cyclic hemiketal is nearly identical; simply replace the explicit hydrogen with a methyl group.