[bizkett]

Hello,

I am studying for the MCAT and my book states that acetals can exist in cyclic form. However in the practice question it says that only hemiacetals and hemiketals are cyclic. Can anyone please clarify this?

thank you

[Babcock_Hall]

Consider the structure of β-methylglucopyranoside.  What do you think?

[spirochete]

Here's what your book is getting at:

Hemi-acetals in general are not very stable, and they form reversibly. So most hemi-acetals will break down to their parts: a ketone/aldehyde and 2 equivalents of alcohol. This apparently stems from the strength/stability of the C=O pi bond.

There is one exception to this: hemi-acetals where the carbon in the O-R group is connected to a ring. In this case, the equilibrium favors hemi-acetal formation, although there's also significant amounts of the open chain form. This kind of thing is present in carbohydrates like glucose, fructose, etc.

Reasoning for this is entropy: it iess entropically disfavored to make a ring than it is to combine three molecules in to two molecules.

In contrast, regular "full-blown" acetals can do whatever they want, although acetal formation is mostly favored at equilibrium only with little or no water present, AFAIK. A full-blown acetal can be part of a ring or not part of a ring. It matters much less in this case.

[bizkett]

I see, the chapter was on carbohydrates so I suppose that makes sense. Thank you