Why is an acetal less reactive than an aldehyde?

[pup329]

Why is an acetal less reactive than an aldehyde?

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[AdaptPrep]

Hi, pup329--

An aldehyde has a carbonyl group; focus on that. The carbonyl has resonance:

The resonance structure on the right makes it much more reactive than an acetal. An acetal cannot form a carbocation through resonance.
Acetals readily undergo hydrolysis reactions, but aldehydes/ketones can undergo many types of reactions.

Good luck studying!

 

[PlsLetMeIn21]

I was taught that pi bonds are weaker than sigma bonds, so carbonyl pi bonds are more reactive than sigma bonds in acetals and ketals.

 

[CommyO]

Surprised no one menti0ned hindrance

 

[PositivePhotoresist]

Adding onto @AdaptPrep 's answer, that resonance structure shows you that the carbon of the carbonyl is unusually electrophilic because the electron density is being drawn away from that carbon. I should clarify that the resonance structure in that answer is a minor contributor because the carbon does not have a full octet (resonance structures with all atoms with complete octets will be the most stable); nonetheless, it tells us the strong inductive effect of a very electronegative atom next to a double-bonded carbon. Since that carbon essentially has a lot less electron density in general, that carbon will be really electrophilic. On the other hand, the oxygen on the carbonyl will essentially gain electron density and be really nucleophilic. For this reason, carbonyl bonds are typically characterized by all sorts of reactions.

Now, consider the general structure of an acetal. In basic conditions or strong nucleophilic conditions, you'd have a bad leaving group for a substitution reaction. In most acetals, there would also be no reasonable proton for a base to abstract. However, if you had acidic conditions, you can get the reverse reaction of what you got to form the acetal. To conclude, acetals are stable in basic conditions, but they still react in acidic conditions.