As I’m sure you’ve noticed by now, accurate bookkeeping of your carbon-count is seemingly petty but actually important: an error in bookkeeping can lead you to an incorrect answer even if your logic is correct.
With this in mind I thought it’d be helpful to share four common abbreviations that fluster many students.
The prefix “Ac” denotes 2-carbons in the acetyl group. The acetyl group also contains a carbonyl. Pay attention to where the carbonyl is located.
The phenyl group is simply a benzene directly connected to the carbon skeleton bearing the group.
The benzyl group is a phenyl group plus one carbon. The additional carbon is called a benzylic carbon because it is “benzylic to” the aromatic ring.
This is where students get confused. Look at the additional letter in this group’s name: O. The “o” in the name benzoyl can be thought of as an indicator that this group contains an oxygen (carbonyl).
The “benz” part of the name benzoyl can be used as a reminder that this carbonyl resides on a benzylic carbon.
Okay–so you’ve straightened out these four groups. What now? Please, for the sake of your grade, “blow-up” these abbreviations if you ever come across them on an exam: you may reveal something that wasn’t so obvious before blowing them up.
For example, consider blowing-up a phenyl group attached to a carbocation. If you do this, it’ll be more obvious that the carbocation of interest is benzylic to a benzene ring and quite stabilized by resonance.
Next, consider blowing-up a benzyl group attached to a carbocation. If you do this, you’ll see that the benzyl group’s aromatic ring is one-too-many carbons away from the carbocation to stabilize the carbocation.
Moral of the story? Understanding and blowing-up these groups will help you avoid careless errors!
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