Q15-15-32E - Organic Chemistry (Mcmurry)

Question

Cycloheptatrienone is stable, but cyclopentadienone is so reactive that it can’t be isolated. Explain, taking the polarity of the carbonyl group into account.

Cycloheptatrienone                                                Cyclopentadienone

Step-by-Step Solution

Answer

                                       Cycloheptatrienone

                                             Cyclopentadienone

Carbonyl group

When a carbonyl group is polarized it places a positive charge on the ring carbon which makes cycloheptatrienone aromatic and cyclopentadienone anti aromatic based on the huckle’s rule of aromaticity.

Aromatic and antiaromatic

The cycloheptatrienone ring in B contains six pi electrons and is aromatic according to Huckel’s rule whereas the cyclopentadienone ring in D contains four pi electrons and is antiaromatic. The resonance forms of cycloheptatrienone and cyclopentadienone are represented as:

                                           Cycloheptatrienone

                                            Cyclopentadienone