The compound shown is 1,2-dimethylcyclobutane (no configuration specified). Consider all dimethylcyclobutanes (1,1-, 1,2- and 1,3-) with the ring treated as planar. Which statements are correct?

A1,2-Dimethylcyclobutane has three stereoisomers
B1,3-Dimethylcyclobutane exists as a pair of enantiomers
C1,1-Dimethylcyclobutane has no stereoisomers
DThe total number of dimethylcyclobutane stereoisomers, over all three constitutions, is six

Answer: A, C, D

1,2-Dimethylcyclobutane: cis is meso and trans is a pair of enantiomers, so 3 stereoisomers. 1,3-Dimethylcyclobutane: C1 and C3 are not stereocentres (two identical

CH X_{2}

paths); cis and trans exist but both have a mirror plane through C1 and C3, so 2 achiral stereoisomers and no enantiomers. 1,1-Dimethylcyclobutane: 1 form only. Total = 3 + 2 + 1 = 6.

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The compound shown is 1,2-dimethylcyclobutane (no configuration specified). Consider all dimethylcyclobutanes (1,1-, 1,2- and 1,3-) with the ring treated as planar. Which statements are correct?

Answer: A, C, D

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