The functional group that is most similar to an ester in this case is the amide. Draw a Newman projection showing the most stable conformation of the molecule shown below. View the molecule along the C1-C2 bond, with C2 in front. The conformation is more stable when the chlorine is gauche to the smaller methyl group instead of the larger isopropyl group. Be careful to retain the absolute configuration of the C2 stereocenter.
Ring strain is strain that is produced because the molecule has a ring.
It is usually a combination of torsional and angle strain. Draw a molecule that obviously has more ring strain than cyclopropane. There are many possible answers. Cyclopropene below has more ring strain than cyclopropane because two of the sp 3 carbons of cyclopropane have been replaced with sp 2 carbons. Clearly draw the single most stable conformation of the molecule shown below.