What is meant by a spacer group,?
I’ve seen this term used in a number of different contexts. First, generally, if a molecule is being designed with more than one functional group, and each group on the molecule has a distinct “job” to do in mind, it is sometimes necessary to chemically and physically isolate the functional groups from each other. This is most easily done with a spacer chain of methylene groups (I have also seen CF2 groups). The methylene chain shouldn’t be too long, as the chain can bend around, resulting in the interaction that was unwanted in the first place. Another context would be as a synonym for “tethering” groups. If we want a reactive molecule to be physically stuck somewhere, say on a surface, but still be locally mobile so as to maintain reactivity, we could attach it to the surface via a methylene chain spacer, or tether, group. In still another context, I have seen the term “spacer” applied to polymers, in which certain structures, not necessarily methylene, are used to align chain repeat
