[orgo814]

In trying to identify a pair of enantiotopic and diastereotopic atoms in 4-methylheptane I am having some confusion. Diastereotopic I consider any two hydrogen on any of the other carbon atoms (carbon atoms other than C4) since replace of each of them with a substituent such as D would lead to a different diastereomer. However I'm having more issues with enantiotopic. Would we consider the methyl group and hydrogen on C4 to be enantiotopic? I can't see where any other enantiotopic pair would be  on this. Thanks.

[mjc123]

Diastereotopic I consider any two hydrogen on any of the other carbon atoms (carbon atoms other than C4) since replace of each of them with a substituent such as D would lead to a different diastereomer.

On the right lines, but not any other carbon - not the CH₃ groups, as substitution won't make them chiral.

Would we consider the methyl group and hydrogen on C4 to be enantiotopic?

No. Substitution on C4 or methyl would give different structural isomers, not stereoisomers.
Consider the molecule as drawn below. As the propyl groups are the same, C4 is not chiral, but it becomes so if you make a substitution on one of the propyl groups. So does the substituted carbon if it is a methylene (not methyl) group.
What is the relationship between, for example:
An H on C1 and an H on C7?
H2a and H2b?
H2a and H6a?
H2a and H6b?
(C3-C5 are analogous to C2-C6)

[orgo814]

So if you say 2A and 2B are enantiomeric, how would a pair here be diastereotopic? Since there's no chiral stereocenter here.

[mjc123]

So if you say 2A and 2B are enantiomeric

I didn't say that. They aren't. Nor are they enantiotopic (enantiomeric refers to whole molecules). They are, as you said, diastereotopic. If you substitute on C2, C4 becomes S (I think, as I have drawn it). If you replace H2a with D, you get 2S, 4S. If you replace H2b you get 2R, 4S. What do you get if you replace H6a or H6b?

[orgo814]

So 2a and 6a would be enantiotopic (since 6a would lead to 2R 4R)?

[mjc123]

Yes

[orgo814]

Thank you for your help you were very clear