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1 Answer
Al E.
May 22, 2018

A diastereomer is a non-superimposable non-mirror image of a molecule with more than one chirality center.

A simple heuristic I use is: Look for molecules that have at least one chirality center in common and one or more inverted.

I find the method with the highest certainty is using RS configuration. I've noted each's configuration below (the more practice you get, the quicker you will get).

puu.sh

Hence,

(iii) and (iv) are the same molecule, as well as enantiomers of (i).

(i) and (ii) are diastereomers. (1)

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