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The first pair is not stereoisomers. You have correctly explain why. When you are dealing with stereoisomers, that's what you have to do first: Identify stereo centers. Accordingly, each compound of second pair has two stereo centers. Then, mark (R,S) configurations of all stereo centers. As I marked, the first compound has (R,S) configuration, while second ...


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Your first set of compounds are identical. In the Fischer projection of a chiral centre, you can rotate three of the connected groups in a clockwise/anti-clockwise direction without changing the configuration of the chiral centre. Upon performing this operation (keeping the top fixed, rotating bottom three clockwise) on the first compound in the first set, ...


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