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126 Organic Chemistry Solutions Manual Example (c) might look like the example (b) except that the product has three 'chiral centres' and looks more complicated. In fact, the starting material is C₂ symmetric and the two suggested by the wiggly line to the OH group are identical. Check for yourself. The product is a single diastereoisomer (anti-) but there is no meaning in any stereochemical designation of the central (OH) centre. H rotate about dotted line by 180° Problem 12 How many diastereoisomers of compound 1 are there? State clearly whether each diastereoisomer is chiral or not. If you had made a random mixture of stereoisomers by a chemical reaction, by what types of methods might they be separated? Which isomer(s) would be expected from the hydrogenation of compound 2? Me Me Me Me 1 2 CO2H Purpose of the problem A more difficult example with a chemical reaction. Suggested solution The simplest way to do the first part is to say that the two methyl groups can either be on the same or opposite sides of the ring and that in each case the CO₂H group can be up or down. Let's see what we get from that. Me Me Me Me Me Me Me Me methyl groups methyl groups on the same on opposite side of the ring sides of the ring CO₂H CO₂H The first two compounds (methyls cis) are obviously different diastereoisomers and both have a plane of symmetry so neither is chiral. The second pair (methyls trans) are slightly more tricky - they are the same! And this isomer does not have a plane of symmetry and is chiral. In fact this is like the last example in Problem 11. There are three diastereoisomers and only one of them is chiral. Reduction of a double bond by catalytic hydrogenation puts two hydrogen atoms on the same side of the alkene. This can give only the first two compounds but we might expect hydrogen to add more easily to the side of the alkene opposite the CO₂H group. Our prediction is: Me Me Me Me Me Me catalyst + major product? minor product?