Problem

3,4-Dimethylpent-1-ene has the formula When pure (R)-3,4-dimethylpent-1-ene is treated...

3,4-Dimethylpent-1-ene has the formula When pure (R)-3,4-dimethylpent-1-ene is treated with hydrogen over a platinum catalyst, the product is (S)-2,3-dimethylpentane.

(a) Draw the equation for this reaction. Show the stereochemistry of the reactant and the product.

(b) Has the chirality center retained its configuration during this hydrogenation, or has it been inverted?

(c) The reactant is named (R), but the product is named (S). Does this name change imply a change in the spatial arrangement of the groups around the chirality center? So why does the name switch from (R) to (S)?

(d) How useful is the (R) or (S) designation for predicting the sign of an optical rotation? Can you predict the sign of the rotation of the reactant? Of the product? (Hint from Juliet Capulet: “What’s in a name? That which we call a rose/By any other name would smell as sweet.”)

Step-by-Step Solution

Solution 1

Step #1 of 4

(a)

3, 4 – Dimethyl -1- pentene has one chiral centre. So the molecule shows optical activity.

When pure (R) – 3, 4 – dimethyl -1- pentene is treated with hydrogen over platinum catalyst the product is (S) – 2, 3 – dimethyl pentane.

Step #2 of 4

(b) There was an inversion in the configuration of the chiral centre during the hydrogenation.

Step #3 of 4

(c) According to the Cahn – Ingold – Prelog convention the reactant get “R” configuration because the priority order gives the clockwise rotation. But when the molecule is hydrogenated the vinyl group becomes ethyl group which has less priority than the iso propyl group. So the priority order gives anticlock wise rotation which means ‘S’ configuration. There is no change in the spatial arrangement.

Step #4 of 4

(d) The (R) and (S) nomenclature is only an artificial way of describing how the atoms are arranged in space. The R – S nomenclature gives the absolute configuration of a compound. But there is no obvious correlation between the sign of rotation and absolute configuration. The polarimetre can measure the sign of rotation of a particular enantiomer.

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