A D-Aldopentose Forms Two Aldohexoses, a and B, After a Kiliani-Fisher

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The Answer is The Kiliani-Fischer synthesis adds a carbon to an aldopentose to generate the two aldohexoses. The aldopentose and aldohexoses can be oxidized to aldaric acids, where the top and bottom carbons of the Fischer projections are both carboxylic acids. The aldopentose gives an optically inactive aldaric acid, so it must have been either D-ribose or D-xylose, since both aldaric acids formed from these have a plane of symmetry bisecting carbon 3.

The Kiliani-Fischer synthesis adds a carbon to an aldopentose to generate the two aldohexoses. The aldopentose and aldohexoses can be oxidized to aldaric acids, where the top and bottom carbons of the Fischer projections are both carboxylic acids. The aldopentose gives an optically inactive aldaric acid, so it must have been either D-ribose or D-xylose, since both aldaric acids formed from these have a plane of symmetry bisecting carbon 3. The two aldohexoses formed from the Kiliani-Fischer synthesis of ribose would be D-allose and D-altrose. The aldaric acid made from D-allose would be optically inactive, while the aldaric acid made from D-altrose would be optically active. This matches the data given. Let's look at the other possibility. The two aldohexoses made from the Kiliani-Fischer synthesis of xylose would be D-gulose and D-idose. The aldaric acids made from gulose and idose will both be optically active, which doesn't match the data given, so we can conclusively identify the aldopentose as D-ribose. Aldohexose A is D-allose and aldohexose B is D-altrose.

The two aldohexoses formed from the Kiliani-Fischer synthesis of ribose would be D-allose and D-altrose.

The aldaric acid made from D-allose would be optically inactive, while the aldaric acid made from D-altrose would be optically active. This matches the data given. Let's look at the other possibility.
The two aldohexoses made from the Kiliani-Fischer synthesis of xylose would be D-gulose and D-idose.

The aldaric acids made from gulose and idose will both be optically active, which doesn't match the data given, so we can conclusively identify the aldopentose as D-ribose. Aldohexose A is D-allose and aldohexose B is D-altrose.

A D-Aldopentose Forms Two Aldohexoses, a and B, After a Kiliani-Fisher

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A D-Aldopentose Forms Two Aldohexoses, a and B, After a Kiliani-Fisher

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