Count the stereoisomers of four sugars, each with an unbranched carbon chain: an aldotetrose (4…
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Count the stereoisomers of four sugars, each with an unbranched carbon chain: an aldotetrose (4 carbons), an aldopentose (5 carbons), an aldohexose (6 carbons) and a 2-ketohexose (6 carbons). Count the D and L forms together. Then say how many of the aldohexoses belong to the D series.
Need a reminder? The key ideaShow
A carbohydrate is a polyhydroxy aldehyde or ketone. An aldose has stereocentres and a ketose has , where is the number of carbons in the chain. The number of stereoisomers is , and half of them belong to the D series. An empirical formula comes from the moles of each element in 100 g.
The worked solution
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Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Plan. Count the stereocentres, then use .
Step 1. Stereocentres. Aldotetrose: . Aldopentose: . Aldohexose: . Ketohexose: .
Step 2. Stereoisomers. , , and .
Step 3. The D series. Half of the 16 aldohexoses are D sugars: 8.
Answer: 4, 8, 16 and 8 stereoisomers, and 8 D aldohexoses.
Check. A ketohexose has one stereocentre fewer than an aldohexose, so it has half as many stereoisomers.
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