Homework 7 Answers

Carey, pages 318 -325 :
Problems 7.25 - 7.48

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1. Write complete names for each of the following compounds, including designation of stereochemistry:

1a) (3S,6R)-3-bromo-2,3,6,7,7-pentamethylnonane

1b) (2S,3S)-1,1-dichloro-2,3-dimethylcyclopropane

1c) (3R,4R)-4-chloro-3-fluoro-1,1-dimethylcyclohexane

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2. All of the naturally occuring amino acids are called L because they have a stereochemistry that was historically correlated with one stereoisomer of glyceraldehyde, shown below.

L-Glyceraldehyde and the natural amino acids all have the S absolute configuration. The two exceptions are glycine and cysteine. Look up their 3-dimensional structures and explain why they are different.

Glycine does not have a stereocenter (R = H).

Cysteine has a sulfur substituent (R = CH2SH), but it does have the same relative configuration as all the other L-amino acids. Because of the S atom, the priority for the R group is higher than for the COOH group, which is different than for all the other amino acids (where the R group priority falls between the COOH and H).