Draw and compare all the possible stereoisomers for 2,3-pentanediol and 2,4- pentanediol. Which stereoisomers are chiral? Which are achiral?

1 Answer
anor277
Dec 27, 2017

Well, you have two chiral centres per molecule....

Explanation:

And in principle there are #2^2# stereoisomers per molecule...of course, there is a catch....

For #"2,4-pentandiol"#, we mark the potential chiral centres with a star...

We gots ....#H_3C-stackrel"*"CH(OH)CH_2stackrel"*"CH(OH)CH_3#

And thus there are #"RR"#, #"SS"#, and #"RS"#, and #"SR"# stereoisomers. But #"RS"#, and #"SR"# are optically inactive stereoisomers in that there is an internal mirror plane, i.e. #"RS"# #-=# #"SR"#.

For #"2,3-pentandiol"#, #H_3C-CH(OH)CH(OH)CH_2CH_3#
#RS!=SR#, because there is no internal plane of symmetry...and here there are four stereoisomers...

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