Exercício de Química Sintética e Organica Mista (169)

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(c) Both stereoisomers have two equivalently substituted stereogenic centers, and so we must be
alert for the possibility of a meso stereoisomer. The structure at the left is chiral. The one at the
right has a plane of symmetry and is the achiral meso stereoisomer.
(d) The first structure is achiral; it has a plane of symmetry.
The second structure cannot be superposed on its mirror image; it is chiral.
7.25 There are four stereoisomers of 2,3-pentanediol, represented by the Fischer projections shown. All
are chiral.
There are three stereoisomers of 2,4-pentanediol. The meso form is achiral; both threo forms are
chiral.
meso-2,4-Pentanediol
CH3
CH3
H
H
H
OH
OH
H
CH3
CH3
H
H
HO
OH
H
H
CH3
CH3
H
H
HO
OH
H
H
Enantiomeric threo isomers
Enantiomeric erythro isomers
CH2CH3
CH3
H
H OH
OH
CH2CH3
CH3
HO
HO
H
H
Enantiomeric threo isomers
CH2CH3
CH3
HO
H OH
H
CH2CH3
CH3
HO
H
H
OH
Cl Cl
Cl
Reference structure Mirror image Reoriented mirror image
�
Cl
1
4
7
Plane of symmetry passes through C-1, C-4, and C-7.
H2N
NH2
CH3
CH3
H
H
Chiral
Plane of symmetry
NH2
NH2
CH3
CH3
H
H
Meso: achiral
164 STEREOCHEMISTRY