Chapter 5

Stereochemistry

The Arrangement of Atoms in Space;

The Stereochemistry of Addition Reactions

Molecular Chirality: Molecular Chirality: EnantiomersEnantiomers

A molecule is A molecule is chiralchiral if its two mirror image if its two mirror image forms forms are not are not superimposable upon one superimposable upon one another. another.

A molecule is A molecule is achiralachiral if its two mirror if its two mirror image image forms forms areare superimposable. superimposable.

ChiralityChirality

BrBrClCl

HH

FF

Bromochlorofluoromethane is chiralBromochlorofluoromethane is chiral

It cannot be It cannot be superimposed point superimposed point for point on its mirror for point on its mirror image.image.

BrBrClCl

HH

FF

Bromochlorofluoromethane is chiralBromochlorofluoromethane is chiral

HH

ClClBrBr

FFTo show To show nonimsuperposability, nonimsuperposability, rotate this model 180° rotate this model 180° around a vertical axis.around a vertical axis.

Chirality & nonsuperimposable mirror images Chirality & nonsuperimposable mirror images

EnantiomersEnantiomers

QuickTime™ and aSorenson Video decompressorare needed to see this picture.

IsomersIsomers

stereoisomersstereoisomersconstitutionalconstitutionalisomersisomers

IsomersIsomers

stereoisomersstereoisomersconstitutionalconstitutionalisomersisomers

diastereomersdiastereomersenantiomersenantiomers

Physical Properties of Physical Properties of EnantiomersEnantiomers

Physical properties are the same:Physical properties are the same: melting point, boiling point, density, etcmelting point, boiling point, density, etc

Some others are differentSome others are different: : properties that depend on the shape of molecule properties that depend on the shape of molecule

eg. biological-physiological and optical properties. eg. biological-physiological and optical properties.

Properties of enantiomersProperties of enantiomers

OO OO

CHCH33 CHCH33

HH33CC HH33CCCHCH22 CHCH22

OdorOdor

(–)-Carvone(–)-Carvonespearmint oilspearmint oil

(+)-Carvone(+)-Carvonecaraway seed oilcaraway seed oil

IbuprofenIbuprofen is chiral, but normally sold as is chiral, but normally sold as

a racemic mixture. The enantiomer belowa racemic mixture. The enantiomer below

is the one responsible for its analgesic is the one responsible for its analgesic

and antiinflammatory properties.and antiinflammatory properties.

Chiral drugsChiral drugs

CHCH22CH(CHCH(CH33))22

HHHH33CC

CC

OO

CC

HOHO

A Sedative or a Potent Teratogen?

a carbon atom with foura carbon atom with fourdifferent groups attached to itdifferent groups attached to it

also called:also called:chiral center; chiral carbonchiral center; chiral carbonasymmetric centerasymmetric centerasymmetric carbonasymmetric carbonstereocenterstereocenterstereogenic centerstereogenic center

The chiral carbon atomThe chiral carbon atom

ww

xx yy

zz

CC

A molecule with a single stereogenic A molecule with a single stereogenic center is chiral.center is chiral.

2-Butanol is an example.2-Butanol is an example.

Chirality and chiral carbonsChirality and chiral carbons

CHCH33

OHOH

HH

CC CHCH22CHCH33

Examples of molecules with 1 chiral carbonExamples of molecules with 1 chiral carbon

CHCH33

CC

CHCH22CHCH33

CHCH22CHCH22CHCH22CHCH33CHCH33CHCH22CHCH22

a chiral alkanea chiral alkane

Examples of molecules with 1 chiral carbonExamples of molecules with 1 chiral carbon

Linalool, a naturally occurring chiral alcoholLinalool, a naturally occurring chiral alcohol

OHOH

Examples of molecules with 1 chiral carbonExamples of molecules with 1 chiral carbon

1,2-Epoxypropane: a chiral carbon1,2-Epoxypropane: a chiral carboncan be part of a ringcan be part of a ring

OO

HH22CC CHCHCHCH33

attached to the chiral carbon are:attached to the chiral carbon are:

——HH

——CHCH33

——OCHOCH22

——CHCH22OO

Examples of molecules with 1 chiral carbonExamples of molecules with 1 chiral carbon

Limonene: a chiral Limonene: a chiral carbon can be part of a carbon can be part of a ringring

CHCH33

HH CC

CHCH33

CHCH22

attached to theattached to thechiral carbon are:chiral carbon are:

——HH

——CHCH22CHCH22

——CHCH22CH=CH=

——C=C=

Examples of molecules with 1 chiral carbonExamples of molecules with 1 chiral carbon

Chiral as a result of isotopic substitutionChiral as a result of isotopic substitution

CHCH33CCDD

TT

HH

A molecule with a single chiral carbonA molecule with a single chiral carbonmust be chiral.must be chiral.

But, a molecule with two or more But, a molecule with two or more

chiral carbons may be chiralchiral carbons may be chiral

or it may not. or it may not.

We’ll return to this when we consider molecules We’ll return to this when we consider molecules with more than one chiral carbon atom.with more than one chiral carbon atom.