Organic Chemistry

A shortcut—called a Fischer projection—for showing the three-dimensional
arrangement of groups bonded to an asymmetric carbon was devised in the late 1800s
by Emil Fischer. A Fischer projecton represents an asymmetric carbon as the point of
intersection of two perpendicular lines; horizontal lines represent the bonds that pro-
ject out of the plane of the paper toward the viewer, and vertical lines represent the
bonds that extend back from the plane of the paper away from the viewer. The carbon
chain always is drawn vertically with C-1 at the top of the chain.

To draw enantiomers using a Fischer projection, draw the first enantiomer by ar-
ranging the four atoms or groups bonded to the asymmetric carbon in any order. Draw
the second enantiomer by interchanging two of the atoms or groups. It does not matter
which two you interchange. (Make models to convince yourself that this is true.) It is
best to interchange the groups on the two horizontal bonds because the enantiomers
then look like mirror images on paper.
Note that interchanging two atoms or groups gives you the enantiomer—whether
you are drawing perspective formulas or Fischer projections. Interchanging two atoms
or groups a second time, brings you back to the original molecule.
A stereocenter(or stereogenic center) is an atom at which the interchange of two
groups produces a stereoisomer. Therefore, both asymmetric carbons—where the in-
terchange of two groups produces an enantiomer and the carbons where the inter-
change of two groups converts a cis isomer to a trans isomer (or a Zisomer to an E
isomer)—are stereocenters.

PROBLEM 7

Draw enantiomers for each of the following compounds using:

a. perspective formulas

b. Fischer projections

Br

1. CH 3 CHCH 2 OH

CH 3

2.

CH 3

OH

ClCH 2 CH 2 CHCH 2 CH 3 3. CH 3 CHCHCH 3

H

H

Br

H 3 C C

H

CH 3 CH 2

C C

H H

CH 3 CH 2 CH 3

Cl

Br

a stereocenter a stereocenter

a stereocenter

Br

CH 2 CH 3

H

Fischer projections of the enantiomers

of 2-bromobutane

CH asymmetric carbon

3 CH 3

CH 2 CH 3

HBr

perspective formulas of the enantiomers of 2-bromobutane

H C

Br

CH 3

CH 2 CH 3

C

H 3 C

Br

H

CH 3 CH 2

Section 5.5 Drawing Enantiomers 187

The solid wedges represent bonds that

point out of the plane of the paper to-

ward the viewer.

The hatched wedges represent bonds

that point back from the plane of the

paper away from the viewer.

Make certain when you draw a perspec-

tive formula that the two bonds in the

plane of the paper are adjacent to one

another; neither the solid wedge nor

the hatched wedge should be drawn

between them.

In a Fischer projection horizontal lines

project out of the plane of the paper

toward the viewer and vertical lines ex-

tend back from the plane of the paper

away from the viewer.

Emil Fischer (1852–1919)was born

in a village near Cologne, Germany.

He became a chemist against the

wishes of his father, a successful mer-

chant, who wanted him to enter the

family business. He was a professor

of chemistry at the Universities of

Erlangen, Würzburg, and Berlin. In

1902 he received the Nobel Prize in

chemistry for his work on sugars.

During World War I, he organized

German chemical production. Two of

his three sons died in that war.