Cis / Trans Isomerism in Capsaicin Stereoisomers are "compounds that have the same atomic connectivity but differ in the spatial arrangement of constituent atoms"

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John Iyiola

Cis/Trans Isomerism in Capsaicin
Stereoisomers are "compounds that have the same atomic connectivity but differ in the spatial arrangement of constituent atoms" (, p. 93). In other words, stereoisomers are molecules that 1. are composed of the exact same atoms, and where 2. each atom in one molecule is connected to the same neighbors as its twin in the other molecule, but 3. the three-dimensional orientation of the molecules is different.

Sounds impossible, right? Well, consider a pair of leather gloves. Each is composed of identical components- four fingers, a thumb, and a palm section. In each case, the components are connected the same relative to each other- the thumb is connected to the palm on the outside, the index finger is connected to the palm between the thumb and the middle finger, etc. Yet, the 3-D orientation of the two gloves is different. You can prove this to yourself by trying to put a glove on your right hand. The right glove will fit, but the left glove won't. The connectivity of the parts is the same, but their 3-D orientation is different.

One type of stereoisomerism common to compounds with C=C bonds is called
cis/trans isomerism. Cis/trans isomerism occurs because a double bond prevents the internal rotation that would ordinarily occur if the double bond was a single bond. The atoms in a molecule are not static objects that are fixed in space, rather, they are constantly engaged in all sorts of internal motions, such as stretching and bending of bonds and rotation of parts of the molecule around different bonds.

To picture internal rotation about a bond, think about two apples impaled on either end of a pencil. Both apples can easily be rotated perpendicular to the pencil by twisting them; atoms bonded together by a single bond are free to rotate in the same manner. If you added a second pencil between the apples, they would no longer be free to rotate; a second bond between two atoms inhibits rotation in the same manner. Thus, the two atoms joined by a double or triple bond are frozen in space relative to each other.

Because the two double-bonded atoms cannot rotate relative to one another, isomerism occurs. Consider the molecules below:

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cis-2-butene and trans-2-butene


The two molecules are definitely stereoisomers- they are made of the same atoms, the atoms have the same connectivity, and the 3-D orientation of the molecules is different. In the
cis isomer, the methyl groups (-CH3) are both below the double bond, while in the trans isomer, one is above the plane of the bond, and the other is below it. Because the double bond prohibits rotation about itself, the molecule is stuck in one conformation or the other, and cannot switch between them under ordinary conditions.

Cis/trans isomers are distinctly different compounds, and exhibit differing properties (although the differences may not ...

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