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Cis/Trans Isomerism in Capsaicin
Stereoisomers are "compounds that have the same atomic connectivity but differ in the spatial arrangement of constituent atoms" (Ref 10, 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:
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 be large). They will melt at different temperatures, boil at different temperatures, may behave differently in the presence of other chemicals, and (most importantly) they will have different standard free energy values.
The free energy difference between cis and trans isomers is the root cause of many of their property differences. A higher-energy molecule contains more energy, and is thus less stable or more reactive. Why do the stabilities of the two isomers differ? Consider the picture above again. In the cis isomer, the two methyl groups are fairly close to each other. In fact, they are close enough that they tend to get in each other's way, causing them to repel each other slightly.
This steric hindrance does not exist in the trans isomer. The additional strain imposed on the cis isomer by the proximity of the methyl groups causes it to be a less stable arrangement than the trans isomer.
The Capsaicin Molecule
Taking another look at our good friend capsaicin, it is obvious that capsaicin is the trans conformation. In four different studies cited by Krajewska (Ref 9, p. 902) no cis isomers of capsaicinoids having C=C bonds were found in peppers.
All of the chemical structures and reaction mechanism diagrams used in this
section are my own creations. None of the chemical structures that I've seen on any other Chile Head sites show unshared electrons, nor do they properly depict the unsaturation in the
capsaicin molecule, both of which are major determinants of the chemical behavior of capsaicin. Due to the use of a really cheesy red-gold Chrome effect, all of my
technical artwork is readily identifiable (and hopefully too ugly to steal). If you see someone else using my artwork without giving proper credits, please 1) give them a boot in the
rear via email, and/or 2) let me know about it so I can do the same.
Last updated 31 January 2015.
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