Cis and trans Geometric Isomers tutorial

Cis and trans isomers may exist in two types of  molecules:

1) Cycloalkanes with substituents

2) Alkenes

1) Cis and Trans Isomers in cycloalkanes

Rotation around the bonds in a cyclic structure is limited. The formation of cis-trans isomers or geometric isomers, is a consequence of the absence of the free rotation. Geometric isomers are a type of stereoisomer.  The term "cis" is derived from Latin and means "on the same side."  The term "trans" is also derived from Latin and means "across from."

•The limited rotation of the carbon-carbon single bonds in cycloalkanes has an interesting side effect in that it allows for the existence of stereoisomers, molecules that:

•have the same molecular formula

•have the same atomic connections

•have a different 3-dimensional shape

•are interchanged only by breaking bonds

2) Cis and Trans Isomers in alkenes

 

In the case of 2-butene, there are two ways in which the hydrogen atoms can be arranged on the double-bonded carbons. Both hydrogens can be placed on the same side of the chain or they can be placed on opposite sides.


(Black dots are C atoms)

Since the chain cannot rotate about the double bond, one form has quite distinct properties from the other. For example, their melting points are thirty degrees apart, and chemists exploit this fact to separate one isomer from the other.

Call the form which has both hydrogens on the same side a cis isomer ("cis" rhymes with "miss"). The one with hydrogens on opposite sides is called a trans isomer ("trans" means "across"). Generally, cis and trans are written in italics or underlined in printing.

Thus, the names of the two isomers are cis-2-butene and trans-2-butene.

(Does 1-butene have cis and trans forms? Why?)

Notice that cis and trans isomers occur across a double bond only. They never occur across single bonds because single bonds can rotate freely.

Naming cis and trans Alkenes

The naming of alkenes is very similar to the naming of alkanes, with two exceptions.

In alkenes the double bonds take precedence over the substituted groups and the chain is numbered so that the double bonds have the lowest location numbers possible (left side of example).

Always check double bonds to determine if the molecule is a cis isomer, trans isomer, or neither. For example, study the following two molecules.

                                                                Molecule A                                             Molecule B

Molecule A is neither a cis or a trans isomer because the hydrogen has the same relation to the CH3 groups no matter which side it is on. Molecule B is a trans isomer since the two hydrogens are clearly on opposite sides of the double bond.

Example 1:

Naming some structural formulas of alkenes.

There are five carbons in the longest chain, so the root name begins with "pent."

There is a carbon-to-carbon double bond, so this is an alkene and the name ends in "ene."

The carbons are numbered from right to left since the double bond is closest to the right end.

The double bond starts on carbon 1.

The name must contain a number to indicate the position of the double bond.

There are no cis or trans isomers since the double bond starts on carbon 1.

Answer: The name is 1-pentene.

Drawing Alkene Structures Given the Name

Drawing structures from a name is pretty much the opposite process of naming alkenes. One of the best ways to learn is by looking at some examples.

Example 1:

4-methyl-1-hexene

The "hex" in the name indicates that the longest carbon chain contains 6 carbons.

The "ene" ending indicates that the compound is an alkene and must contain a carbon-to-carbon double bond.

The "1" immediately preceding the hexene indicates the double bond starts on carbon 1.

There is a methyl (CH3) group on carbon 4.

The structural diagram is:

Note that the methyl group could also be pointed up and that there is no cis or trans isomer since the double bond is on carbon 1 and it holds two hydrogens. The structure can be flipped around left to right or top to bottom, and it is still the same molecule.

Note that this structure can be flipped around left to right.

Example 3:

trans-5-ethyl-3-heptene

The "hept" in the name indicates that the longest carbon chain contains 7 carbons.

The "ene" ending indicates that the compound is an alkene and must contain a carbon-to-carbon double bond.

The "3" immediately preceding the heptene indicates the double bond starts on carbon 3.

There is one ethyl group on carbon 5.

This is a trans isomer, so the hydrogens on the carbons involved in the double bond are on opposite sides of the molecule.

The structural diagram is:

Note that this structure can be flipped around left to right.