Geometric Isomerism

This is an introduction to Geometric Isomerism, also know as Cis- Trans Isomerism. If you think you're in the wrong place or are unprepared for the horrors that lie ahead, then RUN! RUN AS FAST AS YOUR LEGS WILL CARRY YOU!

Now that we've weeded out the weaker and spindlier of you aspiring chemists, we'll get down to business with a simple definition of Geometric Isomerism:

Geometric Isomerism

A type of stereoisomerism that involves a molecule with no possible rotation about the Carbon-Carbon bond (i.e. double bonds or alkene groups)

Because these compounds are not able to rotate, two different forms of the same compound can be formed. A cis- compound, where the offending molecules are placed on the same side, and a trans- compound, where said molecules are placed on opposing sides.

cis-1,2- dibromoethene; trans-1,2-dibromoethene

As you can see from the example above, these are versions of the EXACT same compound, with the only exception being the placement of the bromine. Of course, this cis-trans type of isomerism can only happen if there are two DIFFERENT types of molecules or alkyl groups on the left-hand carbon, and two different types of molecules or alkyl groups on the right hand carbon. However, if all four groups are different, as is the case below, cis and trans lose all their meaning.

But, both fortunately and unfortunately for you, these compounds will not be covered in this site.

Now you may be asking yourself, "Why is this important to the real world? Does it even make a difference if some molecules are switched around?" The answer, unfortunately, is yes. The differences between the cis and trans formations have the same compound have many physical differences between them, including but not limited to density, melting points, and boiling points.

Optical Isomerism

Optical isomerism is a phenomenon in which two molecules have the same chemical formulae, however, the individual molecules are arranged in such a way that the images of both molecules cannot be superimposed. This causes the molecules to rotate a plane of light either clockwise or counter-clockwise. Molecules that exhibit this behavior are called chiral (ky-ral) the different forms are called enantiomers. Optical isomerism belongs to stereo isomerism along with geometrical isomerism.

Structural Isomerism

Structural isomerism: The same molecular formula of a structure but a different structural formula.

• A molecular formula defines how many atoms of each element there are in a compound
• A structural formula defines how each of the atoms are arranged in a compound.

Structural Isomerism of Alkanes:

• Same number of carbons and hydrogens
• Different arrangement of carbons and hydrogens

Example of Structural Isomerism

The isomerism of a molecule represents completely different organic compounds: not only are their atoms bound differently but they also have different chemical properties. An example in the change of physical properties is the difference in boiling point due to the molecular bonding of the isomers. For example, CH₃CH₂CH₂CH₃ boiling point is -0.6°C and the boiling point of CH(CH3) is 10.2°C. Same chemical formula, but different boiling points.