HS Chemistry - Introduction to Organic Chemistry

Isomers

Overview of The Page

This page will cover:

• What are isomers?
• What are structural isomers? What are the different types of structural isomers?
• What are stereoisomers? What are the different types of stereoisomers?

The previous page gave an introduction to organic compounds, their naming system, and their structures and formulae. In particular, it explained that the molecular formula didn't give much information about how the atoms are bonded to each other. You might be wondering why this is important to distinguish. The reason is because of isomers.

Isomers are different organic molecules that have the same chemical formula. For example, consider the alkane C₄H₁₀.

It could represent butane:

Butane has 4 Carbon atoms and 10 Hydrogen atoms, so it has a molecular formula of C₄H₁₀.

The following molecule also has 4 Carbon atoms and 10 Hydrogen atoms, and therefore a molecular formula of C₄H₁₀:

But this is not butane. It has a main chain of three Carbon atoms, and then there is another Carbon atom "branching" off the middle Carbon atom. This is 2-methylpropane.

Yet 2-methylpropane is also made of 4 Carbon atoms and 10 Hydrogen atoms. Both butane and 2-methylpropane have the molecular formula C₄H₁₀. Butane and 2-methylpropane are isomers of each other.

There are two main kinds of isomers, each with their own subtypes:

• Structural isomers:

  • Functional group isomers
  • Chain isomers
  • Position isomers

• Stereoisomers $aka geometric isomers$:

  • Cis-trans isomers
  • Optical isomers

Structural Isomers

Structural isomers are molecules that have the same atoms, but they are bonded to each other in different ways. There are three kinds of structural isomers: functional group isomers, chain isomers, and position isomers.

1. Functional group isomers are molecules that have the same chemical formula but different functional groups. This is often seen when an atom is moved from a side chain to the main chain $though not always$. This can commonly be seen between aldehydes, ketones and carboxylic acids $covered in Unit 16$.

2. Chain isomers are molecules that have the same chemical formula and functional group, but different main chains. An example of this is butane and 2-methylpropane. Although they both have the same functional group $alkanes$, butane:

   

   Has 4 Carbon atoms in its main chain, while 2-methylpropane:

   

   Has 3 Carbon atoms in its main chain. Although both molecules have the same functional group and the same molecular formula, they have different main chains, which is why they are called chain isomers.

3. Position isomers are molecules that have the same chemical formula, functional group and main chain, but that have a different arrangement of elements. An example of this is 1-iodobutane and 2-iodobutane.

   1-iodobutane has the Iodine attached to the first Carbon atom:

   

   While 2-iodobutane has the Iodine attached to the second Carbon atom:

   

   The molecular formula $C~4~H~9~I$, functional group, and main chain are the same for both molecules. Nothing has changed other than the location of the Iodine atom in the molecule. Therefore, 1-iodobutane and 2-iodobutane are position isomers.

   When dealing with position isomers, something that must be kept in mind is that Carbon atoms can freely rotate around single bonds. What that means is it doesn't matter whether you draw hexane as:

   

   Or as:

   

   They are still the same molecule, and not considered isomers of each other, because the single bonds allow for atoms to rotate around each other without changing the structure.

Stereoisomers

Stereoisomers are molecules that have the same intramolecular bonds $the same atoms are bonded to each other$, but the atoms are arranged differently. There are two kinds of stereoisomers: cis-trans isomers and optical isomers.

1. Cis-trans isomerism arises out of the fact that while Carbon atoms can rotate around single bonds, double and triple bonds "lock" them into place relative to one another, which leads to interesting combinations.

   For this, we'll introduce alkenes. An alkene functional group is present when two Carbon atoms are double-bonded to one another. Ethene is the most basic alkene:

   

   Alkenes $including ethene$, as well as all other functional groups, follow the same naming conventions as alkanes. Thus, 1,2-dichloroethene looks like how you'd imagine:

   

   Note that even though the Chlorine atoms are bonded to the Carbon atoms with single bonds, 1,2-dichloroethene is still classified as an ethene. This is because the alkene functional group has a double bond, while the halogenoalkane functional group has a single bond. Double- and triple-bond functional groups take precedence over single-bond functional groups when classifying organic compounds.

   If we rotate the 1,2-dichloroethene molecule $rotating the entire molecule so that we don't change the molecule$, we get:

   

   Because the two Carbon atoms are double bonded, their position relative to one another is locked, and they can't rotate independently of one another. However, the following molecule is 1,2-dichloroethene:

   

   And if we rotate it $rotating the entire molecule so that we don't change the molecule$, we get:

   

   And since it has the same atoms bonded to each other, it isn't a structural isomer.

   **This is cis-trans isomerism. When both Chlorines are on the same level $both on top or both on bottom$, the molecule is a cis-isomer; when they are on different levels $one on top and one on the bottom$, the molecule is a trans-isomer.

   Thus, this:

   

   Is cis-1,2-dichloroethene, and this:

   

   Is trans-1,2-dichloroethene.

2. Optical isomerism arises out of the fact that when a Carbon atom is bonded to 4 other atoms, it is possible to switch the positions of two of the atoms bonded to the Carbon atoms in a way that cannot be achieved by simply rotating the atoms. This is probably best seen through an example.

   Let's take the molecule 1-bromo-1-chloroethane:

   

   We can represent it in the 3D displayed formula as:

   

   Or as:

   

   Where the Carbon atom in the center is bonded to four different groups. This is necessary for optical isomerism.

   The Carbon and Hydrogen atoms are on the same plane $the plane of the screen$, while the Bromine atom is on a plane in front of the Carbon atom, and the Chlorine atom is on a plane behind the Carbon atom.

   If we took its mirror image:

   

   The result is a molecule that is very similar to the previous one, but has a different intramolecular arrangement. Rotating the previous molecule will not give you this one, and the opposite holds true as well. The Carbon atom in the center is called the chiral center.

   These are two different optical isomers $aka enantiomers$ of 1-bromo-1-chloroethane. These optical isomers have different effects on polarized light, and will rotate it in different directions.