Isomers

= = =**Isomer **= -Two or more chemical compounds that share the same molecular formula but different structural formulas - Generally speaking, isomers do not share the same chemical properties unless they share the same functional groups

=Structural Isomerism (aka branched isomerism, the kind of isomerism on the AP)= -For instructional purposes, the model of the alkane will be considered however, essentially the same technique works consistently with all other hydrocarbons (ie. Alkenes, Alkynes)

Consider a standard straight chain arrangement of Pentane

Note the continuity of the C-C chain. In other words, your finger can connect all the carbon atoms in the chain without ever traversing the same spot more than once. However, organic molecules often assume alternative arrangements where the structures are discontinuous, or branched. In branched isomers, the placement of functional groups on a chain are altered. However, to be considered a structural isomer, the arrangement of atoms achieved in the isomeric state must not be able to be obtained by the simple rotation of the central carbon chain. When examining pentane, there are two isomeric structures that can be achieved: A structure where a single methyl group is branched from the chain, and a structure where two methyl groups are branching from the chain. Those structures are shown below



Naturally, as the number of carbons in the initial alkane structure increases, the number of potential isomeric structures also increases.

=**Nomenclature of Isomeric Structures:**= = = Again, the example of Pentane and its isomers will be used for instructional purposes. And also, the same fundamental principles will apply to all isomers.

Consider the straight chain model of Pentane. Since there are no branches from the chain, that structure is called **n-pentane**. Generally, for any structure in which a single functional group is branched from the chain, add the prefix iso- to the original root. Ergo the naming of the second structure shown, **isopentane**. Again, generally for any structure in which two functional groups are branched from the chain, add the prefix neo- to the original root. Voila - **Neopentane**.

However, for more complex structures, it is necessary to use a more descriptive naming system to identify the isomer. It can simply be broken down into the answers to three basic questions:

1. What's the longest? 2. Where are the branches? 3. What are the branches?

Consider Neopentane for the following example.

1. What's the longest?

First, identify the longest chain of carbons. Name this chain according to standard organic convention. In neopentane the longest chain of carbons is 3 carbon molecules long and is an alkane, leading to the name of propane.

2. Where are the branches?

When looking at Neopentane, count the number of the carbon at which the branch splits from the chain. In cases like neopentane where there is more than one branch, find the carbon at which every functional group branches. In cases like neopentane, both functional groups split at the second carbon. Be careful when identifying sites at which chains diverge not to identify the same site twice to to reflection.

3. What are the branches?

This asks to identify the functional groups that branch from the chain. Again in the case of Neopentane, both functional groups branching from the chain are methyl groups.

Combination of the parts:

First, the bonding sites of the functional chains are listed, followed by the identity of the branches, and then the identity of the longest carbon chain.

In the case of Neopentane

2,2 - Dimethyl Pentane

Note how the prefix di- is used to indicate that there are two methyl groups off the chain. Also note that although the methyl groups branch from the same bonding site, the number is repeated to indicate that there are two individual branches from the central chain.

Supplemental Video: Enjoy. The isomers of hexane.

http://www.youtube.com/watch?v=qOhEJK4Umds

All information supplied on this page came directly from the mind Alex without any outisde influence. Some may say that my life has given Alex this curse. However, in accordance with Alex's new optimistic world view, she chooses to view it as an excuse to avoid unnecessary source citations.

Photo Credits (in order of appearance): http://pedagogie.ac-montpellier.fr:8080/disciplines/scphysiques/academie/ABCDORGA/famille/produit/pentane.gif http://upload.wikimedia.org/wikipedia/commons/thumb/3/35/Isopentane-3D-balls.png/777px-Isopentane-3D-balls.png http://www.mocho.pt/local/local/ciencia3d/quimica/bibmolec/estereoscopia/c5h12-d.jpg