MadSci Network: Chemistry

Re: What are the five isomers of Hexane?

Area: Chemistry
Posted By: Jeremy Starr, Grad Student, Chemistry, California Institute of Technology
Date: Fri Oct 31 17:36:51 1997
Area of science: Chemistry
ID: 878051173.Ch

Hi Karen,

The five isomers of hexane are: hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane, and 2,3-dimethylbutane. They are constitutional isomers because they each contain exactly the same number and type of atoms, in this case, six carbons and 14 hydrogens and no other atoms. For conventional, stable, organic molecules it is necessary that each carbon atom forms four bonds to other atoms and each hydrogen atom forms only one bond. With these "rules" it is fairly straitforward to see that _all_ molecules containing just six carbons and 14 hydrogens will have the following things in common:

  1. A contiguous carbon skeleton and..
  2. Hydrogens will only be bonded to carbons.
With this in mind, constructing all of the isomers of hexane can be accomplished by first ignoring the hydrogens and determining all the unique carbon-carbon bonding arrangements possible with six carbons. Those are:
C-C-C-C-C-C	C-C-C-C-C     C-C-C-C-C	    C-C-C-C	C-C-C-C     
		  |		  |	      |		  | |
		  C		  C	      C		  C C

  hexane         2-methyl      3-methyl    2,2-dimethyl  2,3-dimethyl
		 pentane       pentane     butane        butane
Once that is done then bonding enough hydrogens to each carbon such that each carbon possesses exactly four bonds will require 14 hydrogens in each case and give you the completed structures of _all_ the isomers of hexane. For simplicity I did not depict the possible cyclic and poly-cyclic ring structures because those would require fewer than 14 hydrogens to fulfill the four bond requirement for carbon and thus those are _not_ isomers of hexane.

It is important for chemists to understand that a single molecular formula (ie C6H14) does not represent a single compound. This point is especially important when one considers that the formula, C30H62, represents 4,111,846,763 unique compounds! The reason that this information is important is that it is the foundation for realizing (and comprehending) the vast diversity of _structural_ possibilities within organic chemistry. The ability of carbon to bond to itself in a seemingly endless array of structures makes carbon utterly unique amongst the elements and is the reason why biological systems are built from carbon containing molecules. Utilizing structural diversity is key to being a competent and informed chemist.

I hope this information was helpful!


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