Alkanes

Posted on July 23, 2015 by misterguch

What’s a functional group?

Not every molecule is the same as every other molecule.  You’ve probably noticed this because there’s more than one structure written in your chemistry book.  What’s different about the different types of molecule are the functional groups they contain.

Essentially, you can think of functional groups as being little reactive things that are located somewhere on an organic compound.  For example, if an organic compound contains the -OH functional group, it’s referred to as an alcohol and we can expect it to behave in certain ways.  Compounds can contain more than one functional group, which helps to make organic chemistry a lot more interesting and useful.

Let’s take a look at our friends the alkanes.

Alkanes:  No functional group at all

Alkanes are organic compounds that contain only carbon and hydrogen atoms singly-bonded to one another.  This complete lack of interesting things causes use to treat alkanes as the basis of all organic compounds.

alkane-doggie

Put another way, we usually think of organic compounds as being boring things that have interesting functional groups attached to them.  The boring alkane part is usually pretty unreactive, so it’s usually the case that we can just focus on the other, more interesting functional groups.  Though alkanes can and do react on their own, it’s usually a pretty good bet that any other functional groups on the molecule will react first.

Alkanes are named according to the structures of the n-alkanes (the ones in which all of the carbon atoms are in a straight chain)¹

  • One carbon atom:  Methane
  • Two carbon atoms:  Ethane
  • Three carbon atoms:  Propane
  • Four carbon atoms:  Butane
  • Five carbon atoms:  Pentane
  • Six carbon atoms:  Hexane
  • Seven carbon atoms:  Heptane
  • Eight carbon atoms:  Octane
  • Nine carbon atoms:  Nonane
  • Ten carbon atoms:  Decane
propanes

These may appear different at first glance, but all of them are reasonable ways to draw propane, which contains three carbon atoms.

Naming alkanes

Here’s the main thing that people really want to know:  How to name alkanes.  Without further ado, here are the rules:

  • Draw the molecule.
alkanephoto1

We’ll arbitrarily use this as our example.

  • The carbon atoms in this molecule should be arranged in one or more chains.  Figure out which of these chains is the longest and count the carbon atoms.  This number of carbon atoms tells us which alkane we have.
alkane2

There are several possibilities for the carbon chain, but the longest possible one is shown here in bold.  Because there are eight carbon atoms in a row, this molecule is an “octane” of some kind.

  • Number the carbon atoms in this chain.  The correct way to do this is so that the substituents (i.e. the things that aren’t the main chain) have the lowest possible numbers.
alkane3

In the top figure we start numbering at the left of the chain, so the two substituents are on the 3- and 5- carbon atoms. In the bottom one, we start on the right, so the substituents are on the 4- and 6- carbons. Since 3 and 5 are less than 4 and 6, the top method is correct.

  • Figure out what the substituents should be called.  Fortunately, this is pretty easy because we just need to figure out how many carbon atoms are in a row on each of these.  As a result, the substituent on the 3-position is a methyl group (1 carbon atom), while the one at the 5-position is an ethyl group (2 carbon atoms).

alkane4

  • Name the compound by indicating the substituents (in alphabetical order) and their positions, followed by the name of the longest chain.  The name of this compound, then, is 5-ethyl-3-methyloctane.

Some handy naming tips:

  • If you have more than one substituent of the same type (for example, let’s imagine that the 5-position in the above example contained a methyl group instead of an ethyl group), we’d use a prefix to indicate this.  In the example I just mentioned, this compound would be called “3,5-dimethyloctane.”
  • If you have a molecule where the atoms are in a circle (i.e. a “cycloalkane”), the same thing holds true, except that you add a “cyclo” in front of the name of the molecule to indicate that the atoms are in a circle.  Check out the pictures of cyclopentane and cyclohexane below:

Cyclopentane_and_cyclohexane

 

Footnotes:

  1. The “n” in the term “n-alkane” stands for the word “normal.”  To me, this suggests that branched alkanes are hideous freaks, which should make studying them a little more entertaining.

Image credits:

 

 

 

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