n-Alkanes

[sodiumlamp]

I’m trying to sort out some thoughts about hydrocarbons, and I’ve been neglecting this blog, so let’s try to kill two birds with one stone. 

The hydrocarbons are chemical compounds composed exclusively of two elements: hydrogen and carbon.  People like to whine about chemical nomenclature, but this is one time they got it right.  Hydrocarbons are made of hydrogen and carbon. 

Hydrogen atoms can only form one chemical bond at a time, since each atom of hydrogen only has one electron to work with.  However, carbon atoms have four valence electrons, which can form bonds with up to four other atoms at once.  Consequently, you can make a lot of different molecules with just those two elements, because there’s so many ways to combine them.  The simplest hydrocarbon molecule is methane, CH₄ . 

In the above image, the carbon atom is represented by a letter ‘C’, and the hydrogen atoms are represented by a letter ‘H’.  The lines connecting the letters represent chemical bonds.  One line is dotted to indicate that it’s tilted away from the plane of your computer screen.  Another line is thicker than the others, and tapers out toward the H.  This is to indicate that the bond is angled out of the screen, and towards you.  Chemists use this to show what the molecule is shaped like in three dimensions.

Of course, a lot of times it just isn’t worth the hassle, so you’ll often see the methane molecule drawn to look like a flat cross. 

It’s important to understand that while methane is often depicted this way, it is most definitely not shaped like a flat cross.   Each chemical bond is the result of two atoms sharing electrons with one another.  In this case, each hydrogen atom is sharing its lone electron with one of the four valence electrons on the carbon atom.  So we can think off the lines in these diagrams as pairs of electrons.  And electrons are negatively charged particles, and like charges repel, so when you’re an atom like carbon that can form multiple bonds at once, the most comfortable arrangement is to space those bonds as far apart from one another as possible.   In a two-dimensional world, that would mean having the bonds at right angles to each other, like a cross.  But carbon atoms are three dimensional, so you end up with the bonds pointed at 109.5 degree angles from each other.   The result is a tetrahedron shape, with each hydrogen atom at one of the vertices of the tetrahedron, and the carbon atom in the very center.

But this isn’t the only way to make a molecule of just carbon and hydrogen.  What if you had two carbon atoms?   The hydrogen atoms can only form one bond at a time, so they could only attach to one carbon or the other, but since carbon can form multiple bonds, they can bond together.  Each carbon atom still has three more bonds it can make, so we can throw in six hydrogen atoms to complete the molecule, which is called ethane, C₂H₆. 

Again, we can show the structure of ethane as a flat diagram, or use dotted lines and wedges to show how it looks in three-dimensions.  

We can add another carbon atom to the mix and make a third molecule called propane, C₃H₈.

You might notice that the wedge and dotted line format is getting cumbersome as the molecules get bigger.  This is one reason chemists will use the flat version.

It doesn’t reflect the true shape of the molecule, but it’s a lot easier to read.  Another alternative is to draw only the carbon-carbon bonds, leaving out the carbon-hydrogen bonds.  This way, you only have three points connected by two lines, like this:

This kind of model is called a “skeletal structure”.  It may seem a little drastic for a small molecule like propane, but it’s very useful when dealing with larger hydrocarbons, where the C-H bonds would just get in the way.  When viewing a skeletal structure like this, it’s implied that any bonds not shown are C-H bonds.  Since we know carbon atoms form four bonds, we can infer that the middle carbon has two bonds not shown in the diagram, and these must be two C-H bonds.  The carbon atom on one end only has a single bond show, which must mean that the other three bonds are C-H bonds.  So we can imagine what the rest of the molecule must look like. 

These three molecules are the simplest members of a hydrocarbon family called the alkanes.  Each alkane has a chemical formula of C_nH_2n+2 , where n is any number you can think of.  But let’s just focus on the number four right now.   If we keep going with the pattern we’ve established so far, we can make a fourth alkane using four carbon atoms, like so.

But wait.  We don’t have to line them up like this.   What if we arranged the carbon atoms like this:

In other words, we have two options here.  We can daisy-chain the carbon atoms one after another, or we can have a central carbon atom with the other three carbon atoms bonded to it.  The skeletal structures make this a little easier to digest.  Take a look:

The molecule on the left is called n-butane.  The n stands for “normal”, since it’s customary to think of this as the “default” way to arrange carbon atoms in a hydrocarbon.  Just line them up one after the other in a chain.  The one on the right is called isobutane. The prefix iso- refers to the word “isomer”, which means a molecule that has the same chemical formula as another molecule, but with a different structure.   In this case, n-butane and isobutane are isomers of one another.   Collectively, we could just call them “butanes”.  

The two butanes represent the simplest example of isomerism among the alkanes, because there’s no other way to rearrange the atoms in methane, ethane, or propane.  You need at least four carbon atoms to open up another option, and even then, all we really get is one alternative.  But suppose we had a fifth carbon atom.  Now, there’s even more possibilities… no, I’m just kidding, there’s only three.

The straight-chain version on top is called n-pentane.   The one one the bottom left is isopentane.   The one on the bottom right is called neopentane. 

With six carbon atoms in an alkane, you have five possible structures.   Add a seventh, and you have nine isomers.  An alkane with ten carbon atoms has seventy-five possible structures.  An alkane with twenty carbon atoms has over three hundred thousand isomers.  This is why the daisy-chain pattern is deemed “normal”, because no matter how many carbon atoms there are, no matter how complex the other isomers may be, there’s always that default scheme of just lining up the carbon atoms one after the other.  Thinking about all the alkanes is pretty complicated, but the n-alkanes are easy, because there’s just one n-alkane for each number of carbon atoms. 

Naming the n-alkanes is pretty simple too.  After the first four in the sequence, you just use Greek prefixes to indicate the number of carbons in the molecule.

• C1: methane
• C2: ethane
• C3: propane
• C4: n-butane
• C5: n-pentane
• C6: n-hexane
• C7: n-heptane
• C8: n-octane
• C9: n-nonane
• C10: n-decane
• C11: n-undecane
• C12: n-dodecane
• C13: n-tridecane
  

Oftentimes, you can just drop the n from the front of the name, and people will understand what you mean.  I find with the smaller ones, it’s a good idea to be specific, but it depends on the context.  A lot of times, with the bigger numbers, people will just say “C10′s” or “C25′s”, because they’re referring to any hydrocarbons of that particular size.   Still, there are some applications that try to sort out all these molecules, and the n-alkanes make a sort of handy index.  There may be hundreds of thousands of C25 alkanes, but there’s only one n-C25, which I find strangely comforting…