Basics of Organic Chemistry

I've decided to skip redox after all and talk about Organic Chemistry. Apart from redox being more annoying (well, actually drawing Organic Chemistry diagrams is a rather annoying task as well) I also have a validation test or something like that for Organic Chem next week.

Organic chemistry is the study of organic compounds. I'm not entirely sure what the definition of an organic compound is, but I do know that most of them are hydrocarbons (compounds containing mainly hydrogen and carbon, but some have halogens or other nonmetal elements like oxygen and nitrogen attached to them too). When you study organic chemistry, you're pretty much just dealing with hydrocarbons anyway.

Why hydrocarbons? you may ask. Well, carbon is a pretty special element simply because of the way it bonds. Yeah, it bonds using good ol' covalent bonds, but since it's a group 14 element, it can form 4 bonds. (See Covalent Bonding for more info on covalent bonding.) Being able to form 4 bonds is great- it means carbon can bond to up to 4 other atoms (including other carbon atoms) and it can form single, double or triple bonds (but not quadruple bonds otherwise all the electrons'd throw a fit from all being shoved in the same space together). Now, why carbon and not any other group 14 element? Why don't we have, say, hydrosilicons or hydrogermaniums or... hydrotins? Well, I've been told time and time again that carbon is special because it's not overly fussy about the atoms that it bonds with, and is also able to form long carbon chains and rings. I'm not 100% sure on this one, but I'm guessing that these two attributes must be unique to carbon or carbon does these things better, resulting in more hydrocarbons than hydro- anything else. In fact, there's a hell of a lot of hydrocarbons simply because of the way carbon bonds.

Now there's different types of hydrocarbons, grouped depending on how many bonds they have and whether the carbon atoms are bonded in chains or rings. I've even drawn a diagram with pretty colours to show you how they're categorised and what the groups (which I'm pretty sure are called "homologous series") are called. (These aren't all the homologous series, just the ones studied in Year 11.)

By the way, I'm pretty sure that the criteria that we use to categorise hydrocarbons (e.g. double bonds, triple bonds, presence of benzene etc.) are known as "functional groups." So "homologous series" are the group names and "functional groups" are the defining features of the groups.

As you can see from the diagram above, alkanes are carbon chains with single bonds only, alkenes are carbon chains with at least one double bond, and so on.

One thing that I forgot to add to my diagram is the special names for hydrocarbon chains and hydrocarbon rings. Hydrocarbons in chain formation are known as aliphatic hydrocarbons while hydrocarbons in ring formation are known as alicyclic hydrocarbons.

Right. Now on to drawing some basic hydrocarbons so you can see what they look like.

Meet ethane. It only has single bonds (a line is a shorthand way of writing a bond) and is in a chain. Therefore, it's an alkane. In the name "ethane," the "-ane" comes from "alkane" and the "eth-" indicates that there are 2 carbons.

Why don't they call it biane or diane? I don't know. Chemistry likes to be different, so in naming hydrocarbons there's a few special prefixes telling you how many carbons are in the main hydrocarbon chain. (I say "main" because, as we shall see later, some hydrocarbon chains and rings have branches.) You'll have to learn the special names for small numbers of carbon, but as the number of carbons increases, the names will settle back into normality because evidently the IUPAC's (International Union of Pure and Applied Chemistry) imagination only goes so far.

1: meth-
2: eth-
3: prop-
4: but-
5: pent-
6: hex-
7: hept-
8: oct-

By the way, I've always been interested as to what 11 is, so I googled it. It's undec-. 12 is dodec- and 13 is tridec-. And for you superstitious people who don't like me ending on 13, 14 is tetradec-. Wikipedia has the prefixes for 15, 20, 30, 40 and 50. It's like learning to count in a foreign language...

Now all alkanes have a few things in common. First of all, since they already have their hands (bonds?) full with hydrogen, they're not all too willing to react. They will undergo combustion reactions (just like every other organic compound that I know of), reacting with oxygen to produce carbon dioxide and water (which is usually seen in the form of steam due to the high heat in the highly exothermic combustion reaction), but apart from these they'll only undergo substitution reactions in which one of the hydrogens is substituted for another element. Substitution reactions can only take place if there's a catalyst present, like UV light. (Wait, does UV light count as a catalyst? I don't know.)

Oh wait, I just introduced another standard equation. Let's make it official.

Organic Compound + Oxygen gas -> Carbon Dioxide + Steam

Maybe I should also give an example for a substitution reaction just to make it clearer. How about our dear friend ethane with bromine water?

C2H6 + Br2 -> C2H5Br + HBr

As you can see, one of the hydrogens has swapped places with one of the bromines. If bromine is in excess there are different compounds that can be formed, but I don't know all too much about this.

Another similarity between alkanes is the general formula of alkanes. You see, each carbon has 2 hydrogen atoms bonded to it, while the carbon atoms at either end of the chain have one extra hydrogen each. Therefore, the number of hydrogens is double the number of carbons add 2. This can be summed up in a neat little general formula:

CnH2n+2

Let's meet another hydrocarbon- an alkene this time. Meet ethene- "eth-" for two carbons, "-ene" because it's an alkene.

As you can see, the double bond means that ethene can't contain as many hydrogens as ethane. We could also say that ethene is unsaturated because it doesn't have the maximum amount of hydrogens, while ethane is saturated because it can't hold any more hydrogens.

Alkenes are more reactive than alkanes due to the double bond. The double bond can break to bond with new elements in addition reactions. For example, if ethene was to react with bromine water, this reaction would occur:

C2H4 + Br2 -> C2H4Br2

Easy, no?

Like alkanes, alkenes also undergo combustion reactions. Also like alkanes, alkenes have a special general formula, but theirs is different from that of alkanes. The general formula of alkenes is as follows:

CnH2n

You don't have to learn about alkynes in year 11, but it should be pretty obvious what they'd look like. Ethyne would be like ethene but with an extra bond in the middle and two less hydrogens to compensate. Alkynes also undergo addition and combustion reactions. The general formula of alkynes is CnH2n-2.

Cycloalkanes are basically alkanes arranged in rings. Here's cyclopropane (cyclo- because it's cyclic, prop- because there's three carbons and -ane because it's an alkane):

Cycloalkanes behave in pretty much the same way as regular alkanes, but they have a different general formula. The general formula of cycloalkanes is the same as the general formula of alkenes, that being CnH2n.

Cycloalkenes are cycloalkanes with a double bond and two fewer hydrogens to compensate. Again, they behave like cycloalkenes, but have the same general formula of alkynes, that being CnH2n-2.

Last but not least, we have the aromatic compounds. That requires talking about benzene. Say hi to a simplified version of benzene!

Benzene has the formula C6H6 and it is a very special compound because its bonds aren't a regular length and it has a whole bunch of delocalised electrons in the middle, represented by that circle in the middle. The bonds of benzene are between the length of a single and a double bond. Each carbon atom has 3 bonds: one between the carbon and an adjacent hydrogen atom and the other two between the carbon atom and adjacent carbon atoms. The remaining electron makes up the sea of delocalised electrons in the middle. Unlike graphite, however, which also has delocalised electrons, benzene doesn't conduct electricity because all of the delocalised electrons are still somewhat localised to one molecule, whereas in graphite, the delocalised electrons are free to travel throughout the whole network. (For more info on graphite, see my post on covalent bonding.)

Basically, an aromatic compound is a compound that contains benzene. Since benzene has those weird bonds which are single bonds but are shorter than normal single bond lengths, benzene is only as reactive as an alkane- that is, it only undergoes catalysed substitution reactions or combustion reactions, but not addition reactions.

So there you have it- the basics of Organic Chemistry. Next up is drawing and naming hydrocarbons!