Writing Formulas of Simple Organic Coumpounds

Organic compounds are all made of carbon chains. In the simple organic compounds that we are going to look at, those chains are surrounded by hydrogen atoms (with an occasional oxygen).

Counting carbons

The naming system has two parts. The first part tells you the number of carbon atoms in the chain. Although the first four numbers in the system as not obvious, the rest are pretty straight-forward.

1 - meth
2 - eth
3 - prop
4 - but
5 - pent
6 - hex
7 - hept
8 - oct
9 - non
10 - dec

So, by looking at the beginning of the name, we can immediately determine the number of carbon atoms in the formula.

The second half of the name tells us about the bonding in the carbon chain.

"ane" compounds

ex: ethane, butane, heptane

The simplest of the organic compounds to name are the alkanes. These are single bonded chains of carbon surrounded by hydrogen atoms. Remember that each carbon atom has 4 valence electrons and "likes" to make 4 bonds. So, in any chain of carbon atoms, each carbon will have 2 hydrogen atoms (above and below) and there will be two additional hydrogen atoms - one at each end.

So, a simple way to think about the formula of alkanes is C_xH_{(2x +2)}.

"ene" compounds

ex: ethene, butene, heptene

If the carbon chain contains one double bond the compound is called an alkene.
As you get further along in your chemistry studies you will learn how to determine exactly WHERE in that chain the double bond occurs, but that is more than we are going to concern ourselves with in this text.

What is important to realize is that the double bond between two carbon atoms "uses" up two electrons that will now not be bonded to hydrogen atoms. That means that propene has two less hydrogen atoms than propane.

So the simple formula for a alkene is C_xH_2x.

"yne" compounds

ex: ethyne, butyne, heptyne

If the carbon chain contains a triple bond, the compound is called an alkyne. Again, where the triple bond occurs involves details beyond the scope of this course.

When the triple bond is formed it involves 4 more electrons than a single bond, so an alkyne will have 4 less hydrogen atoms than an alkane with the same number of carbon atoms.

Therefore, the simple formula for an alkyne is C_xH_{(2x -2)}.

"anol" compounds

ex: ethanol, butanol, heptanol

If one of the hydrogens from an alkane is replaced with an OH group, the compound is called an alcohol.  Although the OH group can replace any hydrogen in the structure, we are (again) not going to concern ourselves with this detail.

The OH in this case is called a functional group. There are many functional groups and it is through functional groups (both what the group is and where they attach) that organic compounds differ and behave in interesting ways. For instance, different amino acids are formed by attaching amine groups and acid groups at various places on chains of different lengths. The study of these details is called Organic Chemistry and (at its simplest) involves several semesters of college level study.

For our purposes, we are only going to consider compounds with one OH group.

The simple formula for an alcohol is C_xH_{(2x +1)}OH.

Here are some problems to try: (answers are here)

propanol
decyne
hexene
methanol
nonene
octane
pentyne