Nomenclature of Hydrocarbons

Carbon atoms can link up into chains that are short or long, so the number of possible hydrocarbons is enormous. Learning them one by one would be hopeless, so chemists sort them into families. All the members of one family fit a single general formula, and each member is bigger than the one before it by exactly the same piece.

Look at the first four members of the alkane family: methane \( \mathrm{CH_4} \), ethane \( \mathrm{C_2H_6} \), propane \( \mathrm{C_3H_8} \) and butane \( \mathrm{C_4H_{10}} \). All four fit the general formula \( \mathrm{C_nH_{2n+2}} \), where \( n \) is the number of carbon atoms. Butane has \( n = 4 \), so it has \( 2 \times 4 + 2 = 10 \) hydrogen atoms, which gives \( \mathrm{C_4H_{10}} \). Now see what is added in going from methane to ethane: one carbon and two hydrogens, that is \( \mathrm{CH_2} \). Exactly the same piece is added going from ethane to propane and from propane to butane.

A \( \mathrm{CH_2} \) group is one carbon atom and two hydrogen atoms. Taking the atomic mass of carbon as 12 and of hydrogen as 1, the mass of this group is:

\[ \mathrm{CH_2} = 12 + 2 \times 1 = 14 \]

Check it with methane and ethane. Methane weighs \( 12 + 4 = 16 \) and ethane weighs \( 2 \times 12 + 6 = 30 \). Their difference is:

\[ 30 - 16 = 14 \]

The idea is not limited to alkanes. Alcohols form a series of their own. Methanol has one carbon, ethanol has two and propanol has three. In the table below, one \( \mathrm{CH_2} \) is slipped into the chain each time while the \( \mathrm{OH} \) end stays exactly as it was. Their molecular weights are 32, 46 and 60, so once again each step is 14.

• All members can be written with one general formula.
• Neighbouring members differ by one \( \mathrm{CH_2} \) group.
• The molecular weights of neighbouring members differ by 14.

In methane, \( \mathrm{CH_4} \), all four bonds of the carbon atom are taken up by hydrogen, so it is a complete molecule. If one hydrogen is removed, one bond of the carbon is left free. The \( \mathrm{-CH_3} \) group that remains does not sit on its own. It is ready to join another atom or group through that free bond. Such a group is called an alkyl radical.

An alkane has \( 2n+2 \) hydrogen atoms. Remove one and \( 2n+1 \) are left, which is why an alkyl radical is \( \mathrm{C_nH_{2n+1}} \). The radical from ethane has \( n = 2 \), so it has \( 2 \times 2 + 1 = 5 \) hydrogen atoms and is written \( \mathrm{-C_2H_5} \). A radical is named by changing the ending -ane of the alkane to -yl: methane gives methyl, ethane gives ethyl.

A functional group attaches to the free bond of an alkyl group, and a new kind of organic compound is formed. Think of the alkyl part as the body and the functional group as what decides how the compound behaves. For example, when \( \mathrm{-OH} \) joins a methyl group, the result is methanol, \( \mathrm{CH_3OH} \), which is an alcohol. When \( \mathrm{-COOH} \) joins a methyl group, the result is acetic acid, \( \mathrm{CH_3COOH} \), the acid that makes vinegar sour. The methyl part is the same in both, yet changing the attached group changes the whole family the compound belongs to.

At one time the same compound was called by different names in different places. The gas bubbling up from marshy ground was known as marsh gas, and chemists elsewhere could give the very same gas another name. This led to mistakes in reading and understanding. To end the confusion, scientists set up one system of naming that works the same way all over the world. In that system the gas has a single name: methane.

The name of a hydrocarbon is built from two parts. The first part is the word root, which tells how many carbon atoms are in the chain. The second part is the suffix, which tells what kind of bond joins the carbon atoms. The two rules below show how to find each part.

\[ \text{name} = \text{word root} + \text{suffix} \]

First count how many carbon atoms the molecule has. Then pick the word root for that number from the table below.

Once the word root is found, look at the bonds between the carbon atoms. If every bond is a single covalent bond, add the suffix -ane. If two carbon atoms share a double bond, add -ene, and if they share a triple bond, add -yne. The suffix therefore also tells you which family the compound belongs to.

Example 1: Name \( \mathrm{CH_3-CH_3} \).

Step 1: Counting the carbon atoms. There is one carbon at each end, so two in all, and the word root is:

\[ 2 \ \mathrm{C} \rightarrow \text{eth} \]

Step 2: Looking at the bond between the carbons. It is a single covalent bond, so the suffix is:

\[ \text{single bond} \rightarrow \text{ane} \]

Step 3: Joining the word root and the suffix:

\[ \text{eth} + \text{ane} = \text{ethane} \]

Example 2: Name \( \mathrm{CH_3-CH=CH_2} \).

Step 1: Counting the carbon atoms. \( \mathrm{CH_3} \), \( \mathrm{CH} \) and \( \mathrm{CH_2} \) each hold one carbon, making three, so the word root is:

\[ 3 \ \mathrm{C} \rightarrow \text{prop} \]

Step 2: Looking at the bonds between the carbons. Two of them share a double bond, so the suffix is:

\[ \text{double bond} \rightarrow \text{ene} \]

Step 3: Joining the word root and the suffix:

\[ \text{prop} + \text{ene} = \text{propene} \]

Example 3: Name \( \mathrm{HC \equiv CH} \).

Step 1: Counting the carbon atoms and choosing the word root:

\[ 2 \ \mathrm{C} \rightarrow \text{eth} \]

Step 2: The two carbons share a triple bond, so the suffix is:

\[ \text{triple bond} \rightarrow \text{yne} \]

Step 3: Joining the word root and the suffix:

\[ \text{eth} + \text{yne} = \text{ethyne} \]

The same two rules name every other simple hydrocarbon. The table gives more examples to practise on.

The naming rules also work backwards. Given a name, the word root gives the number of carbon atoms and the suffix gives the general formula. Here is the molecular formula of pentane worked out.

Step 1: Reading the word root. Pent means five carbon atoms:

\[ n = 5 \]

Step 2: Reading the suffix. The ending -ane means an alkane, whose general formula is:

\[ \mathrm{C_nH_{2n+2}} \]

Step 3: Putting \( n = 5 \) to find the number of hydrogen atoms:

\[ 2n + 2 = 2 \times 5 + 2 = 12 \]

Step 4: Writing the molecular formula of pentane:

\[ \mathrm{C_5H_{12}} \]

In the same way, propene has three carbon atoms and is an alkene, so \( \mathrm{C_nH_{2n}} \) gives \( \mathrm{C_3H_6} \). Propyne is an alkyne, so \( \mathrm{C_nH_{2n-2}} \) gives \( 2 \times 3 - 2 = 4 \) hydrogen atoms and the formula \( \mathrm{C_3H_4} \).