Saturated and Unsaturated Hydrocarbons

A hydrocarbon is a compound made of only two elements, carbon and hydrogen. The methane in natural gas, the propane and butane packed into a cooking gas cylinder, and the ethyne burned in a welding torch are all hydrocarbons. A carbon atom always forms four covalent bonds. In a hydrocarbon some of these bonds join carbon to hydrogen, and the rest join carbon to other carbon atoms.

Two carbon atoms can be joined in three ways. If they share one pair of electrons they form a single bond, \( \mathrm{C{-}C} \). If they share two pairs they form a double bond, \( \mathrm{C{=}C} \). If they share three pairs they form a triple bond, \( \mathrm{C{\equiv}C} \). The kind of bond between the carbon atoms decides which family a hydrocarbon belongs to and how easily it reacts.

Watch what happens to the hydrogen. In ethane each carbon uses one bond on the other carbon and three on hydrogen, so ethane has six hydrogen atoms. In ethene each carbon spends two bonds on the other carbon, leaving room for only two hydrogen atoms on each, four in all. In ethyne three bonds lie between the carbons, and just one hydrogen fits on each carbon. Every extra bond between the carbons costs two hydrogen atoms.

Saturated means filled up. In an alkane every carbon atom already holds as many hydrogen atoms as its four bonds allow, so no more hydrogen can be added. Single bonds are strong and there is no spare bond for another substance to attack, so alkanes are very stable and react very little. That is why they are also known as paraffins, from Latin words meaning little affinity.

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

This is the general formula of the alkanes. Here n is the number of carbon atoms, and n = 1, 2, 3, 4 and so on. The number of hydrogen atoms is always two more than twice the number of carbon atoms.

A hydrocarbon can be written in three ways, and each way shows something different:

• Molecular formula: only the number of each kind of atom, such as \( \mathrm{C_3H_8} \).
• Condensed formula: the atoms grouped carbon by carbon along the chain, such as \( \mathrm{H_3CCH_2CH_3} \), so you can see how the chain is built.
• Structural formula: a drawing that shows every atom, with every bond drawn as a line.

Any row can be checked with the general formula. Butane has 4 carbon atoms, so it should have \( 2 \times 4 + 2 = 10 \) hydrogen atoms, and \( \mathrm{C_4H_{10}} \) has exactly 10. In the condensed formula, \( \mathrm{(CH_2)_2} \) means two \( \mathrm{CH_2} \) groups in a row in the middle of the chain.

They are called unsaturated because they are not filled up with hydrogen. If one of the extra bonds in a double or triple bond opened, the carbon atoms there could hold more hydrogen. That extra bond is the weak point of the molecule. It opens easily when another substance reaches it, so unsaturated hydrocarbons are less stable and far more reactive than alkanes. They are known as olefins. The name comes from olefiant gas, an old name for ethene meaning oil-making gas, because ethene reacts with chlorine to give an oily liquid.

A double or triple bond needs two carbon atoms to sit between. So the smallest unsaturated hydrocarbon has two carbons, and there is no unsaturated hydrocarbon with only one carbon atom.

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

This is the general formula of the alkenes. Again n is the number of carbon atoms, but here it starts from 2. An alkene has exactly twice as many hydrogen atoms as carbon atoms. That is two fewer than the alkane with the same number of carbons, and those two missing hydrogen atoms are the price of the double bond.

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

This is the general formula of the alkynes, where n is the number of carbon atoms and starts from 2. A triple bond uses up one more bond on each of its two carbons than a double bond does, so an alkyne has four hydrogen atoms fewer than the alkane with the same number of carbons. Ethyne is better known as acetylene. It burns with a very hot flame and is used to cut and weld metals.

The name of a hydrocarbon tells you two things. The first part counts the carbon atoms, and the ending tells you the kind of bond.

So propyne is prop, three carbons, with yne, a triple bond, which is \( \mathrm{C_3H_4} \). Butene is four carbons with a double bond, which is \( \mathrm{C_4H_8} \).

Example 1: Find the molecular formula of the alkene that has 5 carbon atoms.

Step 1: Writing the general formula of the alkenes:

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

Step 2: Putting n = 5 into the number of hydrogen atoms:

\[ 2n = 2 \times 5 = 10 \]

Step 3: Writing the molecular formula with 5 carbons and 10 hydrogens:

\[ \mathrm{C_5H_{10}} \]

It has five carbons and a double bond, so its name is pentene.

Example 2: Find the molecular formula of the alkyne that has 5 carbon atoms.

Step 1: Writing the general formula of the alkynes:

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

Step 2: Putting n = 5 into the number of hydrogen atoms:

\[ 2n - 2 = 2 \times 5 - 2 = 8 \]

Step 3: Writing the molecular formula with 5 carbons and 8 hydrogens:

\[ \mathrm{C_5H_8} \]

This compound is pentyne.

Example 3: A hydrocarbon has the formula \( \mathrm{C_3H_4} \). Which family does it belong to?

Step 1: Counting the carbon atoms gives the value of n:

\[ n = 3 \]

Step 2: Working out the hydrogen an alkane with 3 carbons would have:

\[ 2 \times 3 + 2 = 8 \]

Step 3: Working out the hydrogen an alkene with 3 carbons would have:

\[ 2 \times 3 = 6 \]

Step 4: Working out the hydrogen an alkyne with 3 carbons would have:

\[ 2 \times 3 - 2 = 4 \]

The compound has 4 hydrogen atoms, and only the alkyne count matches. So \( \mathrm{C_3H_4} \) is an alkyne, and with three carbons it is propyne.

The extra bond of an alkene can open, so hydrogen can be added across it. When ethene and hydrogen are heated with a catalyst such as nickel, the double bond opens, each carbon takes one more hydrogen atom, and ethane is formed:

\[ \mathrm{H_2C{=}CH_2 + H_2 \rightarrow C_2H_6} \]

On the left is an alkene and on the right an alkane. Industry uses the same change on a large scale. Vegetable oils such as soybean and mustard oil contain unsaturated fatty acids, whose long carbon chains carry double bonds, and this keeps the oil liquid at room temperature. Hydrogen gas is bubbled through the warm oil with nickel as the catalyst. The double bonds take up hydrogen, the unsaturated fatty acids become saturated fatty acids, and the liquid oil sets into a soft solid, Vanaspati ghee.