Hydrocarbon

Look along a kitchen shelf and you will find many compounds: salt, sugar, water, cooking oil, baking soda, rice and the steel of a pot. Chemists sort this huge variety into two big groups. One pair of names for these groups is carbonic and non-carbonic compounds. The other, more common pair is organic and inorganic compounds. Both pairs describe the same division.

The division first came from where a compound is found. Compounds obtained from minerals, such as salt dug from rock or lime made from limestone, were called inorganic or non-carbonic compounds. Compounds obtained from plant and animal sources, such as sugar from sugarcane, oil from mustard seeds or ghee from milk, were called organic or carbonic compounds.

Every organic compound contains carbon. A few compounds that contain carbon are still treated as inorganic, though. So the rule chemists use is this: all the compounds of carbon, except its oxides, carbonates, bicarbonates and carbides, are organic compounds. Among the organic compounds, the simplest family is made of just two elements.

A simple test with fire sorts common materials into two groups. Small amounts of wood pieces, sugar, stone, water, cooking oil, ghee, rice, maize, glass and iron pieces are collected. A wood fire is made in an open place, and the materials are put into it one at a time while someone watches what happens to each.

Sugar melts first, turns brown, then black, and catches fire. Oil and ghee burn with a bright flame. Rice and maize char and then burn. Wood keeps the fire going. Stone, glass and iron only get hot and never catch fire. Water does the opposite of burning and puts out part of the flame.

Now read the last column of the table. Every material that burned came from a plant or an animal. Every material that did not burn came from rock, a mineral or another non-living source. On this basis we conclude that combustible substances are organic and non-combustible substances are inorganic.

The fire test also explains why we use certain materials as fuel. All combustible substances consist of carbon and hydrogen, and hydrocarbons are made of exactly these two elements. Think of the places where fire is used around you:

• Kerosene lamps are lit in villages where there is no hydroelectricity.
• Firewood is burnt to cook food in many homes.
• Spirit lamps are used in laboratories as a source of heat.
• Charcoal is burnt to heat and melt metals while making utensils.

All of these fuels contain carbon and hydrogen, and all of them come from plants and animals. Firewood and charcoal come from trees. Kerosene is made from petroleum, which formed deep underground over millions of years from the buried remains of ancient plants and animals. When such a fuel burns, its carbon and hydrogen combine with oxygen from the air and give out heat and light.

Your own body is built largely from organic compounds. The protein, hormones, carbohydrates, fat, enzymes and protoplasm found in the human body are all organic compounds. That is where the word organic comes from: it means belonging to living organisms.

Source and burning make organic compounds easy to recognise, but chemists need a definition based on what is inside the molecule. That definition is about bonds. A carbon atom has four electrons in its outer shell. It completes its octet by sharing these electrons with other atoms, which forms covalent bonds. Carbon can share electrons with other carbon atoms to make long chains and rings, and it can share them with hydrogen atoms.

Carbon bonds with more than hydrogen. It can also form covalent bonds with oxygen, nitrogen, halogens such as chlorine and bromine, sulphur and phosphorus. So organic compounds are generally made of elements like carbon, hydrogen, oxygen, nitrogen, halogens, sulphur and phosphorus. Some organic compounds even contain a metal bonded to them. Chlorophyll, the green pigment of leaves, has a magnesium atom at its centre, and the haemoglobin in your blood contains iron.

Containing carbon does not automatically make a compound organic. The oxides of carbon, carbonates, bicarbonates and carbides all contain carbon atoms, yet they are counted as inorganic compounds. They behave like mineral compounds, and many of them are found in rocks and minerals.

Organic compounds range from tiny molecules with one carbon atom to giant molecules with thousands of atoms. Here are some you will meet often:

• Methane: the main gas in biogas, with one carbon atom and four hydrogen atoms.
• Ethane: a gas found in natural gas.
• Ethene: a gas used to ripen fruit and to make plastics.
• Acetylene: a gas burnt in welding torches that join metals.
• Methanol: a poisonous alcohol used as a solvent.
• Chloroform: a liquid containing chlorine, once used to make patients unconscious.
• Urea: a nitrogen compound found in urine and used as a fertiliser.
• Insulin: a hormone that controls the amount of sugar in the blood.
• Protein: the building material of muscles, hair and skin.
• Oil: cooking oils pressed from seeds, and animal fats.

Hydrocarbons are the parent family of organic chemistry. Replace some of the hydrogen atoms of a hydrocarbon with other atoms and you get other organic compounds. For example, replacing three hydrogen atoms of methane, \( \text{CH}_4 \), with chlorine gives chloroform, \( \text{CHCl}_3 \). Replacing one hydrogen atom with an \( \text{OH} \) group gives methanol, \( \text{CH}_3\text{OH} \).

Petroleum is the main source of hydrocarbons. Crude petroleum pumped from underground is a mixture of many hydrocarbons, and it is separated into petrol, diesel, kerosene, cooking gas and other fuels. A cooking gas cylinder holds hydrocarbons such as propane and butane.

Hydrocarbons are classified into two groups on the basis of the type of bond between their carbon atoms:

• Saturated hydrocarbons: the carbon atoms are joined only by single covalent bonds.
• Unsaturated hydrocarbons: at least one double or triple covalent bond joins two carbon atoms.

A molecule is far too small to see, so a model helps you picture how its atoms are joined. In a ball-and-stick model, each ball is an atom and each stick is a covalent bond. Two rules decide the shape: a carbon atom always forms four bonds, and a hydrogen atom always forms one. In these models the bigger balls stand for carbon and the smaller balls for hydrogen.

Methane, \( \text{CH}_4 \), has one carbon ball with a hydrogen ball on each of its four sticks. Ethane, \( \text{C}_2\text{H}_6 \), has two carbon balls joined by one stick. Each carbon uses its other three sticks for hydrogen, which gives six hydrogen atoms in all.

Propane, \( \text{C}_3\text{H}_8 \), has a chain of three carbon atoms. Each end carbon holds three hydrogen atoms, because only one of its sticks goes to another carbon. The middle carbon uses two sticks for its two carbon neighbours, so it holds only two hydrogen atoms. That makes \( 3 + 2 + 3 = 8 \) hydrogen atoms.

Under each model is its structural formula, in which each letter is an atom and each line is a bond. Count the sticks on any carbon in the models and you will always find four. Count the sticks on any hydrogen and you will always find one. If a model shows a carbon with only three sticks, or a hydrogen with two, it does not show a real molecule.