Hydrocarbons Classification

Hydrocarbons and compounds derived from them generally fall into three large categories.

Aliphatic hydrocarbons consist of chain of carbon atoms that do not involve cyclic structures. They are often referred to as open-chain or acyclic structures, e.g. propane, pentane, hexane.

Alicyclic or simply cyclic hydrocarbons are composed of carbon atoms arranged in a ring or rings, e.g. cyclopropane, cyclopentane, cyclohexane.

Aromatic hydrocarbons are a special group of cyclic compounds that usually have six-membered rings with alternative single and double bonds. They are classed separately from aliphatic and alicyclic hydrocarbons because of their characteristic physical and chemical properties, e.g. benzene, naphthalene.

Addition of hydrogen: hydrogenation. The addition of hydrogen to a carbon-carbon double bond, called hydrogenation, reduces an alkene to an alkane. The process requires the presence of a metal catalyst, and for this reason, it is also called catalytic reduction. Catalytic reduction of alkenes is a very important reaction in the laboratory. In hydrogenation, both hydrogen atoms are added to the same side of the alkene molecule.

Addition of water: hydration. In the presence of an acid catalyst, often 60% aqueous sulfuric acid, water adds to alkenes to produce alcohols. Hydrogen adds to the carbon of the double bond with the greater number of hydrogen; OH adds to alkenes in accordance with Markovnikov^,s rule.

The addition of water to an alkene is called hydration. Hydration of alkenes is a very important reaction both in the chemical industry and in biological systems.

Sources and uses of hydrocarbons. Petroleum and its associated natural gases are now the major source of hydrocarbons. Natural gas is composed principally of methane (CH₄). Ethane (C₂H₆) and propane (C₃H₆) typically represent 5 to 10 per cent of the total, along with traces of C₄ and C₅ hydrocarbons. The gas is freed of various unwanted contaminants and then it is utilized almost exclusively as fuel. Valuable side products of petroleum cracking provide raw materials for the petrochemical industry. Ethylene (C₂H₄) and propylene (C₃H₆) are principal starting points for the manufacture of chemicals, medicines, and polymers.

Revision exercises

Ex.1. Answer the following questions:

1. What are naturally occurring hydrocarbons? 2. What classifications are hydrocarbons divided into? 3. What purposes do the classifications serve to? 4. What are hydrocarbons composed of? 5. What are alkanes? Give examples. 6. What is the chemical formula of alkanes? 7. What are the chemical reactions of alkanes? 8. What are the three classes of hydrocarbons? 9. What are the rules of naming them? 10. What other classifications are hydrocarbons divided into? 11. What is hydrogenation? 12.Why is hydrogenation called catalytic reduction? 13. What is hydration? 14. What are sources and uses of hydrocarbons?

Ex.2. Match the words with their definitions:

1. .liquid	a. a material (such as coal, oil, or gas) that is burned to produce power;
2. medicine	b. to cause to combine until there is no further tendency to combine;
3. fuel	c. a special part or characteristic, quality, structure, form;
4. bond	d. a substance that is used in treating disease or relieving pain, and that is usually in the form of a pill or liquid;
5. feature	e. capable of flowing freely like water: not a gas or a solid;
6. saturate	f. an attractive force that holds together atoms, ions, or groups of atoms in a molecule or crystal;

Ex.3. Say whether the following statements are true or false:

1. The structures and chemistry of individual hydrocarbons do not depend in large part on the types of chemical bonds that link together the atoms of their constituent molecules. 2. The addition of hydrogen to an alkene is called hydration. 3. Petroleum and its associated natural gases are now the major source of hydrocarbons. 4. The addition of hydrogen to a carbon-carbon double bond, called hydrogenation, reduces an alkene to an alkane. 5. Aromatic hydrocarbons are a special group of cyclic compounds that usually have six-membered rings with alternative triple and double bonds. 6. Aliphatic hydrocarbons are divided into three main groups according to the types of bonds they contain: alkanes, alkenes, and alkynes.

Ex.4. Insert the necessary word:

1. Alkanes are described as saturated hydrocarbons, while …, alkynes, and aromatic hydrocarbons are said to be unsaturated. 2. Hydrocarbons with single carbon-carbon bonds are referred to as being … whilst any hydrocarbon that contains a double bond is said to be … . 3. Saturated hydrocarbons are also called the … , while the unsaturated hydrocarbons include both those molecules that contain carbon-carbon double bonds (referred to as the alkenes) and those that contain carbon-carbon triple bonds (referred to as the …). 4. … and … are natural products that have resulted from the … of organic compounds from plants and animals that lived millions of years ago. 5. They are found today as petroleum, which are … of hydrocarbons containing up to 30 or 40 carbon atoms. 6. Different components of … can be isolated by fractional distillation. 7. These hydrocarbons are good sources of fuels, the so-called „fossil fuels‟. 8. As mentioned previously, the global production of such fossil fuels is 3 billion tonnes. 9. As they are produced in such large quantities, … of the environment with these fossil fuels is of concern. 10. The major route of entry into the environment isn’t through spectacular … such as the oil spills from ships, but rather through our daily activities. 11. Pumping fuel into cars, and oil spilled onto the road as a result of old faulty cars are major contributors. (disasters, alkanes (2), mixtures, decay, saturated, alkynes, pollution, alkenes (2), petroleum, unsaturated).