Introduction
The simplest organic compounds are hydrocarbons. All organic compounds can be derived from hydrocarbons by substituting a hydrogen atom with a suitable functional group. Replacing a hydrogen atom by a OH group in a hydrocarbon, gives an alcohol: replacement of H atom in a hydrocarbon by COOH group gives in a hydrocarbon carboxylic acid, and so on. It is therefore appropriate to begin the study of hydrocarbons with its preparation, properties and reactions.
Classification of Hydrocarbons
There are millions of organic compounds and this makes it physically impossible to study each individual compound. To facilitate their study, organic compounds are classified into various groups and sub-groups.
Conformations of Alkanes and Cycloalkanes
The carbon-carbon bonds in alkanes result from sigma overlap of two tetrahedral carbon sp3 orbitals. Sigma bonds result from the head-on overlap of two atomic orbitals, and that they are cylindrically symmetrical. The cross section of a sigma orbital is circular. Due to this sigma bond symmetry, there is free rotation around the carbon-carbon single bond. Bond overlap is exactly the same for all geometric arrangements of the hydrocarbons.
Alkenes
Alkenes are unsaturated hydrocarbons containing a carbon double bond (C=C) in their molecule. They were earlier known as olefins.
Isomerism in Alkenes
Chain isomerism
This is due to the difference in the structure of the carbon chain. For example, butene-1 and 2-methylpropene are chain isomers.
Laboratory Preparation of Alkenes
In the laboratory, alkenes are usually prepared from either alcohols or haloalkanes (alkyl halides).
Physical Properties of Alkenes
The first lower member like ethene, propene and butene are colorless gases. Alkenes with five to fifteen carbon atoms are liquids and higher ones are solids at ordinary temperatures. Alkenes have characteristic smell.
Uses of Alkenes
Lower alkenes are used as fuel and illuminant. These may be obtained by the cracking of kerosene or petrol.
Alkynes
Unsaturated hydrocarbons that contain a triple carbon (C º C) bond in their molecules are called alkynes. Alkynes were earlier called acetylenes. Alkynes can be described by the formula CnH2n - 2 where n is 2,3, ……
Isomerism in Alkynes
Chain isomerism
It is due to the different arrangement of carbon atoms in the chain i.e., straight chain or a branched-chain e.g.,
Laboratory Preparation of Alkynes
Alkynes may be prepared by dehydrohalogenation (elimination) of dihaloalkanes preferably bromo derivatives. There are two methods of preparation for the two types of dihaloalkanes.
Physical Properties of Alkynes
The first three members (ethyne, propyne and butyne) are colourless and odorless gases. Due to the presence of phosphine as an impurity ethyne (acetylene) has garlic smell. The next eight members are liquids, and higher members are solids under normal conditions of temperature and pressure.
General Chemical Properties of Alkynes
Carbon-carbon triple bond, C=C, is a combination of one and two bonds. Alkynes give electrophilic addition reactions as they show reactivity due to the presence of bonds. This property is similar to alkenes but alkynes are less reactive than alkenes towards electrophilic addition reactions due to the compact CC electron cloud.
Stability of Conjugated Dienes
Stability of conjugated dienes can be explained in terms of orbital structure 1,3-butadiene. All the four carbon atoms in 1, 3-butadiene are sp2 hybridized. Each carbon atom also has an unhybridized p orbital, form bond. The hybrid orbitals of each carbon atom form sigma bonds. The p orbital of C-2 can overlap equally with the p orbitals of C-1 and C-3. Similarly, the p orbital of C-3 can overlap equally with the p orbitals of C-2 and C-4. As a result the p electrons in conjugated dienes are delocalised. This makes them more stable as the p electrons feel simultaneous attraction of all the four nuclei in all the four carbon atoms. It may be noted that in isolated or cumulated dienes, this type of delocalisation of p electrons is not possible.
Uses of Alkynes
Alkynes are generally used as the starting materials for the manufacture of a large number of organic compounds of industrial importance such as, chloroprene, vinyl chloride etc.
Arenes
Aromatic hydrocarbons that contain one or more benzene rings are called Arenes. A benzene ring is a six-membered ring containing carbon atoms linked to each other with alternate single and double bonds.
Sources of Aromatic Hydrocarbons
Coal is a complex mixture of hydrocarbons. It also contains some organic compounds containing nitrogen and sulphur in small amounts. It can be approximated to the formula (C3H4)n.
Structure of Benzene
The molecular formula of benzene has been found from analytical data, to be C6H6. Relatively higher proportion of carbon and addition of chlorine to benzene molecule indicate it to be an unsaturated compound. Depending on the various facts available to scientists from time to time, many structures for benzene had been proposed. Some are described below.
Aromaticity (Huckel rule)
Aromatic compounds are those, which resemble benzene in chemical behavior. These compounds contain alternate double and single bonds in a cyclic structure. They undergo substitution reactions rather than addition reactions. This characteristic behavior is called aromatic character or aromaticity.
Isomerism in Arenes
Monosubstituted arenes
Isomerism is not shown by monosubstituted benzene having methyl or ethyl group as the side chain, because all the positions on benzene ring are identical.
Preparation of Benzene
The simplest arene is benzene (C6H6). Benzene is a typical arene and it gives all the typical reactions of arenes. Benzene was first isolated by Faraday (1825) from the cylinders of the compressed illuminating gas obtained from natural sources. In 1845, Hoffmann isolated benzene from coal-tar. Coal-tar is the chief source of benzene.
Properties of Benzene
Physical Properties of Benzene
* Benzene is a colorless liquid (melting point 5.5oC, boiling point 80.4oC) with a characteristic smell of petrol.
* It is immiscible with water, but dissolves in organic solvents.
* It is lighter than water: density 0.87 g cm-3
* Its vapors are toxic.
Uses of Benzene
As a starting material for dyes, drugs, perfumes and explosives and polymers
Polynuclear Aromatic Hydrocarbons and their Toxicity
Polynuclear aromatic hydrocarbons are a class of compounds found throughout the environment in the air, in the soil and in the water. They are found naturally in crude oil, coal tar, and coal. They are man-made during incomplete combustion of hydrocarbons like coal, oil, gas, tobacco, and during forest fires. There are more than 100 different compounds that make up this particular class. Polynuclear aromatic hydrocarbons generally exist as colorless, pale yellow or white solids. Because they do not dissolve easily in water and generally do not burn, they can persist in the environment for months to years. It is difficult to isolate and analyze these hydrocarbons in the laboratory, due to the fact that they exist naturally as mixtures of many compounds.
Petroleum and Petrochemicals
Sources of Hydrocarbons
Until the early years of the nineteenth century, only the plants and animals were the known sources of organic compounds. With the advent of the Industrial Revolution in Europe, fossil fuels like coal and petroleum gained prominence as the sources of organic compounds, particularly hydrocarbons. Today, hydrocarbons are mostly obtained from petroleum. In recent years coal has started gaining prominence, probably due to the uncertain conditions in the world oil market.
Flash Point of a Hydrocarbon
The explosive nature of a hydrocarbon is determined by its volatility. The volatility of any liquid depends upon the temperature.
'The flash point of any liquid is the lowest temperature at which a liquid hydrocarbon gives off enough vapor to form an explosive mixture with air'.
The flash point of any liquid hydrocarbon is so adjusted that it remains safe under the conditions of its use. For example, the minimum flash point permitted in India is 44oC, while in France it is 33oC and in Britain it is 22oC.
Petrol (gasoline additives)
Gasoline additives are compounds added to gasoline, which improve the octane number of a fuel. Such substances are called antiknock compound that improve the fuels combustion in an internal combustion engine. Tetraethyl lead ((C2H5)4Pb) is a main additive and is a common antiknock compound . A small quantity of tetraethyl lead (TEL) improves the anti-knock quality of the gasoline. Gasoline containing tetraethyl lead is called leaded petrol or ethyl gasoline.
Synthetic Petrol (Aliphatic hydrocarbons from coal)
The petrol obtained artificially from coal as a mixture of alkanes resembling petroleum like aliphatic hydrocarbon fuels is called synthetic petrol. Two important methods for producing synthetic petrol are the Fischer-Tropsch process and the Bergius process. These processes were developed in Germany during World War II, when its petroleum supplies were cut off. Germany produced considerable amounts of fuel from coal by the above processes during that period.
