Hydrocarbons

Hydrocarbons: Organic compounds contain only two elements, hydrogen and carbon, and hence are known as hydrocarbons. On the basis of structure, hydrocarbons are divided into two main classes, aliphatic and aromatic. Aliphatic hydrocarbons are further divided into families: alkanes, alkenes, alkynes, and their cyclic analogs (cycloalkanes). Hydrocarbons (aliphatic + aromatic ) Aliphatic (alkanes +alkenes +alkynes + cyclic aliphatic)  

Methane The simplest member of the alkane family and, one of the simplest of all organic compounds is methane, CH4 . We shall study this single compound at some length, since most of what we learn about it can be carried over with minor modifications to any alkane.

structure of methane:   Each of the four hydrogen atoms is bonded to the carbon atom by a covalent bond, that is, by the sharing of a pair of electrons. When carbon is bonded to four other atoms, its bonding orbitals (sp3 orbitals, formed by the mixing of one (s) and three (p) orbitals) are directed to the corners of a tetrahedron.

We shall ordinarily write methane with a dash to represent each pair of electrons shared by carbon and hydrogen (I). To focus our attention on individual electrons, we may sometimes indicate a pair of electrons by a pair of dots (II). Finally, when we wish to represent the actual shape of the molecule, we shall use a simple three-dimensional formula like( III or IV).

Physical properties: The unit of such a non-ionic compound, whether solid, liquid, or gas, is the molecule. Because the methane molecule is highly symmetrical, the polarities of the individual carbon-hydrogen bonds cancel out; as a result, the molecule itself is non-polar. Attraction between non-polar molecules is van der Waal forces; for such small molecules, these attractive forces must be tiny compared with the enormous forces between, sodium and chloride ions.

It is not surprising,then, that these attractive forces are easily represent by thermal energy, so that melting and boiling occur at very low temperatures: (m.p -183 °C, b.p.- 101.5 °C). (Compare these values with the corresponding ones for sodium chloride: m.p. 801 °C , b.p. 1413 °C) As a consequence, methane is a gas at ordinary temperatures.  Methane is colorless and, when liquefied, is less dense than water .In agreement with the rule "like dissolves like," it is only slightly soluble in water, but very soluble in organic liquids such as gasoline, ether, and alcohol. In its physical properties methane sets the pattern for the other members of the alkane family.

Oxidation:( Heat of combustion) Reactions: Typically, it reacts only with highly reactive substances or under very vigorous conditions . At this point we shall take up only its oxidation( by oxygen, by halogens, and even by water).   Oxidation:( Heat of combustion) Combustion to carbon dioxide and water is characteristic of organic compounds ; under special conditions it is used to determine their content of carbon and hydrogen. Combustion of methane is the principal reaction taking place during the burning of natural gas, the important product is not carbon dioxide and water only also heat is evolved.

Burning of hydrocarbons takes place only at high temperatures, as provided,for example, by a flame or a spark. Once started, however, the reaction gives off heat which is often sufficient to maintain the high temperature and to permit burning to continue.  The quantity of heat evolved when one mole of a hydrocarbon is burned to carbon dioxide and water is called (heat of combustion); for methane its value is 213 kcal/mole.

Chlorination: (substitution reaction)   Under the influence of ultraviolet light or at a temperature of (250-400 °C)a mixture of the two gases, methane and chlorine, reacts vigorously to yield hydrogen chloride and a compound of formula CH3Cl. We say that methane has undergo chlorination, and we call the product, CH3Cl, chloromethane or methyl chloride (CH3 = methyl).

Chlorination is a typical example of a wide class of organic reactions known as substitution. A chlorine atom has been substituted for a hydrogen atom of methane, and the hydrogen atom thus replaced is found combined with a second atom of chlorine. The methyl chloride can itself undergo further substitution to form more hydrogen chloride and CH2Cl2 , dichloromethane or methylene chloride (CH2 =methylene).  

In a similar way, chlorination may continue to yield CHCI3 trichloromethane or chloroform, and CCl4 , tetrachloromethane or carbon tetrachloride. Chloroform as an anesthetic, and carbon tetrachloride as a non-flammable cleaning agent and the fluid in certain fire extinguishers.

Transition state: A chemical reaction is a continuous process involving a gradual transition from reactants to products. It has been found extremely helpful, however , to consider the arrangement of atoms at an intermediate stage of reaction as though it were an actual molecule. This intermediate structure is called the transition state; its energy content corresponds to the top of the energy hill (Fig.). Eact = energy of activation