Preparation of Alkanes Methane through -pentane and iso-pentane can be obtained in pure form by fractional distillation of petroleum and natural gas; neo-pentane does not occur naturally. 1.Hydrogenation of alkenes. Hydrogenation of unsaturated hydrocarbons: Alkenes or alkynes react with hydrogen in presence of catalyst (Ni, Pt or Pd) at o C to give alkanes.

Reduction of alkyl halides (a)Hydrolysis of Grignard reagent. When a solution of an alkyl halide in dry ethyl ether, (C 2 H 5 ) 2 O, is allowed to stand over turnings of metallic magnesium, a vigorous reaction takes place: the solution turns cloudy, begins to boil, and the magnesium metal gradually disappears. The resulting solution is known as a Grignard reagent, after Victor Grignard (of the University of Lyons) who received the Nobel prize in 1912 for its discovery. Hydrolysis of Grignard Reagent: Alkyl magnesium halide which is prepared by treating alkyl halides with magnesium in dry ether, on hydrolysis yield alkane

B- Reduction by metal and acid. Alkyl halide on reduction gives alkanes. Reducing agents used for reduction are zinc and acetic acid.

Coupling of alkyl halides with organometallic compounds Corey-House Alkane synthesis: It involves first conversion of alkyl halide to lithium dialkyl copper, LiR 2 Cu and then Coupling takes place in the reaction between a lithium dialkyl copper, R 2 CuLi, and an alkyl halide, R'X. (R' stands for an alkyl group that may be the same as, or different from, R.) USE:-This method is suitable for preparation unsymmetrical alkane. For good yields, R'X should be a primary halide; the alkyl group R in the organometallic may be primary, secondary, or tertiary. For example:

Chemical Reactions Of Alkanes Alkanes are extremely stable and inert substances. This is due to the fact that there is a small difference in the electro negativity of carbon (2.60) and hydrogen (2.10). Thus the bond electron in C-H bond are practically equally shared between them and the bond is almost non polar. the C-H and C-C bonds are strong bonds. Alkanes due to these types of strong bonds remain unaffected by acids, alkalies and oxidizing agents under ordinary conditions Lower alkanes undergo two types of reactions: i) Substitution Reaction ii) Thermal and catalytic Reactions 1) Halogenations: This involves replacement of hydrogen atom by halogen atom.

Depending upon which nydrogen atom is replaced, any of a number of isomeric products can be formed from a single alkane

Orientation of halogenation As we can see, w-propyl radicals are formed by abstraction of primary hydrogens, and isopropyl radicals by abstraction of secondary hydrogens.

Thermal decomposition [Pyrolisis or Cracking]: The decomposition of compound by heat is known as pyrolysis. Thermal decomposition when applied to alkanes it is known as Cracking. This leads to the formation of lower alkanes, alkenes and hydrogen etc. For example In presence of catalyst pyrolysis can be carried out at less higher temperature. This is called Catalytic Cracking.

Oxidation[Combustion]: When burnt in excess of air or oxygen alkanes form carbon dioxide and water with the evolution of heat.

Alkenes Structure and Preparation Alkenes are unsaturated hydrocarbons containing carbon-carbon double bond (C=C) in their molecules. General formula of alkene is C n H 2n. They are commonly known as olefins because lower members form oil products with chlorine or bromine (Latin Oleum=Oil; ficane = to make). Ethylene and propylene are the first and second members of this series respectively.

Double bonded carbon in alkenes is sp 2 hybridised. The Carbon-Carbon double bond in alkene consists of one sigma (σ) and one pi (π) bond. Sigma bond is formed by overlapping of sp 2 hybrid orbital of each carbon and π bond is formed by overlapping of unhybridised p-orbitals. The two carbon atoms which are double bonded in alkanes are held more tightly together than in alkane with the result C=C bond length in alkene is less (1.34A o ) than C-C bond length in alkanes. For example Orbital Structure of Ethene

Names of alkenes The rules of the IUPAC system are: 1. Select as the parent structure the longest continuous chain that contains the carbon-carbon double bond; then consider the compound to have been derived from this structure by replacement of hydrogen by various alkyl groups. The parent structure is known as ethene,propene,butene,pentene, and so on, depending upon the number of carbon atoms; each name is derived by changing the ending -ane of the corresponding alkane name to -ene:

2. Indicate by a number the position of the double bond in the parent chain. Although the double bond involves two carbon atoms, designate its position by the number of the first doubly-bonded carbon encountered when numbering from the end of the chain nearest the double bond; thus 1-butene and 2-butene. 3. Indicate by numbers the positions of the alkyl groups attached to the parent chain.

We should also note that IUPAC changed its naming rules in Prior to that time, the locant, or number locating the position of the double bond, was placed before the parent name rather than before the -ene suffi x: 2-butene rather than but-2-ene, for instance.

Although the interconversion of cis and trans alkene isomers doesn’t occur spontaneously, it can be brought about by treating the alkene with a strong acid catalyst. If we do, in fact, interconvert cis-but-2-ene with trans-but- 2- ene and allow them to reach equilibrium, we find that they aren’t of equal stability. The trans isomer is more favored than the cis isomer by a ratio of 76: 24.

According to the E,Z system, a set of sequence rules is used to rank the two substituent groups on each double-bond carbon. If the higher-ranked groups on each carbon are on opposite sides of the double bond, the alkene is said to have E stereochemistry, for the German entgegen, meaning “opposite.” If the higher ranked groups are on the same side, the alkene has Z stereochemistry, for the German zusammen, meaning “together.” Called the Cahn–Ingold–Prelog rules after the chemists who proposed them, the sequence rules are as follows: