Alkanes

Introduction
Hybridization Of alkanes
Halogenations of Alkanes
Chlorination of Methane by Substitution
General Reaction of Alkanes

1. Alkanes Introduction Hybridization Of alkanes Halogenations of Alkanes Chlorination of Methane by Substitution General Reaction of Alkanes

2. Introduction Alkanes are aliphatic hydrocarbons having only C−H and C−C σ-bonds. They can be cyclic or acyclic. Acyclic alkanes have the molecular formula C n H 2+2 (where n = an integer.) They are also called saturated hydrocarbons because they have the maximum number of hydrogen atoms per carbon. Cyclic alkanes contain carbon atoms joined into a ring. They have molecular formula CnH2n They are also known as Paraffins due to low reactivity. The main source of Compounds are Petroleum.

3. Hybridization Of alkanes The combination of one s and three p orbitals to form four hybrid orbitals of equal energy is known as sp3-hybridization. Example: Methane (CH4) molecule. These sp3-hybridized orbitals are oriented at an angle of 109°28'. When these four sp3 hybrid orbitals overlaps with four 1s orbitals of hydrogen, a symmetrical tetrahedral shaped CH4 molecule form.

4. General Reaction of Alkanes Alkane halogenations is an example of a substitution reaction, a type of reaction that often occurs in organic chemistry. A substitution reaction is a chemical reaction in which part of a small reacting molecule replaces an atom or a group of atoms on a hydrocarbon or hydrocarbon derivative. A general equation for the substitution of a single halogen atom for one of the hydrogen atom of an alkane is

5. Methods of Formartion : The alkanes are prepared by following general methods of Preparation of this homologous series. By Catalytic Hydrogenation of Alkenes and Alkynes: The method involves the hydrogenation of aklene or alkynes in the presence of catalyst, nickel at about 200-300 C. This reaction is known as Sabatier and Senderen’s reaction.

6. By Grignard Reduction: Alkyl Halide react with magnesium metals in the presence of dry ether to form alkyl magnesium halide (R-Mg-X) also called Grignard reagent with subsequently decomposed by water or alcohol to give alkanes. Eg. By Wurtz reaction: When 2 molecule of alkyl halide react with two atoms of sodium in dry ether , a higher alkane is obtained . Other metals like Cu, Ag, in a finely divided state may also be used in place of sodium.

7. Limitation These reaction is not suitable for preparation of unsymmetrical alkanes . Mechanism: Ionic mechanism through intermediate formation Free Radical mechanism

8. B y Corey-House synthesis: The coupling reaction is a good synthetic way to join two alkyl groups together to produce higher alkanes. This versatile method is known as the Corey-House reaction. Mechanism

9. Halogenation of Alkanes A halogenation reaction is a chemical reaction between a substance and a halogen in which one or more halogen atoms are incorporated into molecules of the substance. Halogenation of an alkane produces a hydrocarbon derivative in which one or more halogen atoms have been substituted for hydrogen atoms. Alkanes are notoriously unreactive compounds because they are non-polar and lack functional groups at which reactions can take place. Free radical halogenation therefore provides a method by which  alkanes  can be functionalized. A severe limitation of radical halogenation however is the number of similar C-H bonds that are present in all but the simplest alkanes, so selective reactions are difficult to achieve.

10. General Features of Halogenation of Alkanes Note the following features of halogenation of alkanes. The notation R-H is a general formula for an alkane. R in this case represents an alkyl group. Addition of a hydrogen atom to an alkyl group produces the parent hydrocarbon of the alkyl group. The notation R-X on the product side is the general formula for a halogenated alkane. X is the general symbol for a halogen atom. Reaction conditions are noted by placing these conditions on the equation arrow that separates reactants from products. Halogenation of an alkane requires the presence of heat or light.

11. Chlorination of Methane by Substitution In halogenation of an alkane, the alkane is said to undergo fluorination, chlorination, bromination or iodination depending on the identity of the halogen reactant. Chlorination and bromination are the two widely used alkane halogenation reactions. Fluorination reactions generally proceed too quickly to be useful and iodination reactions go too slowly. Halogenations usually result in the formation of a mixture of products rather than a single product. More than one product results because more than one hydrogen atom on an alkane can be replaced with halogen atoms. Methane and chlorine when heated to a high temperature in the presence of light react as follows.

12. The mechanism for this reaction takes place in three steps. 1. Initiation Step: The Cl-Cl bond of elemental chlorine undergoes hemolysis when irradiated with UV light, and this process yields two chlorine atoms, also called chlorine radicals. 2. Propagation Step: A chlorine radical abstracts a hydrogen atom from methane to produce the methyl radical. The methyl radical in turn abstracts a chlorine atom from a chlorine molecule and  chloromethane  is formed. The second step of propagation also regenerates a chlorine atom. These steps repeat many times until termination occurs.

13. 3. Termination Step: Termination takes place when a chlorine atom reacts with another chlorine atom to generate Cl2, or chlorine atom can react with a methyl radical to form chloromethane which constitutes a minor pathway by which the product is made. Two methyl radicals can also combine to produce ethane, a very minor by product of this reaction. The reaction does not stop at this step, however because the chlorinated methane product can react with additional chlorine to produce polychlorinated products. By controlling the reaction conditions and the ratio of chlorine to methane. It is possible to favour formation of one or another of the possible chlorinated methane products.

14. Uses of Alkanes Alkanes are important raw materials of the chemical industry and the principal constituent of gasoline and lubricating oils. Natural gas mainly contains methane and ethane and is used for heating and cooking purposes and for power utilities (gas turbines). For transportation purposes, natural gas may be liquefied by applying pressure and cooling it (LNG = liqid natural gas). Crude oil is separated into its components by fractional distillation at oil refineries. The different "fractions" of crude oil have different boiling points and consist mostly of alkanes of similar chain lengths (the higher the boiling point the more carbon atoms the components of a particular fraction). The following table provides a short survey of the different fractions of crude oil: Propane and butane can be liquefied at fairly low pressures, and are used, for example, in the propane gas burner, or as propellants in aerosol sprays. Butane is used in cigarette lighters (where the pressure at room temperature is about 2 bar). The alkanes from pentane to octane are highly volatile liquids and good solvents for nonpolar substances. They are used as fuels in internal combustion engines.

15. Alkanes from nonane to hexadecane are liquids of higher viscosity, being used in diesel and aviation fuel (kerosene). The higher melting points of these alkanes can cause problems at low temperatures and in polar regions, where the fuel becomes too viscous. Alkanes with 17 to 35 carbon atoms form the major components of lubricating oil. They also act as anti-corrosive agents, as their hydrophobic nature protects the metal surface from contact with water. Solid alkanes also find use as paraffin wax in candles. Alkanes with a chain length above 35 carbon atoms are found in bitumen (as it is used in road surfacing). These higher alkanes have little chemical and commercial value and are usually split into lower alkanes by cracking.   Methane can co-crystallize with water at high pressures and low temperatures, forming a solid methane hydrate. The energy content of the known submarine methane hydrate fields exceeds that of all known natural gas and oil deposits put together.