Preparation and Properties of Alkane

Preparations and Properties of Alkane

Alkane: Prepartion and Properties

Methods of Preparation

Preparation of Alkanes from Alkenes and Alkynes (Unsaturated Hydrocarbons)

Alkanes can be prepared from alkene and alkyne through hydrogenation. In this process, hydrogen gas is added to alkenes and alkynes in the presence of finely divided catalyst such as nickel, palladium or platinum.

Alkanes can also be prepared from alkenes by hydroboration. Alkene on hydroboration give trialkyl borane as a result of addition of diborane on olefinic bond. This trialkyl borane on treatment with acetic acid or propanoic acid yields alkanes.

Preparation of Alkanes from Alkyl Halides

Alkanes can be prepared from alkyl halides by two ways-
Alkanes can be prepared from alkyl halides (except fluorides) by reduction with zinc and dilute hydrochloric acid.

Alkyl halides treated with sodium metal in the presence of dry ether higher alkanes with an even number of carbon atoms are produced. This reaction is known as Wurtz reaction. Methane can not be prepared by this reaction. The reaction fails in case of tertiary halides

Corey- House Synthesis

Corey-House Reaction is also called as coupling of alkyl halides with organo metallic compounds. It is a better method than Wurtz reaction. In this synthesis, alkyl chloride reacts with lithium in presence of ether to give lithium alkyl then reacts with CuI to give lithium dialkyl cuprate. This lithium dialkyl cuprate again reacts with alkyl chloride (may be same or different from first one) to given higher alkane. This reaction can be used to make symmetrical, unsymmetrical and straight-chained or branched-chain alkanes.

Preparation of Alkanes from Carboxylic Acids

Alkanes can be prepared from carboxylic acids mainly by two processes

Alkanes can be prepared from carboxylic acid via decarboxylation (removal of carbon dioxide) process. It produces alkane with a carbon atom lesser than that present in the carboxylic acid.

Alkane is produced through electrolysis of sodium or potassium salt of carboxylic acid. This process is called Kolbe's Electrolysis.

Preparation of Alkanes from Grignard Reagent

Grignard reagent on double decomposition with water or with other compounds having active H(the hydrogen attached on O, N, F or triple bonded carbon atom are known as active hydrogen) give alkane.

Reduction of Carbonyl Compounds

Preparation of Alkanes from Aldehydes, Ketones and Carboxylic Acids
Aldehydes, ketones and carboxylic acids reduced to alkanes by amalgamated zinc and conc. HCl. This is known as Clemmensen reduction.
Aldehydes and ketones reduced to alkanes by hydrazine in basic medium. This is known as Wolf Kishner reaction.

Properties of Alkanes

Physical Properties

First four alkanes (i.e. methane to butane) are gases. The next seventeen members from pentane to heptadecane are liquid and the higher members are waxy solids. The gasesous and liquid alkanes have characteristic odour while the solid members are generally odourless.

Structures of Alkanes

General Formula: C_nH_2n+2 and C_nH_2n
Hybridization: sp³
Structure: Tetrahedral
Bond Angle: 109.5°

Solubility of Alkanes

Alkanes are generally non-polar molecules due to very little difference in electronegativity between carbon and hydrogen and the covalent nature of C-C bond or C-H bond. Generally polar molecules are soluble in polar solvents whereas non-polar molecules are soluble in non-polar solvents. Hence, alkanes are hydrophobic in nature, that is, alkanes are insoluble in water. However, they are soluble in organic solvents as the energy required to overcome the existing Van Der Waals forces and the energy required to generate new Van Der Waals forces is quite comparable.

Boiling Point of Alkanes

The boiling point of alkanes increases with increasing molecular weight due to the intermolecular Van Der Waals forces increase with the increase of the molecular size or the surface area of the molecule. The straight-chain alkanes are observed to have a higher boiling point in comparison to their structural isomers.
The melting point of alkanes follows the same trend as their boiling point, that is, it increases with an increase in molecular weight due to the fact that higher alkanes are solids and it's difficult to overcome intermolecular forces of attraction between them. It is observed that even-numbered alkanes have a higher melting point in comparison to odd-numbered alkanes as the even-numbered alkanes pack well in the solid phase, forming a well-organised structure which is difficult to break.

Density

The density of alkanes increases with the size of the molecule and tends to approach a constant value of 0.8 with hexadecane. Thus all the alkanes are lighter than water.

Chemical Properties

Combustion

Complete combustion (with sufficient oxygen) of any hydrocarbon produces carbon dioxide and water.
C₃H₈ + 5O₂ → 3CO₂ + 4H₂O + Energy
This is an exothermic reaction so it is used as fuels.

Substitution Reaction

One or more hydrogen atom(s) of alkane are substituted by either atoms or group of atoms.

Halogenation of Alkane

In halogenation of alkane, hydrogen is replaced by halogen atom.
The order of reactivity of halogens in this regard is-
F₂ > Cl₂ > Br₂ > I₂

Chlorination and Bromination of Alkane

Chlorination of alkane takes place in the presence of heat (300-400°) or diffused sunlight or UV light and gives chlorine derivative of alkane.

Bromie reacts with alkane in a same manner but less vigorously.

Iodination of Alkane

Iodine reacts with alkanes reversibly. The HI formed as the by-product is a strong reducing agent and is capable of reducing the iodoalkane to the alkane.
R-CH₃ + I₂ ⇌ R-CH₂I + HI
However, alkanes can be iodinated in the presence of an oxidizing agent such as HNO₃ or HIO₃ which destroys the HI. 5HI + HIO₃ → 3I₂ + 3H₂O.

Fluorination of Alkanes

Pure fluorine reacts with alkanes explosively under most conditions. Fluoroalkanes can be obtained from alkanes by the action of fluorine diluted with nitrogen.

Why is the reaction of alkanes with fluorine difficult to control ?

Hints: The reaction of alkanes with fluorine is difficult to control because the activation energy for hydrogen abstraction is very low.

Nitration of Alkanes

At ordinary temperature, alkanes do not react with nitric acid. However, when a mixture of an alkane and nitric acid vapour is heated at 400° to 500°C. One H-atom is replaced by -NO₂ group. This process is called vapour phase nitration.

The reaction occurs at high temperature so carbon-carbon bonds breaks during the reaction. Thus mixture of nitroalkanes are obtained.
Nitration of propane yields a mixture of four nitroalkanes- 1-nitropropane, 2-nitropropane, nitroethane and nitromethane.

Sulphonation of Alkanes

At ordinary temperature, neither concentrated nor fuming sulphuric acid reacts with alkanes. However, when alkanes are subjected to a prolonged reaction with fuming sulphuric acid, one hydrogen atom on the alkane is replaced by a sulphonic group.

The lower alkanes do not react unless they have a tertiary hydrogen atom because the ease of replacement of hydrogen atoms is-
tertiary > secondary > primary

Pyrolysis (Cracking)

When alkanes are heated at very high temperature (500-800°C) in the absence of air, pyrolysis or thermal decomposition occurs. Large alkanes are broken down to give a mixture of smaller alkanes, alkenes and hydrogen. In the presence of a catalyst (finely divided silica-alumina) reactions can be occurs at less higher temperature and this is called catalytic cracking.
3CH₃CH₃ → 2CH₂=CH₂ + CH₄ + H₂

Isomerization

Normal alkanes are converterd into their branched chain isomers in the presence of aluminium chloride and HCl at room temperature.

Aromatization

Alkanes with six or ten carbon atoms are converted into benzene and its homologues at high temperature in the presence of a catalyst. This process is called aromatization and innvolvs simultaneously dehydrogenation and cyclization to give aromatic hydrocarbon with the same carbon atoms.