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# Alkanes

Top
 Sub Topics A straight chain hydrocarbon is a hydrocarbon in which each carbon atom is attached to one or two other carbon atoms only, at the ends of the chain can be seen in the structural formula. A branched chain hydrocarbon is a hydrocarbon where at least one carbon atom is attached to three or four other carbon atoms, thus forming a "branch" off the main chain. The first analogous series of hydrocarbons to examine is the series of compounds known as the alkanes. In this series, the names of all the compounds end in -ane. Many of them will sound familiar because they are all very common and may have previously encountered them.

## Definition

Alkanes are compounds that contain only C-C and C-H bonds. No double or triple bonds, nitrogen, sulfur, oxygen and the halogens are not present. Just C-C and C-H bonds. The general formula for alkanes are CnH2n+2. For example, if an alkane has 5 carbon atoms, it will have 12 hydrogen atoms, giving it the formula C5H12

Alkanes generally obtained from fossil fuels, particularly natural gas and crude oil. Natural gas is usually a mixture of small alkanes with low boiling points. The first ten alkanes line structure is shown below.

## Alkanes and Cxycloalkanes

Alkanes belong to the calss of hydrocarbons where all the carbons present in the molecule is linked through single bonds.and thus referred as saturated hydrocarbons. It follows the general formula CnH2n+2. The examples of alkanes are as follows.

Cycloalkanes are the simplest type of alicyclic compounds. The general formual for cycloalkanes is CnH2n. The first few members of the series are

The boiling point of alicyclic hydrocarbons increase regularly as the ring size increases and their boiling points are higher than the corresponding alkanes.

## Nomenclature and Numbering

1. Simple alkanes

The names of the first ten simple alkanes are methane, ethane, propane, butane, pentane, hexane, heptane, octane, nonane and decane. The names of the simplest straight chain alkanes are shown below.

2. Branched alkanes

Branched chain are alkanes with alkyl substituents branching off from the main chain. The procedure for naming the branched chain alkanes are
• Identify the longest chain of carbon atoms.
• Number the longest chain of carbons.
• Identify the carbon with the branching group.
• Identify and name the branching group.
• Name the structure by first identifying the substitute and its position in the chain, then naming the longest chain.
For example,

3. Cyclo alkanes

Cycloalkanes are named simply by identifying the number of carbons in the ring and prefixing the alkane name with cyclo.

## Preparation of Alkanes

Alkenes can be prepared by a number of methods.

### Synthesis of Alkanes

1. Catalytic hydrogenation of alkenes and alkynes

An unsaturated hydrocarbon does not react with hydrogen as such but in the presence of a metal catalyst, addition of hydrogen occurs to form alkanes.

Examples

2. From haloalkanes (by reduction)

Haloalkanes are used as a versatile source to produce alkanes. It is reduced in the presence of different reducing agent resulting in the formation of alkane.

R-X $\overset{reducing\ agent}{\rightarrow}$ R-H

Different reducing agent are H2/Pt, Pd or Ni, Zn/HCl or CH3COOH, Na/ethanol, Zn-Cu couple or LiAlH4. For example,

## Properties

Physical properties

Lower alkanes are gases from C1 to C4, alkanes from C5 to C17 are liquids with petrol like odor whereas higher alkanes that is from C17 onwards exist as colorless, odorless and waxy solids. Alkanes are less dense than water.
• Boiling point - The boiling point of alkanes increases with an increase in molecular mass, that is the highest number of carbons in an alkane, higher is its boiling point.
• Melting point - Melting point depends on lattice structure. In highly branched but symmetrical alkanes the melting point is exceptionally high.
Chemical properties

Chemical properties of alkanes include substitution reaction, oxidation reactions, nitration and sulfonation etc.

1. Halogenation

The alkanes react with halogens at high temperature or in the presence of light to form haloalkanes.

R-H + X2 $\xrightarrow[h\nu ]{\Delta}$ R-X + HX

2. Nitration

The reaction of alkanes with nitric acid results in the formation of nitroalkanes. Nitration may be carried out at a high temperature of 400-500oC or it may be carried out at a low temperature.

R-H + HONO2 $\overset{450^{o}C}{\rightarrow}$ R-NO2 + H2O

3. Sulfonation

Alkanes react with fuming sulfuric acid to form alkanesulfonic acid.

R-H + HOSO3H $\rightarrow$ RSO3H + H2O

## Reactions

1. Combustion

The main type of reaction that alkanes undergo is combustion reactions. In this reaction, alkanes react with oxygen in an exothermic reaction to form carbon dioxide and water vapor. A general formula for combustion of hydrocarbons is

Examples

Incomplete combustion of methane results in the formation of carbon black.

2. Dehydrogenation

Lower alkanes on heating in presence of catalysts such as Al2O3, ZnO or V2O5, undergo dehydrogenation to produce alkenes.