Thursday, January 17, 2008

Ch.21 Alkanes - Core Points for Revision


Preparation, properties and reactions of alkanes:

Homologous series,
Preparation of alkanes by Wurtz reaction
Preparation of alkanes decarboxylation reactions.
physical properties of alkanes (melting points, boiling points and density); Combustion and halogenation of alkanes;

Alkanes: Introduction

Alkanes are saturated hydrocarbons containing only carbon-carbon single bonds in their molecules.

Thye are also called paraffins (meaning little affinity or reactivity, we will see later why it is so).

Alkanes are divided into 1. Open chain or acyclic Alkanes and 2. CycloAlkanes or cyclic alkanes.

The general formula of alkanes is CnH2n+2

Preparation of alkanes

1. From unsaturated hydrocarbons (alkenes and alkynes)
2. From alkyl halides
3. From carboxylic acids and their salts

1. From unsaturated hydrocarbons (alkenes and alkynes)
By catalytic hydrogenation alkenes and alkynes are converted into alkanes (Note that this point will come in alkenes and alkynes chapter as reactions of them).
Ni, Pt or Pd in the form of fine powder are used as catalysts. A temperature of 523-573 K needs to be employed.

Methane cannot be prepared by this method because alkenes or alkynes will have two carbons at their lowest level.

2. Wurtz reaction (From alkyl halides)
When an alkyl halide (usually bromide or iodide) is treated with sodium in dry ether, a symmetrical alkane containing both twice the number of carbon atoms of alkyl halide is obtained.

3. Decarboxylation reaction
When sodium salt of a monocarboxylic acid is heated with soda lime (amixture of NaOH and Cao in the ratio of 3:1) at about 630 K, alkane is formed.

Physical properties of alkanes

1. State: CH4 to C4H10 are gases, C5H12 to C17H36 are liquids and higher ones are solids
2. Boiling point: Boiling point increases with molecular mass. Branched isomers have a lower boiling point than normal alkanes.
3. Melting point
4. Solubility: Being nonpolar, these are insoluble in water.
5. Density: Liquid alkanes lighter than water

Large quantity of heat generated in the combustion of alkanes

Halogenation of alkanes

This involves substitution o fhydrogen atom by halogen atom. The order of reactivity is F2>Cl2>Br2(>I2). The mechanism of chlorination and bromination involves free radicals.

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