AIEEE Chemistry UNIT 21 HYDROCARBONS

Classification, isomerism, IUPAC nomenclature, general methods of preparation, properties and reactions:
Alkanes - Conformations: Sawhorse and Newman projections (of ethane); Mechanism of halogenation of alkanes.
Alkenes - Geometrical isomerism; Mechanism of electrophilic addition: addition of hydrogen, halogens, water, hydrogen halides (Markownikoff’s and peroxide effect); Ozonolysis, oxidation, and polymerization.
Alkynes - Acidic character; Addition of hydrogen, halogens, water and hydrogen halides; Polymerization.
Aromatic hydrocarbons - Nomenclature, benzene - structure and aromaticity; Mechanism of electrophilic substitution: halogenation, nitration, Friedel – Craft’s alkylation and acylation, directive influence of functional group in mono-substituted benzene.



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.

Nomenclature of alkanes

Examples:

2,2-Dimethylpropane
2-Methylpentane
3-Methylhexane
3-ehtyl-2-methylhexane
4-ehtyl-2,4-dimethylhexane
4-(1-methyl ethyl) heptane or 4-Isopropylheptane

Straight chain alkanes or normal alkanes (n-alkanes): All the carbon atoms are attached by covalent bonds in a continuous chain.

Branched alkanes:

Iso-alkanes: In these, one carbon chain is attached to the second carbon atom of the long chain (parent chain).

Neo-alkanes: In these two single carbon branches are attached to the second carbon atom of the long chain.

Preparation of alkanes

General methods

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. From alkyl halides

a) Wurtz reaction (specially in syllabus)
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. The equation of the reaction will make the statement more clear.

RX + 2Na + XR ---> R-R + 2NaX catalyst sodium in dry ether

In the reaction different alkyl halides can also be used in stead of a single halide. If two different halides are taken with the aim of preparing an alkane with odd number of carbon atoms, a mixture of products is obtained in stead of a single alkane. This is because in this case three reactions takes place and three different products are obtained.

b) Reduction of alkyl halides

Reducing agents can be used to add hydrogen to the halide and remove the halogen atom.
i) Zinc + HCl is one reducing agent.
ii) Catalytic hydrogenation using Pd or Pt as catalyst
iii) Hydrogen iodide (halogen acid) in the presence of red phosphorous also acts as reducing agent. In this reaction phosphorous combines with iodine to form phosphorous triiodide.
iv) zinc copper couple and alcohol


c) By the use of Grignard reagent

Alkyl halides react with magnesium metal in diethyl ether to form alkyl magnesium halides which are called as Grignard reagents. (This reaction will come in alkyl halides chapter also)

Grignard reagetns are highly reactive and are easily decomposed by water or alcohol to form alkanes

RMgX + HOH (H2O) ---> RH + Mg(OH)X


3. From carboxylic acids and their salts
a) Decarboxylation reaction
b) Kolbe's reaction
c) Reduction of carboxylic acid


a) 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.

RCOONa + NaOH -->RH + Na2CO3

In this reaction a CO2 group is removed from carboxylic acid and therefore the reaction is called decarboxylation.


b) Kolbe's reaction

When an acqueous solution of sodium or potassium salt of carboxylic acid is eletrolysed alkane is evolved at the anode.

Kolbe's reaction can also be used like wurtz reaction for preparing alkanes with even number of carbon atoms.

c) Reduction of carboxylic acid
Carboxylic acids are reduced to alkanes by hydroiodic acid (HI). In this reaction COOH group in the carboxylic acid is reduced to CH3 group.

The methods in this section can be summarised as

R-COONa ---> RH
R-COONa ---> R-R
R-COOH---> R-CH3


Industrial method: Petroleum provides the natural source of alkanes.

Physical properties of alkanes

1. State
2. Boiling point
3. Melting point
4. Solubility
5. Density

Chemical properties or reactions

1. Substitution reactions of alkanes
2. Oxidation
3. Action of steam
4. Isomerisation
5. Aromatization
6. Thermal decomposition or fragmentation