Chemistry For HSC Part 2 - Chapter No.7 - Hydrocarbons (Questions And Answers)

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Chapter No.7
Chemistry Of Hydrocarbons
Hydrocarbons (Questions And Answers)

By Sir Asif Izhar (Anees Hussain)

Hydrocarbons

HYDROCARBONS:

"Organic compounds containing carbon and hydrogen only are called "Hydrocarbons".
On the basis of structure and bonding, hydrocarbons are divided into two main classes:

1. Aliphatic Hydrocarbon
2. Aromatic Hydrocarbon

1. ALIPHATIC HYDROCARBONS OR OPEN CHAIN AND CLOSED CHAIN HYDROCARBONS:

• These were so named because of the fact that first compounds of this class to be studies were fatty acid (Greek : aliphos = fat).
• It is a fundamental class of organic compounds and deals with alkanes, alkene and their cyclic analogs.
• These compounds under go chiefly addition and free radical substitutions.
• In aliphatic hydrocarbons, the carbon atoms are attached to another carbon to form chains.
• These chains may be either open or closed (rings) chains.

A. Open Chain Hydrocarbons:
The open chains hydrocarbons, on the basis of saturation of valence are classified as

1. saturated and
2. unsaturated hydrocarbons.

(i) Saturated Hydrocarbons:

• The compounds in which all the valencies of carbon atoms are fully satisfied by single bonds, are called saturated hydrocarbons.
• These are called alkane for an open chain and cyclo alkane for a close chain.
• The examples of saturated hydrocarbons are alkanes.
• (i) Methane CH₄
• (ii) Ethane C₂H₆
• These are chemically stable and non polar molecules.
• These compounds do not decolorize KMnO₄ and Br₂.
• The physical state, B.P, M.P, density etc increase by increasing the carbon chain.

(ii) Unsaturated Hydrocarbon:

• The compounds in which all the valencies of carbon atoms are not fully utilized, are called unsaturated hydrocarbons.
• In these hydrocarbons one or more double or triple bond is present in the carbon chain.
• Alkenes and alkynes are the examples of unsaturated open chain hydrocarbons.
• The examples of unsaturated hydrocarbons are:
  
  
  
• These are chemically reactive and polar molecules.
• Their important reactions are addition reaction, oxidation reaction etc.
• These compounds have tendency to decolourize KMnO₄ and Br₂.

B. Cyclic Hydrocarbon OR Closed Chain Hydrocarbons:
The hydrocarbons which contain closed chain of carbon atoms are called cyclic hydrocarbons.
In these compound the terminal of the chain join together to give a cycle or ring.
The examples are cyclo alkanes and cyclo alkenes.

2. AROMATIC HYDROCARBONS:

• Aromatic hydrocarbons are special class of  cyclic compounds containing Benzene ring.
• Benzene ring is a six-member ring of carbon atoms with alternative double bonds.
• Other aromatic hydrocarbons are mostly related to benzene and its derivatives (homologs) and are called as "Arenes".
• These compounds are further divided into homo-nuclear and hetero-nuclear aromatic compounds.

Alkanes. Alkenes, Alkynes and Cyclic-Hydrocarbons

Continuation of Physical Properties of methane:
5. Liquefies at 161.4 °C and solidifies at -184 °C.
6. It is more soluble in organic solvents like acetone, ether and alcohol.

4. It solidifies at -172 °C.

Continuation of Physical Properties of ethene:
(v) Liquid ethene boils at - 150 °C and solid hydrocarbon melts at -169 °C.

(v) Liquid acetylene boils at -82 °C.
(vi) It is more soluble in alcohol and very soluble in acetone.