1. Preparation of alkynes from alkenes :
This process involves two steps:
(i) Halogenation of alkenes to form vicinal dihalides
(ii) Dehalogenation of vicinal dihalides to form alkynes.
\({ CH }_{ 2 }={ CH }_{ 2 }+{ Br }_{ 2 }\rightarrow \overset { { CH }_{ 2 } }{ \underset { Br }{ | } } \overset { -{ CH }_{ 2 } }{ \underset { Br }{ | } } \overset { Alcoholic\quad KOH }{ \longrightarrow } \overset { CH={ CH }_{ 2 } }{ \underset { Br }{ | } } \overset { Na{ NH }_{ 2 } }{ \underset { \underset { -{ NH }_{ 3 } }{ -NaBr } }{ \longrightarrow } } CH\equiv CH\)
                               1,2-Dibromoethane              Bromoethane             Acetylene
2. Preparation of alkene from gem dihalides:
A compound containing two halogen atoms on the same carbon atom is called gem dihalide (Latin word 'Gemini' means twins). On heating with alcoholic KOH, gem dihalides give alkynes.

3. Preparation of alkenes from electrolysis of salts of Unsaturated Dicarboxylic Acids. (Kolbe's Electrolytic Method)
Electrolysis of sodium or potassium salt of maleic or fumaric acid yields alkynes.
\(\overset { CHCOOK }{ \underset { CHCOOK }{ | } } \overset { Eletrolysis }{ \longrightarrow } \overset { { CHCOO }^{ - } }{ \underset { { CHCOO }^{ - } }{ || } } +{ 2K }^{ + }\)
Potassium maleate
\(\overset { { CHCOO }^{ - } }{ \underset { CH{ COO }^{ - } }{ || } } \longrightarrow \overset { CH }{ \underset { CH }{ ||| } } +2{ CO }_{ 2 }+{ 2e }^{ - }\)
                    Acetylene
4. Industrial preparation of ethyne:
Ethyne can be manufactured in large scale by action of calcium carbide with water.

(i) To begin draw the carbon backbone of the straight chain isomer
C-C-C-C-C
(ii) To determine the carbon backbone structure of the other isomers, arrange the carbon atoms in the other way:
(a) \(C-\underset { \overset { | }{ C } }{ C } -C-C\) 
(b) \(C-\overset { \underset { | }{ C } }{ \underset { \overset { | }{ C } }{ C } } -C\)
(iii) Fill in all the hydrogen atoms so that each carbon forms four bonds
(a) CH_3-CH₂-CH₂-CH₂-CH₂
(b) \({ CH }_{ 3 }-\underset { \overset { | }{ { CH }_{ 3 } } }{ C } H-{ CH }_{ 2 }-{ CH }_{ 3 }\) 
(c) \(CH_{ 3 }-\overset { \underset { | }{ { CH }_{ 3 } } }{ \underset { \overset { | }{ { CH }_{ 3 } } }{ C } } -{ CH }_{ 3 }\)

Huckel proposed that aromaticity is a function of electronic structure. A compound may be aromatic, if it obeys the following rules
(i) The molecule must be co-planar
(ii) Complete delocalization of \(\pi\) electron in the ring
(iii) Presence of (4n+2) \(\pi\) electrons in the ring where n is an integer (n=0,1,2 .... ) is known as Huckle's rule.

1.	Cyclo penta diene	(i) It has a planar Structure (ii) It has four \(\pi\) electron but the elecron are not delocalised and hence it is not an aromatic compound.
2.	Cyclooctatetraene	Molecule is non-planar and hence it is not an aromatic compound.
3.	Cyclopropenylcation	(i) Cyclopropenyl cation has planar structure. (ii) It has 2 delocalised \(\pi\) electron. (iii) 4n + 2 = 2 4n = 0 n = 0 (an interger) and hence it is aromatic compount.

Polymerisatlon:
A polymer is a large molecule formed by the combination of larger number of small molecules. The process in known as polymerisation. Alkenes undergo polyrnerisation at high temperature and pressure, in the presence of a catalyst.

The exothermic nature of alkane combustion reaction explains the extensive use of alkanes as fuels. Methane present in natural gas is used in home heating. Mixture of propane and butane are known, as LPG gas which is used for domestic cooking purpose. GASOLINE is a complex mixture of many hydrocarbons used as a fuel for internal-combustion engines.
Carbon black is used in the manufacture of ink, printer ink and black pigments. .It is also used as fillers.

The purple KMnO₄ becomes colourless producing acetone.
 

Benzene undergoes nitration reaction becasuse it is an elecron - rich system due to delocalised \(\pi\) electron. So it is easily attacked by electrophiles and gives substituted products.

Mechanism:
Step - I : Formafion of the electrophile:

Step - II : Formation of carbocation intermediate:
The electrophile attacks the aromatic ring to form a carbocation intermediate which is stabilised by resonance.

Step - III: Formation of nitro benzene:
The electrophile attacks the artomatic ring to fo

CH₃CH₂CH₂COONa + NaOH \(\overset { CaO }{ \underset { \Delta }{ \rightarrow } } \)CH₃CH₂ - CH₃+Na₂CO₃
Sodium salt of butanaic acid           Propane
Heating sodium salt of butanoic acid (Sodium butanoate) with soda lime gives propane.

(i)\({ CH }_{ 3 }-\overset { \overset { { CH }_{ 3 } }{ | } }{ { CH }-\underset { \overset { | }{ { CH }_{ 3 } } }{ CH } - } CH_{ 2 }-{ CH }_{ 2 }-\overset { \overset { { CH }_{ 2 }-\overset { \underset { | }{ { CH }_{ 3 } } }{ CH } -{ CH }_{ 3 } }{ | } }{ CH- } { CH }_{ 2 }-{ CH }_{ 2 }-{ CH }_{ 2 }-{ CH }_{ 3 }\)
(ii) 

(iii) \({ CH }_{ 3 }-\underset { \underset { { CH }_{ 3 } }{ | } }{ CH } -{ CH }_{ 2 }-{ CH }_{ 2 }-\underset { \overset { | }{ \overset { CH-CH_{ 3 } }{ \overset { | }{ { H }_{ 3 }C-CH-{ CH }_{ 3 } } } } }{ CH } -{ CH }_{ 2 }-{ CH }_{ 2 }-{ CH }_{ 2 }-{ CH }_{ 3 }\)

Markovnikoff's rule: "When an unsymmetrical alkene reacts with hydrogen halide, the hydrogen adds to the carbon that has more number of hydrogen and halogen add to the carbon having fewer hydrogen".
Eg: Addition HBr to unsymmetrical alkene: In the addition of hydrogen halide to an unsymmetrical alkene, two products are obtained.

Conformations of n-Butane: n-Butane may be considered as a derivative of ethane, as one hydrogen on each carbon is replaced by a methyl group
Eclipsed conformation: In this conformation, the distance between the two methyl group is minimum. So there is maximum repulsion between them and it is the least stable conformer.
Anti or staggered form: In this conformation, the distance between the two methyl groups is maximum and so there is minimum repulsion between them. And it is the most stable conformer.