1) Write notes on preparation of alkanes, alkenes , alkynes..

2) Write notes on chemical properties of alkanes, alkenes , alkynes..

3)Write notes confirmation of alkanes, alkenes , alkynes..

4) Differentiate between alkanes and alkenes..

5) Differentiate between alyphatic and aromatic compunds..

6) Write notes on hybridisation in acetylene..

7) Differentiate between benzenoid and non-benzenoid....

Preparation of alkanes:

• From unsaturated hydrocarbon. Example:

CH₂=CH₂ +H₂ → (in presence of Pt or Pd, 523-573 K) CH₃-CH₃

• From alkyl halides by using Grignard’s reagent, by Wurtz reaction and by reduction of alkyl halides.

Grignard’s reagent: Allkyl halides are reacted with Magnesium in presence of dry ethoxyethane to form alkylmagnesium halides. This then reacts readily with water to give alkane.

R-X + Mg → (in dry ether) R-Mg-X

    (Grignard’s reagent)

R-Mg-X + H₂O → R-H + Mg(OH)X

• Wurtz reaction: An alkyl halide is treated with metallic sodium in presence of dry diethyl ether and an alkane with double the number of carbon atoms in the alkyl halide is formed.

R-X + 2Na + X-R → (dry ether) R-R + 2NaX

• Reduction of alkyl halides: Reduction of alkyl halides (preferably bromides or iodides) in presence of nascent hydrogen yields alkanes. Nascent hydrogen is prepared by:

Zn + 2HCl→(in presence of alcohol) ZnCl₂ + 2[H] (nascent hydrogen)

R-X + 2[H] → R-H + HX

• From carboxylic acid by decarboxylation:

Carboxylic acid is heated with a mixture of sodium hydroxide and CaO in 3:1 ratio (called Soda Lime) at 630K, CO₂ is lost and an alkane with one less carbon atom than the acid is formed.

R-COOH + NaOH→ R-COONa +H₂O

R-COONa + NaOH→ (CaO, 630 K) R-H + Na₂CO₃

Preparation of alkenes:

• From alkyl halides or haloalkanes: Dehydrohalogention of alkyl halides take place when they are heated in presence of a strong base like sodium ethoxide or alcoholic solution of potassium hydroxide.

•  From 1,2- dihaloalkanes: alkenes can be prepared by heating a suitable 1,2-dihaloalkane in presence of zinc dust in methanol or ethanol.

CH₂-Br + Zn→ (CH₃OH, ∆) CH₂=CH₂ +Zn Br₂

• Alkenes can also be prepared from monohydric alcohol: When monohydric alcohols containing a beta hydrogen is heated with conc. Sulphuric acid eliminate a water molecule to form alkenes.

Preparation of alkynes:

• By the action of water on calcium carbide:

CaC₂ + 2H₂O → HC≡CH + Ca(OH)₂

• By dehydrohalogenation of dihaloalkanes:

• By dehalogenation of tetrahalides:

• By dehalogenation of haloform by heating:

CHCl₃ + 6Ag + Cl₃CH →HC≡CH + 6AgCl

• Synthesis from carbon and hydrogen: Berthelot Synthesis:

2C + H₂→( electric arc, 3270K) HC≡CH                  

2.    Chemical properties of alkane

• They combust in oxygen to form carbon dioxide, water and heat.
• They undergo substitution reactions with halogens in the presents of UV light.

        Chemical properties of Alkene

• Alkenes readily undergo halogenation—the addition of halogens. 
• Addition reaction between an alkene and water to form an alcohol. This reaction is called as hydration

        Chemical properties of Alkyne 

• Ethyne burn in air with a luminous, smoky flame, (forming carbon dioxide and water).
• Ethyne is oxidised by a dilute aqueous solution of potassium permanganate to form oxalic acid.
• Due to unsaturated nature of ethyne addition reactions can occur across the triple bond.
• Polymerisation of Alkynes due to Triple Bond

3.     Conformation of Alkane

Ethane: This conformation of ethane is the staggered form. The various hydrogen atoms on the two carbons form a dihedral angle of 60º

Conformation fo Alkene

Shows the designations cis and trans, Steroisomers

4. Differentiate between alkanes and alkenes

We can use Bromine water or acidified permanganate solution

Alkane: No reaction with acidified permanganate.

Alkene Will form diol and turn color from purple to colourless.

In Bromine water + Sunlight : Alkanes will undergo slow substitution reaction. Where as Alkene undergo addition reaction, it will turn blue litmus to red.

5. Differentiate between alyphatic and aromatic compound 

Aromatic hydrocarbons contain benzene rings.Aliphatic hydrocarbons do not. They can even have double and triple bonds, as long as they are not cyclic and conjugated as in benzene.

Standard process such as Freidel-Crafts type test  of presence of testing of benzene will tell us what is aromatic or what is aliphatic hydrocarbons.

6. Acetylene contains two elements, carbon and hydrogen, and the molecular formula of acetylene is C₂H₂.  Like methane, ethane, and ethylene, acetylene is a covalent compound.

The two carbon-hydrogen bonds in the acetylene molecule are identical.  According to VSEPR theory, the geometry at each carbon atom in the acetylene molecule is linear

The coordination number of each carbon atom in acetylene is 2 giving us idea that acetylene molecule could not be sp3  or sp2 hybridized.

It shows sp-hybrized C-atom

7. Benzenoids are aromatic compounds containing benzene ring. Example:

Non-benzenoids are compounds which are aromatic but don’t not contain benzene ring. Example: