Alkynes

 In this triple bond is present between carbon atoms (C  C  bond).The hybridisation is sp and the general formula involved is C_nH_2n -2.

•  The lowest member of alkynes is: simplest compound that is CH  CH (ethyne).
• The common name of it is acetylene.
• It has high Bond dissociation energy, as there is triple bond present therefore the energy is high of the order 823 KJ/mol.
• Due to this reason the acetylene is mixed with O₂ and the flame is used for welding purpose.

 Structure of ethyne

The geometry is linear structure with bond angle 180⁰ and sp hybridization.

Nomenclature: Suffix used is “yne”

          CH₃ – CH₂ -CH₂-C    CH                                              CH₃-C C-CH₃

                          Pentyne                                                           but – 2 – yne

         CH₂= CH -CH₂– C     CH

               Pent – 1 – en – 4 yne

    Isomerism: There is restricted rotation around C C bond therefore no geometrical isomerism is  seen. .

1. Chain isomerism (converting straight chain to branched).

CH₃ -CH₂-CH₂CH₂ – C CH

                       Hexyne

 CH₃– CH₂– CH(CH₃) – C CH

          3 methyl pent – 1 – yne

2. Position isomerism (change in the position of triple bond).

CH₃– CH₂-CH₂ -CH₂ -C   CH           CH₃ – CH₂-C C-CH₂– CH₃

               Hexyne                                  Hex – 3 – yne

3. Functional isomerism: It involves conversion of one triple bond to two double bonds.

CH₃ CH₂– C  CH                                CH₂ = CH- CH = CH₂

   (C₄ H₆)                                                          (C₄ H₆) same molecular formula

Ring Chain Isomerism: In this Open chain is converted to ring structure.

Preparation of Alkynes

1. Laboratory preparation –It is prepared in by action of H₂O on Calcium carbide i.e. CaC

• First we need to prepare Calcium carbide CaC

                   CaO +  3C     à     CaC₂ +  Co   

                     Lime         (273K)

The reaction involved is:

 This reaction can lead to explosion so, in order To avoid explosion we need to remove air and for  that we take oil gas.

1. By dehydrohdogenation of vicinal dihalide
2.  

By action of Zinc on Tetra halogen derivatives

We can prepare higher alkyne from acetylene: This is also called up-gradation reaction.

4. Kolbe’s electrolytic reduction

            In this electric current is passed through metal carboxylic salt as shown:

            It can be in cis form or trans form:

Reaction involved:

Dehalogenation of haloforms: In this 2molecules of chloroform reacts with silver to form alkyne.

 7. Synthesis from Carbon & Hydrogen: In this electric discharge is passed through carbon and hydrogen mixture to form alkynes.

Physical properties of alkynes

 1. Existence: Acetylene, propyne, butyne are gases and other with carbon 4 to 12 are liquids, more than 12 are solids.

C₄ – C₁₂  liquids                        

> C₁₂  solids.

• They all have no smell, except garlic like smell of acetylene.

2. Solubility: They are weekly polar in nature and not soluble in H₂O but soluble in organic solvents.
3. Melting point and boiling point: Because of triple bond they have higher melting point and boiling point.

• With increase in number of carbon atoms the boiling point increases.
• With increase in branching the boiling point decreases.

Chemical properties of alkynes

1. Acidic Character – This is shown by replacement of one of the hydrogen atom by metal atom.
2.  

Alkynes are acidic because of sp hybridisation of Carbon atom. The “s” character in case of alkynes is 50%. Due to more “s” character, electrons in sp hybrid orbitals are held more tightly by nucleus and are quite electronegative. As a result, the H – C bond get more displaced towards carbon atom and proton is easily released. Due to this, the bond easily breaks and the Hydrogen is easily released.

            Application: By this we can upgrade number of carbon atoms in alkynes.

2. Addition reaction: The basic general reaction involved in addition is similar to that of alkenes as shown.
3.  

 Addition of halogens

• Addition of halogen acids (hydro halogenations Reaction ) : The order of reactivity of this reaction is:

                                       HF  <  HCl <  HBr  < HI

The reaction involved is:

 Anti-markoniov’s rule is followed in case of addition of HBr:

• Addition of H₂O ( hydration Reaction)

Addition of HCN

Addition of H₂

Please note:

When we use lindlar catalyst that is Pd/BaSO₄ we get Cis form and whenever we use catalyst that is Na in liquid NH₃, then it is called birch reduction and we get trans form as shown below :

2. Oxidation of alkynes

The oxidation is carried out with:

• Cold and dilute alkaline potassium permanganate
• Hot and concentrated potassium permanganate
• With ozone

 With Hot and Concentrated KMnO₄

Please note that if we carryout oxidation of butyne with acidic potassium permanganate under low and high temperature we get different products as shown below:

With Ozone

Polymerisation

 Cyclic Polymerisation