Classwork Series and Exercises {Chemistry- SS3}: The Alkynes

ALKYNES

An alkyne is a molecule made up entirely of carbon and hydrogen where on or more carbon atoms are connected by triple bonds. The general formula for an alkyne is C_nH_2n-2 where n is the number of carbon atoms in the molecule. Alkynes are named by adding the the -yne suffix to the prefix associated with the number of carbon atoms present in the molecule.

The first three are:

C₂H₂ – Ethyne

C₃H₄ – Propyne

C₄H₆ – Butyne

Laboratory Preparation of Ethyne (Acetylene)

Acetylene is prepared in laboratory by the action of water on calcium carbide.

                        CaC_2(s) + 2H₂O_(l) → Ca(OH)_2(aq) + C₂H_2(g)

 The air of the flask in which acetylene is to be prepared is displaced with oil gas as acetylene forms an explosive mixture with air. Acetylene evolved is collected over water. The acetylene so prepared is contaminated with small amounts of impurities such as PH₃, H₂S, AsH₃ and NH₃ which are removed by passing the mixture through CuSO₄ solution before its collection over water.

Physical Properties of Alkynes

(i)     Lower members (C₂ to C₄) are gases; middle one (C₅ to C₁₂) are liquids; higher are solids.

(ii)    The boiling and melting point of alkynes show a regular increase with increase in molecular weight; however less volatile than alkene.

(iii)    All are colourless & pure ethyne is a sweet smelling gas, however C₂H₂ has garlic odour due to the impurities of PH₃, H₂S etc.

(iv)   Soluble in organic solvents like acetone, alcohol & sparingly soluble in water.

(v)     It is slightly lighter than air

 Chemical Properties of Ethyne

   Uses of Alkyne

• Acetylene is used in the form of oxy-acetylene flame in welding and cutting tools .
• Used in the preparation of vinyl plastics .
• Used in the preparation of several carbon compounds like acetaldehyde , acetic acid , ethyl alcohol , benzene  etc .
• Used in the preparation of solvents acetylene tetrachloride or trichloroethylene.
• Used in Hawker’s lamp and in light houses for illumination purposes.

EXERCISES

Lets see how much you’ve learnt, attach the following answers to the comment below

1. When chlorine is mixed with ethane at room temperature, the product is (a) 1,2- dichloroethane (b) 1,2- dichloroethene (c) 1,1- dichloroethane (d) 1,1- dichloroethene
2. The functional group contained in the compound CH₂=CH₂ is a(n) (a) ester (b) ketone (c) alkene (d) alcohol
3. The molecular formula of __________ fit the general formula (C_nH_2n-2). (a) alkynes (b) alcohol (c) alkenes (d) alkanes
4. For which of the following is cis and trans isomerism possible? (a) 1,2-dibromoethyne (b) 1,1-dibromoethane (c) 1,1-dibromoethene (d) 1,2-dibromoethene
5. The product of the reaction (CH₃–CH=CH–CH₂–CH₂–CH₃ + Br₂) is (a) CH₃–CHBr–CHBr–CH₂–CH₂–CH₃ (b) CH₃–CHBr₂–CHBr₂–CH₂–CH₂–CH₃ (c) CH₂=CHBr–CHBr–CH₂–CH₂–CH₃ (d) CH₃–CHBr =CHBr–CH₂–CH₂–CH₃ 

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