Alkynes and Alcohols chemistry Assignment Help




Alkynes include the class of organic compounds when at least one or more carbon-carbon triple bond is present. In this case, “yne” is added to the name of the parent chain.

Another class of compounds called “alcohols” is formed when one or more hydrogen atoms are replaced by the hydroxyl group (OH -), we get an alcohol. While naming, “ol” is added to the chain name.

Introduction to preparation of alkynes

Alkynes ( Acetylene  or  Ethyne )

Unsaturated hydrocarbons containing a carbon - carbon triple bond ( - Cimg C - ) are called alkynes . A triple bond consists of one sigma       () and two pi ( img ) bonds . Carbon at triple bond shows 'sp' hybridization in these compounds . The  - C img C -  bond length in acetylene is 1.20 A which is less when compared to the C = C bond length in alkenes ( 1.34 A ) and C - C distance in alkanes ( 1.54 A ) . The                - Cimg C - bond energy ( 827.64 kJ mol-1 ) is more when compared with the bond energy of C = C and C-C bond energy ( img 10.7 kJ mol-1) ,   ( 341.1 kJ mol-1) respectively . The C - H bond length in acetylene ( 1.06 A ) is less than C - H bond length in ethylene ( 1.09 A ) because of the sp hybridisation . Greater 's' character ( 50 % ) is associates with the sp hybrid orbital hence acetylenic hydrogens are acidic in nature . The general formula of alkynes is CnH2n-n .

Methods of Preparation of Alkynes

Industrial method : It is prepared by the hydrolysis of calcium carbide .

                                  CaC2    +   2 H2O    `->`   H - C img C - H  +  Ca(OH)2

Laboratory methods of preparation

1. Dehydrohalogenation : Acetylene is obtained when 1 , 2 or 1 , 1- dihaloethane is treated with alcoholic KOH .

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2. Dehalogenation of  1 , 1 , 2 , 2 - tetrahaloethane :  1 , 1 , 2 , 2 - tetrabromoethane on heating with Zn dust gives acetylene .

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3. From haloform : Acetylene is obtained by heating iodoform , CHI3 with silver powder .

          CHI3    +   6 Ag    +    I3HC    img H - C img C - H  +  6 AgI

4. Kolbe's electrolysis :  Acetylene is prepared by the electrolysis of a concentrated aqueous solution of potassium maleate or fumarate .

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Uses of Alkyne

  • Acetylene is used in the form of oxy-acetylene flame in welding and cutting tools .
  • Used in th epreapretion 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 westron ( acetylene tetrachloride ) westrol ( trichloroethylene ) .
  • Used in Hawker's lamp and in light houses for illumination purposes .

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