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316 CHEMISTRY CaO + 3C CaC 2 + CO (9.56) the sp hybridised carbon2atoms whereas Calcium they are attached to sp hybridised carbon atoms in ethene and sp hybridised carbons carbide CaC + 2H O Ca(OH) + C H in ethane. Due to the maximum percentage of 2 2 2 2 2 (9.57) s character (50%), the sp hybridised orbitals of carbon atoms in ethyne molecules have 2. From vicinal dihalides :Vicinal dihalides on treatment with alcoholic potassium highest electronegativity; hence, these attract hydroxide undergo dehydrohalogenation. the shared electron pair of the C-H bond of ethyne to a greater extent than that of the One molecule of hydrogen halide is sp hybridised orbitals of carbon in ethene eliminated to form alkenyl halide which 3 on treatment with sodamide gives alkyne. and the sp hybridised orbital of carbon in ethane. Thus in ethyne, hydrogen atoms can be liberated as protons more easily as compared to ethene and ethane. Hence, hydrogen atoms of ethyne attached to triply bonded carbon atom are acidic in nature. You may note that the hydrogen atoms attached to the triply bonded carbons are acidic but not all the hydrogen atoms of alkynes. HC ≡ CH + Na HC ≡ C Na + H 9.4.4 Properties 2 Monosodium Physical properties ethynide Physical properties of alkynes follow the same (9.59) trend of alkenes and alkanes. First three – + – – + members are gases, the next eight are liquids HC ≡ C Na + Na Na C ≡ C Na + H 2 and the higher ones are solids. All alkynes Disodium ethynide are colourless. Ethyene has characteristic odour. Other members are odourless. Alkynes (9.60) CH – C ≡ C – H a NH – are weakly polar in nature. They are lighter 3 2 than water and immiscible with water but – + soluble in organic solvents like ethers, carbon CH 3– C ≡ C Na + NH 3 tetrachloride and benzene. Their melting Sodium propynide (9.61) point, boiling point and density increase with These reactions are not shown by alkenes increase in molar mass. and alkanes, hence used for distinction Chemical properties between alkynes, alkenes and alkanes. What Alkynes show acidic nature, addition reactions about the above reactions with but-1-yne and and polymerisation reactions as follows : but-2-yne ? Alkanes, alkenes and alkynes A. Acidic character of alkyne: Sodium follow the following trend in their acidic metal and sodamide (NaNH ) are 2trong behaviour : bases. They react with ethyne to form sodium i) CH ≡ CH > H C –CH > CH –CH acetylide with the liberation of dihydrogen 2 2 3 3 gas. These reactions have not been observed ii) HC ≡ CH > CH –C≡ CH >> CH –C≡C–CH in case of ethene and ethane thus indicating 3 3 3 that ethyne is acidic in nature in comparison B. Addition reactions: Alkynes contain a to ethene and ethane. Why is it so ? Has triple bond, so they add up, two molecules of it something to do with their structures dihydrogen, halogen, hydrogen halides etc. and the hybridisation ? You have read that Formation of the addition product takes place hydrogen atoms in ethyne are attached to according to the following steps. Reprint 2026-27