to ethene and ethane. Why is it so ? Has it something to do with their structures and the hybridisation ? You have read that hydrogen atoms in ethyne are attached to the sp hybridised carbon atoms whereas they are attached to sp hybridised carbon atoms in ethene and sp hybridised carbons in ethane.
Due to the maximum percentage of s character ( %), the sp hybridised orbitals of carbon atoms in ethyne molecules have highest electronegativity; hence, these attract the shared electron pair of the C-H bond of ethyne to a greater extent than that of the sp hybridised orbitals of carbon in ethene 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 Monosodium ethynide ( . ) HC ≡ C – Na + Na → Na + C – Na + ≡ C – Na + + / H Disodium ethynide ( . ) CH – C ≡ C – H + Na + NH – ↓ CH – C ≡ C – Na + + NH Sodium propynide ( . ) These reactions are not shown by alkenes and alkanes, hence used for distinction between alkynes, alkenes and alkanes.
What about the above reactions with but- -yne and but- -yne ? Alkanes, alkenes and alkynes follow the following trend in their acidic behaviour : i) CH ≡ CH > H C – CH > CH –CH ii) HC ≡ CH > CH –C ≡ CH >> CH –C ≡ C–CH B . Addition reactions : Alkynes contain a triple bond, so they add up, two molecules of dihydrogen, halogen, hydrogen halides etc. Formation of the