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Give brief note on acidic nature of alkyne. |
Answer» Solution : s-orbit is more negative than p-orbit, so electronegativity increases as s-orbit increases. So electronegativity order : `SP C gt sp^(2) C gt sp^(3) C` Due to this reason, sp carbon attract mximum bonded electron towards itself. And thus, hydrogen attached directly to the sp carbon has more acidity than hydrogen of ALKANE and alkene. `underset("Acidic H " larr "Neutral H")(-=C-H gt =C-H gt- C-H)` "H attached to the carbon having triple bond of alkyne is more acidic and rest hydrogens are not acidic in nature". Only H attached to triple bond of `HC -= CH, CH_(3)C-=CH, CH_(3)CH_(2)C-=CH` is acidic in nature. So in `R-C-=C-H` only terminal H is of acidic natured. And H of R is not acidic in nature. Also, `R-C-=C-R` no H is of acidic nature. Chemical reaction of acidic hydrogen or reaction which shows acidic nature of H attached to Carbon having triple bond : Both terminal H of acetelene is weak acidic in nature. ETHYNE on reaction o with strong base of sodium metal at high temperature and sodamid `(NaNH_(2))` it gives ethynide (acetelide) product. `underset("ETHYN")(H-C-=C-H)+Na overset(475 K)rarr underset("etheynide")underset("Monosodium")(H-C-=C^(-)Na^(+))+ (1)/(2)H_(2)`(Eq. (i)) `H-C-=C^(+)Na+Na overset(475 K)rarr underset("Disodium ethynide")(Na^(+)C^(-)-=C^(-)Na^(+))+(1)/(2)H_(2)`(Eq. (ii)) `H-C-=C-H+underset("Sodamide")(NaNH_(2)) overset(NH_(3))rarr H-C-=CNa + (1)/(2)H_(2)`(Eq. (iii)) `H-C-=C-H+2NaNH_(2)overset(NH_(3))rarr ""(+)NaC^(-) -=C^(-)Na^(+)+H_(2)` (Eq. (iv))  Acidic order of hydrogen attached with carbon : (i) `underset(~10^(-25))(HC-=CH) gt underset(~10^(-85))(H_(2)C=CH_(2)) lt underset(~10^(-40))(H_(3)C-CH_(3))` (ii)`HC-=CH gt CH_(3)C-=CH gt CH gt gt CH_(3)C-=C CH_(3)` There is no acidic hydrogen present in `R-C-=CR, CH_(3)C-=C-CH_(3)`, so they do not show any reaction with Na or `NaNH_(2)` |
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