The correct decreasing order of acidic character of hydrogen
in ethane, ethene and ethyne is :
(ay ethane > ethene > ethyne
(b) ethyne > ethene > ethane
(c) ethyne > ethane > ethene
(d) ethene > ethane > ethyne
Answers
option b is correct but this acidic character is for carbon as carbon in ethyne is sp hybridized with increase of s character acidity increases so for sp^2 in ethene and sp^3 in methane
Option (b)
In general the more s-character the closer the electron is to the nucleus. (an electron is negatively charged a nucleus positively hence the closer the electron to the nucleus the more stable it is)
e.g. take typical carbon hybridization. sp3 (alkane), sp2 (alkene) and sp (alkyne).
If we would compare 3 simple carbon molecules with different hybridization.
sp3: Ethane,
sp2: Ethene
sp: Acetylene (Ethyn)
in acetylene the sp hybridization partially polarizes the C-H bond this makes the hydrogen more available for donation (alternatively if the hydrogen is donated the resulting anion is more stabilized)
Therefore acetylene will be more acidic in comparison with ethene and an ethane.
As you can see the acetylene has a pKa of ca 25 compared to alkane with a pKa of ca 50 (please remember that pKa are logarithmic so this is a huge difference !)
You still need a very strong base to deprotonate an acetylene.
It also explains the difference between propionic acid sp3, acrylic acid (propenoic acid) sp2 and propiolic acid (propynoic acid) sp: An effect comparible with electronegativity.
Similar explanation holds for basicity.
e.g. the nitrogen in compounds like piperidine sp3, pyridine sp2, and acetonitrile sp, Due to the sp hybridization the lone pair in acetonitrile is closer to the nucleus, hence acetonitrile is a very weak base.