A terminal alkyne (like HC≡CH) has its C-H bond on an sp carbon. An sp carbon has 50% s-character, so it pulls the shared electrons very close to itself. This makes it easy to remove the H as H+, so the alkyne acts as a weak acid. Memory hook: "More s-character = more acidic," so sp (alkyne) beats sp2 (alkene) beats sp3 (alkane).
Acidity of a C-H bond rises with the s-character of the carbon. sp alkynes (50% s) hold the electron pair closest, so they lose H+ most easily and are the most acidic, followed by sp2 alkenes and sp3 alkanes.
Your doubts, answered
Why does an sp carbon make the C-H bond acidic?
An sp carbon is made from 1 s-orbital and 1 p-orbital, so it has 50% s-character. The s-orbital is closer to the nucleus than the p-orbital. More s-character means the electron pair of the C-H bond sits closer to the carbon nucleus. So the carbon holds those electrons more tightly. When the H leaves as H+, the leftover negative charge stays on this carbon and is held stably. Easy to lose H+ = acidic.
What is the correct acidity order: alkane, alkene, alkyne?
The order is Alkyne > Alkene > Alkane. In numbers: HC≡CH > CH2=CH2 > CH3-CH3. This follows the s-character: sp (50%) > sp2 (33%) > sp3 (25%). More s-character = more acidic. This exact order was asked in NEET 2017.
Why is ethyne (acetylene) more acidic than ethene?
Ethyne has sp carbons (50% s-character). Ethene has sp2 carbons (33% s-character). Higher s-character in ethyne pulls the C-H bonding electrons closer to carbon, so its H is easier to remove as H+. That is why ethyne is more acidic than ethene.
Are terminal alkynes strong acids like HCl?
No. Terminal alkynes are only WEAK acids. Acetylene has a pKa of about 25. Compare: water pKa about 15.7, ethanol about 16. So alkynes are weaker acids than water and alcohols. They are 'acidic' only compared to alkenes and alkanes, not compared to real acids.
Why are only TERMINAL alkynes acidic and not internal ones?
Acidity needs an H attached directly to the sp (triple-bond) carbon. A terminal alkyne has this H (R-C≡C-H). An internal alkyne like CH3-C≡C-CH3 has no H on the triple-bond carbons, so it has no acidic hydrogen. Only the H on the sp carbon can leave as H+.
How do we prove terminal alkynes are acidic in the lab?
React the terminal alkyne with a base like sodium metal, sodium amide (NaNH2), or ammoniacal silver nitrate / ammoniacal cuprous chloride. The acidic H is removed and a metal acetylide forms. For example, with ammoniacal AgNO3, acetylene gives a white precipitate of silver acetylide. Alkenes and alkanes do NOT give this test.
⚠️ The NEET trap ✗ Choosing that ethene is more acidic than ethyne because the double bond 'looks less crowded', or picking the reverse order alkane > alkene > alkyne. ✓ Acidity follows s-character of the carbon holding the H. sp (alkyne, 50%) > sp2 (alkene, 33%) > sp3 (alkane, 25%). So HC≡CH > CH2=CH2 > CH3-CH3. 🧠 More s-character pulls electrons closer = holds negative charge better = more acidic. Alkyne always wins.
Real NEET questions
NEET 2017
Which one is the correct order of acidity?
A · CH2=CH2 > CH3-CH=CH2 > CH3-C≡CH > CH≡CH
B · CH≡CH > CH3-C≡CH > CH2=CH2 > CH3-CH3 ✓
C · CH≡CH > CH2=CH2 > CH3-C≡CH > CH3-CH3
D · CH3-CH3 > CH2=CH2 > CH3-C≡CH > CH≡CH
Solution: Acidity depends on s-character of the carbon holding the H. sp carbons (in ≡CH) have 50% s-character, sp2 (in =CH2) have 33%, sp3 (in CH3) have 25%. More s-character = more acidic. Both CH≡CH and CH3-C≡CH have terminal sp C-H, so they are the most acidic. CH≡CH is more acidic than CH3-C≡CH because the CH3 group pushes electron density in and slightly reduces acidity. Then come the sp2 alkene (CH2=CH2) and finally the sp3 alkane (CH3-CH3). Order: CH≡CH > CH3-C≡CH > CH2=CH2 > CH3-CH3. Answer B.
NEET 2025
For benzene (aryl sp2 C-H, I), phenylacetylene C6H5-C≡C-H (terminal alkynyl sp C-H, II) and a methylene sp3 C-H (III), the correct order of C-H bond dissociation... choose the order of bond dissociation energy.
A · III > II > I
B · II > III > I
C · II > I > III ✓
D · I > II > III
Solution: A C-H bond gets SHORTER and STRONGER as the s-character of the carbon increases, because higher s-character holds the bonding electrons closer and tighter. sp (in the terminal alkyne, II) has the most s-character, so it has the highest bond dissociation energy. sp2 (benzene, I) is next. sp3 (methylene, III) has the least s-character, so it is the weakest. Order: II (sp) > I (sp2) > III (sp3). Answer C. Note: strong short bond and easy loss of H+ both come from the same cause - high s-character pulling electrons to carbon.
NEET 2018
Which molecule shows the order of hybridisation sp2, sp2, sp, sp from left to right for its carbon atoms?
A · CH2=CH-CH=CH2
B · CH2=CH-C≡CH ✓
C · HC≡C-C≡CH
D · CH3-CH=CH-CH3
Solution: Read left to right in CH2=CH-C≡CH. The first two carbons are part of a double bond, so they are sp2. The last two carbons are part of a triple bond, so they are sp. This gives sp2, sp2, sp, sp - exactly the asked order. Answer B. This links to acidity: the sp carbons at the end carry the acidic terminal H, which is why this molecule is a terminal alkyne and reacts with ammoniacal AgNO3.
Solved Hydrocarbons NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
No. Acetylene has a pKa of about 25, while water has a pKa of about 15.7. A smaller pKa means a stronger acid, so water is more acidic. Terminal alkynes are acidic only compared to alkenes and alkanes.
What acidic hydrogen means in alkynes?
It means the H attached to the sp (triple-bond) carbon can leave as an H+ ion when a strong base is added. This H is called acidic because it can be removed, forming a stable acetylide ion (carbanion).
Which reagents detect a terminal alkyne?
Ammoniacal silver nitrate gives a white silver acetylide precipitate, and ammoniacal cuprous chloride gives a red copper acetylide precipitate. Sodium metal and NaNH2 also remove the acidic H. Internal alkynes, alkenes and alkanes give no such reaction.
Does more s-character always increase acidity?
Yes, for C-H bonds on the same type of carbon skeleton. More s-character holds the electron pair closer to carbon and stabilises the negative charge left after H+ leaves. So sp > sp2 > sp3 in acidity.
Why is CH≡CH slightly more acidic than CH3-C≡CH?
Both have a terminal sp C-H. But the CH3 group in propyne is electron-donating (+I effect). It pushes electron density toward the alkyne carbon and slightly destabilises the acetylide ion, making propyne a bit less acidic than acetylene.
