# The major product of the following reaction

Here,1,2-dibromobutane is reacted with
i) alc. $$\ce{KOH}$$
ii) $$\ce{NaNH2}$$ in liq. $$\ce{NH3}$$

According to me, since alc. $$\ce{KOH}$$ causes dehydrohalogenation, using Saytzeff's rule the product obtained after reaction (i) should be one of the either:

And after reaction (ii), since $$\ce{NH2-}$$ is a strong base, an E2 reaction proceeds and again using Saytzeff's rule the final product should be:

However, according to the answer key the final product obtained is given as:

Can someone help by explaining where and how I went wrong in my attempt to solve the question?

• An allylic bromide will react with KOH. What happens to terminal alkynes in the presence of NaNH2? Check pKa's! Apr 7 '19 at 18:38
• Hey, all of your compounds except the first one are wrong as they have 5 carbons. Apr 7 '19 at 20:03
• @AnubhabDas Yeah sorry, I've corrected them now. Apr 8 '19 at 3:28

## 1 Answer

When you form the allene, the methylene hydrogen has enhanced acidity because deprotonating it leads to a carbanion in which the negative charge is delocalized:

$$\ce{CH3-CH=C=CH2} - \ce{H^+ -> CH3-CH=C=CH^- <-> CH3-C^-H-C#CH}$$

The proton then reattaches to the opposite end of the allene chain to form a more stable product. In effect this is a (strong) base catalyzed tautomerization.

• Thanks @zhe. How to get my resonance structures lined up properly? Apr 7 '19 at 22:45
• Honestly, I'm not the best person to ask. I liked you what did you with $\equiv$, and that might be the best approach here. @OscarLanzi
– Zhe
Apr 7 '19 at 23:46
• @OscarLanzi For a triple bond mhchem uses # symbol, and yes, it looks somewhat misaligned and I guess one should just accept it. I also tried to improve the location of the negative charge on carbons with \overset{...}{...} so that it doesn't distance hydrogens away. Apr 8 '19 at 0:19
• Thanks @andselisk. Apr 8 '19 at 1:20
• Thanks @OscarLanzi, but since the methylene H of the allene is acidic, shouldn't it react with NaNH2 instead of undergoing tautomerisation? And another thing, are alkynes, in general, more stable compared to allenes? Apr 8 '19 at 3:26