Chemistry Stack Exchange is a question and answer site for scientists, academics, teachers, and students in the field of chemistry. It only takes a minute to sign up.
Sign up to join this community
1,2-Dibromopropane and KOH react together to form propyne. But I wondered if 1,2-propadiene was also being produced since the there is also a proton on the $\ce{CH3}$ group to be used in the elimination reaction.
Does it just come down to Zaitzev’s rule (in an elimination reaction the major product is the more stable alkene with the more highly substituted double bond) that all I see in propyne or is there something I am missing?
The concept here is that after the formation of the alkene (propene, in this case), it is much more easier to abstract the vinylic hydrogen as compared to the allylic hydrogen and thus preferring the formation of alkyne (propyne, in this case) over the formation of a diene (prop-1,2-diene, in this case).
Remember, $\mathrm pK\mathrm a$ of vinylic systems is around $43$ and that of allylic systems is about $50$. This webpage might be helpful to you in future.
It turns out that propyne and allene do equilibrate with the equilibrium mix in favor of the triple bond. I looked it up and think that Keq is about 20. Both geminal and vicinal dihalides seem to give predominately the alkyne. It might be possible that this is a situation of thermodynamic control. Maybe these reactions should be examined a bit more carefully?
