Answer:
I expect to observe a change in colour from reddish brown to a colourless solution
Explanation:
Bromine (Br2) attacks the electron rich carbon-carbon triple bond in but-2-yne, an alkyne to form an initial product 2, 3 dibromobut-2-ene; which reacts with excess bromine to form a final product 2,2,3,3 -tetrabromobutane.
The reaction occurs in two steps. On approaching but-2-yne, bromine molecule becomes polarised forming an induced dipole containing a bromonium ion.
Br - Br → Br+ - Br-
The bromonium ion (Br+) formed then attacks the carbon - carbon triple bond to form the initial product
2,3- dibromobut-2-ene
CH2-C≡C-CH2 + Br+ →
CH2 - CBr =CBr-CH2
(2,3- dibromobut-2-ene)
Which in the presence of excess bromine gives the final product
2,2,3,3 - tetrabromobutane.
CH2 - CBr =CBr-CH2 + Br2 →
CH3 -CBr2-CBr2 - CH3
2,2,3,3 - tetrabromobutane.
A visible change in colour from the reddish-brown colour of Bromine to a colourless solution is observed during the reaction.
