Alkenes and bromine. Starter. Attempt the reaction mechanism sheet. Learning outcomes. • Describe how heterolytic fission leads to electrophilic addition in alkenes, using bromine as an example. • Recall the reactions of alkenes. Addition of the hydrogen halides.
• Describe how heterolytic fission leads to electrophilic addition in alkenes, using bromine as an example.
• Recall the reactions of alkenes.
Bromine is non-polar but when it approaches a double bond, the electrons in the π-bond repel the electrons in the Br-Br bond. This inducesa dipole in the Br2 molecule.
or you could draw this...
Finally, because the positively charged carbocation is unstable, it quickly reacts with the bromide ion to form the organic product 1,2-dibromoethane
dipoles shown correctly on the Br–Br and curly arrow from the Br–Br 1
bond towards the Brδ
correct intermediate shown 1
curly arrow from the lone pair or the negative charge on the Br - to the C+
(b) (i) Hs are diagonal to each other in the trans/ 1
difference clearly shown in a diagram
(ii) (the product is saturated hence) there is no restricted rotation/single 1
bonds allow rotation/because C=C prevents rotation