Question

10.50 (*.) In this chapter (Section 10.9.1), you learned that alkenes react more quickly with electrophiles than do the corresponding alkynes Kalkene Kalkyne > 1). Provide an explanation for the fact that there is a greater disparity in the alkene vs. alkyne reactivity in the addition of H-Br as compared to the addition of Br2. [The rate data is not real, but is meant to illustrate a real trend.] relative rate Br relative rate Br Br2 10 Н-Br 100 Br Br Br Br2 Н-Вr н Br Kalkene Kalkene .= 100 =10 Кatyne Какуne 10.52 () Trans-addition is heavily favored for the addition of Br2 and Cl2 to alkynes. However, with chlorination, more of the syn-addition product is formed. Rationalize this fact in light of your answer to 10.50 and 10.51 above Br (exclusive) Br2 Br CI CI CI Cl2 (minor) (major)

Answer 1

10.50)

Alkenes reacts rapidly with electrophiles as compared to alkynes, this is because the carbon atom in alkyne is sp hybridises whereas in alkenes it is sp² hybridised as a result the alkynes are highly electronegative, their pie electrons are not available so easy to be attacked by an electrophile as they are so tightly held, whereas in alkenes the pie electrons are readily available to be attacked by any electrophile.

Therefore, the relative rate of reaction in the above mentioned reaction is more for alkenes as compared to alkynes.

10.52)

Bromination and chlorination of the given alkynes occurs via the formation of a three membered bromonium or chloronium cyclic transition states respectively. Bromination gives exclusively anti product only whereas chlorination don't, this is attributed to the fact that the activation energy barrier in the energy profile diagram for chlorination is very low as compared to bromination, i.e. less amount of energy is required to form a chlorinated product as a result both transition states (syn and anti) are equally favourable but with more of the syn addition product.