a) Write a detailed mechanism for the depletion of ozone in the stratosphere wit

Question

a) Write a detailed mechanism for the depletion of ozone in the stratosphere with CFC compounds such as CCl3F.

(b) The bromination of 1-bromopropane gives the following products. Br Br Bro, 200°C Br Br Br Br Br 90 1.5% 8.5% Calculate the relative reactivities of the hydrogens on each of the three carbons toward bromine atoms Compare these results with the bromination of propane taking into account that the reactivity of the hydrogens on C2 is four times more reactive than those on C1 or C3. Suggest an explanation to justify the difference observed. /8 marks

Explanation / Answer

The reactions involving Oxygen and ozone are :

O2---(hv)-------> O+ O

O + O2 -------->O3

When CFC's are present in the stratosphere,

(1) CCl3F ----hv ------> CCl2F + Cl

The released chlorine atoms react quickly with O3

(2) Cl + O3 ---------> ClO + O2

The ClO has an odd number of electrons and is very reactive. When it meets a free O atom, the O in the ClO is highly attracted to the O atom, and breaks away to form a new O2 molecule.

(3) ClO + O --------> Cl + O2

The net effect of the reactions above is:

O + O3 ---> O2 +O2

Finally, ozone gets depleted into 2 molecules of Oxygen.

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