Which of the compounds cannot be prepared directly?
(i). Fluoroarenes
(ii). Chloroarenes
(iii). Bromoarenes
(iv). Iodoarenes

Options

A. (i),(ii)
B. (ii),(iii)
C. (i),(iii)
D. (i),(iv)

Answer

4

Explanation

Benzene3 cannot be iodinated with `I_(2)` directly because (a) iodination with `I_(2)` is slow `(F_(2)gtCl_(2)gtBr_(2)gtI_(2))` and (b) somewhat reversible `(DeltaH=12` kcal `mol^(-1))`. To iodinate benzene with `I_(2)`, we use an oxidizing agent such as `HNO_(3)` which affect the reaction by oxidizing `I_(2)` to a more powerful electrophile hydrated.
In direct iondination `I^(-)` would be a product and its presence could reverse the reaction An oxidizing agent coverts `I^(-)` to `I_(2)`.
Note that the electrphile `overset(delta)(HO)-overset(delta-)(I)` is formed when `H_(2)O_(2)` is the oxidant.
the iodination of an arene is possible with ICl as ICl is more electrophilic than `I_(2)` because it is polarized. i.e., `overset(delta+)(I)-overset(delta-)(CL)`. The aromatic ring more easily displaces `Cl^(-)` from `overset(delta+)(O)-overset(delta-)(Cl)`. Than it displaces `I^(-)` from the nonpolar `I-I`
Fluoroarenes cannot be prepared by direct reaction of `F_(2)` because fluorine is highly reactive. Both addition and substitutions take place. to make fluorobenzene, we make use of a process called thallation where benzene is reacted with `TI(OCOCF_(4))_(3)`. Followed by the addtion of KF and `BF_(3)`.

`PhH+overset(+)(T)l(OCOCF_(3))_(2)overset(-H^(+))toPhTl(OCOCH_(3))_(2)underset(BF_(3))overset(KF)toPhF`