Question

In friedel-crafts alkylation of benzene with an alkyl halide and AlCl3:

1. In terms of intermediate and product stabilities explain why the ortho and para isomers would be favored under kinetic conditions and why the meta isomer would be favored under thermodyanmic conditions?

2. What function does the nitromethane serve in the kinetic reaction?

3. What reactants would you employ for the preparation of the following compounds by reactions of the Friedel-crafts type?

- a. diphenylmethane

- b. 4-bromophenyl phenyl ketone

Answer 1

!. In general, Thermodynamic reaction control or kinetic reaction control in a chemical reaction can decide the composition in a reaction product mixture when competing pathways lead to different products and the reaction conditions influence theselectivity or stereoselectivity. The distinction is relevant when product A forms faster than product B because the activation energy for product A is lower than that for product B, yet product B is more stable. In such a case A is the kinetic product and is favoured under kinetic control and B is the thermodynamic product and is favoured under thermodynamic control.

In general, short reaction times favour kinetic control, whereas longer reaction times favour thermodynamic reaction control. Low temperatures will enhance the selectivity under both sets of conditions, since T is in the denominator in both cases. The ideal temperature to optimise the yield of the fastest-forming product will be the lowest temperature that will ensure reaction completion in a reasonable amount of time. The ideal temperature for a reaction under thermodynamic control is the lowest temperature at which equilibrium will be reached in a reasonable amount of time. If needed, the selectivity can be increased by then slowly cooling the reaction mixture to shift the equilibrium further toward the most stable product. When the difference in product stability is very large, the thermodynamically controlled product can dominate even under relatively vigorous reaction conditions.

The second factor that becomes important in reactions of substituted benzenes concerns the site at which electrophilic substitution occurs. Since a mono-substituted benzene ring has two equivalent ortho-sites, two equivalent meta-sites and a unique para-site, three possible constitutional isomers may be formed in such a substitution. If reaction occurs equally well at all available sites, the expected statistical mixture of isomeric products would be 40% ortho, 40% meta and 20% para. For the benzene, for the resonance of the moleculae, the positives charges are activated, thus, they are stable in kinetic conditions.

2. The nitro group is an electron atractor thus the methyl carbon can be atack by the ring.

3.

a)

b)