[Schrödinger]

Hey guys

I was going through Friedel Crafts alkylation and acylation reactions.

There was this order of reactivity of compounds given:

For FC(Friedel Crafts) alkylation, RF>RCl>RBr>RI
For FC acylation, ROI>ROCl>ROBr>ROF

There is no explanation offered in any of my books. I tried searching the web, but the topic is too specific...(couldn't get any relevant search results.)

I gave it a shot :
FC acylation, RCOI>RCOCl>RCOBr>RCOF, maybe because of the relative strength of C-X bond (I maybe wrong)

Can someone give me a hint?

P.S. I don't know if what the book says is correct. So, please give the correct order of reactivity and an explanation as to why.

[cck05]

In my opinion, the larger size of the atom is, the weaker of the bond it has
so, for the FC alkylation, i think the rate is RI>RBr>RCl>RF since the bond of RI is weaker and it is more easily to form R+ cation

for FC acylation, i can't figure out the rate because there is C=O, so partial double bond may occurs between the delocalization of pie electrons in C=O and the halogen atoms(I,Br,Cl,F) . This question remains for the professionals =]

[Schrödinger]

You are most certainly right about the bond strengths, but they are not broken during the rate determining step (RDS) of the reaction are they?

That's what caused all the confusion for me. I thought I could neglect all the steps that occur before the RDS, as they are fast steps.

[cck05]

You are most certainly right about the bond strengths, but they are not broken during the rate determining step (RDS) of the reaction are they?

That's what caused all the confusion for me. I thought I could neglect all the steps that occur before the RDS, as they are fast steps.

as i know, the rds step is the broking of RX (ie. X=halogen)
if i am not correct, welcome to correct me =]

[srihari]

The rds is the attack of electrophile as far as I know.
btw good question by schoridnger

[Schrödinger]

It is only logical that the RDS is the attack of the electrophile, as pointed out by Srihari, as there is temporary loss of aromaticity in the benzene ring. This increases the potential energy of the transition state/intermediate formed by the addition.

That's what's complicating things for me, as mentioned earlier.

[movies]

This is an interesting question, and not one that I have a great answer for.

There are some comments I do want to add as far as the kinetics go.  The RDS is almost certainly the attack of the aromatic system onto the electrophile.  However, this step is bimolecular so it will depend both on the concentration of the aromatic piece (usually very high concentration, sometimes even the solvent) and the concentration of the electrophile.  Remember the concentration of the electrophile is the active carbocation or acylium ion, not the alkyl or acyl halide itself, so the cation concentration is likely very low, although that concentration will depend on the equilibrium between the cation and the alkyl/acyl halide, which will not be the same for each of these electrophiles.

The way I see it, there are two possibilities here: first, the equilibrium between alkyl/acyl halide and free cation is significantly different in these cases and the difference is enough to bear out in the observed rate despite the very large concentration of aromatic nucleophile; second, the rate at which the nucleophile adds to the electrophile may not be the same in each case.  The latter could be due to a close proximity of the counterion to the cationic intermediate.

It is probably a lot more complicated than that.  I haven't come across any extensive kinetic studies of the F–C reaction, but I haven't really looked either!

[AWK]

RCOI>RCOCl>RCOBr>RCOF

This is only for acetyl halides and probably benzene - original report is in japanese