[dvp331]

Hello,

I have a question about the attached molecule... I understand that tri-halide groups are strongly deactivating and meta directors, as on the left side... but on the right side, would this be considered a halogen and ortho para director even though it is not directly attached to the benzene ring?

Also, out of the two, I thought that if mono bromination was to occur it would happen on the right side, para (or ortho) to the flourine group... however this is not the case and it adds meta on the right benzene, anybody know why this is?

Thank you

[enahs]

These are 2 carbons away from the ring, so everything stems from the electronegativity of the fluorine atoms. The ring on the left is more electron deficient because it has more fluorine's closers to it.

[dvp331]

Ok, that makes sense, but what is it then that determines that the Br will attach Meta?

[opsomath]

Since the CF3 group and F group are not attached to the ring, they do not follow the classic directing group pattern usually associated with these groups.

The right-hand ring is less deactivated than the one on the left, because a CH2F group is less electron-withdrawing than a CF3. However, it is still a very little deactivating, hence the meta-directing effect.

Edit: Like the CF3 group, the CH2F group must be a mild deactivator even one carbon unit away...like, for instance, trifluoroethanol is a little more acidic than ethanol. I wouldn't have known that if you hadn't told me, though.

[Honclbrif]

It's my understanding that classic directing effects are a resonance phenomenon. When an electron donating group is in conjugation with the ring it pushes electron density to the ortho and para sites*, activating them toward electrophiles. When an electron withdrawing group is in conjugation with the ring it removes electron density from the ortho and para sites*, deactivating them toward electrophiles. The only site which you can't draw a + charge on is meta, so by default it is the only one which is reactive toward electrophiles but since the ring has had density inductively withdrawn it is still overall deactivated.

Halogens seem to put a wrinkle in this because they are deactivators, but ortho/para directing. Again, if you draw the resonance structures it all makes sense. The electronegative halogen inductively withdraws density from the ring resulting in overall deactivation, but the lone pairs can be sent to the ring via resonance resulting in regions of relatively high density at the ortho and para positions.

The structure in question has its halogens removed from conjugation by several bonds, so I wouldn't expect a huge effect on activation or directing. I would expect to see the same effect you would from a carbon in the benzyl position (toluene, etc), but slightly weaker due to the halogens near it. Again, there's no resonance at all, and the inductive effect should be pretty dampened due to distance.

*Draw the resonance structures of aniline and nitrobenzene and you'll see

[opsomath]

I would expect to see the same effect you would from a carbon in the benzyl position (toluene, etc)

Yeah, we all understand what effects resonance donors (pi-donors) have. The OP's molecule doesn't have any of these. Everything is sigma-bonded. The question is, is an alkyl group with a fluorine alpha to the benzyl carbon a sigma-acceptor or a sigma-donor? It appears that the answer is "acceptor," although I would have guessed the other way.

[orgopete]

This looks like an error to me. Although I cannot access the paper for the nitration of benzyl fluoride, it should be far more directing than a fluorophenylethane. See ref 5, http://www.orgsyn.org/orgsyn/prep.asp?prep=cv6p0835. (Perhaps someone could post the data, if given.) Even ethyl phenylacetate is an ortho-para director. Could someone do a search for the bromination of 2-fluorophenylethane?