Chemistry - Directive Effects

[vanilla chai]

I'm kind of confused on how it works... I get how the electrophiles attack the places with higher electron density but I don't understand how the electron density changes. This is what I know so far, so please help me out. X)

+I effect results in 2,4,6 directives because it pushes the electron density down into the benzene ring towards the 2,4,6 positions and increases electron density there relative to the 3,5 positions

And the -I effect pushes the electron density out of the benzene ring? I think it makes the activation energy higher, but what effect does that have on the directive...?

The lone pair effect is like the +I effect because it pushes electron density into the benzene ring? So would it also be 2,4,6 directive?

Yeah... I'm confused. Hahaha help much appreciated

[Sandwich]

No idea how helpful this is as I have cleverly blanked all of this hideous stuff, but on the eve before my own Further Organic Chem exam when I DID used to understand all this stuff, I made the following notes which may possibly help you...

Methylbenzene and its positive inductive effect (activating group) 2-4directing

The methyl group has a positive inductive effect, thanks to the methyl group. Therefore

the electron density within the ring will increase. This makes it far more reactive than

benzene in terms of electrophilic reactions (i.e. all of the reactions just discussed) than

benzene under the same conditions.

The big thing about the methylgroup is that it DIRECTS the location of subsequent

substituent groups. It sends everything to 2 and 4 positions. This is because of the

stability of the intermediate carbocations. The positive charge can go onto the carbon

bonded to the methyl group in 2 and 4, but in 3 (and 5, but they’re the same thing) it can’t

be moved onto that carbon. That carbon is most stable as the positive inductive effect

causes the pushing of the electrons towards the positive charge.

OH and NH2 groups on the benzene ring (activating group) 2-4directing

The non-bonded pairs on these groups can contribute to the electron ring. So instead of 6

pi electrons spread over 6 atoms, now there are 8 spread out over 7 atoms. Hence the

electron density greatly increases and so the benzene becomes MORE reactive as phenol,

or with an amine group in phenylamine. They are 2 and 4 directing as an extra resonance

form is available (4 resonance forms as opposed to only 3 resonance forms for the 3

version).

Phenol is so amazingly reactive, you can just stick it in some chlorine at room

temperature, no electron carrier needed and you’ll get 2,4,6trichlorophenol.

NO2 on the benzene ring (deactivating group) 3directing

NO2 is special because it has three very electronegative atoms and no lone pair. So it

can’t stick any electrons into the ring, and it can’t push electrons towards the ring. All it

can do is attract electrons out towards itself. Therefore it is deactivating. It is 3-directing.

If that IS helpful, incidentally, all my further organic notes are actually available to download off here if you have a VIP subscription / IBsurvivalplug