Same thing has been posted on RA chem blog (see my links)
What is an Arene?
The homologous series Arenes are compounds based on the benzene ring C6H6. This ring, and its stability confer distinctive properties to any molecule that contains it.
Arene bonding and structure
Given benzene’s formula, C6H6, one would expect three C=C double bonds and three C-C single bonds. Hence it can exist as either of 2 resonant forms (left and centre in the diagram below), depending on the position of the double bonds.
In actual fact it exists as a mixture of both – each carbon atom uses its 2s and two of its 2p orbitals to form the σ framework of the benzene molecule (bonding to each adjacent C or H atom). The remaining 2p orbitals combine to form delocalised π orbitals, shared equally by each carbon atom (hence each bond is neither a single nor a double bond, but something in between), emphasized by the diagram on the right. Hence all carbon-carbon bond lengths in benzene are the same (0.139nm).
Delocalisation lowers the energy of the structure significantly (this is known as the delocalization enthalpy, about 150 kJ/mol). This electronic configuration and stability has a dominant influence on the properties of benzene.
For instance, even though benzene is unsaturated, it does not decolourise bromine water (the usual test for unsaturation). It does not readily undergo halogenation (common for alkenes) because addition of halides would destroy the delocalised π cloud.
Electrophillic substitution
As you read through this section, pay special attention to the position of electrons, it is the principle concern affecting further substitution reactions.
The most important reaction of arenes is called electrophilic substitution: an atom or group of atoms (called electrophile) replaces a hydrogen atom of the benzene ring (i.e. positively charged ions attack the cloud of π electrons).
Thus:C6H6 + E+ (an electrophile) --> C6H5E + H+
The mechanism of electrophile attack starts with attack by an electrophilic cation (e.g. Cl+ after heterolytic cleavage of Cl2). A Wheland intemediate (extreme right in the figure below) forms, in which the positive charge is shared by the five other carbon atoms in the ring. To stabilise the ring further, the intermediate then loses a proton, hence the net result is not an addition but a substitution.
Activating and deactivating groups
Existing functional groups attached to arene rings have effects on further substitution reactions.
Activating groups make further reactions more likely and faster, while deactivating groups make further reactions less likely and slower. Thus whether C6H5E would react faster or slower with another electrophile (say E’) will depend on whether E is an activating or deactivating group.
The first mechanism holds that activating groups must donate electrons to the arene ring (hence positively-charged electrophiles are now more attracted to the ring, relative to the hydrogen already attached to the ring), and deactivating groups withdraw electrons.
Alternatively, activating groups must stabilise the Wheland intemediate, hence lowering the activation energy and making the reaction more feasible.
Some activating groups
Halogen substituents such as Cl is more electronegative than benzene, hence withdrawing electrons and deactivating the ring.
A hydroxyl group (OH) is strongly activating. Oxygen (like carbon in the ring) has an filled but unbonded p orbital approximately perpendicular to the plane of the ring, hence when an OH group is attached to the benzene ring, the delocalised π electron cloud extends to this oxygen in the OH group.
Inductive effect: when electrophiles (positively charged functional groups) add to the arene molecule via electrophillic substitution, a cation (the wheland intemediate) is first formed (the electrophile attaches before the H breaks off). Usually the positive charge is stabilised by sharing this charge over all the C atoms. When the delocalisation is extended to the O atom, the positive charge can be shared by it as well, hence the positive charge on each atom is lessened, lowering the reaction's activationg energy (hence faster / more probable reaction). Hence we say that the OH group activates the ring.
The same inductive mechanism goes for methyl (CH3) or other alkyl groups, but these are less activating.
Some deactivating groups
Nitro (NO2) groups and ketone (COCH3) groups are deactivating because they withdraw electrons from the ring. The highly electronegative oxygen atom withdraws electrons from the atom substituting for hydrogen (i.e. N or C), such that delocalisation of electrons does not extend onto this atom, and electron density may be withdrawn from the ring.
Nitro groups are more deactivating than ketone groups, because of its 2 oxygen atoms (versus ketone’s 1), and because N is more electronegative than C, hence further withdrawing electron density from the ring
Directing effects
Activating groups are ortho and para directing (position 2, 4, and 6 on the ring), while deactivating groups are meta directing (position 3 and 5 on the ring).
Here's why. An incoming electrophile’s positive charge is concentrated on carbon 2, 4, 6 (where the original functional group is no. 1). If an electron donating group is already present in these positions, it will donate electron density, lessening the positive charge on and stabilizing this wheland intemediate. Hence the wheland intemediate leading to the 1,2 (ortho) and 1,4 (para) isomers have lower energy than the wheland intemediate leading to the 1,3 (meta) isomer, and therefore the ortho and para isomers are likely to form are therefore more likely to form.
