6.1.1 (k,l) Directing Effect
Syllabus
(k) the 2- and 4-directing effect of electron-donating groups (OH, NH2) and the 3-directing effect of electron-withdrawing groups (NO2) in electrophilic substitution of aromatic compounds
{Learners will not be expected to know further electron-donating or electron-withdrawing groups; relevant additional data will be supplied in examinations.}
(l) the prediction of substitution products of aromatic compounds by directing effects and the importance to organic synthesis
What does this mean?
Some groups donate electrons to a Benzene ring (like the OH group in Phenols), others withdraw electrons from the ring.
These two actions can affect how, and importantly where, further substitutions will happen.
Electrophiles (E+) can be mono-substituted onto this aromatic compound in three places on the molecule: 2 (Ortho in older books), 3 (Meta) or 4 (Para).
Clearly, Carbon 5 is equivalent to 3 and Carbon 6 is equivalent to 2.
2,4 directing (Activating) groups
We saw with Phenols that Electron donating groups (which activate the ring) direct the electrophile towards 2 and 4.
2 is likely to be around twice as common as 4 because there are two equivalent Carbon atoms in the 2 position (2 and 6) but only one Carbon 4.
The reason for this is not examinable at A level, though there are plenty of internet sites that will explain it to you if you're interested.
The syllabus only mentions -NH2 and -OH groups specifically so we have to assume that this is the only activating groups you are supposed to recall (this is a new addition to the syllabus so past papers aren't much guide).
You could reasonably expect a question that told you that Chloro/Bromo groups (or some other group) has the same directing effect as an -OH/-NH2 group
3 directing (Deactivating) groups
It won't come as a surprise to learn that electron withdrawing groups (which deactivate the ring) direct electrophiles to position 3.
The only example that you're expected to recall is -NO2 group.
Because the NO2 group in Nitrobenzene is deactivating the ring we would still have to use Conc. Sulphuric/Conc Nitric mixture to achieve the second nitration - almost all of which will form 1,3-dinitrobenzene