[Edher]

Saludos,

        How can I tell which will be the major product of the following reaction?

[Yggdrasil]

If there is an equal probabliliy of reacting at the ortho/para positions, there will actually be a 2:1 ratio of o-nitrichlorobenzene and p-nitrochlorobenzene because there are two ortho positions and one para position.  However, my ochem professor actually used the nitration of chlorobenzene as an example, and he says that the rates of reaction (relative to the rate of nitration of benzene) are:

ortho (2) - 0.03
meta (3) - 0.0009
para (4) - 0.14

In general, you will get signficant amounts of both ortho and para product when reacting an aromatic system with an ortho/para dirrecting substituent.  One exception is when you have a large, bulky o/p dirrecting group which can sterically hinder substitution at the ortho position.

[timeless]

also, keep in mind that electrons like to be as far away from each other as possible. putting something in the ortho position is fine but para is more favored, as shown by the rates of rxn from above.

[victor]

I can say that ortho direction is more faourable because as you see the first group is Cl which has high electronegativity...so, the tendency of electrons will be drawed nearer to Cl group. We also know that the formal charge of NO₂ group is +1, so it's an electrophilic group.
Correct me if I'm wrong..