ortho, para directing

Usually, if there is a bulky group near the ortho or para position, then the other position will be more favored because the attack at that position can occur more readily. Also, of course, if both ortho positions or the para position are blocked, then the other position will be more favored. If there are groups near both positions, the position with the least steric bulk will be favored.

 

If the the other substituent on the ring is activating, the rxn is directed ortho/para.
Activating = Electron density is donated to the ring. You have two options for determining whether a group is activating or not: Either memorize a big list of things that are activating/not or determine based on the electronegativity/lewis structure of the substituent. Alkyl groups are slightly activating--just remember that one...just like they stabilize carbocations by donating e density. alkyloxy (ie methoxy) and NH2 groups are activating...this is easily determined for any N or O containing group...just draw the lewis structure including lone pairs. If a lone pair is on the atom that connects to the benzene then it is usually an activating group (there are probably exceptions to this....NO2 maybe?...cant remeber if it has a double or single bong off hand).

Halogens are the wierd exception. They are deactivating by inductance (their electronegativity sucks electrons out of the ring), but activating by resonance (can donate a lone pair to the ring). Thus they are deactivating (slightly) but direct meta.

If the ring has multiple substituents, the activating groups direct, just ignore the deactivating ones. The affects of various groups can be either additive or in conflict. If no activating group is present or activating groups oppose...then you cant really predict what will happen...many products will likely be formed.

Hope this helps!