How is Free rad substitution different from halogenation of
arenes? How do i tell?
Also,do the state matter? like in alcohol,liquid...
The mechanisms are totally different. You can tell in two
ways : the different mechanisms themselves (if you wanna really
understand Chemistry, you gotta be prepared to go beyond what JCs
teach you, particularly in the area of mechanisms in which JCs only
teach the basic syllabus requirements, which does not allow the
student to understand Chemistry more deeply), as well as the
different reactants/products. For instance, whenever benzene is
directly involved, then the benzene is the (guy) nucleophile, while
the substituent is the (girl) electrophile.
When the exam qn asks "what kind of reaction is this?"
you must specify "halogenation via free
radical substitution" (ie. using X2 and UV
light, there will be initiation, propagation and termination steps)
or "halogenation via electrophilic aromatic
substitution" (ie. using X2 with anhydrous Lewis
acid catalyst if benzene ring is not activated, or if benzene ring
is activated by electron donating by resonance groups such as in
phenylamine or phenol, then using X2 in CCl4 for monohalogenation
or X2 (aq) for trihalogenation*) or "halogenation via
nucleophilic substitution" (eg. using PCl5, SOCl2, etc, I
teach my BedokFunland JC students how to draw the
mechanisms of using SOCl2 and PCl5 to convert alcohols
and carboxylic acids to alkyl halides and acyl halides
respectively, to give them a deeper understanding and appreciation
of organic chemistry).
* Cambridge can ask you to explain why and how the different
solvents results in different extents of electrophilic
aromatic substitution. The answer is "the strongly
polar protic water solvent induces a stronger temporary dipole in
the non-polar X2 molecule, hence X2 (aq) is significantly more
electrophilic compared to X2 (in CCl4)."
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Are you in JC1 or JC2? If the latter, then you're woefully
under-prepared for the 'A' levels in 2 months time. If JC1, then
you should do one or more of these 3 things (because online
forums like this one, can't teach you everything, there's too
much to teach).
1) more extensive consultation with your school teachers,
and be willing to ask more qns during tutorial classes.
2) self-learn more using online 'A' level Chemistry websites.
Links to such websites can be found on my website :
http://infinity.usanethosting.com/Tuition/#H2_Planning_and_other_goodies
3) consider tuition if your family finances allow it;
group/class tuition is always cheaper than solo individual tuition.
But there are all types of different tutors with different teaching
styles, and all types of different students with different learning
styles. You may need to try out a few before you find one best
suited for yourself.