Chemistry Question--Benzene rings Watch

charco
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#21
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(Original post by Loz17)
Reactions involving a benzene ring always involves substitution. The hydrogens have to go somewhere
Well not strictly true. The ring can also undergo free radical addition...
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LadySmythe
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#22
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(Original post by charco)
Well not strictly true. The ring can also undergo free radical addition...
Hehe ok

At A-level on the OCR (salters) board, benzene reactions are always substutution :p:, according to my teacher
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chemicalguy
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#23
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Blimey guyz
Sometimes you need to separate out the "pure" chemistry from the "a-level" ways of understanding which is simpler
In all a-level boards they key points are these.

Benzene undergoes an electrophilic substituion mechanism
Primarily this is because the overlap of p-obitals causes the formation of the pi-delocalised ring which is stable because of it.
This stability (evidenced by deltlaH of hydrogenation) affects its reactivity in that the ring will want to be maintained during reactions ...
hence it prefers to undergo substitution to maintain the delocalised ring (rather than addition which would break it)
It undergoes Elect-Sub with Cl+ (using Fe/AlCl3 etc) NO2+ (HNO3/H2SO4) , and friedel-crafts alkylation (CH3+) and acylation (CH3CO+) using AlCl3 catalyst
Some boards (Nuffield is one) that discusses the extreme cause of free radical addition - but note that the free radical is an overkill reactant partical hence it can break the ring.

I accept that some of these A-level "truths" are a grey area when the subject is taken to a higher level or scrutinised too much, but an a-level mark scheme is just that ... and learning it in another eway wil NOT get you the marks ... take it from a former examiner!
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charco
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#24
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(Original post by chemicalguy)
Blimey guyz
Sometimes you need to separate out the "pure" chemistry from the "a-level" ways of understanding which is simpler
In all a-level boards they key points are these.

Benzene undergoes an electrophilic substituion mechanism
Primarily this is because the overlap of p-obitals causes the formation of the pi-delocalised ring which is stable because of it.
This stability (evidenced by deltlaH of hydrogenation) affects its reactivity in that the ring will want to be maintained during reactions ...
hence it prefers to undergo substitution to maintain the delocalised ring (rather than addition which would break it)
It undergoes Elect-Sub with Cl+ (using Fe/AlCl3 etc) NO2+ (HNO3/H2SO4) , and friedel-crafts alkylation (CH3+) and acylation (CH3CO+) using AlCl3 catalyst
Some boards (Nuffield is one) that discusses the extreme cause of free radical addition - but note that the free radical is an overkill reactant partical hence it can break the ring.

I accept that some of these A-level "truths" are a grey area when the subject is taken to a higher level or scrutinised too much, but an a-level mark scheme is just that ... and learning it in another eway wil NOT get you the marks ... take it from a former examiner!
I believe free radical halogenation in the presence of UV light is required for several boards.
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chemicalguy
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#25
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(Original post by charco)
I believe free radical halogenation in the presence of UV light is required for several boards.
Agreed...

Thanks Charco I forgot that some boards do dicriminate between doing halogenations in the dark (sub) or visible/UV light

And I forgot to add that the first step in the accepted mechanism of electrophilic sub is attraction of a formally positive of delta positive centre by a double-hook arrow that originates from ring and end at a point close to the +ve part of the elctrophile

Hope helps
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=gabriel=
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#26
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(Original post by Loz17)
Reactions involving a benzene ring always involves substitution. The hydrogens have to go somewhere
Addition-elimination concerns substituted benzenes possessing a good leaving group. Overall it is a substitution - kind of an SN(Ar) - but the reaction goes through the Meisenheimer intermediate, product of an addition. Any undergraduate organic textbook would cover this.

In this case the hydrogens don't do anything actually. Follows that C6H6 does not undergo addition elimination.
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