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Disubstituted Benzene Names

To indicate relative position on a benzene ring: ortho- (o) on adjacent carbons (1,2) meta- (m) separated by one carbon (1,3) para- (p) separated by two carbons (1,4)

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Disubstituted Benzenes

Ortho, meta, and para can also describe reaction patterns (“occurs at the para position”)

Label positions on the ring relative to the –X substituent:

Reaction at the para position:

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Benzene with more than Two Substituents Choose numbers to get lowest possible values

List substituents alphabetically with hyphenated numbers

Names like “toluene” and “phenol” can serve as root name(note that C1 is fixed by the -CH3 or OH group)

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15.3 Structure and Stability of BenzeneRecall the reaction of regular alkenes with Br2.

You get rapid addition at RT to give the dibromo cmpd:

RTBr

Br+ Br2

.

.

rapid rxn

But under these conditions, benzene gives no reaction:

RT+ Br2 NR

.

.

alkenepi bonds

arereactive

Benzenepi bondsunreactive!

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15.3 Structure and Stability of Benzene

Benzene will react with Br2 to give bromobenzene, but it

requires heat and iron catalyst (slow).

This is substitution rather than addition as seen with regular alkenes, suggesting that benzene has special stability

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Benzene - Aromatic StablilityHeats of hydrogenation demonstrate the extra stability of the benzene molecule.

hypothetical

actual

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Benzene’s Unusual Structure All its C-C bonds are the same length: 139 pm

This is halfway between regular C-C single bond (154 pm) and C-C double bond (134 pm)

Electron density in all six C-C bonds is identical

Structure is planar, hexagonal with 120° bond angles