Chapter 4 - Aromatic Compounds Flashcards
Molecular Structure of Benzene
C6H6
Benzene mainly reacts by what reaction?
Substitution
True or False:
Benzene is planar
True
Symbol for benzene where
• electrons are distributed evenly around the ring
• perhaps the more accurate of the two
Delocalized pi cloud
True or False
Resonance hybrid is always more stable than any of its contributing structures.
True
Chemical reactivity of Benzene
Electrophilic substitution
involve substitution of other atoms or groups for a ring hydrogen on the aromatic unit
Electrophilic Aromatic Substitution
Carbon atom undergoing electrophilic aromatic substitution becomes what?
Sp3-hybridized
It is similar to an allylic carbocation but the positive charge is delocalized over three carbon atoms instead of only two
Benzenonium ion
It is the completed by loss of a proton from the sp3 carbon atom, the same atom to which the electrophile became attached.
Substitution
What is the catalyst used in halogenation?
Iron halide
The electrophile in nitration
Nitronium ion
It protonates the nitric acid, which then loses water to generate the nitronium ion (NO2+), which contains a positively charged nitrogen atom
Sulfuric acid catalyst
A carbocation, which can be formed either by removing a halide ion from an alkyl halide with a Lewis acid catalyst example
Electrophile
Electrophile of acylation
Acyl cation
It provides a useful general route to aromatic ketones.
Friedel-Crafts reaction - acylation
What groups are more electron donating than hydrogen? (2)
Hydroxyl and methyl groups
What groups are more electron withdrawing than hydrogen? (2)
Chloro and nitro groups
• places the positive charge on the hydroxyl-bearing carbon
• Shift of an unshared electron pair from the oxygen to the positive carbon allows the positive charge to be delocalized even further, onto the oxygen
Ortho, para-directing groups
• has two adjacent positive charges
• a highly undesirable arrangement because like charges repel each other
Meta-directing groups
Activating or Deactivating?
if the rate of electrophilic aromatic substitution is faster for the substituted benzene than for unsubstituted benzene.
Activating
Activating or Deactivating?
if the rate of reaction is slower than for benzene
Deactivating
True or False:
If we brominate first and then nitrate, we will get a mixture of meta isomers.
False
(ortho and para isomers)
Is bromine atom ortho,para directing or meta directing?
Ortho, para directing
Is nitro group ortho, para directing or meta directing?
Meta directing
The unusual stability of certain fully conjugated cyclic systems
Aromaticity
Contain at least 2 benzene rings; each ring shares two carbon atoms with at least one other ring
Fused polycyclic hydrocarbon
