Aromatic Chemistry Flashcards

1
Q

What is an aromatic compound?

A

A compound which contains at least 1 benzene ring

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2
Q

What is an aliphatic compound?

A

A compound which has no benzene rings

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3
Q

What is the kekulé structure of benzene?

A

Exists as C6H6

Hydrocarbon ring with alternating double bonds

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4
Q

Why did kekulé suggest his structure for the benzene ring?

A

Suggested it due to the isomers formed when groups were added to the benzene ring.

1 group added formed 1 isomer

2 groups added formed 3 isomers

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5
Q

Why was kekulé’s structure for the benzene ring wrong and what evidence was there to prove this?

A

Would expect benzene to have similar reactions to cyclic alkenes

Would have expected a different shape of benzene

Would expect the enthalpy of hydrogenation be similar to that of cyclic alkenes at -360Kjmol^-1

Kekulé structure is similar to a cyclic Alkene and would therefore expect similar reactions (addition reactions)

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6
Q

Why is the experimental enthalpy of kekulé structure different to the actual enthalpy?

A

Kekulé structure has a different enthalpy change to the experimental enthalpy of benzene. Kekulé treated his structure as a cyclic alkene and therefore would have expected the hydrogenation to be -360kjmol^-1 however the actual enthalpy was only -208kjmol^-1 meaning that the real structure was more stable than kekulé’s. This gives an explanation of the delocalised system of electrons which provides stability to the molecule

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7
Q

Why were the problems with the shape of benzene?

A

Benzene is a planar molecule (all atoms lie in one plane) which would also be true about the kekulé structure of benzene. However C-C bonds are different lengths to C=C bonds meaning that kekulé’s structure would be an irregular hexagon.

Real benzene has the same size bond lengths across all C-C bonds creating a perfect hexagon. This gives evidence for the electrons being share equally across all the carbon atoms in the delocalised system of electrons having a resonating C=C double bond.

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8
Q

How do we name aromatic molecules?

A

Smallest number combination

Prefix in alphabetical order

Ending of benzene

Eg. 1-Bromo-2-Chlorobenzene

If benzene is not the main functional group the. The prefix is Phenyl

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9
Q

What happens when you heat benzene in air?(combustion)

A

Like all hydrocarbons benzene will burn in air

C6H6 +7 1/2O2 ——> 6CO2 + H2O

Burns with a smoky flame

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10
Q

What happens when a halogen is added to benzene?

And give the catalyst used

A

Halogenation reaction ( electrophilic substitution)

An electrophile is attracted to the electron rich delocalised system

Uses an AlCl3 catalyst

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11
Q

What happens when we nitrate benzene?

How is the electrophile generated and what catalyst is used?

A

Nitration is an electrophilic substitution reaction where a nitro (NO2+) group is the electrophile

Electrophile generation:

HNO3 + H2SO4 ——> NO2+ + HSO4- + H2O

Uses the HSO4- as the catalyst

Catalyst regeneration:
H+ + HSO4- ——> H2SO4

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12
Q

What kind of reaction is used to add alkyl branches or carbonyl groups to aromatic molecules?

A

Friedel crafts reactions

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13
Q

How is an alkyl group added to benzene?

A

Using haloalkanes

This is an alkylation reaction by substituting in the haloalkane in for a hydrogen atom

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14
Q

How are carbonyl groups added to benzene molecules?

A

Using acyl chlorides to attach carbonyl groups

Type of acylation reaction

Occur under reflux

Use anhydrous conditions to ensure the electrophiles do not react with water during reactions

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