Aromatic Compounds Flashcards
What are aromatic compounds
Benzene and structurally related compounds
What are common aromatic compounds
Phenol (hydroxybenze, OH), Aniline (aminobenzene, NH2), Toulene (methylbenzene, CH3), Benzaldehyde (HC=O)
Describe bonding in benzene
Benzene is planar and all the carbon bonds are the same length. Bonds are between a single and double bond (1.39A)
What are the canonical forms of benzene
Due to resonance electrons arranged in 2 ways,Real structures is a mixture of the two canonical forms.
Why does benzene not react like an alkene
It does not do electrophilic additions
What is benzene more stable to than alkenes
Hydrogenation
Why is benzene more stable than expected
P orbitals on carbon atoms interact with each other to form pi bonds. Benzene has a doughnut shaped electron cloud above and below the ring- electrons are delocalised
What is the Huckel rule for aromatic compounds
There are 4n+2 pi electrons, where n= an integer. 4n + 2 = 2, 6, 10, 14
How can you tell if cyclic conjugated alkenes are aromatic
- They are planar
2. There are 4n+2 pi electrons
How many pi electrons are 1 double bond or 1 lone pair of electrons
2 pi electrons
Is cyclobutane (2 double bonds) aromatic
No. Has 4 pi electrons which does not follow the Huckel rule. Very unstable
Is benzene (3 double bonds) aromatic
Yes. Has 6 pi electrons (3 double bonds x 2 = 6 pi electrons), follows the Huckel rule. Very stable
Is cyclooctatetraene (4 double bonds) aromatic
No. Has 8 pi electrons so does not follow the Huckel rule. Reacts like an alkene
Is pentalene (4 double bonds) aromatic
No. Has 8 pi electrons so does not follow the Huckel rule. Never been isolated
Is napthalene (5 double bonds) aromatic
Yes. Has 10 pi electrons so follows the Huckel rule. Reacts like benzene
What happens to lone pairs of electrons substituents (e.g. NH2) of aromatic rings
Lone pair becomes partially delocalised (decreases reactivity of lone pair, but increases reactivity of aromatic ring towards electrophiles). E.g. nitrogen lone pair is partially delocalised (makine aniline less basic and less nucleophilic than alkylamine)
Why is phenol more acidic than methanol
The negative charge is stabilised by delocalisation in the phenolate anion
What reactions do aromatic compounds undergo with electrophiles
Electrophilic substitution
Describe bromination
Benzene (+Br2 and FeBr3) -> bromobenzene
Why is the FeBr3 catalyst required in bromination of benzene
It makes Br2 a better electrophile as it polarises it.
What is nitration
Reaction with concentrated nitric acid and sulfuric acid
What is the electrophile in nitration of benzene
NO2+
How is NO2+ formed
HO-NO2 + H2SO4 H2O(+)-NO2 + HSO4- H20 + NO2+ + H2SO4-
Why is nitration a useful reaction for making substituted benzenes
Nitro (NO2) groups can be converted into a variety of other functional groups
What does activated mean
More reactive than benzene
What does deactivated mean
Less reactive than benzene
What are electron donating groups
Activating
What are electron withdrawing groups
Deactivating
Why are alkybenzenes activiating
The alkyl group is electron donating (inductive effect) and is a activator
Why are phenol (-OH substituent) activating
Oxygen donates a lone pair to the benzene ring that stabilizes the intermediate (so activating)
Why are carbonyl and nitro groups deactiviating
These substituents withdraw electrons and hence slow the rate of reaction.
