Chem 227
in-class sheet - Chapter 18 continued
AROMATICITY CAN EXPLAIN SOME NOT SO OBVIOUS EFFECTS ON STRUCTURE
AND REACTIONS
Remember that:
- If a structure is aromatic, the π system is less reactive than otherwise expected
- If an aromatic structure is formed in a reaction, it forms more readily than otherwise
expected
Questions:
Logic:
1. Which is more basic?
H
N
N
2. Which has the lowest pKa?
H2
N
NH
3. Which is more acidic?
4. Which reacts faster in SN1 reactions?
Br
Br
BIRCH REDUCTION – a reaction on the benzene ring itself
What it does: Reduces arenes to nonconjugated dienes
Reagents: Group 1 metal (sodium or lithium) in alcohol (methanol or ethanol) and liquid ammonia
Examples:
O
Na, CH3OH,
NH3
O
Na, CH3OH,
NH3
Mechanism: involves formation of ____________________________________ intermediates
(A single electron transfers from the metal, then H+ transfers from the alcohol)
REACTIONS ON A CARBON SIDE CHAIN ATTACHED TO BENZENE RING
Types of influence of benzene ring
1. Stabilizes benzylic cation or radical by resonance
2. Conjugation to double bond next to ring (stabilization by resonance)
CH2
Reactions on the side chain
1. Oxidation (New this chapter!)
Reagent examples: Na2Cr2O7, H2O, H2SO4, heat or KMnO4, H2O, heat
What happens: When a benzylic H is present, the side chain can be oxidized, making the benzylic
carbon into CO2H
Na2Cr2O7, H2O
H2SO4
Br
2. Free radical halogenation (Review)
Br2 hv
or NBS, benzoyl peroxide, CCl4
3. Rates of SN1 and SN2 reactions of benzylic halides are faster than expected for their degree of
substitution. (Review)
In lab last semester: benzyl bromide + AgNO3 in ethanol – instant reaction
1-bromobutane + AgNO3 in ethanol – 81 seconds
benzyl bromide + NaI in acetone – 1 second
1-bromobutane + NaI in acetone – 20 seconds
4. Dehydration (Review)
OH
5. Dehydrohalogenation (Review)
Br
6. Addition to an alkene (Review)
HCl
HBr, peroxides
K. Long Sp2013