A summary of topic 18, organised so the notes are easy to understand. The notes are on slides, so they can be printed out and used as revision cards or posters, for revision on the go. The notes cross-reference the specification so it is easy to see where each bit of information has come from. They...
Benzene: Naming aromatics
- London forces ∴ low BPT (80°C) Aromatics = containing benzene
- Insoluble in H2O Derivatives of benzene ----------->
Kekule structure vs new model; Benzene can also be used as a side group = use phenyl
= phenyl amine =phenyl acetate
- Each C has 1e- in a p orbital Reactions of benzene:
- Instead of 3π bonds, 1 large delocalised π bond Combustion
containing all 6e-’s - Smokey flame due to high C:H ratio (lots of incomplete
Problem 1 - benzene should decolourise combustion ∴ lots of soot)
Br water due to 3x C=C bonds, however a substitution Reactivity of benzene:
reaction occurs suggesting there are no double bonds. Benzene = quite stable
- No individual c=c bonds ∴ no reaction w/ Br Struggle to take part in addition reactions due to not forming a
Problem 2 - dihalobenzene isomers, should be 4 however dipole to form an electrophile
only 3 were found, meaning 1-2 and 1-6 must be the same. - In benzene, 6e- delocalised over 6C’s
- 1-2 & 1-6 the same bond because halogens on - In c=c, electrons are delocalised over 2C’s ∴ can induce
adjacent C’s have no difference - no different c-c and dipole
c=c bonds. Instead of addition, benzene takes part in electrophilic
substitution:
Problem 3 - bond lengths suggest the c-c bonds in benzene
are the same, and an intermediate species between c-c & - Generally needs a catalyst to make E+ (need dipole)
c=c
- All c-c bonds same bond type ∴ same length
Problem 4 - enthalpy for hydrogenation (reacting w H 2 to fully
saturate) is lower than expected (less than 3x that of
cyclohexane)
- Charge spread out ∴ ↑stability ∴enthalpy change
, Reactions of benzene (continued) Friedel-crafts reactions Phenols:
Nitration: = attaching extra C atoms to a benzene ring = an OH on a benzene
- Reagents = conc H2SO4 & conc HNO3 Alkylation: ‘aromatic alcohols’
- Conditions = 50°C
Phenol = pink solid at
Acylation: room temp
- Conditions = under reflux Benzene rings in
phenol:
The non-bonding e-’s on
the O in OH are drawn
into the ring
→
↑ e- density in the ring
∴↑ susceptible to
electrophilic attack
Reduction of nitrobenzene: Further substitution of arenes (containing
Properties of phenols:
- Reagents = tin & conc HCl (reducing agents) benzene)
- high MPT (H bonds)
- Conditions = heat under reflux Electron withdrawing groups - soluble-ish (H bonds)
- Substitution of nitrobenzene - weak acid:
= harder than w/ benzene
- Takes e-’s out of the ring
- ∴ harder to react
- Produces are 1,3-disubstituted product
Halogenation: Electron donating groups (eqm lies on LHS ∴weak)
- Reagents = AlCl3 or FeBr3 - Substitution of methylbenzene - ↑ reactive than benzene
- Conditions = room temp - Reacts w/ HNO3
= easier than w/ benzene
without catalyst
- Pushes e- density into the ring - Reacts w/ Br without
- ∴ easier to react a catalyst
- Produces a mix of 1,2 and 1,4
disubstituted products
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