6.1.1 Aromatic Compounds Flashcards
What are arenes?
Aromatic compounds that contain a benzene ring
Structure of benzene
Ring of 6 carbon atoms each forming 3 sigma bonds to 2 other C atoms in the ring and an H atom (12 sigma bonds in total). The 4th electron of C is used to form a ring of 6 delocalised pi electrons. This makes benzene very stable
Evidence for benzene
- X ray studies show bond lengths are the same (between single and double bonds) and bond angles are the same
- Benzene does not decolourise bromine water (an alkene would)
- Benzene is more stable than expected (enthalpy of hydrogenation is a less negative number than expected)
- Benzene does not do electrophilic addition reactions like an alkene
Type of reaction benzene does
Electrophilic substitution
3 steps in electrophilic substitution of benzene
- Formation of electrophile using a catalyst
- substitution of benzene ring
- Regeneration of catalyst
Catalyst for halogenation
halogen carrier e.g. FeBr3 or AlCl3
Equation for formation of electrophile for bromination of benzene using AlBr3
Br2 + AlBr3 –> Br+ + AlBr4-
Mechanism for bromination
- pair of electrons from benzene ring attacks Br+
- Unstable intermediate formed (benzene ring with delocalised section from C1 to C5, C6 has C-H bond and C-Br bond, + charge in ring)
- curly arrow from C-H bond to reform ring
(Practice drawing this separately)
Conditions for nitration of benzene
conc. H2SO4
conc. HNO3
50-55 oC
Electrophile for nitration of benzene
nitronium ion NO2+
Equation for formation of nitronium ion
HNO3 + H2SO4 –> NO2+ + HSO4- + H2O
Mechanism for reaction of benzene with nitronium ion
Check notes
Evidence that sulfuric acid is a catalyst in nitration of benzene
It is regenerated (not used up) HSO4- + H+ –> H2SO4
Overall equation for nitration of benzene
C6H6 + HNO3 –> C6H5NO2 + H2O
Overall equation for bromination of benzene
C6H6 + Br2 –> C6H5Br + HBr
Why is a low temperature needed for nitration of benzene
So only mono-substution occurs. Higher temperatures allow multiple substitutions e.g forming 1,3,5-trinitrobenzene
Name of reaction between acyl chloride and benzene
Friedel-Crafts Acylation
General formula of acyl chloride
RCOCl
Equation for formation of electrophile in Friedel-Crafts acylation
RCOCl + AlCl3 –> RCO+ + AlCl4-
Mechanism for Friedel-Crafts Acylation
Check notes
Overall equation for Friedel-Crafts Acylation
C6H6 + RCOCl –> C6H5COR + HCl
Product of Friedel-Crafts Acylation
phenylketone e.g. phenylethanone
Observation in bromination of benzene
bromine decolourised and production of a white solid (bromobenzene)
Test for an alkene
Decolourises bromine water
What is a phenol
Compound containing a benzene ring with 1 OH group attached
Acidity of phenol
Weak acid - will neutralise a strong base e.g. NaOH, will not react with a carbonate
Method to tell apart phenol, ethanol and ethanoic acid including equations
Add NaOH and test pH. If no reaction has occurred and the pH is still 14, then ethanol. Phenol and Ethanoic acid will neutralise NaOH (pH7).
1. CH3COOH + NaOH –> CH3COO-Na+ + H2O
2. C6H5OH + NaOH –> C6H5O-Na+ + H2O
Add sodium carbonate to samples of phenol and ethanoic acid. The sample with effervescence is ethanoic acid.
3. 2CH3COOH Na2CO3 –> 2CH3COO-Na+ + H2O + CO2
Comparison of electrophilic substitution reaction between benzene and phenol
phenol more reactive. A lp on O adds 2 more electrons to the delocalised ring making it more electron dense and more reactive. phenol reactions don’t need a catalyst and multiple substitutions are common
e.g. C6H5OH + 3Br2 –> 2,4,6-tribromophenol + 3HBr
The bromine water is decolourised and a white precipitate is seen
Directing effects
OH and NH2 direct other electrophiles to 2 and 4 positions. NO2 directs to 3 position
