3: Aromatic Compounds Flashcards
What is an aromatic compound
A compound containing a benzene ring
What can an aromatic hydrocarbon also be called
arene
Does benzene decolourise bromine in the dark
No
What reasons are there to reject benzene having 3 localised pi bonds
- Benzene does not easily undergo the typical addition reactions of alkenes (Does not decolourise bromine in the dark)
- All the bonds between C atoms around the ring are the same length
- Hydrogenation of benzene is less exothermic than expected by comparison with alkenes
Description of benzene’s structure
- Benzene is a planar molecule
- All six carbon-carbon bonds lengths are the same
- Each carbon atom forms 3 sigma bonds, two to neighbouring carbon atoms and one to a hydrogen atom
- Each carbon atom also has 1 electron in a p orbital, which extends above and below the plane of the molecule, these orbitals overlap to form a delocalised π-electron system
What is a delocalised π-electron system
A continuous π cloud of electrons above and below the plane of the molecules
How many electrons are in a delocalised π-electron system
6
Why does benzene have low reactivity
Delocalisation spreads out the π-electrons more than they would be in a localised π-bond. Hence the electron density is lower in aromatic compounds and they are consequently less attractive to electrophiles
Why are the carbon-carbon bond lengths in benzene all the same
π-electrons are evenly distributed around the ring
Why is enthalpy change for hydrogenation of benzene lower than expected
Delocalisation lowers the energy of the π-electrons so they are in a more stable state than electrons in localised π bonds.
What is an electrophile
an electron pair acceptor
Why are aromatic compounds less reactive than alkenes
They have lower π-electron density so electrophiles are less strongly attracted. Consequently, reactions of aromatic compounds tend to occur much more slowly and often require catalysts
Do aromatics tend to react by substitution or addition
substitution
Why do aromatics tend to react by substitution
Because it is favourable to keep the delocalised π-electron system
What is the formula of a nitro group
-NO₂
What type of reaction is nitration of an aromatic
Electrophilic substitution
What does the aromatic react with in nitration
conc. nitric acid and conc. sulphuric acid
Equation for formation of the electrophile in nitration of an aromatic
HNO₃ + H₂SO₄ ⇌ NO₂† + H₂O + HSO₄¯
Describe the mechanism of an aromatic reacting with the NO₂† electrophile in nitration reaction
2 electrons move from the delocalised ring to the nitrate ion (curly arrow).
Now the NO₂ is bonded to the aromatic but the ring is now “horse-shoe” shaped and is positive (this is the carbocation).
The hydrogen bonded to the same carbon as NO₂ is eliminated and its bonding pair is put into the horse-shoe to regain the ring
So NO₂ is bonded to the aromatic and there is a separate H† ion
Equation for regen of the catalyst in nitration of aromatics reaction
HSO₄¯ + H† → H₂SO₄
What is needed for aromatic compounds to react with halogens
halogen carriers
Examples of halogen carriers
AlCl₃ or FeCl₃
AlBr₃ or FeBr₃
Observations of the halogenation of aromatic compounds
Bromine is decolourised (orange → colourless)
Steamy fumes of HBr
Mechanism of reaction between halogen and halogen carrier
in halogenation of an aromatic compound
Br₂ + AlBr₃ ⇌ Br† + AlBr₄¯
Br† is a powerful electrophile
Describe the mechanism of an aromatic compound reacting with Br+ in halogenation reaction
2 electrons move from the delocalised ring to the Br† ion (curly arrow).
Now the Br is bonded to the aromatic but the ring is now “horse-shoe” shaped and is positive (this is the carbocation).
The hydrogen bonded to the same carbon as Br is eliminated and its bonding pair is put into the horse-shoe to regain the ring
So Br is bonded to the aromatic and there is a separate H† ion
Equation for the regen of the catalyst (halogen carrier) in halogenation of an aromatic compound
H† + AlBr₄¯ → AlBr₃ + HBr
Equation for generation of electrophile for aromatic reacting with haloalkane (Friedel - Crafts Alkylation)
RCl + AlCl₃ ⇌ R† + AlCl₄¯
Describe the mechanism of an aromatic compound reacting with a positive alkane ion in Friedel-Crafts alkylation
2 electrons move from the delocalised ring to the positive alkane ion (curly arrow).
Now the alkane is bonded to the aromatic but the ring is now “horse-shoe” shaped and is positive (this is the carbocation).
The hydrogen bonded to the same carbon as the alkane is eliminated and its bonding pair is put into the horse-shoe to regain the ring
So the alkane is bonded to the aromatic and there is a separate H† ion
Equation for regen of catalyst in Friedel-Crafts alkylation
H† + AlCl₄¯ ⇌ HCl + AlCl₃
Why are Friedel-Crafts reactions very important in organic synthesis
They form C-C bonds and therefore allow the carbon framework of an organic molecule to be extended.
Equation for generation of the electrophile for aromatic reacting with an acyl chloride
CH₃COCl + AlCl₃ ⇌ CH₃CO† + AlCl₄¯
Describe the mechanism of an aromatic compound reacting with CH₃CO† in Friedel-Crafts Acylation
2 electrons move from the delocalised ring to the C† in the CH₃CO† (curly arrow).
Now the CH₃CO† is bonded to the aromatic but the ring is now “horse-shoe” shaped and is positive (this is the carbocation).
The hydrogen bonded to the same carbon as the CH₃CO† is eliminated and its bonding pair is put into the horse-shoe to regain the ring
So the CH₃CO† is bonded to the aromatic and there is a separate H† ion
Equation for regen of catalyst in Friedel-Crafts Acylation
H† + AlCl₄¯ ⇌ HCl + AlCl₃
What is a benzene ring called when it is in a molecule
It’s a phenyl group
eg. phenylethene is a benzene ring bonded to ethene
What is an aliphatic
Any compound that does not contain a benzene ring
What is an alicyclic
Any aliphatic compound that contains a non-aromatic ring
