3.10 Aromatic Chemistry Flashcards
What is the molecular formula of benzene?
C6H6
State the structure of benzene
planar cyclic
(6 carbons are joined together in a flat ring)
Draw the skeletal formula for benzene
Describe the structure of benzene
- Each C atom forms single covalent bonds to C on either side and to 1 H
- Final unpaired electron on each C atom is located in p-orbital that sticks out above and below the plane of ring
- P-orbitals on each C atom combine to form ring of delocalised electrons
Describe the carbon-carbon bonds in benzene
All carbon-carbon bonds in ring are same = so same length (140 pm)
Lies between the length of a single C-C bond (154 pm) and a double C=C bond (135 pm)
Why is benzene very stable?
∵ delocalised ring of electrons
State how you can prove that benzene is far more stable than theoretical compound cyclohexa-1,3,5-triene (where ring would be made up of alternating single and double bonds)
By comparing enthalpy change of hydrogenation for benzenes with enthalpy change of hydrogenation for cyclohexene
Cyclohexene has 1 double bond and when it’s hydrogenated, the enthalpy change is -120 kJ mol-1. State what the theoretical enthalpy of hydrogenation would be if benzene had 3 double bonds.
-360 kJ mol-1
Describe how the experimental enthalpy of hydrogenation of benzene is different to the theoretical value of -360 kJ mol-1
- Amount of energy is less
- far less exothermic than expected
- (Experimental enthalpy of hydrogenation of benzene is -208 kJ mol-1)
Explain the difference between the theoretical enthalpy of hydrogenation of benzene and the experimental value.
- Energy is put in to break bonds and released when bonds are made
- More energy must have been put in to break bonds in benzene than would be needed to break bonds in a theoretical cyclohexa-1,3,5-triene molecule
- Difference indicates benzene is more stable than cyclohexa-1,3,5-triene would be
- Thought to be due to delocalised ring of electrons
What are aromatic compounds or arenes?
Compounds containing a benzene ring
Name the compound
Name the compound
Name the compound
Name the compound
Name the compound
In other molecules the benzene ring can be regarded as a substituent side group on another molecule, like alkyl groups are. The C6H5 - group is known as the phenyl group
Explain why arenes attract electrophiles
Benzene ring is a region of high electron density so it attracts electrophiles
Explain why arenes undergo electrophilic substitution opposed to electrophilic addition
- As benzene ring’s so stable, doesn’t undergo electrophilic addition reactions, which would destroy delocalised ring of electrons
- Instead undergoes electrophilic substitution reactions where one of H atoms (or another functional group) is substituted for electrophile
Give 2 examples of useful chemicals that contain benzene rings
Dyes and pharmaceuticals
Why is it tricky to make chemicals that contain benzene
∵ benzene is so stable = fairly unreactive
What does Friedel-Crafts acylation do to benzene rings?
- Adds acyl group (RCO-) to benzene ring
- Once acyl group has been added, side chains can be modified using further reactions to make useful products
Why can’t most electrophiles attack a benzene ring?
Most aren’t polarised enough
How can electrophiles be made into stronger electrophiles?
By using a catalyst called a halogen carrier
State what electrophile Friedel-Crafts acylation uses
Acyl chloride
Give an example of halogen carrier that Friedel-Crafts acylation could use
AlCl3
Describe how AlCl3 makes the acyl chloride electrophile stronger
- AlCl3 accepts lone pair of electrons from acyl chloride
- As lone pair of electrons is pulled away, polarisation in acyl chloride increases and it forms a carbocation
- Makes it stronger electrophile and gives it a strong enough charge to react with benzene ring
Draw a diagram showing how AlCl3 makes an acyl chloride electrophile stronger
Draw the mechanism for Friedel-Crafts acylation (electrophilic substitution)
State the conditions for Friedel-Crafts acylation to occur
Reactants need to heated under reflux in non-aqueous solvent (like dry ether) for reaction to occur
Describe nitration briefly
When you warm benzene with concentrated nitric and sulfuric acids = nitrobenzene
State the catalyst used in nitration
Sulfuric acid
Nitration
Why is sulfuric acid used as a catalyst?
Helps to make nitronium ion NO2+ = electrophile
Nitration
Write equations showing how the nitronium ion, electrophile, is made
Draw the electrophilic substitution mechanism for nitration of benzene
Nitration
State what you can do, if you want only 1 NO2 group (mononitration) substituted?
Need to keep temperature below 55°C
Above this temperature = get lots of substitutions
Name 2 uses of nitration reactions
- Nitro compounds can be reduced to form aromatic amines
- Use to manufacture dyes and pharmaceuticals
- Some nitro compounds can be used as explosives
- e.g. 2,4,6-trinitromethylbenzene (TNT)
What happens when methylbenzene is nitrated with a mixture of nitric and sulfuric acid?
Produces mixture of 1-methyl-2-nitrobenzene and 1-methyl-4-nitrobenzene
(More vigorous conditions + more nitro-groups substituted)
How can a mixture of 1-methyl-2-nitrobenzene and 1-methyl-4-nitrobenzene be separated?
By thin layer chromatography
Describe how methylbenzene reacts compared to benzene
Reacts similarly to benzene but is more reactive than benzene towards electrophilic substitution
Explain why methylbenzene is more reactive than benzene towards electrophilic substitution
∵ methyl group releases electrons onto benzene ring = electrophile attracted more
Explain why it’s harder to substitute the chlorine atom in 1-chloromethylbenzene than in phenylchloromethane
- C-Cl bond in 1-chloromethylbenzene is stronger than in phenylchloromethane
- ∵ electrons in filled p-orbitals on Cl atom interact with delocalised electrons on benzene ring
How do substituents affect the reactivity of benzene ring towards electrophiles?
By releasing electrons onto ring or by withdrawing them
Friedel-Crafts Acylation
Write an equation to show the role of aluminium chloride as a catalyst in this reaction (with ethanoyl chloride)
CH3COCl + AlCl3 → CH3CO+ + AlCl4-
Friedel-Crafts Acylation
State the product formed
Phenylketone
Explain why methylbenzene does not react easily with nucleophiles such as ammonia (2)
- Delocalised ring is electron rich
- Repels nucleophiles
The formula of the ester is (CH3)C6H4COOCH2CH2CH3. State why this is not a structural formula. (1)
the CH3 group can be in different positions on the (aromatic) ring
