3.3.10 - 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
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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
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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
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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)
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Explain the difference between the theoretical enthalpy of hydrogenation of benzene and the experimental value.
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- 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
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Name the compound
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Name the compound
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Name the compound
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Name the compound
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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
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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