CD deg: Aromatic compounds & their reactions Flashcards
Describe and draw the Kekulé structure of benzene.
Six-membered ring of carbon atoms with alternating single + double bonds.
Name 2 characteristics of benzene which were accurately predicted by the Kekulé model.
- Bond angles 120o
- Planar
Name three issues with the Kekulé model of the structure of benzene.
- Unexpected unreactivity with bromine water
- Discrepancy between observed and expected bond lengths (shapes)
- Discrepancy between observed and expected enthalpies of hydrogenation
Explain why benene’s unreactivity with bromine water was not accounted for by the Kekulé model of its structure. State the conclusion that this issue pointed towards.
- Kekulé model contains 3 C=C bonds, so benzene was expected to react with + decolourise bromine water
- Unreactivity suggests either that benzene has no double bonds, or that something makes those present less reactive
Explain why benene’s shape was not accounted for by the Kekulé model of its structure. State the conclusion that this issue pointed towards.
- Kekulé structure contains C-C + C=C bonds. C=C bonds are shorter so bonds in benzene expected to be different lengths
- All bonds in benzene are same length, so Kekulé bonding is incorrect
Explain why benzene’s enthalpy of hydrogenation was not accounted for by the Kekulé model of its structure. State the conclusion that this issue pointed towards.
- Enthalpy of hydrogenation of cyclohexa-1,3-diene is approximately twice that of cyclohexene, since it has twice as many double bonds, so that of benzene was expected to be 3 x that of cyclohexene
- Enthalpy of hydrogenation of benzene is lower than that of cyclohexa-1,3-diene
- Benzene’s structure is stabler than suggested by Kekulé model
Describe and draw the delocalised model of benzene.
- Carbon p-orbitals perpendicular to ring overlap, so electrons delocalise
- Forms circular π-orbitals above and below ring, containing 6 electrons overall
Like a bagel sandwich. Each carbon has 4 valence e-: 1 goes to C-H bond, 2 go to C-C bonds, 1 left over. 6 spare e- delocalise
Explain how the issues with the Kekulé model of benzene’s structure were rectified by the delocalised model.
Unreactivity with bromine water: carbon p-orbitals perpendicular to ring overlap, forming circular π-orbitals above and below ring. Delocalisation makes benzene stable, so doesn’t readily react by addition
Discrepancy between observed + expected shape: all bonds are equal length due to delocalisation
Discrepancy between observed + expected enthalpies of hydrogenation: delocalisation makes benzene stable, so hydrogenation doesn’t make benzene much more stable, so is less exothermic than predicted
Name this molecule:
1-ethyl-4-methyl benzene
Minimisation of numbers takes priority over alphabetical order (in this case, it would be 1-4 either way, so is in alphabetical order)
Name the following molecules:
Chlorobenzene, nitrobenzene, benzoic acid, benzene sulfonic acid
-NO2 = nitro group, -SO3H = sulfonic acid group
Name this molecule:
2-phenylbut-2-ene
Phenyl prefix is used to represent benzene when suffix is occupied
Name this molecule:
Phenylamine
Phenyl prefix is used to represent benzene when suffix is occupied
Name the following:
- C6H5OOCCH3
- C6H5COOCH2CH(NH2)CH3
- Phenyl ethanoate (molecular formula was written backwards)
- 2-amino propylbenzoate
What is an arene?
A molecule containing a benzene ring.
All arenes are aromatic, but this isn’t reversible
What are the conditions required for aromaticity?
Molecule must:
- Be cyclic
- Be planar
- Contain 4n + 2 delocalised π-electrons (where n = non-zero integer)
Is this molecule aromatic?
Yes, because:
- Cyclic
- Planar
- 4n + 2 delocalised π-electrons, where n = 3
Is this molecule aromatic?
Yes, because:
- Cyclic
- Planar
- 4n + 2 delocalised π-electrons, where n = 1
- State the name of the mechanism by which aromatic compounds replace hydrogen atoms with other groups
- Draw a general mechanism to represent this
Electrophilic substitution
Initial attack of electrophile is slow + often unfavourable, so this step is catalysed
- Write the equation, using structural formulae, for the bromination of benzene
- State the conditions required
- Name the organic product
- FeBr3 (or form in situ from Fe), reflux
- Bromobenzene
Catalyst used to form Br+, which is a good electrophile
- Write an equation, using structural formulae, for the nitration of benzene
- State the conditions required
- Name the organic product
- c. HNO3 (reactant), c.H2SO4, below 55oC
- Nitrobenzene
At temperatures above 55oC, the 1,3-dinitro and 1,3,5-trinitro products form, which are highly explosive (TNT is trinitrotoluene, a.k.a. trinitro methylbenzene)
- Write the equation, using structural formulae, for the sulfonation of benzene
- State the conditions required
- Name the organic product
- H2SO4 (reactant), reflux
- Benzenesulfonic acid
- Describe how the electrophile in the sulfonation of benzene is formed
- Explain why it is a good electrophile
- Conc sulfuric acid dissociates: H2SO4 → SO3 + H2O
- 3 polar S=O bonds give central sulfur a large partial positive charge, making it susceptible to attack
Draw the mechanism for the sulfonation of benzene.
- Write an equation showing the formation of the electrophile during the reaction between 2-chloropropane and benzene.
- Draw the mechanism for the reaction between benzene and the electrophile.
- Write an equation showing the reformation of the catalyst.
- Name the product.
Product: methyl ethylbenzene / 2-phenylpropane
- Write an equation showing the formation of the electrophile during the reaction between 2-ethanoyl chloride and benzene.
- Draw the mechanism for the reaction between benzene and the electrophile.
- Write an equation showing the reformation of the catalyst.
- Name the product.
Product: 1-phenyl propanone
Fill in the table summarising the substitution reactions of benzene.
Fill in the mechanism summary; in each box:
- Describe briefly what the mechanism applies to
- Draw the general mechanism
