Chapter 25 - Aromatic Compounds Flashcards
What does the Kekule model suggest?
It suggests that benzene was based on a six members ring of carbon atoms joined by alternate single and double bonds
What evidence is there to disprove the kekule model? (Lack of reactivity)
If it contains a C=C double bond it should decolourise bromine and undergo electrophilic addition but benzene does not do either
What evidence is there to disprove the kekule model? (Length of C-C bonds)
When finding the length of the bonds by X-ray diffraction all it’s bonds were between the length of a double and a single.
What evidence is there to disprove the kekule model? (Hydrogenation enthalpies)
If the structure was cyclohexa-1,3,5-triene (kekule) it would have a hydrogenation enthalpy 3 times that of cyclohexene (which is -120kjmol) but it is -208kjmol.
Describe the structure of benzene
6 carbon atoms and 6 hydrogen atoms - hexagonal.
Therefore each carbon only used three of four electrons in bonding.
Each carbon has one electron in a p orbital and adjacent p orbitals overlap sideways and form a ring of electrons density.
This creates a pi-system which spreads over all 6 carbons.
The 6 electrons in the system are said to be delocalised.
How do you name aromatic compounds (with benzene ring)?
Halogens and nitro groups are prefixes
When a benzene ring is attached to an alkyl chain of 7 or more or an alkyl chain with functional group the prefix phenyl is used (benzene is seen as a substituent)
Ring is numbered using lowest combination of numbers
What are the conditions needed for nitration of benzene?
50 Celsius and a H2SO4 catalyst
- If above 50 Celsius further substitutions may take place
Reaction mechanism of the nitration of benzene
1) electrophile NO2+ is produced by reaction of conc nitric acid and sulfuric acid
2) electrophile accepts pair of electrons from benzene ring.
3) dative covalent bond formed
4) Pair of electrons transferred from carbon hydrogen bond to positive charge forming pi system.
5) Breaks down into nitrobenzene and H+ ions. H+ ions react with HSO4- to regenerate catalyst.
What are the two halogen carriers?
Al and Fe
Describe the bromination of benzene
1) bromonium Ion formed when halogen carrier reacts with bromine
2) Bromonium ion accepts a pair of electrons from benzene ring. Dative covalent bond formed.
3) Pair of electrons transferred from carbon hydrogen bond to positive charge reforming pi system.
4) Breaks down into bromobenzene and H+ ion. H+ ion reacts with FeBr4- ion to regenerate FeBr3 catalyst
Describe the alkylation reaction of benzene.
Alkyl group replaces hydrogen atoms in the presence of AlCl3
This reacts with the group e.g C2H5 + AlCl3 -> C2H5Cl
Reactivity of cyclohexene
Pi bond in cyclohexene contains localised electrons (between the bonding C atoms) above and below the plane. This high electron density induces dipole in non-polar bromine and the slightly positive side makes it act like a good electrophile
Reactivity of benzene
Has delocalised pi electrons above and below the plane of the carbon atoms. It’s electrons are delocalised so the electron density is less than a double bond and insufficient to polarise bromine. No reaction can take place unless a halogen carrier is present.
Solubility of phenol vs alcohol
Phenol is less soluble in water than alcohols due to non-polar benzene ring but it does dissociate in water.
Compare the reactivity of phenols, carboxylic acids and ethanol.
Phenol is more acidic than alcohols but less acidic than carboxylic acids.
Phenols and carboxylic acids react with solutions of strong bases (NaOH)
Ethanol doesn’t react with strong or weak bases
Carboxylic acids are strong enough to react with weak bases (NaCO3)
