Aromatic compounds Flashcards
Write molecular formula for benzene
C6H6
Describe the Kekule model for benzene
- 6 membered ring of carbon atoms joined by alternative single and double bonds.
Evidence against the Kekule model for benzene
- Lack of reactivity of benzene- if it contains C=C bonds it should decolourise bromine in an electrophilic addition reaction. BUT it doesn’t undergo electorphilic addition reactions and does not decolourise bromine.
- Lengths of carbon-carbon bonds- Using X-ray diffraction it was found that the bond lengths of benzene are all equal and between the bond length of a single and double C-C bond. Kekule’s structure would have 3 long single bonds and 3 shorter double bonds.
- Benzene has a less exothermic enthalpy change of hydrogenation than expected- much more stable than Kekule’s model suggested.
Descrie the delocalised model of benzene
- Benzene is a planar, cyclic, regular, hexagonal hydrocarbon containing 6 carbon atoms and 6 hydrogen atoms- molecular formula of C6H6
- Each carbon atoms uses 3 of its available four electrons in forming 2 sigma bonds with 2 other carbons and a sigma bond with 1 hydrogen atom.
- Each carbon atom has one electron in a p-orbital perpendicular to the plane of the molecule.
- Adjacent p-orbital electrons overlap sideways, in both directions above and below the plane of the carbon atoms to form a ring of electron density.
- This overlapping of p-orbitals creates a system of pi-bonds which spread over all 6 carbon atoms in the ring structure.
- The 6 electrons occupying this system of pi-bonds are said to be delocalised.
Describe how to name simple benzene molecules
Methylbenzene, Chlorobenzene, Nitrobenzene, Benzoic acid
- Methyl benzene- CH3 group is attached to benzene
- Chlorobenzene- Cl is attached to benzene
- Nitrobenzene- NO2 group is attached to benzene
- Benzenecarboxylic (benzoic) acid- COOH group attached to benzene
What would 2-hydroxybenzenecarboxylic acid look like
- benzene ring with COOH attached
2. OH group on carbon after the carbon with the COOH attached
What would 3,5-dinitrobenzenecarboxylic acid look like
- Benzene ring with COOH group attached
2. NO2 on the 3rd carbon and 5th carbon counting from the COOH group.
What is the name for a C6H5 group and when is it used.
- Phenyl group
- Used when the benzene ring is regarded as a substituent of a larger molecule.
- e.g phenylethene- ethene with a benzene ring attached to the 1st carbon
Draw the structures for phenyl amine, phenyl ethanoate and methyl benzenecarboxylate
- Phenyl amine- NH2 group attached to benzene
2. Phenyl ethanoate- COOCH3 group attached to benzene
What is phenol
- Benzene ring with OH group attached
Write the balanced equation, state any reaction conditions and name the type of reaction:
Benzene with chlorine
- Halogenation of benzene
- Benzene (hexagon with circle) + Cl2 → Chlorobenzene + HCl
- Anhydrous AlCl3 needed
When is benzene considered to be a subsituent
- When a benzene ring is attached to an alkyl chain with a functional group or to an alkyl chain with seven or more carbon atoms.
- Gets the prefix phenyl
What is the general formula for the substitution reaction of benzene and name the type of substitution
- Electrophilic substitution
- Benzene + E+ → Benzene( attached to E) + H+
- The benzene is reacting with an electrophile which replaces a hydrogen atom on the benzene ring.
Describe the nitration of benzene including conditions and balanced equations
- Benzene reacts with nitric acid to form nitrobenzene
- Reaction is catalysed by concentration sulfuric acid and heated to 50 degrees to obtain a good rate of reaction.
- Benzene + HNO3 → Nitrobenzene + H2O
How can 1,3-dinitrobenzene be produced
- React benzene with excess nitric acid
2. Temperature above 50 degrees so further substitution can happen.
Draw the mechanism for the nitration of benzene and state the electrophile
- Electrophile is nitronium ion NO2 +
- Step 1- Electrophile is produced:
HNO3 + H2SO4 →NO2+ + HSO4- + H2O - Step 2- Electrophile accepts a pair of electrons from the benzene ring to form a dative covalent bond. (Curly arrow from benzene circle to NO2+ )
- The organic intermediate formed (Benzene with smiley face and + in the middle)i s unstable and breaks down to form nitrobenzene and the H+ ion (Curly arrow from bond connecting H and Benzene to + in middle of benzene)
- Step 3- H+ + HSO4- → H2SO4
Describe the halogenation of benzene
- Halogens don’t react unless there is a catalyst called a halogen carrier present- AlCl3, FeCl3, AlBr3, FeBr3- generated in situ.
- React at room temperature and pressure.
- One of the hydrogen atoms in benzene is replaced by a halogen atom.
- Benzene + X2 → Xbenzene + HX
Draw the mechanism for the reaction of benzene with Bromine
- Electrophile- Bromonium ion Br+ (generated when the halogen carrier catalyst reacts with Bromine)
- Step 1- Formation of electrophile- Br2 + FeBr3 → FeBr4- + Br+
- Step 2- Bromonium ion accepts a pair of electrons from the benzene ring to form a dative covalent bond (Curly arrow from benzene ring to Br+)
- The organic intermediate formed (Benzene with smiley face and + in middle attached to H and Br) is unstable and breaks down to form the organic product bromobenzene and H+ (Curly arrow from bond connecting H and Benzene to + in middle of benzene)
- Step 3- Regeneration of catalyst- H+ + FeBr4- → FeBr3 + HBr
Describe Alkylation reactions with benzene and give the equation to form ethylbenzene
- Substitution of a hydrogen atom in the benzene ring with an alkyl group.
- The reaction is carried out by reacting benzene with a haloalkane in the presence of AlCl3 which acts as a halogen carrier catalyst- generating the electrophile
- Benzene + C2H5Cl → Ethylbenzene + HCl
Describe the Acylation reaction with benzene and give the equation between benzene and ethanoyl chloride
- When benzene reacts with an acyl chloride in the presence of AlCl3 catalyst an aromatic ketone is formed.
- Benzene + ethanoyl chloride (CH3COCl) → Phenylethanone + HCl
What is the difference between phenols and alcohols
- If the OH group is bonded directly to the benzene then it is a phenol
- If the OH group is bonded to a carbon side chain rather than the aromatic ring it is a alcohol
- The alcohols and phenols have some common reactions
- But many reactions are different as the proximity of the delocalised ring influences the OH group
Describe what happens when phenol is dissolved in water.
- Phenol is less soluble in water (described as slightly soluble) than alcohols due to the presence of the non-polar benzene ring.
- When dissolved in water phenol dissociates forming the phenoxide ion and a proton
- Because it can partially dissociate it is classified as a weak acid.
- Phenol ↔ Phenoxide ion (Benzene attached to O-) + H+
Compare the reactivity of phenol with alcohols and carboxylic acids
- Phenol is more acidic than alcohols but less than carboxylic acids- can be seen by comparing Ka values
- Ethanol does not react with sodium hydroxide (strong base) or sodium carbonate (weak base)
- Phenols and Carboxylic acids react with Sodium hydroxides (strong bases) Phenol dissolves.
- Only Carboxylic acids are strong enough acids to react with sodium carbonate (weak bases) Carboxylic acid produces CO2
- Sodium Carbonate can be used to distinguish between the phenol and Carboxylic acid.`
Describe the reaction and write the equation for phenol with NaOH
- Phenol reacts with sodium hydroxide solution to give a colourless solution containing the salt sodium phenoxide and water.
- Phenol + NaOH → sodium phenoxide (benzene attached to O-Na+) + H2O