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
Why are phenols acidic even though alcohols have almost no acidic properties
- In phenol electrons in the O-H bond are drawn into the aromatic ring weakening the bond and making it easier for the H+ to be released
- Once the phenoxide ion is formed it is more stable as the electrons are dispersed across the pi cloud.
State what is observed when bromine water is added to an aqueous solution of phenol at room temperature
- Bromine water decolourises (orange to colourless) and a white precipitate
Write the equation for the reaction of Phenol with Bromine and state the type of reaction
- Electrophilic substitution
- Phenol + 3Br2 → 2,4,6-tribromophenol + 3HBr
- At room temperature and a halogen carrier catalyst is not required
Write the equation for the reaction of Phenol with Nitric acid and state the type of reaction
- Electrophilic substitution
- Phenol + HNO3 → 2-nitrophenol or 4-nitrophenol + H2O
- At room temperature and a halogen carrier catalyst is not required
Compare the reactivity of phenol and benzene
- Bromine and nitric acid react more readily with phenol than benzene
- Phenol is nitrated with dilute nitric acid rather than needing concentrated nitric acid and sulfuric acids as with benzene.
- The increased reactivity is caused by a lone pair of electrons form the oxygen p-orbital of the OH group being delocalised to the delocalised pi-system of electrons on the benzene ring.
- This increases the electron density of the benzene ring in the phenol- attracts electrophiles more strongly than benzene
- For bromine the electron density in the phenol ring structure is sufficient to polarise bromine molecules and so no halogen carrier catalyst is required.
Describe what is meant by activating and deactivating groups
- Activating groups- If the group activates the ring it means the aromatic ring reacts more readily with electrophiles.
- Deactivating groups- If the group deactivates the ring it means the aromatic ring reacts less readily with the electrophiles
List the 2,4 directing groups and state whether they are activating or not
All activating accept halogens
- NH2 or NHR
- OH
- OR
- R or C6H5
- F, Cl, Br, I
List the 3,5 directing groups and state whether they are activating or not
All deactivating
- NO2- main
- COOR
- SO3H
- CHO
- COOH
- CN
- NR3+
