Aromatic Compounds and Carbonyls Flashcards
What are the main features of the delocalised model of benzene?
- Each carbon atom uses three of its 4 available electrons to bond to two other carbons and one hydrogen.
- The fourth electron one each carbon occupies a p-orbital at right angles to the plane of the ring.
- Adjacent p-orbitals overlap sideways, in both directions, above and below the plane of the ring, to form a ring of electron density which spreads across all 6 carbon atoms.
- The six electrons in this π system are delocalised.
How do the bond lengths in the benzene ring disprove Kekule’s model?
Single C-C bonds (0.154 nm) are longer than C=C double bonds (0.133 nm). However, all the C-C bonds in benzene are the same length (0.139 nm), suggesting that there are no alternating single and double bonds.
How does the hydrogenation enthalpy of benzene disprove the Kekule structure?
If benzene did have 3 double bonds, its enthalpy of hydrogenation should be 3x greater than that of cyclohexene which has only one double bond, and has a hyrogenation enthalpy of -120 kJmol-1. The actual hydrogenation enthalpy of benzene is -208 kJmol-1, 152 kJmol-1 less exothermic than it should be. Therefore, the actual structure of benzene is more stable than is suggested by the Kekule model.
How are aromatic compunds named?
- Alkyl groups with <7 carbons, halogens & nitro (NO2) groups are all used as prefixes.
- When the ring is attached to an alkyl group with 7 or more carbons or to an alkyl group with a functional group, it is considered a substituent and the prefix phenyl is used.
What happens when benzene or a derivative undergoes a substitution reaction and what is the typical mechanism?
- A hydrogen atom on the benzene ring is replaced by another atom/group.
- Electrophilic substitution.
What are the conditions and reagents in the nitration of benzene?
- Conc. sulphuric acid catalyst, heat to 50ºC (no higher because further substitution can occur, creating explosive products)
- Benzene + conc. nitric acid → nitrobenzene + H2O
What is the electrophile in the nitration of benzene?
NO2+ formed from the reaction of nitric acid and sulphuric acid.
What is the mechanism for nitrogenation of benzene?
What is the electrophile in the halogenation of benzene and how is it made?
- X+
- Reaction of pure halogen with a halogen carrier, to form X+ & MX4-
What is the mechanism for halogenation of benzene?
How is alkylation carried out?
Reacting benzene with a haloalkane in the precense of a halogen carrier, which generates the electrophile.
What is the mechanism for alkylation?
What is formed in an acylation reaction?
An acyl chloride reacts with benzene in the presence of AlCl3 catalyst to form an aromatic ketone where the oxygen is always bonded to the carbon joined to the ring. Hydrochloric acid is also formed.
What is the mechanism for an acylation reaction?
Why is benzene less reactive with halogens than alkenes?
- The double bond in an alkene has a π-bond w/ localised electrons above and below the σ-bond.
- Benzene has a system of delocalised π electrons, therefore the electron density around each σ-bond is lower.
- The double bonds have a high enough electron denisty to polarise halogen molecules whereas benzene requires a halogen carrier to generate electrophiles.
- Alkenes can do electrophilic addition by themselves but benzene requires a catalyst to do electrophilic substitution.
What is the evidence for the weak acidity of phenol?
- Phenol is more acidic than normal alcohols, but less acidic than carboxylic acids.
- Carboxylic acids & phenol react with solutions of strong bases such as NaOH, whereas ethanoldoesn’t react with strong or weak bases.
- Phenol can’t react with a weak base like sodium carbonate however carboxylic acids can (used as a test to distinguish phenols and carboxylic acids: CO2 produced if carboxylic acid).
How soluble is phenol in water and what does it form when it does dissolve?
- Less soluble than normal alcohols due to the presence of the non-polar benzene ring.
- Partially dissociates into a phenoxide ion and a proton (H+).
Describe the reaction of phenol with sodium hydroxide.
Neutralisation reaction: sodium phenoxide and water formed.
Why does phenol undergo electrophilic substitution more readily than benzene?
- Oxygen donates lone pair of electrons from p-orbital into the π-system, increasing the electron density of the benzene ring.
- Phenol therefore attracts electrophiles more strongly than benzene therefore the aromatic ring is more susceptible to attack from electrophiles.
Describe the reaction of phenol with bromine water.
- White ppt. of 2,4,6-tribromophenol formed (+ 3HBr), decolourising the bromine water.
- Reaction is carried out at room temp.
What is a carboxylic acid derivative?
A compound that can be hydrolysed to form the parent carboxylic acid. They all include an acyl group attached to an electronegative atom/group of atoms (to make hydrolysis at that point possible):
Draw the mechanism for acid hydrolysis.
Draw the mechanism for alkaline hydrolysis.
Describe the reaction between acyl chlorides and ammonia/amines.
- Ammonia/amines can act as nucleophiles by donating the lone pair of electrons on the nitrogen atom
- Acyl chloride + ammonia → primary amide + ammonium chloride
- In a primary amide, the nitrogen atom is attached to only one carbon.
- Acyl chloride + primary amine → secondary amide + alkylammonium chloride
- Primary amine: R-NH2
