2023_JC2_arenes lecture_notes_tutors and student

Cyclohexane

Anhydrous FeBr3 catalyst is used in the bromination of benzene.

30°C

The resonance energy of benzene is 151 kJ mol –1.

Concentrated HNO3 and concentrated H2SO4 catalyst

60°C

The carbon atoms in the delocalized cloud of pi electrons in methylbenzene are sp2 hybridized.

Fe can be used as a catalyst in halogenation because it is converted to iron(III) halide in situ by reaction with the halogen used as the reagent.

Free Radical Substitution is a reaction where a hydrogen atom in an alkane is replaced by a halogen atom, typically initiated by UV light.

Room temperature

FeCl3 acts as a Lewis acid by accepting a lone pair of electrons from the halogen, thereby inducing polarity in the halogen and eventually generating a strong electrophile, Cl+.

It contains only one tetrahedrally-bonded carbon atom.

The product is a carboxylic acid, specifically 2,2-dimethylpropanoic acid, formed through the oxidation of the alkene group.

A strong electrophile, Cl+, is generated.

Arenes burn with a smoky and luminous flame due to their relatively high carbon content, with a carbon to hydrogen ratio close to one.

A 2,4-directing group is a substituent on a benzene ring that directs incoming electrophiles to the ortho (2) and para (4) positions relative to itself.

The nitronium ion (NO2+) attacks benzene to form a resonance-stabilized carbonium ion.

Carbon atoms in the ring are numbered 1 to 6, beginning with the carbon that is bonded to the first group and continuing in such a direction as to lead to the lowest numbers for the remaining ones.

The oxidation of CH3 using KMnO4 (aq) and H2SO4 (aq) with heat produces CO2 and H2O.

The salt of carboxylic acid must be first acidified before solid benzoic acid can be obtained.

Intermediate (I) is a resonance-stabilized carbonium ion and is a resonance hybrid of structures II, III, and IV.

Excessively high temperatures (100°C or above) will lead to the formation of di-substituted or tri-substituted products.

The propagation step involves the reaction of C6H5CH3 with a Cl radical to form C6H5CH2 and HCl, followed by the reaction of C6H5CH2 with Cl2 to form C6H5CH2Cl and another Cl radical.

The substituent group controls the position of entry of the electrophile, with positions 2, 3, and 4 corresponding to ortho, meta, and para positions relative to the substituent.

The halogenated products of benzene side-chain substitution reactions can be hydrolyzed using aqueous NaOH and heat to give alcohols or carbonyl compounds.

The strong electrophile generated in the nitration reaction is NO2+ (nitronium ion).

HSO4- abstracts H+ from the carbonium ion to yield the substitution product.

2-bromophenol

Reagents: Cl2, anhydrous FeCl3 catalyst. Condition: room temperature.

Benzene undergoes substitution rather than addition because addition would destroy the resonance-stabilized structure.

The reaction needs to be heated at 60°C under reflux.

Halogenation is a substitution reaction where benzene reacts with a halogen (Br2 or Cl2) in the presence of a halogen carrier catalyst (e.g., anhydrous AlBr3/AlCl3, FeBr3/FeCl3) at room temperature, producing a halobenzene and hydrogen halide (HX).

Cyclohexane.

A σ bond is formed by the overlap of an sp3 hybrid orbital of C1 with a 1s orbital of H in methylbenzene.

The phenyl group is a hydrogen atom removed from benzene, represented as –C6H5.

The termination step involves the recombination of any two free radicals, such as 2 C6H5CH2 radicals forming C6H5CH2CH2C6H5.

Arenes are liquids or low melting point solids with characteristic 'aromatic' odours. Their vapours are toxic and inhalation should be avoided.

The strong electrophile generated in the alkylation reaction is CH3+.

Evidence includes benzene having the same carbon-carbon bond lengths of 0.139 nm, which are intermediate between C–C and C=C bonds, and a lower heat of hydrogenation than expected.

1,2-dimethylbenzene

UV light provides the energy needed to break the bond in the halogen molecule, forming free radicals that initiate the substitution reaction.

Benzene evolves 151 kJ mol -1 less energy than predicted.

The product is CO2H OCH3 + CH3 CO2H.

The typical reaction of benzene is electrophilic substitution.

The main product is bromomethylbenzene, formed through the substitution of a hydrogen atom on the methylbenzene ring with a bromine atom.

Benzene can only be attacked by strong electrophiles.

Hydrogenation is the reaction where benzene reacts with hydrogen in the presence of a nickel catalyst at 150°C to form methylcyclohexane.

Further substitution can produce C6H5CHCl2 and C6H5CCl3.

Excess benzene is used in Friedel-Crafts Alkylation to prevent further alkylation, which occurs even more readily.

H2SO4 acts as a Bronsted–Lowry acid by donating a proton, which leads to the generation of the strong electrophile, NO2+.

Nitrobenzene undergoes further substitution to form 1,3-dinitrobenzene because the -NO2 group is 3-directing and deactivating.

Chlorobenzene

2,6-dinitromethylbenzene

Reagents: RX, anhydrous AlCl3 catalyst. Condition: Room temperature, excess methylbenzene.

The purpose of the condenser is to prevent the escape of volatile organic compounds through vaporization, thus increasing the yield of the product obtained.

The delocalisation of π electrons renders the benzene ring less nucleophilic compared to the C=C bond in alkenes.

The final step is the formation of the substitution product and the regeneration of the catalyst, FeCl3.

The initiation step involves the homolytic cleavage of Cl2 under uv light to form two Cl radicals.

In the second step, the electrophile (Cl+) reacts with benzene to form a chlorobenzene and a proton (H+).

Benzene can be prepared by passing phenol vapour over heated zinc powder, which reduces phenol to benzene and forms ZnO.

An electron-withdrawing inductive effect decreases the electron density of the benzene ring, making it less reactive towards electrophilic substitution reactions.

Benzene undergoes substitution instead of addition to preserve its resonance stabilized ring system.

AlCl3 acts as a catalyst in the alkylation reaction, facilitating the formation of the electrophile.

The first step is the generation of a strong electrophile with the use of acid catalysts.

A weak oxidizing agent in side-chain oxidation reactions can be CrO2Cl2 or MnO2 with 65% H2SO4.

AlCl3 acts as a Lewis acid by accepting a lone pair of electrons from Cl in CH3Cl, thereby generating a strong electrophile, CH3+.

An activating group is a substituent that increases the electron density of the benzene ring, making it more reactive towards electrophilic substitution reactions.

A deactivating group is a substituent that decreases the electron density of the benzene ring, making it less reactive towards electrophilic substitution reactions.

The heat of hydrogenation is the heat evolved when 1 mole of an unsaturated compound is hydrogenated. Benzene has a lower magnitude of heat of hydrogenation than expected.

Nitrobenzene

Benzoic acid

Potassium dichromate (VI), K2Cr2O7, is incapable of effecting side-chain oxidation.

In intermediate (I), the positive charge is delocalized over 5 carbon atoms, forming a resonance-stabilized carbonium ion.

Nitration is the reaction where benzene reacts with concentrated HNO3 and concentrated H2SO4 at 30°C to form nitrobenzene.

In the nomenclature of monosubstituted benzene compounds, benzene is regarded as a substituent and the compound name will have the prefix 'phenyl'.

The molecular formula of benzene is C6H6.

No, the extreme resonance forms (I) and (II) of benzene do not exist.

-CH3 group in methylbenzene is 2,4-directing and activating, meaning it directs incoming substituents to the 2nd and 4th positions on the benzene ring and makes the ring more reactive.

1,4-dimethylbenzene can be oxidized to benzene-1,4-dicarboxylic acid.

Deuterium, D, is a heavy isotope of hydrogen.

A σ bond is formed by the overlap of an sp2 hybrid orbital of C2 with an sp2 hybrid orbital of C3 in methylbenzene.

A substituent group that makes the ring more reactive than benzene is an activating group, while one that makes the ring less reactive is a deactivating group.

Strong oxidizing agents like KMnO4 (aq) / H2SO4 (aq) can oxidize longer side-chains with a benzylic hydrogen (i.e., H attached to C adjacent to the benzene ring).

The product formed is methylbenzene.

An electron-donating inductive effect increases the electron density of the benzene ring, making it more reactive towards electrophilic substitution reactions.

Electrophilic substitution

Anhydrous FeX3 or AlX3 catalyst

The reagents used are concentrated nitric acid with concentrated sulfuric acid as a catalyst.

Nitrobenzene is a pale yellow liquid with an almond smell.

Benzene is a colourless liquid with a boiling point of 80°C and a melting point of 5.5°C. Continued inhalation can induce anaemia and even leukaemia.

The extreme resonance forms of benzene are theoretical structures with localized pi electrons, often represented as forms (I) and (II).

H2SO4 is considered a catalyst because it is regenerated at the end of the reaction.

Bromobenzene

Methylbenzene

3-bromo-5-chloronitrobenzene

Addition reactions result in the destruction of the ring system in benzene.

Prolonged nitration of benzene leads to the formation of trinitrotoluene (TNT), also known as 2, 4, 6-trinitromethylbenzene.

The reagents for the side-chain substitution reaction of benzene are Cl2 or Br2, and the conditions include UV light.

Arenes are insoluble and less dense than water but are soluble in organic solvents. Benzene and methylbenzene are useful solvents themselves.

A resonance hybrid in benzene is the actual structure, which is a combination of the extreme resonance forms, with pi electrons delocalized over the whole ring.

The circle inside the ring of benzene represents the delocalised electrons.

Methylbenzene undergoes further alkylation to form 1,2-dimethylbenzene because the -CH3 group is activating and 2,4-directing.

Phenol

The first step is the formation of the electrophile: Cl-Cl + AlCl3 → Cl+ + AlCl4-.

Under prolonged conditions and in the presence of a higher proportion of the halogen, 1,2-dihalide and 1,4-dihalide are formed.

Friedel-Crafts Alkylation is a reaction where benzene reacts with an alkyl halide (RCl or RBr) in the presence of a catalyst such as anhydrous AlCl3/AlBr3 or BF3 at room temperature, producing alkylbenzene and hydrogen halide (HX).

A strong oxidizing agent in side-chain oxidation reactions is KMnO4 (aq) / H2SO4 (aq) heated under reflux.

The product of oxidizing CH2CH3 with KMnO4 (aq) / H2SO4 (aq) under reflux is CO2H.

Methylbenzene is a colourless liquid with a boiling point of 111°C. Its fumes are considerably less toxic than those of benzene.

Benzene can be prepared by the fusion of sodium salt of benzoic acid with soda-lime, resulting in the release of CO2 and formation of Na2CO3.

A 3-directing group is a substituent on a benzene ring that directs incoming electrophiles to the meta (3) position relative to itself.

The carbon-carbon bond length in benzene is 0.139 nm.

3-chlorophenylamine

The boiling point of arenes increases with an increase in relative molecular mass due to a higher number of electrons, leading to stronger induced dipole–instantaneous dipole attractions.

Resonance occurs when p orbitals overlap extensively, resulting in the delocalisation of pi electrons over 3 or more atoms, leading to added stability.

The carbon-carbon bond length in benzene is intermediate between the C–C bond length of 0.154 nm and the C=C bond length of 0.132 nm.

Benzaldehyde