Arenes Chemistry

Arenes

• Arenes chemistry is exemplified by reactions of benzene and methyl benzene.
• Benzene structure can be described, and different benzene compounds named.
• The mechanism of electrophilic substitution in arenes can be described.
• Halogenation in arenes can be predicted to occur either in the side-chain or in the aromatic ring, depending on reaction conditions.
• In the electrophilic substitution of arenes, different substituents direct to different ring positions.

Definitions

• Aromatic hydrocarbons contain sigma bonds and delocalized pi electrons between carbon atoms in a ring.
• Electrophilic substitution is a chemical reaction where a functional group attached to a compound is replaced by an electrophile.
• The Friedel-Crafts reaction is an organic coupling reaction where substituents attach to aromatic rings through electrophilic aromatic substitution.
• Alkylation and acylation reactions are the two primary types of Friedel-Crafts reactions.
• Nitration is electrophilic substitution where a hydrogen atom is replaced by a nitro group (-NO2).

Benzene Structure

• Benzene structure consists of six carbon atoms arranged in a regular hexagon, joined to a hydrogen atom and to neighboring carbons by σ bonds.
• Carbons in benzene form 3 bonds instead of 4.
• P orbitals form sp2 hybrid bonds.
• Remaining p-orbitals are non-hybridized and have an unpaired electron, forming part of the central delocalized electron cloud.
• Six spare p orbitals, one on each carbon atom, are parallel and perpendicular to the ring's plane.
• Each carbon has a 120° bond angle and a trigonal planar shape.
• Carbon-carbon bonds are equal in length and strength.
• Six delocalized electrons at the center create a planar, regular hexagonal shape and prevent normal addition reactions.
• The π bond also makes benzene more stable.

Aromatic compounds

• Aromatic compounds have rings of delocalized electrons.
• Arenes consist of aromatic compounds like: Benzene, methylbenzene, and naphthalene.
• Benzene is the simplest arene.

Naming Arenes

• When naming arenes, the ring is numbered so you can indicate the positions of substituents on the benzene ring.
• The lowest order of priority ensures that the lowest number is achieved.
• Priority of functional groups: CO2H > OH > NH2 > Alkyl > Halogen > Aldehyde > NO2.
• If the benzene ring is a substituent, it is named phenyl.

Physical Properties

• Non-polar
• Liquid at room temperature due to strong intermolecular forces
• Colourless liquid with a characteristic odor
• Insoluble in polar solvents but soluble in polar solvents
• Boiling point is similar to equivalent cycloalkanes
• Poor conductor as non-polar

Reactivity

• Benzene's π bond has high electron density, which attracts electrophiles.
• Benzene requires very powerful electrophiles because of its stability, due to delocalized electrons.
• Bromine water and aqueous acid have no effect on benzene.
• Alkenes react by electrophilic addition, while arenes react by electrophilic substitution.
• In benzene, the carbocation intermediate loses a proton to reform the ring of π electrons after an electrophile attacks.

Reactions

• Combustion
• Addition
• Electrophilic substitution

Combustion

• Benzene and methylbenzene are components of unleaded petrol.
• They completely burn to carbon dioxide and steam in sufficient oxygen.
• C6H6 + 7.5O2 → 6CO2 + 3H2O
• Liquid arenes produce very smoky flames when lit, due to insufficient oxygen for complete combustion.
• A smoky flame indicates a compound with a high carbon:hydrogen ratio.
• C6H6 + 1.5O2 → 6C(s) + 3H2O
• The enthalpy of combustion of benzene is less exothermic than expected due to the stability of the six delocalized pi electrons.

Electrophilic Substitution

• A powerful electrophile is attracted to benzene's π bond, breaks the ring of electrons, and forms a σ bond to one of the carbon atoms.
• Two of the six π electrons are used to form the dative bond to the electrophile.
• The other four π electrons spread over the remaining five carbon atoms of the ring in a five-center delocalized orbital.
• The intermediate carbocation loses a proton, which reforms the sextet of π electrons.

Halogenation-bromination

• Benzene reacts with non-aqueous bromine on warming in the presence of anhydrous iron (III) bromide to form bromobenzene.
• The bromine electrophile is formed by reacting anhydrous iron (III) bromide with Br2.
• FeBr3 reacts with bromine by accepting a lone pair of electrons on bromine, causing strong polarization and weakening the bond, which leads to heterolytic breaking.
• Br - Br + FeBr3 → Br⁺ + [FeBr4]⁻
• The final stage regenerates the catalyst, by the reaction between the proton H⁺ formed with the [FeBr4]⁻: H⁺ + (FeBr4)⁻ → HBr + FeBr3

Halogenation-chlorination

• A similar electrophilic substitution reaction occurs when chlorine gas is bubbled through benzene at room temperature.
• A catalyst such as iron (III) chloride or aluminum chloride may be used.
• These catalysts are known as halogen carriers, e.g., FeBr3, AlCl3, and FeCl3.

Friedel-Crafts Alkylation

• When benzene is heated under reflux with chloroalkane in the presence of aluminum chloride, the alkyl group attaches to the benzene ring.

Friedel-Crafts Acylation

• A similar reaction occurs when benzene is refluxed with acyl chloride, ethanoyl chloride, and aluminum chloride as a catalyst.

Nitration

• When benzene reacts with a mix of concentrated nitric and sulphuric acid heated under reflux to around 50 °C, nitrobenzene forms.
• 2H2SO4 + HNO3 → 2HSO4⁻ + H3O⁺ + NO2⁺

Addition

• Benzene reacts with hydrogen gas and nickel catalyst to form cyclohexane at 200°C.

Reactions of Substituted Benzene Ring

• Substituted benzene rings undergo electrophilic substitution.
• Position and reaction rate are determined by the substituent's nature relative to unsubstituted benzene.
• Ortho and para substitutions occur at positions 2, 4, and 6.
• Meta substitutions occur at positions 3 and 5.
• Orientations depend on current substituent, not the electrophile.

2,4-Directing

• Groups that are either capable of donating electrons to the ring by the inductive effect, or they have a lone pair of electrons on the atom joined to the ring.
• This lone pair can incorporate into the π system by sideways overlap of p orbitals.

3-Directing

• Substituents favoring 3-substitution have a + atom joined directly to the ring.

Rate of Electrophilic Substitution

• Normally in 2,4-directing groups cause substitution faster than benzene, and are called activating groups (chlorine is an exception).
• 3- directing groups cause slower substitution than benzene, and are called deactivating groups.
• In electrophilic substitution, the rate-determining step is the attack of the electrophile on the ring.
• An activating group donates electrons into the ring, making it more negative and attracting the electrophile more strongly causing a faster reaction.
• A deactivating group withdraws electrons, the electrophile is less strongly attracted and the reaction occurs slowly.

Methyl Benzene (Toluene)

• Methyl benzene also reacts via electrophilic substitution, and is an activating group.
• Reaction conditions are milder than those of benzene's reactions.
• The methyl group donates electron density into the benzene ring, positive inductive effect.
• Increases electron density which increases attraction to electrophiles for a faster reaction.
• The methyl group is a 2,4- directing group.

Reactions

• Side chain reactions
  • Free radical substitution
  • Oxidation with aq KMnO4
• Ring reactions-electrophilic substitution
  • Halogenation
  • Alkylation
  • Acylation
  • Nitration

Side Chain Reactions

• Free Radical Substitution
  • When methyl benzene reacts with chlorine in UV light, side chain substitution occurs.
  • A hydrogen atom in the methyl group is substituted by a Cl atom.
  • The mechanism involved is similar to free radical substitution in alkanes.

Oxidation

• When alkylbenzenes react with hot acidified potassium manganate (IV), then oxidation of the whole side chain occurs.
• The carbon closest to the ring is left as a carboxylic acid group.

Ring Reactions

• Halogenation -- Methyl benzene reacts with chlorine and a halogen carrier catalyst AlCl3 (anhydrous) at room temperature.
  • The products are 2-chloromethylbenzene and 4-chloromethylbenzene.

Nitration

• Methylbenzene reacts faster than benzene in nitration: The reaction is about 25 times faster.
• Lower temperatures can prevent substitution of more than one nitro group (30°C rather than 50°C).
• Using the same nitrating mixture of concentrated sulphuric and nitric acids is a must.
• The main products are: 2-nitromethylbenzene and 4-nitromethylbenzene.
• Higher temperatures (100°C) cause trinitration producing 2,4,6-trinitramethyl benzene
• Nitro groups are directed toward 2nd, 4th and 6th positions.

Friedel-Crafts Acylation

• The Friedel-Crafts Acylation reaction is similar with methylbenzene reaction except the acyl group attaches to the ring relative to the methyl group.
• Normally, the methyl group in methylbenzene directs new groups into the 2- and 4- positions (assuming the methyl group is in the 1- position).
• In acylation virtually all the substitution takes place in the 4- position.

Points to Note

• Arenes are hydrocarbons that contain one or more benzene rings.
• The benzene ring contains a delocalized group of six p electrons, which confers great stability.
• Despite their unsaturation, arenes do not undergo the same addition reactions as alkenes
• The preferred reaction type is electrophilic substitution.
• Aromatic compounds can be halogenated either in the ring or in the side chain, depending on the conditions.
• Aromatic compounds with alkyl side chains can be oxidized by potassium manganate (VII) to benzenecarboxylic acids.
• Under strong conditions with hydrogen, the benzene ring can undergo addition rather than substitution.