Benzene & Aromaticity

Aromatic Hydrocarbons

• 3rd type of unsaturated hydrocarbon together with alkenes and alkynes.

• Do NOT undergo electrophilic addition.

• Considering the structure of benzene ring, the structures has a special type of bonding depicting both localized and delocalized bonds

C6H6

Benzene

Benzene

• Simplest aromatic hydrocarbon with a special type of bonding that stabilizes the structures

• A general aromatic hydrocarbon substituent is called “aryl group” (Ar)

For monosubstitution

• Benzene is used as parent name

• methylbenzene, chlorobenzene, hydroxybenzene

For disubstitution

-ortho, -meta, and -para are terms used to refer to the position of one of the substituents in reference to the position of the other

ortho (o-)

1,2-substitution on the ring

meta (m-)

1,3-substitution on the ring

para (p-)

1,4-substitution on the ring

For trisubstitution or more

• Numbers are assigned to refer to the position of the substituents in the ring.

• The lowest set of numbering system is used and substituents are arranged in alphabetical order.

Aromatic compounds

Represented by benzene (simplest) are cyclic, conjugated, stable, planar and undergoes substitution reaction.

Hückel’s Rule

Aromatic compounds follow this rule

Hückel’s Rule

Predicts that a compound which is planar, cyclic, has a conjugated double bonds will have total of 4n + 2 pi-electrons.

Polar Mechanism

Aromatic compound (benzene) reaction follow this which creates an electrophile (R+).

Electrophilic substitution reaction (SE)

When the reaction follows a polar mechanism that creates an electrophile.

Reaction Catalyst

This aids in the polarization of the reagents forming an electrophile product.

Pi bond in the benzene

This is cleaved as the electrons are attracted to accommodate the incoming electrophile.

Neighboring Carbon Cleves

This happens when —— the H attached to it, takes the electrons from the bond and delocalizes it back to the carbocation — restoring the benzene double bonds.

Common electrophilic substitution reaction (SE)

• Halogenation

• Nitration

• Sulfonation

• Friedel-Crafts Alkylation

• Friedel-Crafts Acylation

Halogenation

Substitution of halogens (X2) to a benzene ring.

Electrophile (+X)

Aromatic compound can be substituted with a polarized halogen which serves as an ——-

Catalyst

Aids in creating a polarized halogen (X+). This creates the electrophile to be added to the aromatic compound.

Nitration

• Substitution of a nitro group (+NO2) to a benzene ring

• Presence of Acid Catalyst; H₂SO₄

• Form nitro substituted ring product (ArNO₂)

• Reducing Agent Fe/SnCl₂

• Nitro substituted product is reduced into an arylamine (ArNH2) product

Sulfonation

• Substitution of +SO3H (sulfonic acid) to a benzene ring.

• Reagent & Catalyst; Mixture of SO₃ and H₂SO₄

• Sulfuric acid (H₂SO₄) reacts with sulfur trioxide (SO₃) forming the electrophile (+SO3H, sulfonic acid).

Friedel-Crafts Alkylation

• Substitution with +R (alkyl group) to a benzene ring.

• Done with alkyl halides (RX) with aluminum halide (AlX3) catalyst to produce the alkyl electrophile (R+, alkyl).

• Hydride shift / Alkyl shift may occur to form more stable product

Hydride/Methide shift

When a negatively-charged hydrogen (-H, hydride) or a negatively-charged alkyl, usually a methyl (-CH3, methide) is translocated to a nearby carbocation to form a more stable carbocation.

Friedel-Crafts Acylation

• Substitution with +COR (acyl group) to a benzene ring

• Done with acyl halides (RCOX) with aluminum halide (AlX3) catalyst to produce the alkyl electrophile (+COR, acyl)

• The RCOX that are commonly used are RCO-Cl and RCO-Br with AlCl3 and AlBr3 respectively

Does NOT follow the SE mechanism.

• Hydrogenation

• Bromination of alkyl side chain

• Oxidation of Alkyl side chains

• Reduction of Aryl Alkyl ketone

Hydrogenation

• Addition of hydrogen (H2) to a benzene ring to create a saturated product.

• Follows the same mechanism of alkene hydrogenation.

• It adds H2 atoms in the presence of Pt or Rh (Rhodium) metal catalyst under high pressure

Bromination

• Addition of bromine (Br2) to an alkyl side chain of a benzene ring

• Follows an SR mechanism similar to alkanes

• Occurs on the benzylic position with an alkylbenzene treated with N-bromosuccinimide (NBS) and benzoyl peroxide (Ph(CO2)2) acting as radical initiator.

Oxidation reaction

• Complete oxidation of alkyl side chain on a benzene ring

• The alkyl side chains (benzylic position) is rapidly affected by this, and converts to -COOH, regardless of the initial alkyl group attached to the ring.

• The alkyl side chain will only be oxidized if there is a benzylic hydrogen (H attached to the benzylic carbon) present in the structure

Reduction reaction

• The reduction oxygen bonds in a ketone (C=O) and nitro group (-NO2).

• Both aryl alkyl ketones (product of FC Acylation) and nitro substituted (product of nitration) can be reduced via a process called catalytic hydrogenation with a Pd as catalyst (H2 / Pd).

Substituents

Can affect SE reactions in 2 ways: reactivity and orientation

Reactivity

• Can be affected by any substitution.

• It can activate (increase reactivity) and deactivate (decrease reactivity) of the ring.

• Can affect whether subsequent reactions can still take place on the ring

Orientation

• Consequent substitutions attached on the ring are affected by pre-existing substituents.

• Substitutions can be directed at the ortho-,meta-, and para- positions of the ring.

Ortho-/Para-directing activators

Meta-directing deactivators