Chapter 25- aromatic compounds

aromatic compound

molecule that contains a stable ring of atoms with delocalised electrons, often based on a benzene ring 

Explain why the kekule structure is no longer accepted

• x-rays were used to determine the carbon-carbon bond lengths (if kekule was correct double bond would have shorter angles) but they are all the same in benzene

• Benzene does not undergo addition reactions/requires a catalyst for Subsitution reactions

• Enthalpy change of hydrogenation is less (exothermic) for benzene than kekule model

When does benzene become phenyl

when attached to a alkyl chain with a functional group, or an alkyl chain with 7 or more carbons

exceptions for benzene becoming a phenyl

Benzoic acid, phenylamine, benzaldehyde 

Molecular formula for benzene

C₆H₆

Bonding of benzene

Planar, cyclic hexagonal hydrocarbon

Delocalised model of benzene 

adjacent p orbitals overlap sideways, in both directions above and below the plane of C atoms to form a ring of electron density above and below the ring            

Nitration of benzene reagant and conditions

reagent: sulfuric acid

Conditions: must not go above 50 degrees else further Subsitution occurs

Nitration of benzene mechanism

step 1: formation of electrophille ( NO₂⁺)

Step 2: nitrate ion replaces replaces H and forms H+

halogenation of benzene reagent and conditions

reagent: halogen carrier (AlBr₃ or AlCl₃ or

alkylation of benzene reagent and conditions 

reagent:AlCl₃ halogen carier

Conditions: room temp

alcylation of benzene reagent and conditions

reagent: AlCl₃

Conditions: room temp

Formation of NO₂⁺ electrophile

H₂SO₄ + HNO₃ → HSO₄^- + NO₂⁺ + H₂O

Regeneration of NO₂⁺ Catalyst

HSO₄^- + H⁺ → H₂SO₄

Formation of Br⁺ electrophile

Br₂ + FeBr₃→ Br⁺ +.

Regeneration of Br⁺ catalyst

AlCl₄^- + H+ → AlCl₃ + HCl

Phenol

benzene with a -OH group directly bonded

Is phenol a weak or strong acid

Weak

Delocalisation in phenols

• oxygen atom in -OH group donates lone pair of P-orbital electrons to delocalised pi system

• Increases the electron density around ring

• Attracts electrophile more strongly, more susceptible to attack

• Can induce dipole in bromine molecules

• H+ ion more readily lost tha

Phenol as a weak acid

Not as soluble as alcohols as benzene ring is polar and partially dissociates  but more acidic than alcohols but less acidic than carboxylic acids 

Phenol reacting with sodium hydroxide

Forms a salt which is much more soluble than phenol

bromination of phenol

reagent: bromine water

Condition: room temperature

Bromine water decolourises and white precipitate forms 

Nitration of phenol

Reagent: dilute HNO₃

Conditions: room temperature

Forms mixture of 2-nitrophenol and 4-nitrophenol

Alkylation of phenol

add a haloalkane

Does not require halogen carrier

Acyclation of phenol

esterfication 

Phenol reacts with acyl chloride (no catalyst needed) to make ester

Why is phenol more reactive than benzene

• Benzene electrons/pi bonds are delocalised

• O atom in -OH donates lone pair of p-orbital electrons to delocalised pi statement increase electron density around ring

• Makes phenol more attractive to electrophiles and more susecptible to attack than benzene which has lower electron density

• Phenols can lose H+ molecules and replace them with the Br₂ than benzene as phenols can induce dipoles in molecules

2,4- directing groups

further Subsitution of a benzene ring containing any of these groups wil primarily occur on the 2 or 4 positions

Examples of 2,4 directing groups

• OH

• NH₂

Activating groups

OH and NH₂ can activate benzene ring to make it react more readily with electrophiles this is because they are electron donating- benzene donates one pair of electrons to the delocalised pi system making ring more electron dense

3-directing groups

further Subsitution of a benzene ring containing any of these groups wil primarily occur at position 3

Examples of 3-directing groups

NO₂

Deactivating groups

NO₂ deactivates the benzene ring causing it to react more slowly with electrophiles- they are electron-withdrawing so they attract electron density away from the ring and electrophiles are less strongly attracted

Ortho (o)

two

Meta (m)

3

Para (p)

4