Chap 15C - Arenes (Methylbenzene)

Describe menthylbenzene

• Prepared by Friedel-crafts alkylation of benzene 

• One methyl group bonded to benzene ring 

• Aka toluene 

• Structural formula: C6H5CH3

Describe effect of substituent groups on methylbenzene

• Any substituent group attached to the benzene ring will affect the reactivity of the benzene ring and determine the orientation of the substitution

Describe ways electrons can be donated into or withdrawn from the benzene ring

1. Resonance effect 

• Donation or withdrawal of electrons through the overlap of p orbital on the substituent with a p orbital on the C atom of benzene ring

2. Inductive effect 

• Donation or withdrawal of electrons through a sigma bond due to electronegativity difference between the atom

Describe electron-donating substituent on methylbenzene + examples

• Electron-donating group such as –NH2, –OH, –R (R = alkyl group) increases the electron density in the benzene ring

• The monosubstituted benzene is more reactive towards electrophilic substitution (benzene ring is activated) 

• Reaction conditions used for electrophilic substitution are milder

1. –OH, –OR, –NH2, –NHR, –NR2, –NHCOR (resonance effect)

• O and N atoms have a lone pair of electrons (usually in a p orbital) available for donation to the benzene ring

2. –alkyl (inductive effect) 

• Inductively donate electrons through the sigma bond linking the substituents to a benzene ring

Describe electron-withdrawing substituent on methylbenzene + examples

• Such as –Cl, –CHO, –CO2H, decreases the electron density in the benzene ring

• Monosubstituted benzene is less reactive towards electrophilic substitution (benzene ring is deactivated towards electrophilic substitution)

• Reaction conditions used for electrophilic substitution are harsher

1. –CHO, –COR, –CO2H, –CO2R, –NH3+ ,–NO2, –CN (resonance effect) 

• More electronegative N and O atoms are able to withdraw electrons from the benzene ring

2. –Cl, –Br, –I (inductive effect) 

• Inductively withdraw electrons through the sigma bond linking the substituents to a benzene ring

State what determine position of incoming electrophile

1. The substituent group on the benzene ring determines the position of the incoming electrophile after the reaction (NOT the incoming group) 

• G is either 2,4-directing and directs the electrophile onto positions 2 and 4 or is 3-directing and directs the electrophile onto position 3

State what determine position of incoming group E

1. The position of the incoming group E, is determined by the nature of the group G, already bonded to the ring, and NOT by the nature of the incoming group E (given in the Data Booklet)

• If multiple substituents are present, the more activating group will dominate the orientation of substitution

Describe nitrobenzene and methylbenzene and substituent groups

• Nitrobenzene undergoes further substitution to form 1,3-dinitrobenzene and 1,3,5-trinitrobenzene at higher temperatures as –NO2 group is 3–directing and deactivating

• Further alkylation of methylbenzene occurs more readily to give 1,2-dimethylbenzene and 1,4-dimethylbenzene as –CH3 group is 2,4–directing and activating

• Major product: If 1.2-disubstituted and 1,4-disubstituted products are the major products, substituent G stabilises the 1,2-disubstituted and 1,4-disubstituted intermediate cations to a greater extent over the 1,3-disubstituted intermediate

Explain why halogens are electron-withdrawing but 2.4-directing

• Halogen atoms are electronegative -> electron-withdrawing such that they deactivate the benzene ring towards electrophilic substitution.

• BUT: the halogen atom directly bonded to the benzene ring also has a lone pair of electrons which enables them to stabilise the positively-charged intermediate formed during the electrophilic substitution reaction -> This makes halogen atoms electron-withdrawing but 2,4-directing

Describe halogenation + general reaction

• Reagents and conditions: Cl2, anhydrous AlCl3 catalyst (or FeCl3 catalyst), excess methylbenzene

• Observations: Similar to benzene

Draw halogenation mechanism

Describe nitration reagents and conditions

• Reagents and conditions: concentrated HNO3, concentrated H2SO4 catalyst, 30C

• The temperature of reaction is lower than that for benzene as –CH3 is activating

Prolonged nitration at higher temperature leads to formation of the common explosive, trinitrotoluene, TNT (or 2,4,6-trinitromethylbenzene)

Describe general reaction of nitration

Describe friedel-crafts alkylation + general reaction

Reagents and conditions: RCl, anhydrous AlCl3 catalyst (or FeCl3 catalyst), excess methylbenzene

Draw mechanism for friedel crafts alkylation