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chem 104 Lecture 2
electrophilic aromatic substitution
28
Chemistry
Undergraduate 1
02/22/2013

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Term
a reaction in which one group displaces another is known as?
Definition
substitution
Term
name the five major electrophilic substitutions of benzene?
Definition
  • halogenation
  • nitration
  • sulphonation
  • friedel-crafts alkylation
  • friedel-crafts acylation
Term
what is  a species (usually cationic) that can form a bond to a carbon by accepting an electron pair?
Definition
electrophile
Term
what is a species (anionic or neutral) that can form a bond to a carbon by donating an electron pair?
Definition
nucleophile
Term
name the reaction whereby an intial electrophile attack leads to an addition reaction.
Definition
electrophilic addition
Term
name the reaction whereby an electrophile attack leads in a sustitution reaction?
Definition
electrophilic substitution
Term
electrophilic aromatic substitution (EAS) is a how many step process?
Definition

3 step process

 

1. a postively charged electrophile generated

2. benzene ring attacks electrophile generating a resonance carbocation

3. the carbocation lose a h+ to give an aromatic product

 

Term

in generating the electrophile from Br-Br for EAS, how is the bond cleaved?

 

what facilitates this cleavage and how does it help?

Definition

heterolytic cleavage (uneven sharing) 

 

FeBr3 catalysts helps by acting as a Lewis base and accepting a bromide ion.

Term
in EAS an intermediate carbocation is formed. is this intermediate carbocation aromatic? over how many electrons is the charge delocalised over?
Definition

the intermediate carbocation is NOT aromatic

 

the charge of the carbocation is delocalised over THREE specific carbons of the ring

Term
what is the name of the delocalised non aromatic intermediate carbocation in EAS?
Definition

arenium ion

 

only FOUR electrons are in the delocalised pi system of an arenium ion

Term
what is the hybridisation of the arenium ion?
Definition
sp3 hybridised.
Term
from the electrophilic addition, resonance stabilised cation and the rearomatisation stage, which of these steps is the fastest?
Definition
the re-aromatisation stage
Term
what is required for halogenation of benzene?
Definition
a lewis acid catalyst - it facilitates the formation of an electrophile by acting as a halid anion acceptor
Term
what reagents are needed for nitration of benzene?
Definition
concentrated nitric and sulphuric acid
Term
what is the electrophile in nitration of benzene?
Definition

NO2+ nitronium ion

 

this is formed from nitric acid by PROTONATION followed by LOSS of H20

Term
what is the role of sulphuric acid in nitration of benzene?
Definition
to force a proton onto nitric acid.
Term
which reagent is used in the sulphonation of benzene?
Definition

SOdissolved in concentrated sulphuric acid.

 

 

this reagent is also known as OLEUM

Term
what is the electrophile used in sulphonation of benzene?
Definition

SO3H+ 

 

the SO3 is protonated by H2SO4

Term
in Friedel-Crafts Acylation, an acyl group is introduced by reaction with an acid chloride in the presence of which catalyst?
Definition
AlCl- this acts as a halide anion acceptor allowing the acyl (electrophile) cation to form.
Term
how are acid chlorides made?
Definition
they are made from the corresponding acid by reacting it with Thionyl Chloride (SOCl2). the products are acid chloride, sulphur dioxide and HCl
Term
in Friedel-Crafts Acylation, the acylated benzene is a .........
Definition
ketone baby! its a ketone!
Term
which reaction is similar to Friedel-Craft Acylation but uses an alkyl chloride instead?
Definition
Friedel-Craft Alkylation. the AlClcatalyst is still used and has the same role to accept a halid ion to allow the formation of an electrophile.
Term
what complications arise from Friedel-Craft Alkylations involving primary carbocations?
Definition
carbocation ion rearrangement. A primary carbocation can undergo rapid HYDROGEN SHIFT to form a more stable secondary carbocation before the aromatic attacks it
Term
if you really wanted to put a linear alkyl chain onto benzene how would you get around the rapid hydrogen shift?
Definition

do a Friedel-Craft acylation and then reduce the C=O group.

 

A clemmensen reduction reduces an acyl group into an alkyl group.

Term
what does a clemmenson reduction do and what reagents does it involve?
Definition

it reduces an acyl group into an alkyl group and involves

 

Zn (Hg) / HCl

Term

what can nitrobenzene be converted into by

Sn/HCl (aq)

followed by alkaline work up?

Definition
Aniline
Term

what can benzene sulphonic acid get converted into by

NaOH (s) and heat

followed by acid work up?

Definition
a Phenol
Term
oxidation of alkylbenzene with acidified dichromate gives which product?
Definition

benzoic acid.

 

acidified dichromate > Na2Cr2O/H2SO4

                              H20 and heat

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