Chem 345 – Organic Reactions Chapter 16 Prepared by José Laboy, MS http: www.chem.wisc.edu/areas /clc (Resource page) Electrophilic Aromatic Substitution #5: Friedel Crafts Alkylation Reaction Electrophile CH2CH3
CH3CH2Cl / AlCl3 (0.1 eq.)
Cl
H3CH2C
Al
Cl
Mechanism Cl
H3CH2C
Cl
Cl
H2C
Cl
Al
Cl
H3C
+ AlCl4
CH2
Cl
CH3
CH2CH3
CH2CH3
CH2CH3
H
H
H
AlCl4
AlCl3 + HCl
CH2CH3
This is a very useful reaction because you can make carbon-­‐carbon bonds to aromatic compounds. Electrophiles that are primary carbocations are very reactive. Careful attention should be paid to side reactions that often occur when the carbocation electrophile can rearrange to a more stable one (see example below). Cl
+
AlCl
Major Product
< 5%
H
Cl
Cl
H
3
Al
Cl
Cl
Cl
Cl
Al
Cl
Cl
H3C
C
C
H
1o carbocation
H
rearrangement
1,2-hydride shift
H
C
H3C
CH3
2o carbocation
The Friedel-­‐Crafts reaction only requires catalytic amounts of the Lewis acid because it is recycled through the reaction. There is a major disadvantage of the alkylation reaction and that is over-­‐
alkylation. This is due to the fact that the product of the reaction is activated toward EAS more so than the starting reagent. One way of avoiding over-­‐alkylation is to have an excess of the aromatic reagent and adding the electrophile slowly. Another way of performing alkylation is by using an alkene with an acid catalyst or an alcohol with a Lewis acid catalyst (see below). In each case an electrophile is formed. CH 3
H 2C
CH
HF
OH
BF3