Friedel-Crafts

MS 913 - Benzene EAS Reactions, EDG/EWG, and ortho/meta/para Directors Submitted by Matt on August 6, 2011.

Textbook and Chapter: Carey and Giuliano 8th Ed. (2010), Chapter 12

Keywords: EAS mechanism, EDG and EWG, Friedel-Crafts

Description: Covers electrophilic aromatic substitution (EAS) reactions in detail.

It includes many, many resonance exercises that show why:

Electron donating groups (EDG) are ortho-para directors
Electron withdrawing groups (EWG) are meta-directors
Pyrrole undergoes EAS at C-2 and not C-3
Pyridine undergoes EAS at C-3 and not C-2 or C-4
Naphthalene undergoes EAS at C-1 and not C-2

Total Problems: 11

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Problem # 722

Show how to prepare vinyl benzene from benzene.

Solution

This is a good synthesis to know, because vinyl benzene is a good starting point for many synthesis problems you will encounter down the road.

An ethyl group can be added to benzene via Friedel-Crafts alkylation.

To add that alkene, we you will have to do some sort of elimination reaction. The easiest way to do this is via bromination. Free radical bromination (with NBS or Br₂/hν) will add a bromine to the position of the most stable radical, which is the benzylic position (due to resonance).

Then adding a strong base like potassium tert-butoxide will do an E2 reaction to form the alkene.

Problem Details

MendelSet practice problem # 722 submitted by Matt on July 24, 2011.

Subject: Organic Chemistry

Subject Topic(s): Electrophilic Aromatic Substitution

Question Type: Synthesis (show how to prepare X from Y..)

Keyword(s): free radical halogenation, EAS mechanism, Friedel-Crafts, C-C bond formation

Problem # 594

A chemist tried to prepare compound A from benzene via Friedel-Crafts alkylation and instead produced compound B.

Why did this happen? How could the chemist prepare compound A?

Solution

Friedel-Crafts alkylation is prone to carbocation rearrangement. In this case, alkylation produced a 1º carbocation which rearranged to a 3º carbocation, leading to compound B.

We can avoid this by instead doing Friedel-Crafts acylation. The intermediate in acylation is the acylium ion, which is stabilized by resonance and so won't rearrange.

But after the acylation reaction we have to get rid of the carbonyl (C=O) group, so we do a Wolff-Kishner reduction (N₂H₄/NaOH, heat).