Textbook: Loudon 5th Ed. (2009)

Chapter 16: The Chemistry of Benzene and Its Derivatives

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Individual Problems

Individual Problems

Problem # 587

Use curved arrows to draw a mechanism for the generic electrophilic aromatic substitution (EAS) reaction below.

Solution

Notice that the electrophile takes the place of the hydrogen.

Problem Details

MendelSet practice problem # 587 submitted by Matt on July 9, 2011.

Subject: Organic Chemistry

Subject Topic(s): Electrophilic Aromatic Substitution

Question Type: Mechanism (show a mechanism using curved arrows..)

Keyword(s): EAS mechanism

Problem # 596

Indicate the eletrophile formed by each set of reagents/conditions below.

Solution

Note that some textbooks don't draw EAS electrophiles with formal positive charges.

Some textbooks instead draw electrophiles with partial positive charges and a leaving group. It's just a convention, and doesn't affect the product of an EAS reaction. See the problem 595 for an explanation of the difference.

Problem Details

MendelSet practice problem # 596 submitted by Matt on July 9, 2011.

Subject: Organic Chemistry

Subject Topic(s): Electrophilic Aromatic Substitution

Question Type: Concept (explain why this is so..)

Keyword(s): EAS mechanism

Problem # 678

Draw the structure of the major organic product from each reaction sequence.

Solution

The purpose of this problem is to illustrate that there are often several routes to prepare a single molecule.

For the left route:

Br₂/FeBr₃ adds a bromine to benzene via EAS.
Mg/ether converts bromobenzene to phenyl Grignard (a nucleophile).
Phenyl Grignard attacks the aldehyde.
A protic workup gets rid of all negative charges (and quenches the Grignard reagent).

For the right route:

Benzene undergoes Friedel-Crafts acylation (an EAS reaction) to form the ketone.
The ketone is then reduced to an alcohol using NaBH₄.

Note that both methods add a carbon chain to benzene, a step that is quite common in synthesis problems!

Problem Details

MendelSet practice problem # 678 submitted by Matt on July 19, 2011.

Subject: Organic Chemistry

Subject Topic(s): Electrophilic Aromatic Substitution, Alcohol Oxidation Reactions (PCC, Jones Reagent)

Question Type: Reactions (predict the product, show the starting material..)

Keyword(s): hydride reduction

Problem # 588

Let's draw resonance forms to see why some groups are EDG or EWG. (I've started you off)

Where are the positive or negative charges placed in EDG/EWG? (ortho/meta/para) Why would this affect EAS reactions?

Note: EDG = electron donating group, EWG = electron withdrawing group

Solution

The resonance forms for EDG add electron density to the ring (and add a negative charge), while the resonance forms for EWG remove electron density from the ring (and leave a positive charge). This is where the terms electron donating group and electron withdrawing group come from.

EAS reactions require benzene to attack something positive (an electrophile), so the more electron density, the better. This is why EDG tend to speed up EAS reactions, while EWG slow down EAS reactions.

Notice that for EDG, the ortho and para positions are partially negative. In EAS reactions benzene attacks a positively charged electrophile, so its not too surprising that the electrophile will want to add at o/p if a EDG is present.

But for EWG, the o/p positions pick up a partial positive charge. So for EAS reactions with a EWG, it's not surprising that the electrophile will avoid the o/p positions, and add meta instead.

This is one explanation for the general trend:

EDG are ortho/para directors and activating.
EWG are meta directors and deactivating.

Problem Details

MendelSet practice problem # 588 submitted by Matt on July 9, 2011.

Subject: Organic Chemistry

Subject Topic(s): Substituted Benzene EAS (EDG and EWG Effects)

Question Type: Concept (explain why this is so..)

Keyword(s): EAS mechanism, EDG and EWG

Problem # 589

-OR is an EDG and an ortho-para director. Let's draw an EAS reaction's cyclohexadienyl cation intermediates to demonstrate why this is true. I've started you off.

What's good about ortho/para? What's bad about meta?

Solution

Short answer: -OR is an ortho/para director because the cyclohexadienyl intermediates that result from ortho and para addtitions are more stable than those that result from meta addition.

Long answer: The cyclohexadienyl intermediates from ortho and para addition include a resonance form where the oxygen is adjacent to the carbocation, while the cyclohexadienyl intermediates from a meta addition do not.

When an oxygen (or anything with a lone pair) is adjacent to a carbocation, it can share its lone pair and stabilize the positive charge. This is only possible when the electrophile adds to the ortho or para positions, so those positions are favored with -OR as a substituent. Hence, -OR is an ortho/para director.

Note that this logic holds for any substituent with a lone pair, so -OH, -OR, -NH₂, -NR₂, -F, -Cl, -Br, and -I are all ortho/para directors.

Problem Details

MendelSet practice problem # 589 submitted by Matt on July 9, 2011.

Subject: Organic Chemistry

Subject Topic(s): Substituted Benzene EAS (EDG and EWG Effects)

Question Type: Concept (explain why this is so..)

Keyword(s): EAS mechanism, cyclohexadienyl cation

Problem # 590

-NO₂ is an EWG and a meta director. Let's draw an EAS reaction's cyclohexadienyl cation intermediates to demonstrate why this is true. I've started you off.

What's good about meta? What's bad about ortho/para?

Solution

Short answer: -NO₂ is an meta director because the cyclohexadienyl intermediates that result from meta addtition are more stable than those that result from ortho or para addition.

Long answer: Addition at both the ortho or para positions lead to a cyclohexadienyl cation that contains two adjacent positive charges. This is very unstable, and so addition at the ortho or para positions is not favored.

The cyclohexadienyl cations that result from meta addition don't have this problem.

This will be the base for any substituent that has a positive charge (or partial positive charge). So -NO₂, -C(O)R, -CF₃, -NH₃⁺, -CN, and -SO₃R are all meta directors.

Problem Details

MendelSet practice problem # 590 submitted by Matt on July 9, 2011.

Subject: Organic Chemistry

Subject Topic(s): Substituted Benzene EAS (EDG and EWG Effects)

Question Type: Concept (explain why this is so..)

Keyword(s): EAS mechanism, cyclohexadienyl cation

Problem # 595

Phenol can be prepared from benzene and hydrogen peroxide in the presence of a really strong acid. Propose a mechanism for this reaction.

Solution

The electrophile we need for this EAS reaction is HO⁺. We can generate it by protonating one of the oxygens in peroxide, which causes it to act as a leaving group.

On the left side of this image I make a note about the structure of the electrophile. Some textbooks use electrophiles with formal positive charges, such as HO⁺. Other textbooks don't use formal positive charges, and instead use partial positive charges with a leaving group, such as HO-OH₂⁺. Using HO⁺as the electrophile is analogous to an S_N1 reaction, while using HO-OH₂⁺. is analogous to an S_N2 reaction. When you draw mechanisms, you should use whichever convention your textbook uses. Both lead to the same product.

Problem Details

MendelSet practice problem # 595 submitted by Matt on July 9, 2011.

Subject: Organic Chemistry

Subject Topic(s): Electrophilic Aromatic Substitution

Question Type: Mechanism (show a mechanism using curved arrows..)

Keyword(s): EAS mechanism

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 # 592

Naphthalene undergoes eletrophilic substitution at C-1.

Why is this the case, even though substitution at C-2 gives more resonance forms?

Solution

Substitution at C-1 and C-2 both lead to many resonance forms. But the resonance forms that arrise from substitution at C-2 are not aromatic, while the resonance forms from substitution at C-1 are aromatic (the benzene ring is maintained).

So the C-1 resonance forms are more stable than the C-2 resonance forms, so substitution occurs at C-1 instead of C-2.

Problem Details

MendelSet practice problem # 592 submitted by Matt on July 9, 2011.

Subject: Organic Chemistry

Subject Topic(s): Substituted Benzene EAS (EDG and EWG Effects)

Question Type: Concept (explain why this is so..)

Keyword(s): EAS mechanism

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).