Textbook: Vollhardt 6th Ed. (2010)

Chapter 16: Electrophilic Attack on Derivatives of Benzene: Substituents Control Regioselectivity

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Individual Problems

Individual Problems

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 # 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.