CHEM 2081 Exam 5

Question 1

Electrophilic Aromatic Substitution

Answer 1

Benzene does not react for an addition reaction, however in the presence of a Lewis acid catalyst it will carry out substitution

The same mechanism is used for all EAS, the only difference is the generation of the electrophile

Question 2

Halogenation of Benzene

Answer 2

Benzene and X₂ in FeX₃ (cat.) (where X=Cl, Br) conditions substitutes on a generated X⁺ electrophile before reforming benzene and H-X and Lewis acid

Question 3

Friedel-Crafts Alkylation

Answer 3

Benzene and R(sp³)-Cl react under AlCl₃ Lewis acid conditions

1,2 hydride shifts will always occur if possible on 1^oR chains to form more stable carbocation

Alcohols (H₂PO₄) and alkenes (H₂SO₄) are good sources for alkyl electrophiles

Question 4

Friedel-Crafts Acylation

Answer 4

Benzene and Cl-(C=O)-R (acyl) react under AlCl₃ Lewis acid conditions, the electrophile of which is an acylium ion (resonance stabilized), therefore not susceptible to rearrangement

Friedel-Crafts Acylation used to add R group chains to benzene in acyl group before reducing C=O

Question 5

Nitration

Answer 5

Adds HNO₃ under H₂SO₄ conditions to form NO₂ electrophile and bonds to benzene

Question 6

Sulfonation

Answer 6

Adds SO₃ under H₂SO₄ conditions to form SO₃H electrophile and bonds to benzene

Question 7

Side Chain Reactions

Answer 7

1. Acyl reduction (via Clemmensen (Zn(Hg) and HCl, or Wolff-Kishner (H₂NNH₂, KOH and heat), reduces acyl to CH₂
2. Alkyl oxidation (via 2-step process of KMnO₃, ^-OH, heat, and H₃O⁺), oxidizes 1^o or 2^o alkyl to carboxylic acid
3. Nitro reduction (via either H₂, Pd or HCl, Fe), reduces NO₂ (deactivating) to NH₂ (strongly activating)

Question 8

Benzene Nomenclature

Answer 8

Benzene serves as base word, substituents named before (e.g., bromobenzene)

Disubstituted benzene rings are named with substituents in alphabetical order either using numerical locators (e.g., 1,2-dibromobenzene) or prefixes (e.g., ortho-dibromobenzene)

ortho: 1,2
meta: 1,3
para: 1,4

Question 9

Substituent Effects on Benzene

Answer 9

Substituents influence the rate of reactions (reactivity of benzene) and orientation of an income electrophile

Substituents are either electron donating or withdrawing, which is dictated by their inductive and resonance effects

Question 10

Benzene Substituent Types

Answer 10

1. Alkyl groups (R) - donating (induction)
2. Z groups (Z = N, O) - donating (resonance)
3. X groups (X = F, Cl, Br, I) - withdrawing (induction)
4. Y=Z groups (Y has full/partial ⁺ charge) - withdrawing (induction and resonance)

Question 11

Orientation of Incoming Electrophile

Answer 11

Generally, electron donating groups are ortho/para directors and electron withdrawing groups are meta directors with an exception of halogens (ortho/para)

If there is a multi-step process, pay attention to the substituent orientation in the final product, as they will give clues as to what order things add in

Question 12

Limitations of Electrophilic Aromatic Substitution

Answer 12

+ Friedel-Crafts reactions (alkylation and acylation) do not occur with meta directing substituents already attached
+ Friedel-Crafts alkylation is easy to overalkylate since R groups are donating (can be prevented with XS benzene or using acylation mechanism and then reducing)
+ Halogenation of strongly activated rings (-OH, -OR, and -NH(R)₂) leads to overhalogenation (can be prevented by not using Lewis acid catalyst (FeX₃))

Question 13

Aniline Ring (Ph-NH(R)₂) Reactivity

Answer 13

Lone pair on N is very basic, reacts with acid catalysts. Avoic by “protecting” group by adding acyl group, which binds to N, and pyridine prior to substitution (ortho/para). Reverts back to NH(R)₂ group by either H⁺/H₂O or ^-OH/H₂O conditions

Question 14

Electrophilic Aromatic Substitution with Disubstituted Rings

Answer 14

When adding a third substituent onto a ring, the stronger activating group/weaker deactivating group directs the addition of the electrophile

Never substitute between 2 groups meta to one another

Question 15

Electrophilic Aromatic Substitution on Other Rings

Answer 15

Other rings have different bonding preferences (due to resonance):
+ Naphthalene: adds to a-position
+ Pyridine: adds to 3 position
+ Pyrrole: adds to 2 position
+ Furan: adds to 2 position

Substitution still reforms acid catalyst following the reaction

Question 16

Electrophilic Aromatic Substitution with Substituted Other Rings

Answer 16

Naphthalene: Has two aromatic rings, if a deactivator is in one, must substitute to other (in a-position), or if activator is in one, substitute to that ring (in a-position)

Disubstituted rings with activator directors will not overhalogenate

Furan/pyrrole will substitute to 2-position regardless of the other substituent