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Organic Chemistry > Chapter 18: Aromatic Substitution Reactions > Flashcards

Chapter 18: Aromatic Substitution Reactions Flashcards

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1
Q

Sigma complex

A

Positively charged, 3 resonant structure intermediate generated in an EAS reaction

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2
Q

Bromination of benzene

A

Reagents

  1. Br2
  2. FeBr3 (iron tribromide) or AlBr3 (aluminum tribromide)

Mechanism

Forms via a sigma complex intermediate and subsequent rearomatization of the benzene ring

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3
Q

Chloronation of benzene

A

Reagents

  1. Cl2
  2. FeCl3 (iron trichloride) or AlCl3 (aluminum trichloride)

Mechanism

Forms via a sigma complex intermediate and subsequent rearomatization of the benzene ring

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4
Q

Sulfonation of benzene

A

Reagents

Fuming H2SO4
H2SO4 & SO3 (sulfur trioxide)

Mechanism

Forms via a sigma complex intermediate and subsequent rearomatization of the benzene ring

Sulfonation is reversible because the reagent is concentration dependent

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5
Q

Nitration of benzene

Installation of a nitro group

A

Mechanism

HNO3 (nitric acid) & H2SO4

Mechanism

Sulfuric acid protonates nitric acid and subsequently forms a nitronium ion which performs the nucleophilic attack

Nitration subsequently occurs via the formation of a sigma complex intermediate and subsequent rearomatization of the benzene ring

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6
Q

Nitration of benzene

Installation of an amine group

A

Reagents

  1. HNO3 (nitric acid) & H2SO4
  2. Fe or Zn & HCl
  3. NaOH

Mechanism

Sulfuric acid protonates nitric acid and subsequently forms a nitronium ion which performs the nucleophilic attack

Nitration subsequently occurs via the formation of a sigma complex intermediate and subsequent rearomatization of the benzene ring

Upon treatment with a metal (Fe or Zn) and HCl the nitro group can be reduced resulting in an ammonium ion (RNH3+) that is subsequently deprotonated by a base

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7
Q

Friedel-Crafts alkylation

A

Reagents

AlCl3 (aluminum trichloride) & alkyl halide

  • Secondary and tertiary alkyl halides are readily converted to carbocations but NOT primary halides EXCEPT ethyl chloride
  • Alkyl halide α-carbon must be sp3 hybridized
  • Beware of possible rearrangements that may occur

Mechanism

The catalyst (AlCl3) converts the alkyl halide into a carbocation; creating a better electrophile

Alkylation subsequently occurs via the formation of a sigma complex intermediate and subsequent rearomatization of the benzene ring

DEACTIVATED rings CANNOT undergo Friedel-Crafts alkylation

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8
Q

Friedel-Crafts acylation

A

Mechanism

AlCl3 (aluminum trichloride) & acyl halide

Mechanism

Treatment of the acyl halide with AlCl3 forms a cationic species called an acylium ion

Acylation subsequently occurs via the formation of a sigma complex intermediate and subsequent rearomatization of the benzene ring

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9
Q

Friedel-Crafts acylation

Installing alkyl groups that are prone to rearrangement

A

Mechanism

  1. AlCl3 (aluminum trichloride) & acyl halide
  2. Zn(Hg) (amalgamated zinc) & HCl, heat

Mechanism

Treatment of the acyl halide with AlCl3 forms a cationic species called an acylium ion

Acylation subsequently occurs via the formation of a sigma complex intermediate and subsequent rearomatization of the benzene ring

A Clemmensen reduction can subsequently be employed to recude the aryl ketone to an alkyl group

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10
Q

Strong activating groups

A

ortho-para directors

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11
Q

Moderate activating groups

A

ortho-para directors

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12
Q

Weak activating groups

A

ortho-para directors

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13
Q

Weak deactivating groups

A

ortho-para directors

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14
Q

Moderate deactivating groups

A

meta directors

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15
Q

Strong deactivating groups

A

meta directors

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16
Q
A

Strong activator

ortho-para director

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17
Q
A

Strong activator

ortho-para director

18
Q
A

Strong activator

ortho-para director

19
Q
A

Strong activator

ortho-para director

20
Q
A

Strong activator

ortho-para director

21
Q
A

Moderate activator

ortho-para director

22
Q
A

Moderate activator

ortho-para director

23
Q
A

Moderate activator

ortho-para director

24
Q
A

Moderate activator

ortho-para director

25
Q
A

Weak activator

ortho-para director

26
Q
A

Weak deactivator

ortho-para director

27
Q
A

Moderate deactivator

meta director

28
Q
A

Moderate deactivator

meta director

29
Q
A

Moderate deactivator

meta director

30
Q
A

Moderate deactivator

meta director

31
Q
A

Moderate deactivator

meta director

32
Q
A

Moderate deactivator

meta director

33
Q
A

Moderate deactivator

meta director

34
Q
A

Strong deactivator

meta director

35
Q
A

Strong deactivator

meta director

36
Q
A

Strong deactivator

meta director

37
Q

Identifying directing effects for di- and poly-substituted benzene rings

A
  1. Identify each group
  2. Select the most powerful activator and identify where it is directing to
  3. Identify the unoccupied position as the most likely site for EAS
  4. Consider steric effects; identify least sterically hindered location
38
Q

Nucleophilic aromatic substitution

A

Reagents

  1. Strong Nuc, 70° C
  2. H3O+

Mechanism

Benzene ring is attacked by a strong nucleophile which creates a resonance-stabilized intermediate called a Meisenheimer complex that exhibits a negative charge throughout the ring

Criteria for reaction to proceed

  1. Ring must contain a strong electron-withdrawling group; typically a nitro group
  2. Ring must contain a good leaving group; typically a halogen
  3. The leaving group must be either ortho or para to the strong deactivating group
39
Q

Elimination-addition reactions

A

Reagents

  1. NaNH2 (sodium amide), NH3 (l)
  2. H3O+

Mechanism

Elimination-addition reaction occurs through a high energy radical/alkyne intermediate called a benzyne

40
Q

Determining reaction type between:

  • EAS
  • SNAr
  • Elimination-addition
A

Organic Chemistry (18 decks)

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