Organic Chemistry > Aromatic Substitution Reaction > Flashcards

Aromatic Substitution Reaction Flashcards

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1
Q

Electrophilic aromatic substitution

A

Reaction in which one of the aromatic protons is replaced by an electrophile & the aromatic moiety is preserved

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2
Q

The presence of ________ in the reaction mixture enhances the electrophilicity of the bromine atom

A

Iron (Fe)

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3
Q

Iron reacts with bromine to make ______________ & is the catalyst in the reaction between benzene & bromine

A

Iron tribromide (FeBr3) (Lewis acid)

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4
Q

What is the mechanism for bromination of benzene?

A
  1. Formation of FeBr3
  2. Sigma complex (Arenium ion)
  3. Nucleophilic Attack
  4. Proton transfer
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5
Q

_____________ can also be used as a catalyst instead of FeBr3

A

Aluminum tribromide (AlBr3)

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6
Q

Substitution is an _____________ process (downhill in energy)

A

Exergonic

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7
Q

Addition is an ___________ process (uphill in energy)

A

Endergonic

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8
Q

_____________ of benzene can occur with Lewis acids such as AlBr3

A

Chlorination

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9
Q

What are the two general steps of benzene substitution reaction?

A
  1. The aromatic ring functions as a nucleophile & attacks ana electrophile to form a sigma complex
  2. Which is followed by deprotonation of the sigma complex to restore aromaticity
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10
Q

When benzene is treated with ____________ a sulfonation reaction occurs & benzene sulfuric acid is formed

A

Sulfuric Acid

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11
Q

The S=O bonds are seen as __________ bonds where sulfur has a partial positive charge & oxygen has a partial negative charge so sulfur is the electrophile that reacts with benzene

A

Single bonds

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12
Q

What is the mechanism of sulfonation of benzene?

A
  1. Nucleophilic attack
  2. Sigma complex
  3. Proton transfer
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13
Q

When benzene is treated with a mixture of nitric acid & sulfuric acid a _________ reaction occurs & nitrobenzene is formed

A

Nitration

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14
Q

A nitration reaction occurs via an electrophilic aromatic substitution in which a ___________ is the electrophile

A

Nitronium ion (NO2+)

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15
Q

What is the mechanism for nitration of benzene?

A
  1. Nucleophilic attack
  2. Sigma complex
  3. Proton transfer
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16
Q

The nitro in the nitrobenzene product can be reduced to give an __________

A

Amino group (NH2)

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17
Q

What are the two ways to install an amino group in a benzene ring?

A
  1. Nitration
  2. Followed by reduction of the nitro group
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18
Q

Friedel -crafts alkylation allows the installation of an __________ group on an aromatic ring

A

Alkyl group

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19
Q

A alkyl halide is treated with a lewis acid like aluminum trichloride to form a ___________ to react with benzene

A

Carbocation

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20
Q

What is the mechanism for Friedel-crafts alkylation?

A
  1. Formation of carbocation
  2. Nucleophilic attack
  3. Sigma complex
  4. Proton transfer
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21
Q

____ & ____ halides are readily converted into carbocation, but primary halides don’t form carbocations but can still react with benzene

A

20 & 30

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22
Q

What are the things to consider with a Friedel- craft alkylation?

A
  1. The carbon in the alkyl halide that is attached to the halogen must be Sp3
  2. Polyalkylation can occur but most of the time it’s under conditions that form monoalkylation
  3. Nitro groups can’t react in a Friedel-craft reaction
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23
Q

What type of substitutent group is this?

A

Acyl group

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24
Q

Acylation

A

Installs an acyl group (Friedel-crafts acylation) where the acyl group is treated with a Lewis acid to form an acylium ion

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25
What is the mechanism for Friedel crafts acylation?
1. Formation of the R-C+=O (Acylium ion) 2. Nucleophilic attack 3. Sigma complex 4. Proton transfer
26
The product of a Friedel-craft acylation is an ___________ which can be reduced using a \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_
Aryl ketone, Clemmensen reduction
27
Nitration of toluene
1. The methyl group in toluene in a nitration reaction is a ortho-para director 2. For the regiochemistry ortho & para products dominate in this reaction
28
What does the methyl group in toluene do in a nitration reaction?
The methyl group on toluene activates the aromatic ring (which causes it to react faster than benzene) because the alkyl group is an electron donating via hyperconjugation & as a result the methyl group donate electron to the ring which stabilize the sigma complex & lower the activation energy
29
Ortho-para director
Means it causes the incoming nitro group into the ortho & para positions
30
Nitration of anisole
In the nitration of anisole, the para product is favored over the ortho product
31
\_\_\_\_\_\_\_\_\_\_\_ is more activating than methyl group
Methoxy group (OCH3)
32
In nitration of anisole there is competition between induction & resonance in anisole, whenever there's competition between resonance & induction, _________ usually wins
Resonance
33
\_\_\_\_\_\_\_\_\_\_\_\_suggest that methoxy group is electron withdrawing & __________ suggest that methoxy group is electron donating
Induction, Resonance
34
All ____________ are ortho- para directors
Activators
35
A _____ group deactivates the ring toward electrophilic aromatic substitution
Nitro
36
The ______ group withdraws electrons because its resonance structures it has positive charges & by removing electron density form the ring it destabilizes the sigma complex & raises the energy of activation
Nitro
37
\_\_\_\_\_\_\_\_ groups in aromatic substitution makes a meta product dominate
Nitro
38
A _______ group deactivates the ring & is a meta director
Nitro
39
Most deactivators are _____ directors
Meta
40
\_\_\_\_\_\_\_\_\_\_ are ortho-para directors & deactivates are meta director
Activators
41
Many halogens such as Cl, Br, & I are __________ directors (They are electron withdrawing)
Ortho-Para
42
When competition between resonance & induction occurs with halogen, ___________ wins because halogens withdraw electron density from the ring, thereby destabilizing the positively charge sigma complex & raising the energy of activation
Induction
43
Both activators & deactivators can be classified as \_\_\_\_\_\_\_\_\_, \_\_\_\_\_\_\_\_\_, \_\_\_\_\_\_\_\_
Strong, moderate, weak
44
Strong activators
Are characterized by the presence of a lone pair immediately adjacent to the aromatic ring
45
Moderate activator
Exhibit a lone pair that is already delocalized outside the ring
46
Alkoxy group are _________ activators
Moderate
47
Alkyl groups are _______ activators because they donate electron density by the relatively weak effect of hyperconjugation
Weak
48
Halogens are weak \_\_\_\_\_\_\_\_\_\_
Deactivators
49
\_\_\_\_\_\_\_\_\_\_ deactivators are groups that exhibit a pie bond to an electronegative atom, where the pie bond is conjugated with the aromatic ring
Moderate
50
How many strong activators are there?
3 (Look in notebook)
51
In some cases the directing effects of all substituents _________ each other
Reinforce
52
Sometimes substitutents can compete with each other where the more powerful (stronger) _________ group dominates
Activating
53
For most monosubstituted aromatic rings the ____ product generally dominates over the ortho product as a result of steric effect
Para
54
The regiochemistry outcome for 1,4-disubstituted aromatic rings is controlled by \_\_\_\_\_\_\_\_, where nitration is more likely to occur at a site that is less sterically hindered (ortho to a methyl group)
Steric effect
55
For 1,3-disubstituted aromatic rings substituent will not occur at the position between ________ substituents because it's the most sterically hindered position
Two
56
A ________ group can be used to direct the bromination towards the ortho position
Blocking group (so blocking group is 1st installed at the para position)
57
\_\_\_\_\_\_\_\_\_\_ is a common blocking group
Sulfonation (SO3H)
58
The simplest kind of synthesis is the formation of ________________ benzene because you don't need a directing affect just need to know what reagents to use for the desired product
Monosubstituted
59
Nitration can't be performed on a ring that contains an __________ group
Amino
60
A friedel-crafts reaction (either alkylation or acylation) can't be accomplished on rings that are either __________ or __________ deactivated. The ring must be either activated or weakly deactivated for a friedel-crafts reaction to occur
Moderately or strongly
61
When dealing with polysusbtituted benzene ring its best to use ____________ approach
Retrosynthetic
62
Electrophilic aromatic substitution
Reaction in which the aromatic ring attacks an electrophile
63
Nucleophile aromatic substitution
A ring is attached by a nucleophile
64
What are the criteria for a reaction to be a nucleophilic substitution reaction?
1. The ring must contain a powerful electron withdrawing (usually a nitro group) 2. The ring must contain a leaving group (usually a halide) 3. The leaving group must be either ortho or para to the electron - withdrawing group. If the leaving group is meta to the nitro group then the reaction doesn't happen
65
For a nucleophilic substitution reaction to occur _________ must be met
All 3 criteria
66
What is the mechanism for nucleophilic aromatic substitution (SNAr)
1. Nucleophilic attack 2. Meisenheimer complex 3. Loss of Leaving group
67
Elimination of H & Cl produces a very high energy intermediate called \_\_\_\_\_\_\_\_\_
Benzyne
68
What is the mechanism for elimination - addition reaction?
1. Proton transfer 2. Loss of LG 3. Nucleophilic attack 4. Proton transfer
69
What are the 3 different mechanism of an aromatic substitution reaction?
1. Electrophilic aromatic substitution 2. Nucleophilic aromatic substitution 3. Elimination- Addition
70
What are the few differences between all 3 mechanisms even though they are accomplish aromatic substitution?
1. The intermediate 2. Leaving group 3. Substituent effect
71
What is the difference between the intermediate in all 3 mechanism?
Electrophilic aromatic substitution occurs via a sigma complex, nucleophilic aromatic substitution occurs via Meisenheimer complex & elimination- addition occurs via benzyne intermediate
72
What is the difference between the leaving group in all 3 mechanisms?
In an electrophilic aromatic substitution, the incoming substituent replaces a proton & the other two mechanism a negatively charge leaving group (Like halide ion) leaves
73
What is the difference in substituent effect between all 3 mechanisms?
In electrophilic aromatic substitution, electron-withdrawing groups deactivate the ring toward attack, while in nucleophilic aromatic substitution an electron-withdrawing group is required for the reaction to occur
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