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CH10136 > Aromatic- Reactivity and Regioselectivity of substituted benzenes in SEAr > Flashcards

Aromatic- Reactivity and Regioselectivity of substituted benzenes in SEAr Flashcards

1
Q

What does substitution of benzene result in

A
  1. Effects both rate and regioselectivity
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2
Q

What is the rate of an SEAr reaction determined by

A
  1. Nucleophilicity of the aromatic
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3
Q

What affect

A
  1. Electron withdrawing groups reduce the rate of reaction either through inductive effects governed by electronegativity (-F) or mesomeric effects determined by conjugation (-NO2)
  2. Very electron-poor aromatics require high temperatures and harsher conditions to get them to react
  3. Electron-donating groups increase the rate of reaction either through inductive effects such as hyperconjugation (alkyl) or mesomeric effects by conjugation (-OME, NMe2)
  4. Very electron-rich aromatic can be too reactive and difficult to prevent multiple substitutions from occurring
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4
Q

What is Hammett constants

A
  1. Shows how electron-donating or electron-withdrawing a specific substituent is
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5
Q

Name some common electron donating groups

A
  1. NH2, NHR, NR2- STRONGEST
  2. OH, OR
  3. NHOCR, OCOR
  4. Ph, CH=CH2
  5. R
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6
Q

Which are have stronger effects- mesomeric or inductive

A
  1. Usually mesomeric effects are stronger

2. EDG with strong +M effects stabilise carbocations more effectively than EDG with weaker +M effects

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7
Q

What are some common deactiviting groups

A
  1. Cl, Br, I
  2. CHO, COR
  3. CO2H, CO2R
  4. SO3H
  5. NO2- STONGEST
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8
Q

What direction do EWGs direct substitution to

A
  1. The meta position

2. Except halogens- ortho or para

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9
Q

What direction do EDGs direct substitution to

A
  1. Ortho or Para position
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10
Q

Why do EWGs direct to the meta position

A
  1. The lower energy pathway
  2. Avoids a disfavoured, high-energy resonance structure where a positive charge is found on the carbon adjacent to the EWG
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11
Q

Why do EDGs direct to the ortho/para position

A
  1. Lower energy pathway
  2. The positive charge can reside on the carbon adjacent to the EDG and can be stabilised either by resonance or hyperconjugation
  3. Meta has least resonance structures so least favoured pathway
  4. Wheland intermediates lower in energy for ortho and para
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12
Q

Why do halogens direct ortho and para

A
  1. Effective orbital overlap of a lone pair of halogen with the Wheland intermediate which helps to stabilise it in a similar manner as a conjugated EDG
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13
Q

Which aromatic halide is most reactive

A
  1. Fluoro>iodo>chloro>bromo
  2. Fluorobenzene than chlorobenzene as improved orbital overlap 2p vs 3p between the pi-cloud and the halogen
  3. As the orbitals get more diffuse, bonds get longer and inductive effects weaker
  4. Iodobenzene has a higher rate of reaction than bromobenzene due to lower electronegativity
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14
Q

Describe ratio of ortho:para for different halogobenzens

A
  1. Ortho:para
  2. Fluoro- 1:9
  3. Chloro- 4:6
  4. Bromo- 4:6
  5. Iodo- 5:5
  6. Fluorine is so selective for para because of strong inductive effect from the highly electronegative fluorine atom
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15
Q

How can reactivity be adjusted

A
  1. Changing the electron density of the aromatic ring
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16
Q

How can regioselectivity be improved

A
  1. Effective use of sterics or blocking groups
17
Q

How can phenols improve their reactivity within SEAr reactions and what is it called

A
  1. Phenols can be turned into their corresponding phenoxide anion through treatment with hydroxide
  2. Allows them to react even with poor electrophiles such as CO2
  3. Ortho is the major product
  4. Reaction known as Kolbe-Schmitt process
  5. Used in industry for synthesis of aspirin
18
Q

How can anilines reduce their activity

A
  1. Temporarily turned into the corresponding acetanilide using a suitable acetylation reagent such as acetyl chloride or acetic anhydride
  2. Prevents lone pair of electrons on the nitrogen atom from being donated into the aromatic ring as much, since it is now in conjugation with the amide
  3. Reduced electron density allows for a single substitution to be performed
  4. Substitution normally at para due to increased steric hindrance at ortho position
  5. The acetyl group can then be easily removed through acid or base hydrolysis to return the free aniline
19
Q

How can you improve the ortho/para selectivity of electron-rich aromatics

A
  1. Blocking groups can be used
  2. Temporarily install functionality at the more reactive position, then remove it after the desired electrophile has been reacted at the less reactive position
20
Q

What acts as a good blocking group

A
  1. Sulfonic acids- reversible nature of the sulfonation SEAr reaction
21
Q

Describe how sulfonic acid can help to product 2-bromophenol from phenol

A
  1. Reacting phenol with bromine would not work as it would lead to 2,4,6-trisubstituted product
  2. Heat phenol with concentrated sulfuric acid - first blocks the para and one of the ortho positions so in subsequent bromination reaction, SEAr only occurs at the remaining ortho position
  3. Heating with dilute sulfuric acid removes the sulfonic acid groups to five the desired ortho-bromophenol product
22
Q

Give examples of how FGIs can change directing effects

A
  1. Meta nitro can be reduced to ortho/para amines
  2. Meta carbonyls can be reduced to ortho/para alkyls
  3. Ortho/para methyl can be oxidised to meta carboxylic acids using KMnO4
23
Q

Describe what happens in poly substituted aromatics

A
  1. Different substituents can direct electorphilic aromatic substitution
  2. Either cooperatively to the same position
  3. Or competitively to different positions
24
Q

What happens with cooperative effects

A
  1. All work together to stabilise the Wheland intermediate when substitution occurs at the preferred position
  2. Improves regioselectivity of the reaction
25
Q

What happens when there are competitive effects

A
  1. Likely position of substitution can be difficult to predict
  2. Both electronic and steric factors need to be considered
  3. Electronic are considered first
26
Q

Describe how to determine position from electronic effects of competitive groups

A
  1. Activating groups take precedent over deactivating groups
  2. When more than one activating group- those directed by conjugation have a much stronger effect and take precedent over those directed by hyperconjugation or inductive effects
27
Q

How can sterics be used to determine position when there are competitive groups

A
  1. When electronic factors cannot easily distinguish between positions, steric effects are considered
  2. Substitution typically occurs at least hindered position
28
Q

What reactions can electron-rich aromatics undergo

A
  1. Iodination
  2. Formylation
  3. Azo-coupling
29
Q

What is a limitation of Friedel-Crafts acylation

A
  1. R cannot = H

2. Useful benzaldehyde products cannot be made by this method

30
Q

How can formylation of electron-rich aromatics occur

A
  1. Gatterman reaction

2. Vilsmeier-Haack reaction

31
Q

Describe the Gatterman reaction

A
  1. Either gaseous HCN or easier to handle but just as toxic zin(II) Cyanide with HCl gas combine to give an imine
  2. In the presence of a Lewis acid catalyst the imine is activated to an iminium cation
  3. This iminium cation is electrophilic enough to be attacked by an electron-rich aromatic- SEAr
  4. he imine product from this is hydrolysed in aqueous work-up to obtain formylated product
32
Q

Describe the Vilsmeier-Haack reaction

A
  1. Dimethylformamide (DMF) and phosphorus oxychloride (POCl3) combine to give an iminium directly
  2. This can be attacked by the aromatic- SEAr
  3. The imine product from this is hydrolysed in aqueous work-up to obtain formylated product
33
Q

How can azo dyes be produced

A
  1. Aryldiazonium salts can be used as electrophiles with SEAr reactions with electron-rich aromatics
  2. Electron-rich aromatics react with them at the terminal nitrogen in a standard SEAr manner.
  3. The products have extended pi-conjugated systems and are therefore highly coloured- dyes and pigments within food and textiles

CH10136 (30 decks)

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