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Aromatic Substitution and Functional Group Transformations – Organic Chemistry Study Guide

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Q1. For each of the following sets of reagents, indicate the major product when reacted with benzene or a substituted benzene ring:

Background

Topic: Electrophilic Aromatic Substitution (EAS) Reactions

This question tests your understanding of how different reagents react with benzene or substituted benzene rings to give major products. You need to know the mechanisms and regiochemistry of EAS reactions such as halogenation, nitration, sulfonation, and Friedel-Crafts alkylation/acylation.

Key Terms and Formulas

  • Electrophilic Aromatic Substitution (EAS): A reaction where an aromatic hydrogen is replaced by an electrophile.

  • Activating/Deactivating Groups: Substituents on the ring that affect reactivity and orientation (ortho/para vs. meta).

  • Common EAS Reactions:

    • Halogenation: or

    • Nitration:

    • Sulfonation:

    • Friedel-Crafts Alkylation/Acylation: or

Step-by-Step Guidance

  1. Identify the starting aromatic compound (benzene or substituted benzene) and the reagents provided for each part (a, b, c, etc.).

  2. Determine the type of EAS reaction based on the reagents (e.g., halogenation, nitration, sulfonation, etc.).

  3. Consider the effect of any substituents already present on the ring (activating/deactivating, ortho/para or meta directors).

  4. Predict the position where the new group will be introduced (ortho, meta, or para) based on the directing effects.

  5. Draw the major product structure, showing the new substituent in the correct position.

Try solving on your own before revealing the answer!

Final Answer:

The major products for each reagent set are as follows (assuming benzene as the starting material unless otherwise specified):

  • Cl2/FeCl3: Chlorobenzene ()

  • HNO3/H2SO4: Nitrobenzene ()

  • SO3/H2SO4: Benzenesulfonic acid ()

  • AlCl3 (with alkyl or acyl halide): Alkylated or acylated benzene (e.g., toluene or acetophenone, depending on the alkyl/acyl group)

For substituted benzenes, the position of substitution depends on the directing effects of the substituent. Activating groups direct to ortho/para, deactivating to meta.

Q2. For each of the following substituted benzenes, predict the major product of reaction with the given reagents:

Background

Topic: Regioselectivity in Electrophilic Aromatic Substitution

This question focuses on predicting the major product when substituted benzene rings undergo EAS reactions. You need to apply knowledge of directing effects and reactivity of substituents.

Key Terms and Formulas

  • Ortho/Para Directors: Electron-donating groups (e.g., , , ) direct new substituents to ortho and para positions.

  • Meta Directors: Electron-withdrawing groups (e.g., , , ) direct to meta positions.

  • Reactivity Order: Activating groups increase reactivity, deactivating groups decrease it.

Step-by-Step Guidance

  1. Identify the substituent(s) already present on the benzene ring and classify them as activating or deactivating, and as ortho/para or meta directors.

  2. Determine the incoming group based on the reagents provided (e.g., from nitration, from bromination, etc.).

  3. Use the directing effects to predict where the new group will be added (ortho, meta, or para to the existing group).

  4. If multiple substituents are present, consider their combined effects (the most activating group usually dominates).

  5. Draw the major product, showing the new substituent in the correct position.

Try solving on your own before revealing the answer!

Final Answer:

The major product for each substituted benzene depends on the substituent's directing effects:

  • For (aniline): New groups add ortho and para to .

  • For (nitrobenzene): New groups add meta to .

  • For : Strongly activating, ortho/para director.

  • Continue this logic for each substituent shown in the question.

Draw the structure with the new group in the correct position for each case.

Q3. For each of the following disubstituted benzenes, predict the major product of reaction with the given reagents:

Background

Topic: Multiple Substituent Effects in EAS

This question tests your ability to predict the outcome of EAS reactions when more than one substituent is present on the benzene ring. You must consider the combined directing effects and possible steric hindrance.

Key Terms and Formulas

  • Disubstituted Benzene: A benzene ring with two substituents, which may have competing or reinforcing directing effects.

  • Major Product: The product formed in the highest yield, based on both electronic and steric factors.

Step-by-Step Guidance

  1. Identify both substituents and classify each as activating or deactivating, and as ortho/para or meta directors.

  2. Determine the incoming group based on the reagents (e.g., for bromination, for nitration, etc.).

  3. Analyze the possible positions for substitution based on the directing effects of both groups.

  4. If the directing effects conflict, the more activating group usually dominates, but steric hindrance may also play a role.

  5. Draw the major product, showing the new group in the most likely position.

Try solving on your own before revealing the answer!

Final Answer:

The major product is determined by the combined directing effects of both substituents. For example:

  • For and : is strongly activating and will direct new groups ortho/para to itself.

  • For and : Both are ortho/para directors; substitution occurs at positions activated by both, unless steric hindrance prevents it.

  • Continue this logic for each disubstituted benzene in the question.

Draw the structure with the new group in the correct position for each case.

Q4. For each of the following, indicate the product(s) of the reaction sequence given (e.g., oxidation, reduction, halogenation, etc.):

Background

Topic: Functional Group Transformations on Aromatic Rings

This question tests your knowledge of how various reagents transform functional groups on aromatic rings, including oxidation, reduction, and halogenation reactions.

Key Terms and Formulas

  • Oxidation: Reagents like or oxidize alkyl side chains to carboxylic acids.

  • Reduction: Reagents like or reduce nitro groups to amines.

  • Halogenation: and light brominate benzylic positions.

  • Clemmensen Reduction: reduces carbonyls to alkyl groups.

  • Wolff-Kishner Reduction: reduces carbonyls to alkyl groups.

Step-by-Step Guidance

  1. Identify the starting material and the functional group(s) present.

  2. Determine the type of reaction based on the reagents (oxidation, reduction, halogenation, etc.).

  3. Predict the transformation that will occur (e.g., alkyl to carboxylic acid, nitro to amine, etc.).

  4. Draw the structure of the product after the transformation.

  5. If there is a sequence of reactions, apply each step in order, updating the structure each time.

Try solving on your own before revealing the answer!

Final Answer:

The products for each reaction sequence are as follows:

  • KMnO4 or H2CrO4: Oxidizes alkyl side chains to benzoic acid ( group attached to the ring).

  • NBS, light: Brominates the benzylic position (next to the ring).

  • H2, Pd/C: Reduces nitro groups to amines ( to ).

  • Sn, HCl: Also reduces to .

  • Zn(Hg), HCl (Clemmensen): Reduces carbonyls to alkyl groups.

  • N2H4, HO- (Wolff-Kishner): Reduces carbonyls to alkyl groups.

Draw the product structure for each transformation as described.

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