Problem 27 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

What is correct order of rate of nitration of the following compounds? (i) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{3}\) (ii) \(\mathrm{C}_{6} \mathrm{H}_{6}\) (iii) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Br}\) (iv) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NR}_{3}\) (v) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NMe}_{2}\) (a) \(\mathrm{iv}>\mathrm{iii}>\mathrm{ii}>\mathrm{i}>\mathrm{v}\) (b) \(\mathrm{v}>\mathrm{iii}>\mathrm{ii}>\mathrm{i}>\mathrm{iv}\) (c) \(\mathrm{v}>\mathrm{i}>\mathrm{ii}>\mathrm{iii}>\mathrm{iv}\) (d) \(v>\mathrm{iv}>\mathrm{ii}>\mathrm{i}>\mathrm{iii}\)

Step-by-Step Solution

Answer

Option (c) is correct: v > i > ii > iii > iv.

Step 1: Identify the Activating and Deactivating Groups

To solve this kind of problem, start by identifying whether each substituent group is an activating or deactivating group, and whether it is an ortho/para or meta director. 1. **Activating Groups** increase the rate of nitration: e.g., alkyl groups like -CH3 increase electron density on the benzene ring, making it more reactive. 2. **Deactivating Groups** decrease the rate of nitration: e.g., halogens like -Br are electron-withdrawing but still direct ortho/para; -NR3 groups are strongly deactivating due to their electron-withdrawing nature. The likely candidates contributing to these modifications are: - (i) -CH3: Activating, ortho/para director. - (ii) No substituent: Baseline reactivity of benzene. - (iii) -Br: Deactivating, ortho/para director. - (iv) -NR3: Strongly deactivating, meta director. - (v) -NMe2: Strongest activating, ortho/para director due to lone pair electron donation from nitrogen.

Step 2: Rank the Compounds Based on Activation/Deactivation

Using the insights from Step 1, we can rank the compounds. The more activating a group, the higher the rate of nitration: 1. **C6H5NMe2 (v):** Most activating due to lone pair donation, providing strong resonance effect. 2. **C6H5CH3 (i):** Less activating than (v) but more than just benzene, due to hyperconjugation and slight resonance stabilization. 3. **C6H6 (ii):** Neutral reference point (no substituents). 4. **C6H5Br (iii):** Deactivating although it directs to ortho/para, it withdraws electron density via induction. 5. **C6H5NR3 (iv):** Strongly deactivating, no lone pair sharing and inductively withdraws electrons, hence slow for nitration.

Step 3: Match with Given Options

Now, match the rate order derived from Step 2 with the given options: - Option (a) does not match our rank. - Option (b) is reversed near the end. - **Option (c):** Correct as it reads: v > i > ii > iii > iv. - Option (d) starts with v but incorrectly ranks the others.

Key Concepts

Activating Groups

In the context of organic chemistry, activating groups are those that enhance the reactivity of a benzene ring towards electrophilic substitution reactions, such as nitration. These groups generally make the aromatic ring more electron-rich, which in turn makes the ring more reactive to electrophiles. Common examples of activating groups include alkyl and alkoxy groups.

• Alkyl groups like -CH3 are classic activating groups. They donate electron density through a process known as hyperconjugation, slightly stabilizing the ring.
• Another potent activator is the -NMe2 group. This group donates electrons via resonance from the nitrogen atom's lone pair, significantly increasing electron density on the benzene ring, thus vastly accelerating the nitration process.

Understanding activating groups helps us predict the order of reactivity in compounds bearing different substituents. For instance, compounds bearing -NMe2 activators would likely undergo nitration faster than those with -CH3.

Deactivating Groups

Deactivating groups, in contrast to activating groups, reduce the reactivity of a benzene ring towards electrophilic substitution reactions. They achieve this by withdrawing electron density from the ring, making it less reactive to electrophiles. These are typically characterized by the presence of electron-withdrawing substituents.

• Halogens such as -Br are peculiar because, while they deactivate through electron withdrawal, they still direct electrophiles to ortho and para positions due to their lone pairs.
• Quaternary ammonium groups (-NR3) are highly deactivating. They pull electron density away from the benzene ring through induction and do not support resonance donation, making them less reactive towards nitration.

Overall, recognizing deactivating groups is crucial for predicting and understanding the order of reactivity in nitration and other electrophilic aromatic substitution reactions.

Ortho/Para Directors

Ortho/para directors are substituents that direct incoming electrophiles to the ortho and para positions on a benzene ring. Whether a group activates or deactivates the ring, its directing effect significantly influences the position of new substituents.

• Activating groups, such as -NMe2 and -CH3, are generally ortho/para directors. They enhance the nucleophilicity of these positions, favoring electrophilic attack there.
• Deactivating halogens like -Br, despite reducing overall ring reactivity, also direct electrophiles to ortho and para positions, thanks to lone pair electron influence.

Knowing if a group is an ortho/para director allows chemists to predict the positions where further reaction might occur, informing the synthesis of more complex aromatic molecules.

Organic Chemistry Problem Solving

Organic chemistry problem solving often centers around understanding the effects of substituents on aromatic compounds. To predict reaction outcomes like the order of nitration rates, consider both the nature of substituents (whether activating or deactivating) and their directional influence (ortho/para or meta).
Here’s a strategy to solve typical problems:

• Identify the type of group: Is the group activating or deactivating? This will guide the expected rate of reaction.
• Consider directing effects: Determine if the group is ortho/para directing or meta directing, impacting where subsequent reactions occur on the benzene ring.
• Rank compounds: Based on the above insights, predict the order of reactivity, matching it against any given options or answer choices.

The ability to draw these logical connections and predict outcomes forms a major part of mastering organic chemistry problem solving, enabling chemists to design reactions and synthesize desired compounds efficiently.