Electrophilic/Nucleophilic aromatic substitutions

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In summary, the speaker is seeking help with understanding how to answer questions involving electrophilic and nucleophilic substitutions. They explain their reasoning for choosing a molecule in one question and it turned out to be correct. However, their reasoning did not work for another problem and they are unsure why. They also did not get the second question right.
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future_vet
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Hello,

I thought I understood how to answer the questions (posted below, please follow the links) but apparently my reasoning is flawed.
Any help is appreciated!

http://www.darkpoetry.com/dp/19/102049
Here is how I answered this question: I looked at all the substituents, and decided whether they were conducive to an electrophilic aromatic subst. reaction. For example, NO2 is really not appropriate, while NH2 is etc. Following this, I found that -OH was a very good choice, and MeO was not bad either and so I picked this molecule. It turned out to be correct.
I did the same for the 2nd question, but inverting the principles since it is a nucleophilic substitution. I correctly chose the last choice.

However, this reasoning did not work for this problem:
http://www.darkpoetry.com/dp/19/102050
I thought in the 1st question, the 3rd choice would be more appropriate because of the alkyl group. Why isn't this correct?
I didn't get the 2nd question right either.

Thanks for your help!
 
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1. What is the difference between electrophilic and nucleophilic aromatic substitutions?

Electrophilic aromatic substitutions involve the addition of an electrophile (a positively charged or electron-deficient species) to an aromatic ring, while nucleophilic aromatic substitutions involve the addition of a nucleophile (a negatively charged or electron-rich species) to an aromatic ring.

2. What is the mechanism of electrophilic aromatic substitution?

In the mechanism of electrophilic aromatic substitution, the electrophile attacks the pi electron system of the aromatic ring, forming a temporary sigma bond. This causes the hydrogen atom on the ring to detach, forming a carbocation intermediate. The intermediate then reacts with a nucleophile to form the substituted product.

3. How do substituent groups affect the reactivity of electrophilic aromatic substitution?

Substituent groups on an aromatic ring can either increase or decrease the reactivity towards electrophilic substitutions. Electron-donating groups, such as alkyl or amino groups, increase the electron density on the ring and make it more nucleophilic. Electron-withdrawing groups, such as nitro or carbonyl groups, decrease the electron density on the ring and make it more electrophilic.

4. What is the purpose of using a catalyst in electrophilic aromatic substitution?

A catalyst is used in electrophilic aromatic substitution reactions to increase the rate of the reaction. It does this by stabilizing the intermediate carbocation and facilitating the formation of the final product. Common catalysts used include Lewis acids, such as aluminum chloride, and Brønsted acids, such as sulfuric acid.

5. What are some common examples of electrophilic and nucleophilic aromatic substitution reactions?

Some common examples of electrophilic aromatic substitution include the nitration of benzene to form nitrobenzene and the Friedel-Crafts alkylation of benzene to form ethylbenzene. Nucleophilic aromatic substitution reactions include the addition of a Grignard reagent to an aromatic ring, which can form a wide range of products depending on the substituents present on the ring.

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