Organic Chemistry : Assigning Ortho, Meta, Para

Study concepts, example questions & explanations for Organic Chemistry

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Example Questions

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Example Question #1 : Assigning Ortho, Meta, Para

Choose the substituent that will direct meta addition on a benzene ring.

Possible Answers:

Correct answer:

Explanation:

 is the correct answer. In most cases, the deactivating substituents direct "meta." Halogens deactivate benzene rings, but are the exception, as they direct "ortho" or "para." A substituent is deactivating when it has a low electron concentration on the atom directly attached to the benzene ring. Carboxylic acid provides resonance, a delocalization of electrons. However, the amine group, for example, sees a lone pair on the nitrogen.

Example Question #2 : Assigning Ortho, Meta, Para

Img 0642

After the electrophilic aromatic substitution reaction occurs, in what position will the bromine be directed?

Possible Answers:

Meta

Ortho

Two of these

Para

Correct answer:

Meta

Explanation:

The functional group that is already on the phenyl is the group that dictates where any other substituent will be directed on the ring. In this case, we have a carboxylic acid as our director. Due to resonance, carboxylic acid deactivates (is an electron-withdrawing group) the benzene ring and directs the bromine to the meta position.

Example Question #3 : Assigning Ortho, Meta, Para

Img 0687

An organic chemist wants to brominate the pictured benzene ring via an electrophilic aromatic substitution (EAS) reaction. On which position will the bromine attach?

Possible Answers:

Correct answer:

Explanation:

Here we have a classic EAS reaction.

If we examine the molecule, we see that the benzene ring already has two substituents on it: , an activating group, and , a deactivating group. Remember, when assigning the position of a new substituent to a benzene ring, activating groups always dictate the position of the new substituent.

We know that activating groups direct new substituents to the ortho or para positions. However, because of sterics, a new substituent will not be directed to the ortho position in between two substituents. Thus, the new substituent will be directed to the para position (or the other ortho position, but that is not one of our answer choices), and the correct answer is position 2.

Example Question #4 : Assigning Ortho, Meta, Para

Which of these functional groups is a meta director?

Possible Answers:

None of these

Correct answer:

None of these

Explanation:

With the exception of halogens, meta directors deactivate a benzene ring. In other words, they make the benzene ring less reactive, especially in an electrophilic aromatic substitution (EAS) reaction. Meta directors have little electron density at the point of contact with the benzene ring. For example, a carboxylic acid is a meta director because it experiences resonance, a delocalization of electrons. All of the answer choices in this problem have a lone pair of electrons on the point of contact with the benzene ring and they all are ortho/para directors.

Example Question #5 : Assigning Ortho, Meta, Para

Which of the following are electron withdrawing substituents?

Screen shot 2015 10 24 at 10.14.50 am

Possible Answers:

2 and 4

1, 4 and 5

1 and 2

4 and 5

3, and 4

Correct answer:

1, 4 and 5

Explanation:

Carbonyls (as in 4 and 5) are always electron withdrawing due to the Oxygen's electronegativity. Similarly, the oxygens on the nitrate (1) are electron withdrawing.

Example Question #6 : Benzene Additions

What is the best explanation for why phenol (shown) directs substitution products ortho and para? 

Screen shot 2015 09 21 at 1.34.45 pm

Possible Answers:

Resonance structures exist that put slight positive charges on meta positions.

Resonance structures exist that put slight negative charges on ortho and para positions.

None of the other answers

Resonance structures exist that put slight positive charges on ortho and para positions.

Resonance structures exist that put slight negative charges on meta positions.

Correct answer:

Resonance structures exist that put slight negative charges on ortho and para positions.

Explanation:

Phenol contains the hydroxide group, which is an electron donor, puts electron density into the benzene ring. Resonance structures are drawn as follows

Screen shot 2015 09 21 at 1.53.32 pm

Example Question #2 : Assigning Ortho, Meta, Para

Which of the following substituents is electron-withdrawing when added to a benzene ring?

Possible Answers:

None of these substituents are electron-withdrawing

Correct answer:

Explanation:

 is the only electron-withdrawing substituent because it contains two electronegative oxygen atoms which pull electrons from the benzene ring towards itself. This effect is electron-withdrawing and makes the ring slightly positive in charge. All the other substituents are electron-donating groups, which activate the ring for electrophilic addition.

Example Question #8 : Assigning Ortho, Meta, Para

Which of the following groups would be added on the para position to phenol?

Possible Answers:

Correct answer:

Explanation:

 is the only electron-donating group listed. Therefore, it will add to the ortho and para positions on phenol. The rest of the substituents are highly electron-withdrawing groups and will add to the meta positions on phenol.

Example Question #3 : Assigning Ortho, Meta, Para

Which position will be most favored when adding chlorine to tert-butylbenzene? 

Possible Answers:

Ortho

Para

Meta and para

Meta

Ortho and para

Correct answer:

Para

Explanation:

A tert-butyl functional group is electron donating and will therefore activate the ortho and para positions. However, the ortho positions are sterically hindered by the bulky tert-butyl group. Therefore, the para position will be favored. 

Example Question #8 : Assigning Ortho, Meta, Para

What is a major product of the reaction shown?

Screen shot 2015 12 26 at 9.12.45 pm

Possible Answers:

II and III

I only

II only

III only

Correct answer:

I only

Explanation:

In the molecule shown, the nitro group will direct incoming substituents to positions meta to it, and the methoxy group will direct incoming substituents ortho or para to it. The only product option shown in which the chlorine substituent is meta (1,3) to the nitro group AND either ortho (1,2) or para (1,4) to the methoxy group is option I.

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