Organic Chemistry : Predicting Benzene Orientation

Study concepts, example questions & explanations for Organic Chemistry

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Example Questions

Example Question #1 : Predicting Benzene Orientation

What is the arrangement of the substituents of the major product when bromine in iron (III) bromide reacts with bromobenzene?

Possible Answers:

Ortho

Substituents will be on the same carbon

Para

Meta

None of these

Correct answer:

Para

Explanation:

When speaking of the arrangemts of two substituents on a benzene, their positions can be ortho, meta, or para. Ortho substituents are bound to adjacent carbons (C1 and C2). Meta substituents are bound to carbons with one intermediary carbon (C1 and C3). Para substituents are opposite one another across the benzene ring (C1 and C4).

The bromines in the given reaction result in a para product because bromine is ortho/para directing. There is a significant amount of ortho product, but because of steric hinderance, the major product is para.

Example Question #2 : Predicting Benzene Orientation

Vt orgo question

Suppose that toluene, whose chemical structure is shown, is reacted with  in the presence of . Which of the following would be expected to be the major product of this reaction?

Possible Answers:

Vt orgo wrong answer 2

Vt orgo wrong answer 3

Vt orgo wrong answer 1

Vt orgo correct answer

Correct answer:

Vt orgo correct answer

Explanation:

This question is essentially testing an understanding of electrophilic aromatic substitution and substituent placement.  first acts as a lewis acid by accepting the chloride from the  reagent, which generates . This, in turn, is susceptible to nucleophilic attack from one of the double bonds in the benzene ring. Due to the disruption of aromaticity in the ring, this double bond is quickly restored by abstraction of an adjacent proton on the ring.

But we also need to be aware of what position this substituent will be directed to. Since the original molecule, toluene, consists of a benzene ring bonded to an electron-donating methyl group, this directs the substitution reaction towards the ortho or para position on the ring. Of the choices shown, only the para position is shown. The ortho position is not shown as an answer choice, and direction to the meta position would not be likely.

Example Question #3 : Predicting Benzene Orientation

1

What is the product of the given reaction?

Possible Answers:

3

4

5

6

2

Correct answer:

5

Explanation:

This is a trick question on Friedal-Craft alkylation. If you didn't pick the starting material, that is because you forgot the rule that alkylation cannot happen on a highly deactivated aromatic ring. Since  is a highly deactivating group the answer is no reaction, or:

5

Example Question #4 : Predicting Benzene Orientation

What is the product for the following reaction?

Screen shot 2015 10 27 at 10.58.21 pm

Possible Answers:

Screen shot 2015 10 24 at 10.10.53 am


Screen shot 2015 10 27 at 10.58.12 pm

Screen shot 2015 10 24 at 10.10.57 am


Screen shot 2015 10 27 at 10.58.15 pm

Screen shot 2015 10 24 at 10.11.01 am

Correct answer:


Screen shot 2015 10 27 at 10.58.15 pm

Explanation:

While both the bromide and the alkyl group are ortho/para directing groups, the alkyl group is more activating and therefore the carbonyl will be added ortho to that substituent. The  and  work to reduce the carbonyl to an .

Example Question #21 : Stereochemistry

Predict the product of the given reaction.

Screen shot 2015 12 26 at 9.13.35 pm

Possible Answers:

I

II

III

IV

Correct answer:

IV

Explanation:

This is an example of a Friedel-Crafts acylation reaction, where an acyl group (an alkyl group with a carbonyl  group) is added at the carbonyl carbon (shown below). Aluminum chloride is used as a catalyst in this reaction.Screen shot 2015 12 26 at 10.36.31 pm

Example Question #22 : Stereochemistry

What is the product of the reaction shown?

Screen shot 2015 12 26 at 9.13.59 pm

Possible Answers:

III

I

II

IV

Correct answer:

II

Explanation:

This reaction is known as a nitration reaction, in which a nitro group ( group) is added to an aromatic ring. Nitric acid is used because this reaction occurs in an acidic solution.

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