Organic Chemistry : Identifying Reaction Mechanisms

Study concepts, example questions & explanations for Organic Chemistry

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Example Questions

Example Question #4 : Other Reaction Mechanisms

Which of these would react fastest with methanol via an SN1 mechanism?

Possible Answers:

All of these will react with similar rates when undergoing an SN1 reaction with methanol

Correct answer:

Explanation:

An SN1 mechanism involves the leaving of the bromine in the first step and the formation of a carbocation on that carbon. The molecule with the most stable carbocation will react most quickly.

The carbocation is most stable on the most highly substituted carbon. All the of the answer options form primary or secondary cations except for one. The correct answer has a carbocation on a carbon bonded to two methyl groups and one ethyl group. The correct answer is .

Example Question #1 : Identifying Reaction Mechanisms


Img 20150723 140242

What is the major product of the reaction shown above?

Possible Answers:

Img 20150723 140319

Img 20150723 140248

Img 20150723 140303

Img 20150723 140255

Correct answer:

Img 20150723 140248

Explanation:

Grignard reagents attack ketones at the site of the ketone's carbon to produce tertiary alcohols.

Example Question #2 : Identifying Reaction Mechanisms

Under which of the following sets of conditions is an SN2 reaction most favored?

Possible Answers:

Ether solvent, primary electrophile, strong nucleophile

Acetone solvent, tertiary electrophile, strong nucleophile

DMSO solvent, primary electrophile, weak nucleophile

Ethanol solvent, primary electrophile, strong nucleophile

Methanol solvent, tertiary electrophile, weak nucleophile

Correct answer:

Ether solvent, primary electrophile, strong nucleophile

Explanation:

SN2 reactions require a strong nucleophile. If the the nucleophile is weak, SN1 is favored over SN2.

For SN2 reactions less substitution in the electrophile is favored (methyl > primary > secondary).

For SN2 reactions polar aprotic solvents are preferred (DMSO, acetone, DMF and ethers are common polar aprotic solvents). Polar protic solvents (ethanol, methanol) favor SN1 reactions.

Example Question #31 : Reaction Mechanisms, Energetics, And Kinematics

Consider the generic reaction shown below.

What type of reaction is occurring here?

Possible Answers:

Substitution

Rearrangement

Addition

Elimination

Correct answer:

Substitution

Explanation:

In this question, we're presented with a hypothetical reaction, and we're asked to identify which type of reaction is occurring. Let's go through each of the answer choices to see which one fits.

Addition reactions are ones in which two atoms or molecules come together. It takes the form of .

Elimination reactions are essentially the opposite of addition reactions. In this case, a molecule splits into two, taking the form of .

Rearrangement reactions are ones in which the reactant is manipulated in a way that gives product, but in the process, no additional reactants are consumed and no additional products are made. It takes the general form of .

Substitution reactions are ones in which there is some sort of exchange of atoms or functional groups between different reactants to give new products. In the reaction shown in the question stem, this qualifies as a substitution reaction.

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