Organic Chem Help ! (paper)

(a)

(b)

Chapter 11: Reactions of Alkyl Halides: Nucleophilic Substitutions and Eliminations A Summary of Reaction Characteristics Book Title: Organic Chemistry Printed By: Joetta Lloyd ([email protected]) © 2016 Cengage Learning, Cengage Learning

A Summary of Reaction Characteristics

The effects on reactions of the four variables—substrate, leaving group, nucleophile, and solvent—are summarized in the following statements:

Substrate The best substrates yield the most stable carbocations. As a result, reactions are best for tertiary, allylic, and benzylic halides.

Leaving group

Good leaving groups increase the reaction rate by lowering the energy level of the transition state for carbocation formation.

Nucleophile The nucleophile must be nonbasic to prevent a competitive elimination of (Section 11-7), but otherwise does not affect the reaction rate. Neutral nucleophiles work well.

Solvent Polar solvents stabilize the carbocation intermediate by solvation, thereby increasing the reaction rate.

Worked Example 11-2

Predicting the Mechanism of a Nucleophilic Substitution Reaction

Predict whether each of the following substitution reactions is likely to be or :

(a)

(b)

Strategy

Look at the substrate, leaving group, nucleophile, and solvent. Then decide from the summaries at the ends of Sections 11-3 and 11-5 whether an or an reaction is favored. reactions are favored by tertiary, allylic, or benzylic substrates, by good leaving groups, by nonbasic nucleophiles, and by protic solvents. reactions are favored by primary substrates, by good leaving groups, by good nucleophiles, and by polar aprotic solvents.

Solution

This is likely to be an reaction because the substrate is secondary and benzylic, the nucleophile is weakly basic, and the solvent is protic.

This is likely to be an reaction because the substrate is primary, the nucleophile is a good one, and the solvent is polar aprotic.

Problem 11-13

Predict whether each of the following substitution reactions is likely to be or :

a.

b.

Chapter 11: Reactions of Alkyl Halides: Nucleophilic Substitutions and Eliminations A Summary of Reaction Characteristics Book Title: Organic Chemistry Printed By: Joetta Lloyd ([email protected]) © 2016 Cengage Learning, Cengage Learning

© 2019 Cengage Learning Inc. All rights reserved. No part of this work may by reproduced or used in any form or by any means - graphic, electronic, or mechanical, or in any other manner - without the written permission of the copyright holder.