Keys to Success Substitution Versus Elimination— Structure Determines Function

KEYS TO SUCCESS SUBSTITUTION VERSUS ELIMINATION-STRUCTURE DETERMINES FUNCTION 

The multiple reaction pathways—Sn2, SnI, E2, and El— that haloalkanes may foUow in the presence of nucleophiles may seem confusing. Given the many parameters that affect the relative importance of these transformations, are there some simple guidelines that might allow us to predict, at least roughly, what the outcome of any particular reaction will be The answer is a cautious yes. This section will explain how consideration of base strength and steric bulk of the reacting species can help us decide whether substimtion or elimination will predominate. We shall see that variation of these parameters may even permit control of the reaction pathway. 

Good nucleophiles that are weaker bases than hydroxide give good yields of Sn2 products with primary and secondary halides and of S l products with tertiary substrates. Examples include I , Br , RS , N3 , RCOO , and PR3. Thus, 2-bromopropane reacts with both iodide and acetate ions cleanly through the 8 2 pathway, with virtually no competing elimination. 

Weak nucleophiles such as water and alcohols react at appreciable rates only with secondary and tertiary halides, substrates capable of following the SnI pathway. Unimolecular elimination is usually only a minor side reaction. 

Strongly basic nucleophiles give more elimination as steric bulk increases 

---

Chapter 3, Section 3-6 KEYS TO SUCCESS USING THE KNOWN MECHANISM AS A MODEL FORTHE UNKNOWN Chapter 6, Section 6-9 KEYS TO SUCCESS CHOOSING AMONG MULTIPLE MECHANISTIC PATHWAYS Chapter 7, Section 7-8 KEYS TO SUCCESS SUBSTITUTION VERSUS ELIMINATION—STRUCTURE DETERMINES FUNCTION... 

---