Halides, elimination reactions of alkyl

Elimination reactions of alkyl halides are important reactions that compete with substitution reactions. hi an elimination reaction the fragments of some molecule (YZ) are removed (eliminated) from adjacent atoms of the reactant. This elimination leads to the creation of a multiple bond  

A widely used method for synthesizing alkenes is the ehminalion of HX from adjacent atoms of an alkyl halide. Heating the alkyl halide with a strong base causes the reaction to take place. The following are two examples  

Reactions like these are not limited to the elimination of hydrogen bromide. Chloroalkanes also undergo the elimination of hydrogen chloride, iodoalkanes undergo the elimination of hydrogen iodide, and, in aU cases, alkenes are produced. When the elements of a hydrogen halide are eliminated from a haloalkane in this way, the reaction is often called dehydrohalogenation  

In these eliminations, as in SnI and Sn2 reactions, there is a leaving group and an attacking Lewis base that possesses an electron pair. 

Chemists often call the carbon atom that bears the leaving group (e.g the halogen atom in the previous reaction) the alpha (a) carbon atom and any carbon atom adjacent to it a beta (/S) carbon atom. A hydrogen atom attached to the j8 carbon atom is called a fi hydrogen atom. Since the hydrogen atom that is eliminated in dehydrohalogenation is from the /3 carbon atom, these reactions are often called /3 eliminations. They are also often referred to as 1,2 eliminations. 

---

Alkyl halides are encountered less frequently than their oxygen-containing relatives alcohols and ethers, but some of the kinds of reactions they undergo—nucleophilic substitutions and eliminations—are encountered frequently. Thus, alkyl halide chemistry acts as a relatively simple model for many mechanistically similar but structurally more complex reactions found in biornolecules. We ll begin in this chapter with a look at how to name and prepare alkyl halides, and we ll see several of their reactions. Then in the following chapter, we ll make a detailed study of the substitution and elimination reactions of alkyl halides—two of the most important and well-studied reaction types in organic chemistry. 

Elimination Reactions of Alkyl Halides Zaitsev s Rule 383... 

Scheme4.25 Substitution and elimination reactions of alkyl halides with base.

Two special cases need to be considered activation by alkyl halide and by a formal cationic center. The interaction diagrams are shown in Figures 10.6a,b, respectively. These are involved in elimination reactions of alkyl halides, E2 and El, respectively. 

Exercise 8-26 a. Why is potassium ferf-butoxide, KOC(CH3)3, an excellent base for promoting elimination reactions of alkyl halides, whereas ethylamine, CH3CH2NH2, is relatively poor for the same purpose ... 

The term stereoselective is often confused with the term stereospecific, and the literature abounds with views as to the most satisfactory definition. To offer some clarification, it is perhaps timely to recall a frequently used term, introduced a decade or so ago, namely the stereoelectronic requirements of a reaction. All concerted reactions (i.e. those taking place in a synchronised process of bond breaking and bond forming) are considered to have precise spatial requirements with regard to the orientation of the reactant and reagent. Common examples are SN2 displacement reactions (e.g. Section 5.10.4, p. 659), E2 anti) elimination reactions of alkyl halides (e.g. Section 5.2.1, p.488), syn (pyrolytic) elimination reactions (Section 5.2.1, p.489), trans and cis additions to alkenes (e.g. Section 5.4.5, p. 547), and many rearrangement reactions. In the case of chiral or geometric reactants, the stereoisomeric nature of the product is entirely dependent on the unique stereoelectronic requirement of the reaction such reactions are stereospecific. 

Elimination reactions of alkyl halides (dehydrohalogenations) are a useful method of synthesising alkenes. For good yield, a... 

In the E2 elimination reaction of alkyl halides (one-step reaction) a base abstracts a proton adjacent to the leaving group, forcing the removal of the leaving group with the formation of a double bond (Scheme 4.1). 

Because elimination reactions of alkyl halides form alkenes, let s review earlier material on alkene structure and learn some additional facts as well. 

The stereochemistry of dehydrohalogenation with DBU has been studied by several authors. Wolkoff (82JOC1944) studied in detail the stereochemical consequences of dehydrohalogenation of secondary and tertiary alkyl and cycloalkyl halides with DBU. A comparison of the product distributions obtained in the elimination reactions of alkyl halides with DBU, with weak bases, and with anionic bases indicated that the elimination reactions with DBU very probably follow an E2C-like mechanism. 

The two main mechanisms for elimination reactions of alkyl halides are El and E2. In both cases, the mechanism involves the loss of HX from RX to give an alkene. 

Chapter 11 covers the elimination reactions of alkyl halides. Because alkyl halides can undergo both substitution and elimination reactions, this chapter also discusses the factors that determine whether a given alkyl halide will undergo a substitution reaction, an elimination reaction, or both substitution and elimination reactions. 

In Chapter 11, we will discuss the elimination reactions of alkyl halides and the factors that determine whether substitution or elimination will prevail when an alkyl halide undergoes a reaction. 

---