Dehydrohalogenation, of alkyl halides

Dehydrohalogenation is the loss of a hydrogen and a halogen from an alkyl halide. It is one of the most useful methods for preparing alkenes by 3 elimination. 

Sodium ethoxide is prepared by the reaction of sodium metal with ethanol. 

When applied to the preparation of alkenes, the reaction is carried out in the presence of a strong base, such as sodium ethoxide (NaOCH2CH3) in ethyl alcohol as solvent. 

FIGURE 5.9 Dehydration of 1-butanol is accompanied by a hydride shift from C-2 to C-1. 

Similarly, sodium methoxide (NaOCHs) is a suitable base and is used in methyl alcohol. Potassium hydroxide in ethyl alcohol is another base-solvent combination often employed in the dehydrohalogenation of alkyl halides. Potassium ferf-butoxide [K0C(CH3)3] is the preferred base when the alkyl halide is primary it is used in either fcrt-butyl alcohol or dimethyl sulfoxide as solvent. 

The regioselectivity of dehydrohalogenation of alkyl halides follows the Zaitsev rule P elimination predominates in the direction that leads to the more highly substituted alkene. 

Write the structures of all the alkenes that can be formed by dehydrohalogenation of each of the following alkyl halides. Apply the Zaitsev rule to predict the alkene formed in greatest amount in each case. 

Dimethyl sulfoxide (DMSO) has the structure (CH3)2 S—or. it is a relatively inexpensive solvent, obtained as a byproduct in paper manufacture. 

Chapter 5 Structure and Preparation of Alkenes Elimination Reactions 

Mechanism 5.4 in Section 5.15 describes this reaction in more detaii. 

The concerted reaction depicted in Equation 10.2 is classified as an E2 process, where E stands for elimination and 2 refers to the molecularity of the ratedetermining step of the reaction. For E2 processes, the rate of the reaction depends upon the concentrations of the organic substrate and the base (Eq. 10.3), so both reactants are involved in the transition state of the rate-determining step. This bimolecularity is illustrated in 4 (Eq. 10.2). 

Because it bears a partial positive charge, the a-carbon atom of an alkyl halide is electrophilic and thus also subject to attack by nucleophiles, which, as Lewis bases, are electron-rich and frequently anionic species. This process produces substitution rather than elimination products (Eq. 10.4), so a possible competition 

Dehydrohalogenation may give a mixture of products if the halogen is unsymmetrically located on the carbon skeleton. Eor example, 2-bromo-2-methylbutane (6), the substrate you will use in this experiment, yields both 2-methyl-2-butene (7) and 2-methyl-l-butene (8) on reaction with strong base (Eq. 10.5). Because such elimination reactions are normally irreversible under these experimental conditions, the alkenes 7 and 8 do not undergo equilibration subsequent to their production. Consequently, the ratio of 7 and 8 obtained is defined by the relative rates of their formation. These rates, in turn, are determined by the relative free energies of the two transition states, 9 and 10, respectively, rather than by the relative free energies of the alkenes 7 and 8 themselves. 

In the absence of complicating factors, the predominant product in an E2 elimination is the more highly substituted alkene, which is 7 in the present example this observation is the source of Zaitsev s rule. The trend is observed because an increase in the number of alkyl substituents on the double bond almost always increases the stability— that is, lowers the free energy—of an alkene. Those factors that stabilize the product alkenes also play a role in stabilizing the respective transition states in which partial double-bond character is developing between the two carbon atoms. Thus, the enthalpy of activation, [Afft], for forming the more stable alkene 7 is less than that for the less stable alkene 8. 

Transition states 9 and 10 serve as an illustrative example of these principles. In 9, which is the transition state leading to the formation of 7, there can be an unfavorable steric interaction between the methyl group on the p arbon atom and the base. There is no comparable steric interaction in 10, which produces the thermodynamically less stable 8. Consequently, as the steric bulk of the base is increased, 8 will be formed in increased amounts. 

Elimination reactions are the most important means for synthesizing alkenes. In this chapter we shall study two methods for alkene synthesis based on elimination reactions dehydrohalogenation of alkyl halides and dehydration of alcohols. 

In an E2 mechanism, a base removes a /3 hydrogen from the j8 carbon, as the double bond forms and a leaving group departs from the a carbon. 

Reaction conditions that favor elimination by an El mechanism should be avoided because the results can be too variable. The carbocation intermediate that accompanies an El reaction can undergo rearrangement of the carbon skeleton, as we shall see in Section 7.8, and it can also undergo substitution by an SnI mechanism, which competes strongly with formation of products by an El path. 

Why Because steric hindrance in the substrate will inhibit substitution. 

When a synthesis must b in with a primary alkyl halide, use a bulky base. Why Because the steric bulk of the base will inhibit substimtion. 

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In addition to being regioselective dehydrohalogenation of alkyl halides is stereo selective and favors formation of the more stable stereoisomer Usually as m the case of 5 bromononane the trans (or E) alkene is formed m greater amounts than its cis (or Z) stereoisomer... 

Dehydrohalogenation of alkyl halides (Sections 5 14-5 16) Strong bases cause a proton and a halide to be lost from adjacent carbons of an alkyl halide to yield an alkene Regioselectivity is in accord with the Zaitsev rule The order of halide reactivity is I > Br > Cl > F A concerted E2 reaction pathway is followed carbocations are not involved and rearrangements do not occur An anti coplanar arrangement of the proton being removed and the halide being lost characterizes the transition state... 

Section 5 15 Dehydrohalogenation of alkyl halides by alkoxide bases is not compli cated by rearrangements because carbocations are not intermediates The mechanism is E2 It is a concerted process m which the base abstracts a proton from the p carbon while the bond between the halogen and the a carbon undergoes heterolytic cleavage... 

Just as It IS possible to prepare alkenes by dehydrohalogenation of alkyl halides so may alkynes be prepared by a double dehydrohalogenation of dihaloalkanes The dihalide may be a geminal dihalide, one m which both halogens are on the same carbon or it may be a vicinal dihalide, one m which the halogens are on adjacent carbons... 

We have previously seen (Scheme 2.9, enby 6), that the dehydrohalogenation of alkyl halides is a stereospecific reaction involving an anti orientation of the proton and the halide leaving group in the transition state. The elimination reaction is also moderately stereoselective (Scheme 2.10, enby 1) in the sense that the more stable of the two alkene isomers is formed preferentially. Both isomers are formed by anti elimination processes, but these processes involve stereochemically distinct hydrogens. Base-catalyzed elimination of 2-iodobutane affords three times as much -2-butene as Z-2-butene. 

Hydrogenation of alkynes to alkenes using the Lindlai catalyst is attractive because it sidesteps the regioselectivity and stereoselectivity issues that accompany the dehydration of alcohols and dehydrohalogenation of alkyl halides. In tenns of regioselectivity, the position of the double bond is never in doubt—it appears in the carbon chain at exactly the sane place where the triple bond was. In tenns of stereoselectivity, only the cis alkene forms. Recall that dehydration and dehydrohalogenation normally give a cis-trans mixture in which the cis isomer is the minor product. 

Dehydrohalogenation of alkyl halides (Section 7.6) General Reaction... 

As has been mentioned previously, one is most likely to find analogies to catalytic reactions on solids with acidic and/or basic sites in noncatalytic homogeneous reactions, and therefore the application of established LFERs is safest in this field. Also the interpretation of slopes is without great difficulty and more fruitful than with other types of catalysts. The structure effects on rate have been measured most frequently on elimination reactions, that is, on dehydration of alcohols, dehydrohalogenation of alkyl halides, deamination of amines, cracking of the C—C bond, etc. Less attention has been paid to substitution, addition, and other reactions. 

In 1,2-elimination, e.g. dehydrohalogenation of alkyl halide, the atoms are removed from adjacent carbons. This is also called -elimination, because a proton is removed from a P-carbon. The carbon to which the functional group is attached is called the a-carbon. A carbon adjacent to the a-carbon... 

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