Dehydrohalogenate

Apparatus. 3-1 Three-necked round-bottomed flask, provided with dropping funnel, stirrer and thermometer + vent for the bromination and acetalization 10-1 wide--necked flask for the dehydrohalogenation, manual swirling. 

This concludes discussion of our second functional group transformation mvolv mg alcohols the first was the conversion of alcohols to alkyl halides (Chapter 4) and the second the conversion of alcohols to alkenes In the remaining sections of the chap ter the conversion of alkyl halides to alkenes by dehydrohalogenation is described... 

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 p elimination... 

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

The regioselectivity of dehydrohalogenation of alkyl halides follows the Zaitsev rule p elimination predominates m the direction that leads to the more highly substi tuted alkene... 

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 cycloalkyl halides lead exclusively to cis cycloaUcenes when the ring has fewer than ten carbons As the ring becomes larger it can accommo date either a cis or a trans double bond and large nng cycloalkyl halides give mixtures of CIS and trans cycloalkenes... 

In the 1920s Sir Christopher Ingold proposed a mechanism for dehydrohalogenation that IS still accepted as the best description of how these reactions occur Some of the mfor matron on which Ingold based his mechanism included these facts... 

On the basis of these observations Ingold proposed a one step bimolecular E2 mechanism for dehydrohalogenation... 

FIGURE 5 12 The El mechanism for the dehydrohalogenation of 2 bromo 2 methylbutane in ethanol... 

There is a strong similarity between the mechanism shown m Eigure 5 12 and the one shown for alcohol dehydration m Eigure 5 6 The mam difference between the dehy dration of 2 methyl 2 butanol and the dehydrohalogenation of 2 bromo 2 methylbutane IS the source of the carbocation When the alcohol is the substrate it is the correspond mg alkyloxonmm ion that dissociates to form the carbocation The alkyl halide ionizes directly to the carbocation... 

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... 

We now have a new problem Where does the necessary alkene come from Alkenes are prepared from alcohols by acid catalyzed dehydration (Section 5 9) or from alkyl halides by dehydrohalogenation (Section 5 14) Because our designated starting material is tert butyl alcohol we can combine its dehydration with bromohydrm formation to give the correct sequence of steps... 

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... 

The most frequent applications of these procedures he in the preparation of terminal alkynes Because the terminal alkyne product is acidic enough to transfer a proton to amide anion one equivalent of base m addition to the two equivalents required for dou ble dehydrohalogenation is needed Adding water or acid after the reaction is complete converts the sodium salt to the corresponding alkyne... 

Double dehydrohalogenation to form terminal alkynes may also be carried out by heating geminal and vicinal dihalides with potassium tert butoxide m dimethyl sulfoxide... 

Double dehydrohalogenation of gemmal dihalides (Section 9 7) An E2 elimination reaction of a gemmal dihalide yields an alkenyl halide If a strong enough base IS used sodium amide for example a second elimination step follows the first and the alkenyl halide IS converted to an alkyne... 

Double dehydrohalogenation of vicinal dihalides (Section 9 7) Dihalides in which the halogens are on adjacent carbons undergo two elimination processes analogous to those of gemmal dihalides... 

As we saw in Chapter 5 dehydrations and dehydrohalogenations are typically regiose lective m the direction that leads to the most stable double bond Conjugated dienes are more stable than isolated dienes and are formed faster via a lower energy transition state... 

Alkenylbenzenes are prepared by the various methods described m Chapter 5 for the preparation of alkenes dehydrogenation dehydration and dehydrohalogenation... 

Acid catalyzed dehydration of benzyhc alcohols is a useful route to alkenylben zenes as is dehydrohalogenation under E2 conditions... 

Dehydrohalogenation of the diastereomenc forms of 1 chloro 1 2 diphenylpropane is stereo specific One diastereomer yields (E) 1 2 diphenylpropene and the other yields the Z isomer Which diastereomer yields which alkene" Why" ... 

Dehydrogenation of alkylbenzenes although useful m the industrial preparation of styrene is not a general procedure and is not well suited to the laboratory prepara tion of alkenylbenzenes In such cases an alkylbenzene is subjected to benzylic bromi nation (Section 11 12) and the resulting benzylic bromide is treated with base to effect dehydrohalogenation... 

Second stage Dissociation of the tetrahedral intermediate by dehydrohalogenation... 

Dehydrohalogenation (Section 5 14) Reaction in which an alkyl halide on being treated with a base such as sodium ethoxide is converted to an alkene by loss of a proton from one carbon and the halogen from the adjacent carbon... 

Double dehydrohalogenation (Section 9 7) Reaction in which a geminal dihahde or vicinal dihahde on being treated with a very strong base such as sodium amide is converted to an alkyne by loss of two protons and the two halogen substituents... 

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