Elimination Reactions of Dihalides

Alkynes can be prepared by elimination reactions under conditions similar to those used to form alkenes. Because an alkyne has two n bonds, two molar equivalents of HX must be eliminated from the starting material. One such suitable reactant is a vicinal dihahde, a compound with halogen atoms on adjacent carbon atoms. We recall that such compounds result from the addition of a halogen to the double bond of an alkene (Section 6.5). A geminal dihahde, which has both halogens on the same carbon atom, can also be used to synthesize alkynes. Geminal dichlorides can be made by reaction of phosphorus pentachloride with an aldehyde or ketone. 

The synthesis of an alkyne from either a geminal dihalide or a vicinal dihalide requires a strong base to eliminate two moles of hydrogen hahde. Two suitable reagents for this reaction are sodium amide in hquid ammonia as the solvent and potassium tert-butoxide in dimethyl sulfoxide. 

A terminal alkyne can be synthesized from a l,2-dihalo compound. We recall that this starting material can be synthesized in an addition reaction of a halogen to a monosubstituted (terminal) alkene. For example, adding chlorine to 1-hexene yields 1,2-dichlorohexane. The double dehydrohalogena-tion of 1,2-dichlorohexane yields 1-hexyne. 

Note that the synthesis of this terminal alkyne requires a total of rAree equivalents of base. The double dehydrohalogenation itself requires two equivalents. However, a third equivalent is required because, as the alkyne forms, its acidic hydrogen atom reacts with the amide ion to convert the alkyne to its conjugate base. Without the extra equivalent of amide ion, the reaction would not go to completion. The water in the workup step acts as an acid to protonate the alkynide and to convert any excess amide ion to ammonia. 

The double dehydrohalogenation occurs in two distinct steps. The second is slower than the first. The reaction can be stopped after the first step by using a weak base such as hydroxide if the vinyl hahde is the object of the synthesis. 

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Naming Alkynes Preparation of Alkynes Elimination Reactions of Dihalides... 

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... 

Reductive 1,3-elimination reaction of alkyl dihalides constitutes one of the classical methods for the preparation of cyclopropyl derivatives and is particularly useful for the synthesis of highly strained polycyclic hydrocarbons. A new preparation method of [l.l.ljpropellane, more versatile than the original Wiberg s method, has been devised3,4. Thus, treatment of l,l-dibromo-2,2-bis(chloromethyl)cyclopropane with alkyllithium or lithium powder affords [1.1. ljpropellane by two successive 1,3-eliminations of halogens by way of 1 -bromo-2-(chloromethyl)bicyclo[l. 1. Ojbutane (equation 1). This method has been... 

Alkynes can be prepared by elimination reactions of 1,2-dihalides, using a strong base. 

Some synthetic methods introduce the carbon-carbon triple bond by functional group transformations such as the elimination reaction of vicinal dihalides. Other methods construct complex alkynes by forming carbon-carbon single bonds between two molecules, one of which already contains a triple bond. We discuss both synthetic methods in this section. 

Alkenes are obtained by the transformation of various functional groups, e.g. dehydration of alcohols (see Section 5.4.3), dehalogenation of alkyl halides (see Section 5.4.5) and dehalogenation or reduction of alkyl dihalides (see Section 5.4.5). These reactions are known as elimination reactions. An elimination reaction results when a proton and a leaving group are removed from adjacent carbon atoms, giving rise to a tt bond between the two carbon atoms. 

Atkenes from sic-dihaiides. The reaction of /c-dihalides with NaSeCH3 or NaSeC6H5 in ethanol or THF-HMPT (3 1) results in alkenes. The reaction involves a formal a rr-elimination in the case of wc-dibromides or uie-chloroiodidcs, but a formal xyn-elimination in the case of w c-dichlorides. Elimination to form alkenes also occurs with /K-haloalkyl phenyl selenides. 

The opposite of an addition to a double bond is a 1,2-elimination reaction. In solution, where the reaction is promoted by solvent or by base, the most common eliminations (and those to which we shall limit our discussion) are those that involve loss of HX, although loss of X2 from 1,2-dihalides and similar reactions are also well known. The mechanisms of eliminations of HX are of three main types (1) The Ex (elimination, first-order), shown in Equation 7.22, which is the reverse of the AdE2 reaction and which has the same first, and rate-determining,... 

A procedure which combines characteristics of the Simmons-Smith reaction with the previously discussed a-eliminations (see Section 4.6.3.1) is the reaction of a geminal dihalide with an alkyllithium re-... 

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