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  Elimination Reactions of Dihalides Homework Help - K-12 Grade Level, College Level Chemistry

Introduction of Reactions of Dihalides

If two halogen atoms are exist in a given compound, reactions with reducing metals might take distinct paths depending on how close the carbon-halogen bonds are to each another.A bis-organometallic compound might be created, If they are separated by four or more carbons, as in the first instance below,. Though, an elimination occurs with formation of a double bond, if the halogens are bonded to adjacent (vicinal) carbons. Because vicinal-dihalides are generally made by adding a halogen to a double bond, this reaction is chiefly helpful for relating structures to each other. The last instance, where two halogens are bonded to similar carbon, considered as as geminal (twinned), gives an not usual reagent which may either react as a carbon nucleophile or, by elimination, as a carbene. Such type of reagents are frequently termed carbenoid.

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The solution structure of the Simmons-Smith reagent is less well recognized than that of the Grignard reagent, but the formula shown here is as helpful as any that have been proposed. Another alpha-halogenated organometallic reagents, like ClCH2Li, BrCH2Li, Cl2CHLi and Cl3CLi, have been prepared, but they are considerably less stable and might be maintained at very low temperature (ca. -100 º C) to avoid loss of LiX. The usefulness and stability of the Simmons-Smith reagent may be attributed in part to the higher covalency of the carbon-zinc bond together with internal and solvation coordination of the zinc. Hydrolysis (reaction with water) provides methyl iodide, verifying the basicity of the carbon; and reaction with alkenes gives cyclopropane derivatives, illustrating the carbene-like nature of the reagent. The latter transformation is depicted by the equation on the right.

Elimination reactions of the stereoisomeric 1,2-dibromo-1,2-diphenylethanes give a nice abstract of the principles discussed above. The following example displays first the meso-diastereomer and below it one enantiomer of the racemic-diastereomer. In each example two conformers are display in parentheses and the anti-relationship of selected vicinal groups in each is colored green in the diagram. The reaction taking place to the left is a dehydrohalogenation induced by treatment with KOH in alcohol. Because this is a stereospecific elimination, each diastereomer gives a distinct stereoisomeric product. The reaction to right is a dehalogenation (the opposite of halogen addition to an alkene), cause by treatment with iodide anion. Zinc dust influence similar reaction, but with a lower degree of stereospecificity. The technique of the iodide anion reaction is displayed by red arrows in the top instance. An identical mechanism depicts the comparable elimination of the racemic isomer. In both reactions an anti-transition state is observed.

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The two stereoisomers of 1-bromo-1,2-diphenylethene (displayed on the left of the picture) go through a second dehydrobromination reaction on more energetic treatment with base, as displayed in the following equation. This elimination generates similar alkyne (carbon-carbon triple bond) from each of the bromo-alkenes. Interestingly, the (Z)-isomer (lower structure) eliminates more quickly than the (E)-isomer (upper structure), again displaying a preference for anti-orientation of eliminating groups.

C6H5CH=CBrC6H5   +   KOH   -->    C6H5C≡CC6H5   +   KBr   +   H2O

Preparation of Alkynes by Dehydrohalogenation

The last reaction displayed above suggests that alkynes may be prepared from alkenes by a two stage technique, consisting first of bromine or chlorine addition to the double bond and secondly base induced double dehydrohalogenation. For an instance, reaction of 1-butene with bromine would give 1,2-dibromobutane and on the treatment with base this vicinal dibromide would be supposed to yield 1-bromo-1-butene followed by a second elimination to 1-butyne.

CH3CH2CH=CH2   +   Br2   -->  CH3CH2CHBr-CH2Br   +   base   -->  CH3CH2CH=CHBr   +   base   -->  CH3CH2C≡CH

In practice this technique works, but it needs care in the selection of the solvent and base. If KOH in alcohol is employes, the first elimination is much faster than the second, so the bromoalkene may be isolated if needed. Under more severe conditions the second elimination occurs, but isomerization of the triple bond also occurs, with the more stable isomer (2-butyne) being created along with 1-butyne, even becoming the major product. To assist the second elimination and avoid isomerization the very strong base sodium amide, NaNH2, may be employed. Because ammonia is a much weaker acid than water (by a factor of 1018) its conjugate base is proportionally stronger than hydroxide anion (the conjugate base of water) and from the bromoalkene, the elimination of HBr may be conducted at relatively low temperature. as well, the acidity of the sp-hybridized C-H bond of the terminal alkyne traps the primarily produced 1-butyne in the form of its sodium salt.

CH3CH2C≡CH   +   NaNH2   -->  CH3CH2C≡C:(-) Na(+)   +   NH3

An additional complication of this technique is that the 1-bromo-1-butene product of the first elimination (see previous equations) is accompanied by its 2-bromo-1-butene isomer, CH3CH2CBr=CH2 and elimination of HBr from this bromoalkene not only gives 1-butyne (base attack at C-1) but also 1,2-butadiene, CH3CH=C=CH2, through base attack at C-3. Dienes of this type, where the central carbon is sp-hybridized, are called allenes and are termed to havecumulated double bonds. They are generally less stable than their alkyne isomers.

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