Introduction to Elimination of Water
In the discussion of alkyl halide reactions we noticed that when treated with strong bases, like hydroxide and alkoxides 2º and 3º-alkyl halides practised fast E2 elimination. Alcohols do not go through such type of base-induced elimination reactions and are, actually, frequently used as solvents for such reactions. However this is other instance of how leaving group stability frequently affects the rate of a reaction. With sodium hydroxide when an alcohol is treated, the following acid - base equilibrium comes into existence. Several alcohols are little weaker acids than water so the left side is favored.
R-O-H + Na(+) OH(-) R-O(-) Na(+) + H-OH
Dehydration is the elimination of water from an alcohol. Remembering that water is a much better leaving group than hydroxide ion, it is reasonable to make use of acid-catalysis rather than base-catalysis to achieve such type of reactions. Four examples of this useful method are displayed below in the diagram. Note: hydrohalic acids (HX) are not generally employed as catalysts because their conjugate bases are good nucleophiles and may give substitution products. The conjugate bases of phosphoric and sulfuric acids are not good nucleophiles and do not give replacement under the general conditions of their use.
The first two illustrations (top row) are typical, and the more facile elimination of the 3º-alcohol implies predominant E1 character for the reaction. With the tendency of branched 1º this agrees and 2º-alcohols to give rearrangement products, as displayed in the last illustration. The last two reactions also depicted that the Zaitsev Rule implies to alcohol dehydrations as well as alkyl halide eliminations. So the more highly-substituted double bond isomer is favored among the products. It should be noticed that the acid-catalyzed dehydrations described here are the opposite of the acid-catalyzed hydration reactions of alkenes. Certainly, for reversible reactions like this the laws of thermodynamics need that the mechanisms in both directions proceed by similar reaction path. This is termed as the principle of microscopic reversibility. To demonstrate, the following picture lists the three steps in each transformation. The dehydration reaction is displayed by the blue arrows in the diagram; the hydration reaction by magenta arrows. The intermediates into these reactions are universal to both, and general transition states are involved.
If their sulfonate ester derivatives are used base induced E2 eliminations of alcohols might be accomplished. This has the benefit of avoiding strong acids, which may be cause molecular rearrangement and / or double bond migration in some cases. Because 3º-sulfonate derivatives are sometimes unstable, this method is best employed with 1º and 2º-mesylates or tosylates. Application of this reaction sequence is displayed here for 2-butanol. Zaitsev Rule favors formation of 2-butene (cis + trans) over 1-butene.
By treatment with phosphorous oxychloride (POCl3) in pyridine, the E2 elimination of 3º-alcohols under comparatively non-acidic conditions may be accomplished. This method is also effective with hindered 2º-alcohols, but for unhindered and 1º-alcohols an SN2 chloride ion substitution of the chlorophosphate intermediate competes with elimination. Some illustrations of these and associated reactions are given in the following diagram. The first equation displays the dehydration of a 3º-alcohol. Predominance of the non-Zaitsev product (less substituted double bond) is supposed because of steric hindrance of the methylene group hydrogens that interferes with the approach of base at that site. The second example depicts two elimination procedures applied to similar 2º-alcohol. The first uses the single step POCl3 technique, which works well in this example because SN2 substitution is retarded by steric hindrance. The second technique is other example in which an intermediate sulfonate ester confers halogen-like reactivity on an alcohol. In each example the anionic leaving group is the conjugate base of a strong acid.
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