In cases where a mixture of isomeric alkenes can be formed, which isomer, if
any, will predominate? Study of many dehydrohalogenation reactions has shown
that one isomer generally does predominate, and that it is possible to predict which isomer this v\ill be- that is, to predict the orientation of elimination -on the basis of molecular structure. Once more, orientation is determined by the relative rates of competing reactions. For .w-butyl bromide, attack by base at any one of three hydrogens (those on C-l) can lead to the formation of 1-butene; attack at either of two hydrogens (on C-3) can lead to the formation of 2-butene. We see that 2-butene is the preferred product that is, is formed faster despite a probability factor of 3:2 working against its formation. The other examples fit the same pattern: the preferred product is the alkene that has the greater number of alkyl groups attached to the doubly-bonded carbon atoms.
In dehydrohalogenation, the more stable the alkene the more easily it is formed.
Examination of the transition state involved shows that it is reasonable tha the more stable alkene should be formed faster: The double bond is partly formed, and the transition state has thus acquired alkene character. Factors that stabilize an alkene also stabilize an incipient alkene in the transition state. Alkene stability not only determines orientation of dehydrohalogenation, but also is an important factor in determining the reactivity of an alkyl halide toward elimination, as shown at the top of the next page.As one proceeds along a series of alkyl halides from 1 to 2 to 3, the structure by definition becomes more branched at the carbon carrying the halogen. This increased branching has two results: it provides a greater number of hydrogens for attack by base, and hence a more favorable probability factor toward elimination; and it leads to a more highly branched, more stable alkene, and hence a more stable transition state and lower act . As a result of this combination of factors,in dehydrohalogenation the order of reactivity of RX is 3 > 2 > 1.
Carbonium ions
To account for the observed facts, we saw earlier, a certain mechanism was
advanced for the halogenation of alkanes; the heart of this mechanism is the fleeting existence of free radicals, highly reactive neutral particles bearing an odd
electron.
Before we can discuss the preparation of alkenes by dehydration of alcohols, we must first learn something about another kind of reactive particle: carbonium ion, a group ofatoms that contains a carbon atom bearing only six electrons.
Carbonium ions are classified as primary, secondary, or tertiary after the carbon bearing the positive charge. They are named by use of the word cation. For exampie: Like the free radical, the carbonium ion is an exceedingly reactive particle, and for the same reason: the tendency to complete the octet of carbon. Unlike the free radical, the carbonium ion carries a positive charge.
One kind of unusually stable carbonium ion (Sec. 12.19) was recognized as early as 1902 by the salt-like character of certain organic compounds. But direct observation of simple alkyl cations should be exceedingly difficult, by virtue of
the very reactivity and hence short life that we attribute to them. Nevertheless, during the 1920 s and 193CTs, alkyl cations were proposed as intermediates in many organic reactions, and their existence was generally accepted, due largely to the work of three chemists: Hans Meerwein of Germany, "the father of modern carbonium ion chemistry;" Sir Christopher Ingold of England; and Frank Whitmore of the United States. The evidence consisted of a wide variety of observations made in studying the chemistry of alkenes, alcohols, alkyl halides, and many other kinds of organic compounds: observations that revealed a basically similar pattern of behavior most logically attributed to intermediate carbonium ions. A sizable part of this book will be devoted to seeing what that pattern is.In 1963, George Olah (now at Case Western Reserve University) reported the direct observation of simple alkyl cations. Dissolved in the extremely powerful Lewis acid SbF5 , alkyl fluorides (and, later, other halides) were found to undergo ionization to form the cation, which could be studied at leisure. There was a RF + SbF5 > R+ SbF6 dramatic change in the nmr spectrum (Chap. 13), from the spectrum of the alkyl fluoride to the spectrum of a molecule that contained no fluorine but instead j -hybridized carbon with a very low electron density. Figure 5.7 shows what was observed for the terf-butyl fluoride system : a simple spectrum but, by its very simplicity, enormously significant. Although potentially very reactive, the to/- butyl cation can do little in this environment except try to regain the fluoride ion and the SbF5 is an even stronger Lewis acid than the cation