Dehydration reactions carbocation rearrangements

Like alcohol dehydrations El reactions of alkyl halides can be accompanied by carbocation rearrangements Eliminations by the E2 mechanism on the other hand nor mally proceed without rearrangement Consequently if one wishes to prepare an alkene from an alkyl halide conditions favorable to E2 elimination should be chosen In prac tice this simply means carrying out the reaction m the presence of a strong base... 

The pinacol rearrangement is a dehydration reaction that converts a 1,2-diol into a ketone. The reaction involves two carbocation intermediates. 

The dehydration reaction has some limitations. Because the mechanism is El and involves a carbocation, rearrangements are possible. Figure 10.7 shows an example of a dehydration involving a carbocation rearrangement. In addition, the reaction is not... 

Carbocation rearrangements involving migration of H or alkyl groups don t just happen in NMR machines. They happen during normal reactions too. For example, acid-catalysed dehydration of the... 

Rearrangements are not unique to dehydration reactions. Rearrangements can occur whenever a carbocation is formed as reactive intermediate, meaning any SnI or El reaction. In fact, the formation of rearranged products often indicates the presence of a carbocation intermediate. 

Dehydration of tert-alcohols.2 This reaction can be effected with BF3 etherate (1-3 equiv.) in CH2CI2 at 25°. Yields are usually higher than those obtained with the Burgess reagent, thionyl chloride/amine, or p-tolucncsulfonic acid. When dehydration could result in two different alkencs, the more thermodynamically stable alkenc predominates. Noepcntyl tert-alcohols result in mixtures of alkencs, some resulting from a carbocation rearrangement. 

The effects of the ethyl and the n-propyl groups at the C atom on the rates of carbocation rearrangements under the long-life conditions (Table 8) are usually close to that of the CH3 group although, as distinct from the available data on the reactions of solvolysis and dehydration, the qualitative constancy of the effects (C,H, > CH3) is not observed here. A considerable decrease in the pushing effect of the ethyl group in the case of ketone in item 9 of Table 8 is probably due to the intervention of conformational factors... 

When alcohols undergo dehydration reactions, alkenes are generated. There is a chance of rearrangement of the carbocation intermediates to form more stable carbocation intermediates, whenever possible, resulting in the formation of more than one type of alkenes. The mechanisms of such rearrangements are discussed in Chapter 21. 

Alcohols undergo acid-catalyzed dehydration reactions to form alkcnes. The reaction is accomplished by the formation of a carbocation intermediate. Thus there is the possibility of rearrangement in the process. First, we will take a look at the mechanism of this reaction. 

Dehydration (loss of H2O) of an alcohol is promoted by treatment with a strong, non-nucleophilic acid (such as H2SO4 or H3PO4) and heat. Alcohol dehydration involves an El mechanism with a carbocation intermediate. Since carbocations can rearrange, the double bond can end up anywhere on the carbon chain, and the most stable, most highly substituted alkene is expected as the major product (follows Zaitsev s rule, with possible rearrangement of the carbon skeleton). The planar carbocation intermediate also results in a loss of stereochemistry so the more stable stereoisomer will be produced as the major product (trans or E isomer). The dehydration reaction is most useful when only a single product is possible otherwise, a mixture of alkene products is likely to be obtained. 

If an elimination reaction is planned, the retrosynthesis of an alkene involves an FGI that adds either HX or water to the alkene to give an alkyl halide or an alcohol starting material, respectively. The stability of the alkene TM will help determine whether an E2 or El elimination would be more suitable. For example, an unstable, terminal alkene cannot be prepared via dehydration because a rearrangement of the carbocation intermediate would be expected. Care must be taken such that the alkyl halide or alcohol starting material would give only the desired TM as the major product. 

In the previous section, we saw that additional steps can accompany an El process. In contrast, an E2 process consists of one concerted step and is rarely accompanied by any other steps. A carbocation is never formed, and therefore, there is no possibility for a carbocation rearrangement. In addition, E2 conditions generally require the use of a strong base, and an OH group cannot be protonated under such conditions. It is therefore not common to see an E2 process with a proton transfer at the beginning of the mechanism. It is much more common to see a proton transfer at the beginning of an El process (a dehydration reaction). All of the E2 processes that we will encounter in this textbook will be comprised of just one concerted step, as seen in Mechanism 8.2. 

Carbocation Rearrangement in Dehydration of 3,3-Dimethyl-2-butanol THE OVERALL REACTION ... 

Be sure to check the structure of the carbocation formed in a dehydration reaction for the possibility of rearrangement. Remember that a carbocation will rearrange if rearrangement produces a more stable carbocation (Section 6.7). For example, the secondary carbocation formed initially in the following reaction rearranges to a more stable tertiary carbocation ... 

The pinacol rearrangement is a dehydration of an alcohol that results in an unexpected product. When hot sulfuric acid is added to an alcohol, the expected product of dehydration is an alkene. However, if the alcohol is a vicinal diol, the product will be a ketone or aldehyde. The reaction follows the mechanism shown, below. The first hydroxyl group is protonated and removed by the acid to form a carboca-tion in an expected dehydration step. Now, a methyl group may move to fonn an even more stable carbocation. This new carbocation exhibits resonance as shown. Resonance Structure 2 is favored because all tire atoms have an octet of electrons. The water deprotonates Resonance Structure 2, forming pinacolone and regenerating the acid catalyst. 

D is correct. This reaction is dehydration of an alcohol and proceeds with rearrangement of the carbocation intermediate from secondary to tertiary. (See page 35.)... 

Like other El reactions, alcohol dehydration follows an order of reactivity that reflects carbocation stability 3° alcohols react faster than 2° alcohols, and 1° alcohols are the least reactive. Rearrangements of the carbocation intermediates are common in alcohol dehydrations. In most cases, Zaitsev s rule applies The major product is usually the one with the most substituted double bond. 

The synthesis of benzomorphans substituted in the 11-position from a 2-tetralone (Scheme 4.2, p. 157) involves either thionyl chloride dehydration of an 11-methylcarbinol (13a)(9) or a pyrolysis of the corresponding acetate perchlorate (136)methylene derivative (14). Carbocation-mediated processes in bridged compounds of this type are likely to give rearrangement byproducts. Kugita and Takeda(124) isolated from the thionyl chloride route an unstable chloride (203), and the indenotetrahydropyridine (204). Other reports(30 96a,125) of unidentified products from these reactions resulted in an investigation 126 127 of both pathways (Scheme 4.20). 

---