Resonance-stabilized carbocation reaction

Either oxygen of the hemiacetal can be protonated. When the hydroxyl oxygen is protonated, loss of water leads to a resonance-stabilized carbocation. Reaction of this carbocation with the alcohol, which is usually the solvent and is present in large excess, gives (after proton loss) the acetal. The mechanism is like an S l reaction. Each step is reversible. 

An electrophilic aromatic substitution reaction takes place in two steps—initial reaction of an electrophile, E+, with the aromatic ring, followed by loss of H+ from the resonance-stabilized carbocation intermediate to regenerate the aromatic ring. 

Initially, the reaction involves protonation of one of the oxygen atoms, followed by loss of this group as a neutral molecule and formation of a resonance-stabilized carbocation. If the oxygen protonated were that of the alkoxy group, then the product would merely be the protonated aldehyde, and the reaction becomes a reversal of hemiacetal formation. Only when the oxygen of the hydroxyl is protonated can the reaction lead to an acetal, and this requires nucleophilic attack of the second alcohol molecule on to the alternative resonance-stabilized carbocation. 

Protonation of the alcohol can be accomplished by using the halogen acids, HC1, HBr, and HI, which also provide the nucleophile for the reaction. These reaction conditions favor the SN1 mechanism, although primary alcohols still follow the SN2 path unless a resonance-stabilized carbocation can be formed. The acids HBr and HI work with most alcohols, but HC1, a weaker acid, requires the presence of ZnCl2 (a Lewis acid) as a catalyst when the alcohol is primary or secondary. Examples are shown in the following equations ... 

Then, reaction of the alcohol with triphenylphosphine under acidic conditions produces the phosphonium salt by an SNI reaclion. A resonance-stabilized carbocation is formed and reacts at the terminal position of the chain. 

In general, we should expect rearrangements in reactions involving carbocations whenever a hydride shift or an alkyl shift can form a more stable carbocation. Most rearrangements convert 2° (or incipient 1°) carbocations to 3° or resonance-stabilized carbocations. 

Figure 5.6 Proposed mechanism for the cyclization of geranyl diphosphate to sabinene and sabinene hydrate under catalysis by monoterpene synthases the reaction begins with the hydrolysis of the diphosphate moiety to generate a resonance-stabilized carbocation (1) the carbocation then isomerizes to an intermediate capable of cyclization by return of the diphosphate (2) and rotation around a single bond (3) after a second diphosphate hydrolysis (4) the resulting carbocation undergoes a cyclization (5) a hydride shift (6) and a second cyclization (7) before the reaction terminates by deprotonation (8) or capture of the cation by water (9). Cyclizations, hydride shifts and a variety of other rearrangements of carbocationic intermediates are a characteristic of the mechanisms of terpene synthases. No known terpene synthase actually produces both sabinene and sabinene hydrate these are shown to indicate the possibilities for reaction termination. PP indicates a diphosphate moiety.

The mechanism of electrophilic addition of HX involves two steps addition of H (from HX) to form a resonance-stabilized carbocation, followed by nucleophilic attack of X at either electrophilic end of the carbocation to form two products. Mechanism 16.1 illustrates the reaction of 1,3-butadiene with HBr. 

In bromination (Mechanism 18.2), the Lewis acid FeBr3 reacts with Br2 to form a Lewis acid-base complex that weakens and polarizes the Br- Br bond, making it more electrophilic. This reaction is Step [1] of the mechanism for the bromination of benzene. The remaining two steps follow directly from the general mechanism for electrophilic aromatic substitution addition of the electrophile (Br in this case) forms a resonance-stabilized carbocation, and loss of a proton regenerates the aromatic ring. 

This reaction is another example of electrophilic aromatic substitution, with the diazonium salt acting as the electrophile. Like all electrophilic substitutions (Section 18.2), the mechanism has two steps addition of the electrophile (the diazonium ion) to form a resonance-stabilized carbocation, followed by deprotonation, as shown in Mechanism 25.4. 

The mechanism of the electrophilic bromination of benzene. The reaction occurs in two steps and involves a resonance-stabilized carbocation Intermediate. 

STEP 2 Reaction of a nucleophile and an electrophile to form a new covalent bond. Reaction of the isopropyl cation with benzene gives a resonance-stabilized carbocation intermediate ... 

That is, the easier it is to form the carbocation, the faster the reaction will proceed. For this reason, S l reactivity is also favored for resonance-stabilized carbocations, such as allylic carbocations (see Sec. 3.15). Likewise, S l reactivity is disfavored for aryl and vinyl halides because aryl and vinyl carbocations are unstable and not easily formed. 

In the reaction with a Br0nsted-Lowry acid or a Lewis acid, a carbonyl reacts similarly to alkenes. The major difference is formation of the oxocarbenium ion products, which are resonance stabilized, whereas a simple noncon-jugated alkene forms a carbocation (a carbenium ion) that is not resonance stabilized. Some alkenes are exceptions to this statement. When the C=C unit is conjugated to another x-bond, as in styrene (phenylethene, 25), reaction with an acid generates a resonance-stabilized carbocation, 26. A conjugated carbonyl compound such as benzaldehyde (27) will also form a conjugated oxocarbenium ion (28) that is resonance stabilized as shown in the illustration. 

Because it is aromatic, benzene does not react directly with reagents such as HBr or HCl, or even with diatomic bromine or chlorine. Benzene reacts with cationic species to ve a resonance-stabilized carbocation intermediate, which loses a hydrogen to give a substitution product. This reaction is called electrophilic aromatic substitution. The most common method for generating reactive cations in the presence of benzene is to treat certain reagents with strong Lewis acids. Lewis acids or mixtures of strong acids can be used to convert benzene to chlorobenzene, bromobenzene, nitrobenzene, or benzenesulfonic acid. 

A crude comparison of the relative stability of cyclohexene (2), cyclohexa-diene (3), and benzene (1 using one Kekule structure implies both resonance forms lA and IB) is available by examining the reaction of each with HBr. The reaction of 2 and HBr is rapid and the product is carbocation 4, which traps bromide to give bromocyclohexane 5 (see Chapter 10, Section 10.2). Cyclohexadiene (3) also reacts rapidly with HBr to give an allylic cation, 6. This is a resonance-stabilized carbocation, and it reacts with bromide to give allylic bromide 7. This reaction is more complicated due to the presence of the second C=C rmit, which leads to resonance stability for 7. Carbocations of this type will be discussed in detail in Chapter 23. 

What is happening in this deuteration And why is the reaction diverted from ordinary addition The answers to these questions will serve to summarize much of the chemistry of aromatic compounds, which will be encountered in detsul in Chapter 14. In deuterio acid, addition of a deuteron gives a resonance-stabilized carbocation, but aromaticity is lost (Fig. 13.63). 

Ionizations of furfuryl alcohols in trifluoroacetic acid solutions lead to the formation of fliranoxonium ions. These resonance-stabilized carbocations have been used in (4 + 3)-cycloaddition reactions with 1,3-dienes. Erker and coworkers have found that 2,5-dimethylthiophene (182) undergoes a novel trimerization reaction in solution of hexanes and CF3S03H. The tetracyclic product (183) is formed in high yield and its structure was confirmed by X-ray crystallography. A mechanism is proposed for the conversion, invoking C(2) protonated 2,5-dimethylthiophene as the initial eleetrophilic species. Subsequent intermediates are sulfur-stabilized carbocations and dications. 

Acylium ion (Section 16.3) A resonance-stabilized carbocation in which the positive charge is located at a carbonyl-group carbon, R—C=0 R—C=0+. Acylium ions are intermediates in Friedel-Crafts acylation reactions. 

The 1" alkyl halide is also allylic, so it forms a resonance-stabilized carbocation. Increasing the stability of the carbocation by resonance, increases the rate of the S l reaction. 

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