Intermediates carbocation

The step is exothermic it leads from the carbocation intermediate to the isolated products of the reaction... 

We will see numerous reactions that involve carbocation intermediates as we proceed through the text so it is important to understand how their structure determines their stability... 

Secondary and tertiary alcohols react with hydrogen halides by a mech anism that involves formation of a carbocation intermediate m the rate determining step... 

Write a structural formula for the carbocation intermediate formed in the dehydration of each of the alcohols in Problem 5 14 (Section 5 10) Using curved arrows show how each carbocation is deprotonated by water to give a mixture of alkenes... 

Like tertiary alcohols secondary alcohols normally undergo dehydration by way of carbocation intermediates... 

One possibility is the two step mechanism of Figure 5 12 m which the carbon-halogen bond breaks first to give a carbocation intermediate followed by depro tonation of the carbocation m a second step... 

Section 5 12 Secondary and tertiary alcohols undergo dehydration by an El mecha nism involving carbocation intermediates... 

Let s compare the carbocation intermediates for addition of a hydrogen halide (HX) to an unsymmetrical alkene of the type RCH=CH2 (a) according to Markovnikov s rule and (b) opposite to Markovnikov s rule (a) Addition according to Markovnikov s rule... 

Give a structural formula for the carbocation intermediate that leads to the major product in each of the reactions of Problem 6 3... 

The second mechanism is the one followed when addition occurs opposite to Markovmkov s rule Unlike electrophilic addition via a carbocation intermediate this alternative mechanism is a chain reaction involving free radical intermediates It is pre sented m Figure 6 7... 

Carbocation intermediates are not involved m hydroboration-oxidation Hydration of double bonds takes place without rearrangement even m alkenes as highly branched as the following... 

Dimerization in concentrated sulfuric acid occurs mainly with those alkenes that form tertiary carbocations In some cases reaction conditions can be developed that favor the formation of higher molecular weight polymers Because these reactions proceed by way of carbocation intermediates the process is referred to as cationic polymerization We made special mention m Section 5 1 of the enormous volume of ethylene and propene production in the petrochemical industry The accompanying box summarizes the principal uses of these alkenes Most of the ethylene is converted to polyethylene, a high molecular weight polymer of ethylene Polyethylene cannot be prepared by cationic polymerization but is the simplest example of a polymer that is produced on a large scale by free radical polymerization... 

We have seen this situation before m the reaction of alcohols with hydrogen halides (8ection 4 11) m the acid catalyzed dehydration of alcohols (8ection 5 12) and m the conversion of alkyl halides to alkenes by the El mechanism (8ection 5 17) As m these other reactions an electronic effect specifically the stabilization of the carbocation intermediate by alkyl substituents is the decisive factor The more stable the carbo cation the faster it is formed... 

Partial but not complete loss of optical activity m S l reactions probably results from the carbocation not being completely free when it is attacked by the nucleophile Ionization of the alkyl halide gives a carbocation-hahde ion pair as depicted m Figure 8 8 The halide ion shields one side of the carbocation and the nucleophile captures the carbocation faster from the opposite side More product of inverted configuration is formed than product of retained configuration In spite of the observation that the products of S l reactions are only partially racemic the fact that these reactions are not stereospecific is more consistent with a carbocation intermediate than a concerted bimolecular mechanism... 

Additional evidence for carbocation intermediates in certain nucleophilic substitutions comes from observing rearrangements of the kind normally associated with such species For example hydrolysis of the secondary alkyl bromide 2 bromo 3 methylbutane yields the rearranged tertiary alcohol 2 methyl 2 butanol as the only substitution product... 

Why does the carbocation intermediate in the hydrolysis of 2 bromo 3 methylbutane rearrange by way of a hydride shift rather than a methyl shift ... 

Carbocation intermediate capable of rearrangement (Section 8 11) No carbocation intermediate no re arrangement... 

When the electrophile attacks at C 3 the positive charge is shared by only two atoms C 2 and O and the carbocation intermediate is less stable and formed more slowly... 

The mechanism for formation of benzaldehyde diethyl acetal which proceeds m two stages is presented m Figure 17 9 The first stage (steps 1-3) involves formation of a hemiacetal m the second stage (steps 4-7) the hemiacetal is converted to the acetal Nucleophilic addition to the carbonyl group characterizes the first stage carbocation chemistry the second The key carbocation intermediate is stabilized by electron release from oxygen... 

The sp hybridized carbon of an acyl chloride is less sterically hindered than the sp hybridized carbon of an alkyl chloride making an acyl chloride more open toward nude ophilic attack Also unlike the 8 2 transition state or a carbocation intermediate m an Stvfl reaction the tetrahedral intermediate m nucleophilic acyl substitution has a stable arrangement of bonds and can be formed via a lower energy transition state... 

Arenium ion (Section 12 2) The carbocation intermediate formed by attack of an electrophile on an aromatic substrate in electrophilic aromatic substitution See cyclohexadienyl cation... 

Elimination unimolecular (El) mechanism (Section 5 17) Mechanism for elimination characterized by the slow for mation of a carbocation intermediate followed by rapid loss of a proton from the carbocation to form the alkene Enamine (Section 17 11) Product of the reaction of a second ary amine and an aldehyde or a ketone Enamines are char actenzed by the general structure... 

Application of Hammond s postulate indicates that the transition state should resemble the product of the first step, the carbocation intermediate. Ionization is facilitated by factors that either lower the energy of the carbocation or raise the energy of the reactant. The rate of ionization depends primarily on how reactant structure and solvent ionizing power affect these energies. 

Jencks has discussed how the gradation from the 8fjl to the 8n2 mechanism is related to the stability and lifetime of the carbocation intermediate, as illustrated in Fig. 5.6. In the 8n 1 mechanism, the carbocation intermediate has a relatively long lifetime and is equilibrated with solvent prior to capture by a nucleophile. The reaction is clearly a stepwise one, and the energy minimxun in which the caibocation mtermediate resides is significant. As the stability of the carbocation decreases, its lifetime becomes shorter. The barrier to capture by a nucleophile becomes less and eventually disappears. This is described as the imcoupled mechanism. Ionization proceeds without nucleophilic... 

Because carbocations are key intermediates in many nucleophilic substitution reactions, it is important to develop a grasp of their structural properties and the effect substituents have on stability. The critical step in the ionization mechanism of nucleophilic substitution is the generation of the tricoordinate carbocation intermediate. For this mechanism to operate, it is essential that this species not be prohibitively high in energy. Carbocations are inherently high-energy species. The ionization of r-butyl chloride is endothermic by 153kcal/mol in the gas phase. ... 

Substitution reactions by the ionization mechanism proceed very slowly on a-halo derivatives of ketones, aldehydes, acids, esters, nitriles, and related compounds. As discussed on p. 284, such substituents destabilize a carbocation intermediate. Substitution by the direct displacement mechanism, however, proceed especially readily in these systems. Table S.IS indicates some representative relative rate accelerations. Steric effects be responsible for part of the observed acceleration, since an sfp- caibon, such as in a carbonyl group, will provide less steric resistance to tiie incoming nucleophile than an alkyl group. The major effect is believed to be electronic. The adjacent n-LUMO of the carbonyl group can interact with the electnai density that is built up at the pentacoordinate carbon. This can be described in resonance terminology as a contribution flom an enolate-like stmeture to tiie transition state. In MO terminology,.the low-lying LUMO has a... 

Stabilization of a carbocation intermediate by benzylic conjugation, as in the 1-phenylethyl system shown in entry 8, leads to substitution with diminished stereosped-ficity. A thorough analysis of stereochemical, kinetic, and isotope effect data on solvolysis reactions of 1-phenylethyl chloride has been carried out. The system has been analyzed in terms of the fate of the intimate ion-pair and solvent-separated ion-pair intermediates. From this analysis, it has been estimated that for every 100 molecules of 1-phenylethyl chloride that undergo ionization to an intimate ion pair (in trifluoroethanol), 80 return to starting material of retained configuration, 7 return to inverted starting material, and 13 go on to the solvent-separated ion pair. 

A significant modification in the stereochemistry is observed when the double bond is conjugated with a group that can stabilize a carbocation intermediate. Most of the specific cases involve an aryl substituent. Examples of alkenes that give primarily syn addition are Z- and -l-phenylpropene, Z- and - -<-butylstyrene, l-phenyl-4-/-butylcyclohex-ene, and indene. The mechanism proposed for these additions features an ion pair as the key intermediate. Because of the greater stability of the carbocations in these molecules, concerted attack by halide ion is not required for complete carbon-hydrogen bond formation. If the ion pair formed by alkene protonation collapses to product faster than reorientation takes place, the result will be syn addition, since the proton and halide ion are initially on the same side of the molecule. 

It is not difficult to incorporate this result into the general mechanism for hydrogen halide additions. These products are formed as the result of solvent competing with halide ion as the nucleophilic component in the addition. Solvent addition can occur via a concerted mechanism or by capture of a carbocation intermediate. Addition of a halide salt increases the likelihood of capture of a carbocation intermediate by halide ion. The effect of added halide salt can be detected kinetically. For example, the presence of tetramethylammonium... 

Even though the rearrangements suggest that discrete carbocation intermediates are involved, these reactions frequently show kinetics consistent with the presence of at least two hydrogen chloride molecules in the rate-determining transition state. A termolecular mechanism in which the second Itydrogen chloride molecule assists in the ionization of the electrophile has been suggested. ... 

The formation of alcohols by acid-catalyzed addition of water to alkenes is a fundamental organic reaction. At the most rudimentary mechanistic level, it can be viewed as involving a carbocation intermediate. The alkene is protonated, and the carbocation is then captured by water. 

This mechanism explains the observed formation of the more highly substituted alcohol from unsymmetrical alkenes (Markownikoff s rule). A number of other points must be considered in order to provide a more complete picture of the mechanism. Is the protonation step reversible Is there a discrete carbocation intermediate, or does the nucleophile become involved before proton transfer is complete Can other reactions of the carbocation, such as rearrangement, compete with capture by water ... 

An interpretation of activation parameters has led to the conclusion that the bromination transition state resembles a three-membered ring, even in the case of alkenes that eventually react via open carbocation intermediates. It was foimd that for cis trans pairs of alkenes tiie difference in enthalpy at the transition state for bromination was greater than the enthalpy difference for the isomeric alkenes, as shown in Fig. 6.2. This... 

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