Alkenes, addition reactions protonation step

This chapter and Chapter 10 continue our cataloging of the standard reactions of organic chemistry. To the SnI, Sn2, El, and E2 reactions we now add a variety of alkene addition reactions. Although there are several different mechanisms for additions, many take place through a three-step sequence of protonation, addition, and deprotonation. The following new problems allow you to practice the basics of addition reactions and to extend yourself to some more complex matters. Even simple additions become complicated when they occur in intramolecular fashion, for example. These problems also allow you to explore the influence of resonance and inductive effects, and to use the regiochemistry and stereochemistry of addition to help work out the probable mechanisms of reactions. 

How does the Hammond postulate apply to electrophilic addition reactions The formation of a catbocation by protonation of an alkene is an endergonic step. Thus, the transition state for alkene protonation structurally resembles the... 

The addition reactions discussed in Sections 4.1.1 and 4.1.2 are initiated by the interaction of a proton with the alkene. Electron density is drawn toward the proton and this causes nucleophilic attack on the double bond. The role of the electrophile can also be played by metal cations, and the mercuric ion is the electrophile in several synthetically valuable procedures.13 The most commonly used reagent is mercuric acetate, but the trifluoroacetate, trifluoromethanesulfonate, or nitrate salts are more reactive and preferable in some applications. A general mechanism depicts a mercurinium ion as an intermediate.14 Such species can be detected by physical measurements when alkenes react with mercuric ions in nonnucleophilic solvents.15 The cation may be predominantly bridged or open, depending on the structure of the particular alkene. The addition is completed by attack of a nucleophile at the more-substituted carbon. The nucleophilic capture is usually the rate- and product-controlling step.13,16... 

Fig. 2 Free energy reaction coordinate profiles for the stepwise acid-catalyzed hydration of an alkene through a carbocation intermediate (Scheme 5). (a) Reaction profile for the case where alkene protonation is rate determining (ks kp). This profile shows a change in rate-determining step as a result of Bronsted catalysis of protonation of the alkene. (b) Reaction profile for the case where addition of solvent to the carbocation is rate determining (ks fcp). This profile shows a change in rate-determining step as a result of trapping of the carbocation by an added nucleophilic reagent.

The relative ease with which hydrogen halides react with alkenes is in the order HI > HBr > HCl > HF. This is the same as their relative acidities (see Section 4.3.2) and indicates that protonation of the alkene is the rate-limiting step for the addition reaction. 

When a stepwise ionic addition reaction involves nucleophilic attack at carbon as a first step, it is described as a nucleophilic addition. Reactions of this type often are catalyzed by bases, which generate the required nucleophile. For example, consider the addition of some weakly acidic reagent HX to an alkene. In the presence of a strong base ( OH), HX could give up its proton to form the conjugate base Xe, which is expected to be a much better nucleophile than HX ... 

The Schlosser Modification of the Wittig Reaction allows the selective formation of -alkenes through the use of excess lithium salts during the addition step of the ylide and subsequent deprotonation/protonation steps. 

Fig. 11.10. Wittig-Horner synthesis of alkenes with stereogenic C=C double bond via condensation of lithiated phosphine oxides B <-> B with aldehydes. In the two-step reaction the initially formed addition products syn- and anti-D are protonated and the resulting alcohols syn- and anti-C are isolated and separated the suitable diastere-omer is then stereoselectively converted into the desired alkene isomer. The one-step reaction leads to the same alkene in a cis,trans mixture.

Markovnikov s rule can be restated by saying that, in the addition of HX to an alkene, the more stable carbocation intermediate is formed. I his result is explained by the Hammond postulate, which says that the transition state of an exergonic reaction step structurally resembles the reactant, whereas the transition state of an endergonic reaction step structurally resembles the product. Since an alkene protonation step is endergonic, the stability of the more highly substituted carbocation is reflected in the stability of the transition state leading to its formal ion. 

The differences in selectivity between catalysts cannot be explained only in terms of the strength of reactant adsorption. A tentative explanation lies in the preference of platinum for concerted addition of protons to adsorbed alkenes with simultaneous electron transfer (25). The electronic structure of the surface intermediate of the concerted step appears to lead to halide cleavage. Palladium, on the other hand, can participate in insertion reactions (305) and promotes surface reaction between hydrogen atoms and adsorbed alkenes 4Sa. It is possible that palladium adsorbs vinyl halides on two different sites or at two different states, dependent on potential, one of which... 

Singlet photosensitized polar addition of methanol to (A )-(>)-limonene (102) in nonpolar solvents afforded a mixture of the diastereomeric ethers 103 and 104 and the rearrangement product 105 (Scheme 6.42).677 The diastereomeric excess (de) of the photoadduct was optimized by varying the solvent polarity, reaction temperature and nature of the sensitizer. The first step of the reaction is the Z E photoisomerization (Section 6.1.1) of 102 to a highly strained /i-isomer, followed by protonation and methanol addition. The initial formation of a carbocation via the protonation step has been excluded under those reaction conditions. The Markovnikov-oriented methanol attack on the less-hindered (Rp)-(E)-102 compared with that of (Sp)-(E)-U)2 explains why 103 can be obtained in up to 96% de upon sensitization with methyl benzoate in a methanol solution. The hypothesis that Z E isomerization of the cyclohexene moiety affords a strained (reactive) alkene, whereas isomerization of the exocyclic double bond does not, was supported by the observation of an exclusive nucleophilic addition to the cyclohexene double bond. 

For the hydronium-ion-catalyzed hydration of bicyclo[4.2.1]non-l-ene (177) and bicyclo[4.2.1]non-l(8)-ene (176), appreciable solvent isotope effects have been observed. Since these correspond to those found for reactions of unstrained olefins, it was concluded that the hydration proceeds as with unstrained alkenes by a two-step mechanism protonation of the double bond is followed by addition of the nucleophile (152b). The strained olefin with its distorted double bond is higher in energy and more reactive than an unstrained alkene. Hence, the transition state for protonation of Bredt-olefins is expected to be an early one (95). 

The addition of HX (X = C1, Br, I) to an alkene, to form alkyl halides, occurs in two steps. The first step involves the addition of a proton (i.e. the electrophile) to the double bond to make the most stable intermediate carbocation. The second step involves nucleophilic attack by the halide anion. This gives a racemic alkyl halide product because the carbocation is planar and hence can be attacked equally from either face. (These addition reactions are the reverse of alkyl halide elimination reactions.)... 

The reactivity of carbon-carbon double bonds toward acid-catalyzed addition of water is greatly increased by ERG substituents. The reaction of vinyl ethers with water in acidic solution is an example that has been carefully studied. With these reactants, the initial addition products are unstable hemiacetals that decompose to a ketone and alcohol. Nevertheless, the protonation step is rate determining, and the kinetic results pertain to this step. The mechanistic features are similar to those for hydration of simple alkenes. Proton transfer is rate determining, as demonstrated by general acid catalysis and solvent isotope effect data. ... 

In the first step of the reaction, the proton can approach the plane containing the double-bonded carbons of the aUcene from above or below to form the carbocation. Once the carbocation is formed, the chloride ion can approach the positively charged carbon from above or below. As a result, four stereoisomers are obtained as products The proton and the chloride ion can add from above-above, above-below, below-above, or below-below. When the two substituents add to the same side of the double bond, the addition is called syn addition. When the two substituents add to opposite sides of the double bond, the addition is called anti addition. Both syn and anti addition occur in aUcene addition reactions that take place by way of a carbocation intermediate. Because the four stereoisomers formed by the cis alkene are identical to the four stereoisomers formed by the trans alkene, the reaction is not stereospecific. 

Why is the transition state for the first step of an electrophilic addition reaction for an alkyne less stable than that for an alkene The Hammond postulate predicts that the structure of the transition state will resemble the structure of the intermediate (Section 4.3). The intermediate formed when a proton adds to an alkyne is a vinylic... 

The first step in the mercuric-ion-catalyzed hydration of an alkyne is formation of a cyclic mercurinium ion. (Two of the electrons in mercury s filled 5d atomic orbital are shown.) This should remind you of the cyclic bromonium and mercurinium ions formed as intermediates in electrophilic addition reactions of alkenes (Sections 4.7 and 4.8). In the second step of the reaction, water attacks the most substituted carbon of the cyclic intermediate (Section 4.8). Oxygen loses a proton to form a mercuric enol, which immediately rearranges to a mercuric ketone. Loss of the mercuric ion forms an enol, which rearranges to a ketone. Notice that the overall addition of water follows both the general rule for electrophilic addition reactions and Markovnikov s rule The electrophile (H in the case of Markovnikov s rule) adds to the sp carbon bonded to the greater number of hydrogens. 

Benzene s aromaticity causes it to undergo electrophilic aromatic substitution reactions. The electrophilic addition reactions characteristic of alkenes and dienes would lead to much less stable nonaromatic addition products. The most common electrophilic aromatic substitution reactions are halogenation, nitration, sulfonation, and Friedel-Crafts acylation and alkylation. Once the electrophile is generated, all electrophilic aromatic substitution reactions take place by the same two-step mechanism (1) The aromatic compound reacts with an electrophile, forming a carbocation intermediate and (2) a base pulls off a proton from the carbon that... 

The addition reaction between HBr and an alkene, for instance, is thought to proceed in two steps. In the first step, which is rate determining (Section 14.6), the HBr attacks the electron-rich double bond, transferring a proton to one of the double-bond carbons. In the reaction of 2-butene with HBr, for example, the first step is... 

Alkenes and alkynes readily undergo addition reactions to the carbon-carbon multiple bonds. Additions of adds, such as HBr, proceed via a rate-determining step in which a proton is transferred to one of the alkene or alkyne carhon atoms. Addition reactions are difficult to carry out with aromatic hydrocarbons, but substitution reactions are easily accomplished in the presence of catalysts. 

Ionic addition reactions of alkenes are quite regioselective. For instance, adding concentrated HCl to 2-methylpropene produces largely 2-chloro-2-methylpropane and a much smaller amount of l-chloro-2-methylpropane. This can be explained by examining the energies of the two carbocation intermediates that can be formed by adding a proton in the first step of the reaction ... 

---