Electrophilic addition hydrogenation

Thus orientation of addition in hydroboration is controlled in fundamentally the same way as in two-step electrophilic addition. Hydrogen becomes attached... 

Among the hydrogen halides only hydrogen bromide reacts with alkenes by both electrophilic and free radical addition mechanisms Hydrogen iodide and hydrogen chlo ride always add to alkenes by electrophilic addition and follow Markovmkov s rule Hydrogen bromide normally reacts by electrophilic addition but if peroxides are pres ent or if the reaction is initiated photochemically the free radical mechanism is followed... 

Markovmkov s rule is obeyed because the mechanism of sulfuric acid addition to alkenes illustrated for the case of propene m Figure 6 8 is analogous to that described earlier for the electrophilic addition of hydrogen halides... 

When formulating a mechanism for the reaction of alkynes with hydrogen halides we could propose a process analogous to that of electrophilic addition to alkenes m which the first step is formation of a carbocation and is rate determining The second step according to such a mechanism would be nucleophilic capture of the carbocation by a halide ion... 

FIGURE 9 5 (a) Curved arrow notation and (b) transition state for electrophilic addition of a hydrogen halide HXto an alkyne... 

Furthermore kinetic studies reveal that electrophilic addition of hydrogen halides to alkynes follows a rate law that is third order overall and second order in hydrogen halide... 

Both resonance forms of the allylic carbocation from 1 3 cyclopentadiene are equivalent and so attack at either of the carbons that share the positive charge gives the same product 3 chlorocyclopentene This is not the case with 1 3 butadiene and so hydrogen halides add to 1 3 butadiene to give a mixture of two regioisomeric allylic halides For the case of electrophilic addition of hydrogen bromide at -80°C... 

When the major product of a reaction is the one that is formed at the fastest rate we say that the reaction is governed by kinetic control Most organic reactions fall into this category and the electrophilic addition of hydrogen bromide to 1 3 butadiene at low temperature is a kmetically controlled reaction... 

The regioselectivity of electrophilic addition is governed by the ability of an aro matic ring to stabilize an adjacent carbocation This is clearly seen m the addition of hydrogen chloride to mdene Only a single chloride is formed... 

Isoprene has sometimes been used as a starting matenal in the laboratory synthesis of ter penes In one such synthesis the first step is the electrophilic addition of 2 moles of hydrogen bromide to isoprene to give 1 3 dibromo 3 methylbutane... 

The aromatic ring of a phenoxy anion is the site of electrophilic addition, eg, in methylolation with formaldehyde (qv). The phenoxy anion is highly reactive to many oxidants such as oxygen, hydrogen peroxide, ozone, and peroxyacetic acid. Many of the chemical modification reactions of lignin utilizing its aromatic and phenoHc nature have been reviewed elsewhere (53). 

Electrophile Addition Reactions. The addition of electrophilic (acidic) reagents HZ to propylene involves two steps. The first is the slow transfer of the hydrogen ion (proton) from one base to another, ie, from Z to the propylene double bond, to form a carbocation. The second is a rapid combination of the carbocation with the base, Z . The electrophile is not necessarily limited to a Lowry-Briiinsted acid, which has a proton to transfer, but can be any electron-deficient molecule (Lewis acid). 

Electrophile Addition. The addition of electrophilic (acidic) reagents HZ involves two steps the slow transfer of hydrogen ion from Z to the butylene to form a carbocation and, a rapid combination of the carbocation with the base Z. 

A mercurinium ion has both similarities and differences as compared with the intermediates that have been described for other electrophilic additions. The proton that initiates acid-catalyzed addition processes is a hard acid and has no imshared electrons. It can form either a carbocation or a hydrogen-bridged cation. Either species is electron-deficient and highly reactive. 

Double bonds in a,/3-unsaturated keto steroids can be selectively oxidized with alkaline hydrogen peroxide to yield epoxy ketones. In contrast to the electrophilic addition mechanism of peracids, the mechanism of alkaline epoxidation involves nucleophilic attack of hydroperoxide ion on the con-... 

Khaiasch proposed that hydrogen bromide can add to alkenes by two different mechanisms, both of which aie regiospecific. The first mechanism is electrophilic addition and follows Maikovnikov s rule. 

Is the electrophilic addition of hydrogen chloride to 2-methyl-propene the reverse of the El or the E2 elimination reaction of fe/t-butyl chloride ... 

Hydroboration-oxidation (Sections 6.11-6.13) This two-step sequence achieves hydration of alkenes in a stereospecific syn manner, with a regiose-lectivity opposite to Markovnikov s rule. An organoborane is formed by electrophilic addition of diborane to an alkene. Oxidation of the organoborane intermediate with hydrogen peroxide completes the process. Rearrangements do not occur. 

Evidence from a variety of sources, however, indicates that alkenyl cations (also called vinylic cations) are much less stable than simple alkyl cations, and their- involvement in these additions has been questioned. Eor example, although electrophilic addition of hydrogen halides to alkynes occurs more slowly than the conesponding additions... 

Electrophilic addition of hydrogen bromide to alkenes follows Markovnikov s rule, leading to the product with halogen on the more-substituted position. However, trace amounts of hydroperoxides (among other impurities ) may initiate a reaction that gives rise to the anti-Markovnikov product, with bromine in the less-substituted position. 

---