214 CHAPTER SIX Reactions of alkenes Addition reactions Addition occurs rapidly in a variety of solvents, including pentane, benzene, dichloro- CH3CH2 CH,CH HBr CH3CH, CH,CHCH, CH cis-3-Hexene Hydrogen bromide 3-Bromohexane(76%) The reactivity of the hydrogen halides reflects their ability to donate a proton. Hydrogen iodide is the strongest acid of the hydrogen halides and reacts with alkenes at the fastest rate Increasing reactivity of hydrogen halides n addition to alkenes HF < HCI< HBr< hI Slowest rate of addition Fastest rate of addition least acidic most acidic We can gain a general understanding of the mechanism of hydrogen halide addi tion to alkenes by extending some of the principles of reaction mechanisms introduced earlier. In Section 5. 12 we pointed out that carbocations are the conjugate acids of alkenes. Acid-base reactions are reversible processes. An alkene, therefore, can accept a proton from a hydrogen halide to form a carbocation H R2C=CR2+“H-x:、R2C一CR2 Alkene Anion (conjugate acid) (conjugate base) Figure 6. 4 shows the complementary nature of the electrostatic potentials of an alkene and a hydrogen halide. We've also seen( Section 4.9) that carbocations, when generated in the presence of halide anions, react with them to form alkyl halides H H RC—CR, R,C—CR Carbocation(electrophile) Halide ion(nucleophile) Alkyl halide Both steps in this general mechanism are based on precedent. It is called FIGURE 6.4 Electro electrophilic addition because the reaction is triggered by the attack of an electrophile static potential maps of HCl (an acid)on the T electrons of the double bond. Using the two TT electrons to form a and ethylene. When the two bond to an electrophile generates a carbocation as a reactive intermediate; normally this tween the electron-rich site is the rate-determining step (red)of ethylene and the electron-poor region(blue) 6.5 REGIOSELECTIVITY OF HYDROGEN HALIDE ADDITION gion of ethylene is associated MARKOVNIKOV'S RULE with the I electrons of the In principle a hydrogen halide can add to an unsymmetrical alkene(an alkene in which double bond, while h is the electron-poor atom(blue)of the two carbons of the double bond are not equivalently substituted)in either of two direc- HCI tions. In practice, addition is so highly regioselective as to be considered regiospecific Back Forward Main MenuToc Study Guide ToC Student o MHHE WebsiteAddition occurs rapidly in a variety of solvents, including pentane, benzene, dichloro￾methane, chloroform, and acetic acid. The reactivity of the hydrogen halides reflects their ability to donate a proton. Hydrogen iodide is the strongest acid of the hydrogen halides and reacts with alkenes at the fastest rate. We can gain a general understanding of the mechanism of hydrogen halide addi￾tion to alkenes by extending some of the principles of reaction mechanisms introduced earlier. In Section 5.12 we pointed out that carbocations are the conjugate acids of alkenes. Acid–base reactions are reversible processes. An alkene, therefore, can accept a proton from a hydrogen halide to form a carbocation. Figure 6.4 shows the complementary nature of the electrostatic potentials of an alkene and a hydrogen halide. We’ve also seen (Section 4.9) that carbocations, when generated in the presence of halide anions, react with them to form alkyl halides. Both steps in this general mechanism are based on precedent. It is called electrophilic addition because the reaction is triggered by the attack of an electrophile (an acid) on the electrons of the double bond. Using the two electrons to form a bond to an electrophile generates a carbocation as a reactive intermediate; normally this is the rate-determining step. 6.5 REGIOSELECTIVITY OF HYDROGEN HALIDE ADDITION: MARKOVNIKOV’S RULE In principle a hydrogen halide can add to an unsymmetrical alkene (an alkene in which the two carbons of the double bond are not equivalently substituted) in either of two direc￾tions. In practice, addition is so highly regioselective as to be considered regiospecific. X  Halide ion (nucleophile) R2C   CR2 H Carbocation (electrophile) Alkyl halide R2C X CR2 H X  Anion (conjugate base) R2C   CR2 H Carbocation (conjugate acid) H X Hydrogen halide (acid) R2C CR2 Alkene (base)  Increasing reactivity of hydrogen halides in addition to alkenes HF HCl HBr HI Fastest rate of addition; most acidic Slowest rate of addition; least acidic C CH3CH2 H H C CH2CH3 cis-3-Hexene  HBr Hydrogen bromide 3-Bromohexane (76%) CH3CH2CH2CHCH2CH3 Br 30°C CHCl3 214 CHAPTER SIX Reactions of Alkenes: Addition Reactions FIGURE 6.4 Electro￾static potential maps of HCl and ethylene. When the two react, the interaction is be￾tween the electron-rich site (red) of ethylene and the electron-poor region (blue) of HCl. The electron-rich re￾gion of ethylene is associated with the π electrons of the double bond, while H is the electron-poor atom (blue) of HCl. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website