Electrophilic addition of hydrogen halides to alkenes

Match each alkene of Problem 6.1 with its correct heat of hydrogenation. 

Addition to the double bond, of which catal5dic hydrogenation is but one example, is the most characteristic chemical property of alkenes. In many of these reactions the attacking reagent is a polar molecule such as a hydrogen halide. Addition occurs rapidly in a variety of solvents, including pentane, benzene, chchloromethane, chloroform, and acetic acid. 

Increasing rate of reaction of alkene with hydrogen halides H2C=CH2 H2C=CHCH3 H2C=C(CH3)2 

Increasing rate of addition of hydrogen halides to alkenes HF HCl HBr HI 

As shown in the following examples, hydrogen halide addition to alkenes can be highly regioselective, even regiospecific. In both cases, two constitutionally isomeric alkyl halides can be formed by addition to the double bond, but one is formed in preference to the other. 

In practice, hydrogenation of a-pinene is observed to be 100% stereoselective. The only product obtained is cw-pinane. None of the stereoisomeric fran -pinane is formed. 

Reactions such as catalytic hydrogenation that take place at the less hindered side of a reactant are common in organic chemistry and are examples of steric effects on reactivity. We have previously seen steric effects on structure and stability in the case of cis and traits stereoisomers and in the preference for equatorial substituents on cyclohexane rings. 

In many addition reactions the attacking reagent, unlike H2, is a polar molecule. Hydrogen halides are among the simplest examples of polar substances that add to alkenes. 

Addition 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 hahdes and reacts with alkenes at the fastest rate. 

The heats of reaction were measured for addition of HBr to cis- and fra/ s-2-butene. 

Increasing reactivity of hydrogen halides in addition to alkenes 

The second step of the mechanism is the same rapid carbocation-anion combination that we saw in Section 4.8 as the last step in the mechanism of the reaction of alcohols with hydrogen halides. 

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Electrophilic Addition of Hydrogen Halides to Alkenes Mechanism and Markovnikov s Rule 340... 

ELECTROPHILIC ADDITION OF HYDROGEN HALIDES TO ALKENES MECHANISM AND MARKOVNIKOV S RULE... 

The intervention of carbocations in the electrophilic additions of hydrogen halides to alkenes has important stereochemical consequences. Consider, for example, the hydrochlorination of cis- and trans-2-hulene. Both give the same product, 2-chlorobutane, as a racemic mixture. We can nnderstand this ontcome by formulating the mechanism. 

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