Ionic addition of hydrogen

When however the ionic addition of hydrogen bromide to 1 3 butadiene is car ried out at room temperature the ratio of isomeric allylic bromides observed is differ ent from that which is formed at — 80°C At room temperature the 1 4 addition product predominates... 

A more complete discussion of the mechanism of ionic addition of hydrogen halides to alkenes is given in Chapter 6 of Part A. In particular, the question of whether or not discrete carbocations are always involved is considered there. 

Organoborane intermediates can also be used to synthesize alkyl halides. Replacement of boron by iodine is rapid in the presence of base.150 The best yields are obtained with sodium methoxide in methanol.151 If less basic conditions are desirable, the use of iodine monochloride and sodium acetate gives good yields.152 As is the case in hydroboration-oxidation, the regioselectivity of hydroboration-halogenation is opposite to that observed for direct ionic addition of hydrogen halides to alkenes. Terminal alkenes give primary halides. 

The ionic addition of hydrogen bromide was discussed in Section 10-4 and will not be considered further at this point. Two questions with regard to the so-called abnormal addition will be given special attention. Why does the radical mechanism give a product of different structure than the ionic addition Why does the radical addition occur readily with hydrogen bromide but rarely with the other hydrogen halides (See Exercise 10-25.)... 

The ionic addition of hydrogen bromide to born-2-ene in ether at — 10°C leads to a mixture containing 50 % of epi-isobornyl bromide (252) and 50 % of the camphyl bromides (253). ... 

Markovnikov s rule is obeyed because the mechanism of sulfuric acid addition to aikenes, illustrated for the case of propene in Figure 6.8, is analogous to that described earlier for the ionic addition of hydrogen halides. 

It is often helpful, especially in a multistep reaction, to focus on the step that creates the stereogenic center. In the ionic addition of hydrogen bromide to 2-butene, for example, the stereogenic center is generated when bromide ion attacks iec-butyl cation. 

With this understanding of the mechanism for the ionic addition of hydrogen halides to alkenes, we can now generalize about how electrophiles add to alkenes. 

Since the ionic addition of hydrogen bromide and the acid-catalyzed addition of water to an alkene proceed via planar carbocations, the formation of products derived from rearranged carbocations is relatively common. In contrast, the bromi-nation of alkenes, which is illustrated by the addition of bromine to cyclopentene (Eq. 10.18), occurs via a cyclic bromonium ion 38 related to 33, so skeletal rearrangements of such cations are not generally observed. Moreover, the intervention of this cyclic ion dictates that the stereochemistry of the addition is completely in the anti sense, as shown. The reddish color of bromine is discharged upon addition to an alkene, making this reaction a useful qualitative test for unsaturation (Sec. 25.8A). 

Devise an experiment that would demonstrate that a phase-transfer catalyst accelerates the rate of reaction between 1-hexene and aqueous hydrobromic acid. Why would it be difficult to perform the ionic addition of hydrogen bromide to 1-pentene using the procedure outlined in this experiment ... 

As pointed out briefly in Chapter 6 (Table 6.1), the ionic addition of hydrogen fluoride (HF) across the carbon-carbon double bond of alkenes produces alkyl fluorides. Generally, the addition is effected by treating a solution of alkene in an ethereal solvent (e.g., tetrahydrofuran [THF]) with a pyridine solution of HF, which must thus contain pyridinium fluoride (Equation 7.1). 

Let s consider specific reaction conditions for the ionic addition of hydrogen bromide to 1,3-butadiene. 

Organomercury compounds have had limited use in synthesis. One promising procedure, based on the availability of the mercury compounds from alkenes via organoboranes, is the synthesis of primary halides from alkenes. This procedure has the opposite regioselectivity to the direct ionic addition of hydrogen bromide... 

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