1 10.1. Addition of X2 to Alkenes and Alkynes to give 1,2-Dihalides. We have mentioned that alkenes and alkynes are not sufficiently good nucleophiles to react with alkyl halides. However, they are sufficiently good nucleophiles to react with elemental halogens, X2. Elemental halogens are fundamentally electrophilic: the two electronegative elements are both tugging on the electrons in the σ bond. However, the bond does not break spontaneously to give Br+ and Br–, because the former is a very high-energy species (an electron-deficient electronegative atom). Instead, a nucleophile attacks Br in Br2 directly. Since each Br already has its octet, the Br-Br bond must break at the same time as the new bond to the nucleophile forms (one of the bromine atoms acts as a leaving group). Nu: Nu+–Br + Br-Br Br Alkenes react with Br2 and Cl2 to give 1,2-dihaloalkanes, or 1,2-dihalides. This addition reaction is usually carried out in a non-nucleophilic solvent like dichloromethane. We can think of the reaction following a mechanism similar to the one we discussed for addition of a strong acid to an alkene: the alkene attacks one Br in Br2, just the same as alkene attack on H+ in HBr. Once attack occurs, a carbocation and Br– is generated. Then Br–, a nucleophile, adds to the carbocation to give the observed product. We can get information about whether mechanisms are correct by considering the experimentally observed regioselectivity and stereoselectivity of the reaction and seeing whether the proposed mechanism explains these aspects of the reaction correctly or at all. In the present reaction, since we are adding the same group to both atoms of the double bond, we needn’t consider regioselectivity. How about stereoselectivity? Let’s see what product we get from the reaction of cyclopentene with Br2. In the first step we would get a carbocation with a Br atom on the neighboring carbon. This2 carbocation could combine with Br– to give two possible diastereomers. The Br– could attack from the same face of the ring as Br resides to give a cis-product, or it could attack from the opposite side to give a trans product. Since a Br atom is larger than a H atom, we would expect to get more trans than cis, but not a whole lot more. H HBr BrH H+Br+ Br: -:::H H+BrH HBrBr HBrBrH Br: -:::abab In fact, we get exclusively trans. Our mechanism doesn't explain this fact. How can we modify the mechanism so that it explains it? It turns out that we had an incorrect structure for the intermediate carbocation. Imagine the Br atom in the carbocation reaching over and forming a bond to the electron-deficient carbon atom using one of its lone pairs. This gives a three-membered ring called a bromonium ion. H H+BrH HBr+ (See Jones Figs 10.12-10.13. Also note that the bromonium ion and the 2-halocarbocation are structural isomers, not resonance structures, since they have different atom-to-atom connections.) The Br atom has a formal positive charge. The C atom is no longer electron-deficient, but it is still electrophilic, because the Br+ atom, which is electronegative, wants to leave and have its lone pair back to itself again. The Br– comes along and attacks the C atom; as its electrons come in to the C atom, other electrons must leave so that the C atom doesn't gain more than an octet; the ones that leave are in the C–Br+ bond, which go back to Br. The Br– attacks from opposite the bond that breaks. We obtain trans-1,2-dibromocyclohexane as product. This is called overall anti addition, because the two Br atoms3 add to opposite sides of the double bond. An acyclic substrate will also undergo anti addition. In the example of bromine addition to cyclopentene, we can see that Br– can attack at either carbon of the bromonium ion intermediate (above). Since the bromonium ion is achiral and the product is chiral, the product is obtained as a racemic mixture. One enantiomer is obtained from attacking one C of the bromonium ion, and the other is obtained from attacking the other C. One synthetic use of the halogenation of alkenes is that the products can be converted to alkynes by two elimination reactions. Thus, alkynes can be prepared from alkenes by a two-step sequence: halogenation, then elimination. Alkynes also react with X2. As in the case with alkenes, the overall addition is anti with the halogen atoms trans to one another and the product is a 1,2-dibromoalkene. A second addition can follow R2R1Br2R2BrR1Br to give the tetrahalide (see Jones fig 10.74). 10.2. Cohalogenation. 2-Haloalcohols, 2-Haloethers. So far we’ve seen nucleophilic Br– attack the electrophilic bromonium ion to give a dibromide. Any other nucleophile that is present might also attack the bromonium ion to give a different product. This addition reaction is called cohalogenation. Cohalogenation is most commonly conducted by adding Br2 to an alkene in water, an alcohol, or a carboxylic acid as solvent. For example, if we add Br2 to an alkene in methanol, we obtain a product in which a CH3O– group is incorporated into the product instead of a Br– group. This product is called a 2-bromoether. The first part of the mechanism is the same as halogenation; a bromonium ion is formed. Then, instead of Br– acting as a nucleophile toward the bromonium ion, the alcohol solvent acts as a nucleophile by using its oxygen to attack C and displace Br- to give a cationic, electron-saturated intermediate. Finally, the oxonium ion intermediate is deprotonated to give the product. Mechanism:4 When we make 2-haloethers and 2-haloalcohols, we are adding one group to one of the C atoms of the double bond and a different group to the other. The question of regioselectivity now arises. For example, what happens if we use 1-methylcyclohexene as a substrate? Remember that we said that 3° carbocations were more stable than 2° ones. We can imagine that in the bromonium ion, the C1–Br+ bond
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