Stereoisomers of 2-butene

Epoxidation of alkenes is a stereospecific syn addition Which stereoisomer of 2 butene reacts with peroxyacetic acid to give meso 2 3 epoxybu tane Which one gives a racemic mixture of (2/ 3/ ) and (25 35) 2 3 epoxybutane ... 

Most, but not all alkenes, also have stereoisomers that are not identical because of different spatial arrangements of the component atoms. Thus there are two stereoisomers of 2-butene that differ in the geometric arrangement of the groups attached to the double bond. In one isomer, both methyl groups are on the same side of the double bond (c/.v-2-butene) and in the other, the methyl groups are on opposite sides of the double bond (rrans-2-butene) ... 

Note from Fig. 22.7 that the p orbitals on the two carbon atoms in ethylene must be lined up (parallel) to allow formation of the -tr bond. This prevents rotation of the two CH2 groups relative to each other at ordinary temperatures, in contrast to alkanes, where free rotation is possible (see Fig. 22.8). The restricted rotation around doubly bonded carbon atoms means that alkenes exhibit cis-trans isomerism. For example, there are two stereoisomers of 2-butene (Fig. 22.9). Identical substituents on the same side of the double bond are designated cis and those on opposite sides are labeled trans. 

Figure 8.2 shows that there is free rotation about the carbon-carbon single bonds of butane, but not around the carbon-carbon double bond of 2-butene. Because of restricted rotation, two stereoisomers of 2-butene are possible. 

Bromine addition to alkenes is an example of a stereospecific reaction. A stereospecific reaction is one in which stereoisomeric starting materials yield products that are stereoisomers of each other. In this case the starting materials, in separate reactions, are the E and Z stereoisomers of 2-butene. The chiral dibromides from (Z)-2-butene are stereoisomers (diastereomers) of the meso dibromide formed from (7 )-2-butene. 

The formation of all three alkenes begins with protonation of the hydroxyl group as shown in step 1 of Mechanism 5.3. Because 1-butanol is a primary alcohol, it can give 1-butene by an E2 process in which a proton at C-2 of butyloxonium ion is removed while a water molecule departs from C-1 (step 2). The cis and trans stereoisomers of 2-butene, however, are formed from the secondary carbocation that arises via a hydride shift from C-3 to C-2, which accompanies loss of water from the oxonium ion (step 2 ). 

---