Neomenthyl chloride

Menthyl chloride and neomenthyl chloride have the structures shown One of these stereoisomers undergoes elimination on treatment with sodium ethoxide in ethanol much more readily than the other Which reacts faster menthyl chloride or neomenthyl chloride" Why" (Molecular models will help here )... 

The elimination of HC1 from the isomeric menthyl and neomenthyl chlorides shown in Figure 11.20 gives a good illustration of this trans-diaxial requirement. Neomenthyl chloride undergoes elimination of HC1 on reaction with ethoxide ion 200 times as fast as menthyl chloride. Furthermore, neomenthyl chloride yields 3-menthene as the major alkene product, whereas menthyl chloride yields 2-nienthene. 

Active Figure 17.20 Dehydrochlorination of menthyl and neomenthyl chlorides, (a) Neomenthyl chloride loses HCI directly from its more stable conformation, but (b) menthyl chloride must first ring-flip before HCI loss can occur. The abbreviation "Et represents an ethyl group. Sign in at www.thomsonedu.com to see a simulation based on this figure and to take a short quiz. 

The difference in reactivity between the isomeric menthyl chlorides is due to the difference in their conformations. Neomenthyl chloride has the conformation shown in Figure 11.20a, with the methyl ancl isopropyl groups equatorial and the chlorine axial—a perfect geometry for L2 elimination. Loss of the hydrogen atom at C4 occurs easily to yield the more substituted alkene product, 3-menthene, as predicted by Zaitsev s rule. 

Treatment of neomenthyl chloride with base rapidly produces two different alkenes, i.e. 2-menthene and 3-menthene. If one considers the three-dimensional shape of neomenthyl chloride, it can be seen that, in the preferred conformer with the two alkyl groups equatorial (see Section 3.3.2), the chlorine is an axial substituent. This means there are two different hydrogen atoms adjacent that are also axial and anfi-periplanar to the chlorine. As a conseqnence, two different E2 eliminations can occnr hence the two observed prodncts. That the two prodncts are not formed in eqnal amonnts will be considered in the next section. 

The E2 elimination of HCl from neomenthyl chloride described above produced two products, namely 2-menthene and 3-menthene in a ratio of about 1 3. 

Most stable conformation of neomenthyl chloride each ft carbon has a proton that is anti to chlorine... 

The anti periplanar relationship of halide and proton can be achieved only when the chlorine is axial this corresponds to the most stable conformation of neomenthyl chloride. Menthyl chloride, on the other hand, must undergo appreciable distortion of its ring to achieve an anti periplanar Cl—C—C—H geometry. Strain increases substantially in going to the transition state for E2 elimination in menthyl chloride but not in neomenthyl chloride. Neomenthyl chloride undergoes E2 elimination at the faster rate. 

The reaction of sodium diphenylphosphide with (+)-neomenthyl chloride (Fig. 14) gives (-)-menthyldiphenylphosphine (MDPP). The overall conversion of (+)-neomenthyl chloride to (-)-MDPP in a typical experiment is 25-30%.s The yield of (-)-MDPP was lower than the yield of (+)-NMDPP because elimination is a more serious competitive process for (+)-neomenthyl chloride than for (-)-menthyl chloride, the MDPP ligand is easily purified by crystallization from ethanol, and a purity of 98% (2% oxide) is attainable with one crystallization. 

The trans-diaxial relationship between the halogen and the hydrogens at C-2 and C-4 accounts for the relatively greater ease with which (+)-neomenthyl chloride undergoes E2 elimination. 

In contrast, neomenthyl chloride, with the opposite configuration at the carbon bearing the chlorine, has both the isopropyl and the methyl groups equatorial in the reactive conformation, with the chlorine axial (see Figure 9.5). Because both of these groups have larger axial strain energies than chlorine (see Table 6.2), the reactive conformation is the more stable one. There is less steric strain in the transition state for elimination,... 

Neomenthyl chloride is a dlastereomer of menthyl chloride, differing only in the configuration of the chlorine. 

The conformation needed for elimination, with the chlorine axial, has both the methyl and the isopropyl groups in equatorial positions. Both the red and the blue hydrogens are anti to the chlorine, so either may be eliminated. Therefore, two alkenes are produced in the elimination reaction. This reaction is more favorable than the reaction of menthyl chloride because the required conformation is the more stable one—the larger methyl and isopropyl groups are both equatorial and there is no extra strain in the transition state. For this reason, neomenthyl chloride undergoes elimination about 40 times faster than menthyl chloride. 

Mechanism and sterochemistry of the E2 elimination reaction of neomenthyl chloride. 

As can be seen in Figure 9.5, neomenthyl chloride has two hydrogens in an anti-periplanar geometry with the chlorine. Either of these hydrogens can be lost in the elimination reaction, resulting in the formation of two alkenes. (All of the examples presented up to this point were chosen so that only one alkene could be formed.) Let s now address this issue of the direction of the elimination and learn how to predict which will be the major product when more than one alkene can be formed. 

Chapter 8 discussed the stereochemistry of substitution reactions—that is, what happened to the stereochemistry when the reaction occurred at a carbon chirality center. This section discusses the regiochemistry of the elimination reaction—that is, what happens when a reaction can produce two or more structural isomers. The structural isomers that can often be produced in elimination reactions have the double bond in different positions. As shown in Figure 9.5, elimination of hydrogen chloride from neomenthyl chloride produces two structural isomers but in unequal amounts. 

In the case of neomenthyl chloride the major product has three carbon groups and one hydrogen as the four groups bonded to the carbons of the double bond, whereas the minor product has two carbon groups and two hydrogens so bonded. 

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