Menthyl 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. 

A final piece of evidence involves the stereochemistry of elimination. (Jnlike the E2 reaction, where anti periplanar geometry is required, there is no geometric requirement on the El reaction because the halide and the hydrogen are lost in separate steps. We might therefore expect to obtain the more stable (Zaitsev s rule) product from El reaction, which is just what w e find. To return to a familiar example, menthyl chloride loses HC1 under El conditions in a polar solvent to give a mixture of alkenes in w hich the Zaitsev product, 3-menthene, predominates (Figure 11.22). 

Figure 11.22 Elimination reactions of menthyl chloride. E2 conditions (strong base in 100% ethanol) lead to 2-menthene through an anti periplanar elimination, whereas El conditions (dilute base in 80% aqueous ethanol) lead to a mixture of 2-menthene and 3-menthene.

Menthyl chloride, El reaction of, 392 E2 reaction of, 390 Mercapto group, 667 Mercuric trifluoroacetate. 

Menthyl chloride (imore stable conformation) Elimination is not possible for this conformation because no hydrogen is anti to the leaving group. 

Nucleophilic displacement of halide ion from a saturated carbon atom by alkali-metal diphenylphosphide reagents occurs with inversion of configuration at carbon, as is found in normal Sn2 displacements.19 Thus menthyl chloride or bromide gives ... 

On the other hand, menthyl chloride is only slowly converted by treatment with base, and into a single prodnct, i.e. 2-menthene. In the preferred conformation of menthyl chloride, all three snbstitnents are equatorial, and no adjacent hydrogen is in a planar relationship to the chlorine leaving gronp. The fact that slow elimination occnrs at all is a resnlt of conformational isomerism into the less-favonred con-former that has all three snbstitnents axial. In this con-former, there is a single hydrogen anft-periplanar with the chlorine, so elimination occnrs giving jnst one product. The conformational eqnilibrinm is slowly disturbed because the elimination removes the small concentration of unfavoured conformer. 

In some cases, especially with cyclic compounds, the more stable olefin forms and Zaitsev s rule applies. For example, menthyl acetate gives 35% of the Hofmann product and 65% of the Zaitsev, even though a cis (3 hydrogen is present on both sides and the statistical distribution is the other way. A similar result was found for the pyrolysis of menthyl chloride.138... 

Preparation 316.—Menthyl Chloride (1-Methyl-i-isopropyl-Z-chloro-cyclohexane). 

Most stable conformation of menthyl chloride none of the three fi protons is anti to chlorine... 

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