Menthol, used to flavor various foods and tobacco, is the most stable stereoisomer of 2−isopropyl−5−methylcyclohexanol. Draw its most stable structure in the chair conformation. draw structure ... Is the hydroxyl group cis or trans to the isopropyl group? The hydroxyl group is to the isopropyl group. Is the hydroxyl group cis or trans to the methyl group? The hydroxyl is to the methyl group.

Interpretation: The most stable chair conformation of menthol is to be drawn labeling each substituent as axial or equatorial. Concept introduction: Among the disubstituted cyclohexanes, 1,2- cis, 1,3- trans and 1,4-cis substituents will have axial and equatorial arrangements.

Explanation:

1 &2. The most stable chair conformation for each menthol is the one which places the bulky isopropyl group in the

equatorial position, thereby avoiding an unfavorable 1,3-diaxial interaction to it. The more stable conformer is

indicated (image attached) by the relative lengths of the reversible arrows. Neoisomenthol probably exists at equilibrium

with substantial amounts of both conformers, since the chair conformation which places the isopropyl group in an

equatorial position places both the methyl and hydroxy groups in axial positions. (next page)

3. Consideration of the most stable chair conformation of each of the diastereomeric menthols suggests that menthol

itself (first diastereomer shown) should be the most stable of the four. Menthol itself is the only diastereomer of the

four in which all three substituents on the cyclohexane ring may assume an equatorial position simultaneously.

4. Yes, all four of the diastereomeric menthols would have of optical activity. The absolute configuration of each

chiral carbon in the menthols has been specified as R or S in the planar projections (next page).

5. In table 4-3, the ΔG° values given are for an equatorial to axial transition (which is why the values are all

positive). The overall ΔG° is the sum of equatorial to axial (+ΔG°) and axial to equatorial (-ΔG°) values. The

estimated ΔG° values for the transitions in the directions shown are: menthol = (1.70 + 0.94 + 2.20) = 4.84 kcal/mol;

isomenthol = (1.70 – 0.94 – 2.20) = -1.44 kcal/mol; neomenthol = 2.96 kcal/mol; neoisomenthol = 0.44 kcal/mol.