[burntbread07]

Compound A (MW=142.19), a 1,4-disubstituted cyclohexane, has the following composition: C, 67.57%; H, 9.92%. The difference in conformational energy for the two chair conformations of A is 0.4 kcal/mol. Using the data in Table 3-6, page 113 (Energy Differences Between . Cyclohexanes), determine the structure of A. Illustrate and explain. What is the conformational energy difference for the stereoisomer of A, ---namely A'. Explain and illustrate. Show the chair comformations of A and A' with the appropriate equilibrium arrows to illustrate the major and minor conformations. Label each conformation with its energy.

The chart in Table 3-6, page 113 reads

Energy Differences Between the Axial and Equatorial Conformations of Monosubstituted Cyclohexanes
-F 0.2 kcal/mol
-CN 0.2 kcal/mol
-Cl 0.5 kcal/mol
-Br 0.6 kcal/mol
-OH 1.0 kcal/mol
-COOH 1.4 kcal/mol
-CH3 1.8 kcal/mol
-CH2CH3 1.9 kcal/mol
-CH(CH3)2 2.1 kcal/mol
-C(CH3)3 5.4 kcal/mol

[burntbread07]

My friend and I attempted this problem earlier.  We thought we could narrow it down to either -F or -CN.  Obviously an element that is not C or H has to be involved since the two percentages given don't add up to 100.

[macman104]

Well, using the two percentages and knowing the molecular weight we can deduce the remaining mass.

142.19 * .6757 = 96.077
142.19 * .0992 = 14.10

This gives us a remaining weight of 32.013.

Just something that might help you along...

[Rabn]

The difference of 0.4 is also a huge clue. Look at the data and find the combinations that provide a difference of 0.4.  That coupled with the information provided by macman should really help elucidate the answer.

[burntbread07]

So far it looks like each substituent has a Carbon, two Hydrogens and probably an oxygen.  But there is no CH2O compound on the list.

or maybe i have just taken a totally messed up path

[Rabn]

There is no CH₂O in the list because it could not exist as a substituent, it would either give C 5 bonds or oxygen 1 bond and neither of those possibilities exist.  The key here is to look at the available amount of mass provided by macman.  I'll help walk you through it. What are the possible combination of atoms that are both in the list of substituents and whose mass adds up to 32.013.  Think in terms of combinations of single atoms.

[burntbread07]

i came up with a carboxyl group and a methyl group.  two oxygens add up to 32.  The carboxyl group has 1.8 kcal/mol and the methyl group has 1.4 kcal/mol, so if one is in the axial and the other in the equatorial position, then the difference between the two conformations is 0.4 kcal/mol?  Sorry for the terrible wording but I'm tired, I hope it makes sense.

Im still worried that I was supposed to find two identical groups.