MS 900 - Conformers and Cyclohexane Practice

Submitted by Matt on August 6, 2011.

Textbook and Chapter: Carey and Giuliano 8th Ed. (2010), Chapter 3

Keywords: conformational analysis, cyclohexane chair forms

Description: Reviews cyclohexane chair forms and conformations of butane:

Which dimethylcyclohexane is more stable 1,2-cis or 1,2-trans? Why?
Drawing chair forms
E2 anti-coplanar conformation and chair forms
Newman projects

Total Problems: 4

Printer-friendly version

Problem # 317

α-D-Glucose is shown below. Draw its two chair forms. Which conformation is more stable? Explain.

I recommend using the common convention wedge = "up" and dash = "down."
Solution
The chair form on the left is more stable because it has no axial substituents, which lead to 1,3-diaxial interactions and higher energies. Glucose is the most stable of the hexose carbohydrates because it is the only one that has al substituents at C2-C5 in the equatorial position.

It's probably no coincidence that the most abundant sugar in the world (glucose) is also the sugar that is most stable!

Problem Details
MendelSet practice problem # 317 submitted by Matt on June 7, 2011.
Subject: Organic Chemistry
Subject Topic(s): Cycloalkanes and Cyclohexane Chair Forms, Carbohydrates
Question Type: Concept (explain why this is so..)
Keyword(s): cyclohexane chair forms
Problem # 318

Two stereoisomers of trimethylcyclohexane are shown below (compounds A and B). Compare cyclohexane chair forms to determine which isomer has a lower heat of combustion. Explain your reasoning.
Solution
A has a chair form that avoids having any substituents in the axial position.

B's most stable chair form has one axial substituent.

So A is more stable than B. Therefore, B is higher eneregy and has a higher heat of combustion than A.

Problem Details
MendelSet practice problem # 318 submitted by Matt on June 7, 2011.
Subject: Organic Chemistry
Subject Topic(s): Cycloalkanes and Cyclohexane Chair Forms
Question Type: Concept (explain why this is so..)
Keyword(s): cyclohexane chair forms, heats of combustion
Problem # 319

For a molecule to undergo an E2 reaction, the leaving group and the beta-proton must be in an anti-coplanar conformation (one atom straight up, the other straight down). Based on this, which compound undergoes E2 reaction with KOtBu faster? Why?
Solution

The methyl group is bulky and so is most stable in the equatorial position.

The 1,4 cis compound's most stable chair form has its leaving group (Br) and beta-hydrogen in an anti-coplanar (anti-periplanar in some textbooks) conformation.

So most of the time, 1,4 cis is in a chair conformation that is able to undergo an E2 reaction.

The opposite is true for the 1,4 trans compound. Most of the time it is in a chair conformation that is unable to undergo an E2 reaction.

Therefore, the 1,4 cis compound will undergo an E2 elimination reaction faster than the 1,4 trans compound.

Problem Details
MendelSet practice problem # 319 submitted by Matt on June 7, 2011.
Subject: Organic Chemistry
Subject Topic(s): Cycloalkanes and Cyclohexane Chair Forms, Alkene Formation: Elimination, Dehydration, and Zaitsev's Rule, Stereochemistry, Chirality, and Optical Activity
Question Type: Concept (explain why this is so..)
Keyword(s): cyclohexane chair forms, E2 stereochemistry
Problem # 320

Let's perform conformational analysis on 2-methylbutane along the C2-C3 bond. We'll use the energy chart given below.

First, draw out the Newman projections along the C2-C3 bond, rotating the front carbon (C-2) by 60 degrees clockwise each time while keeping the back carbon (C-3) stationary.

According to the table above, how much energy does each conformation "cost?"

Second, make a plot of the total energy value for each Newman projection versus its dihedral angle.
Solution

Problem Details
MendelSet practice problem # 320 submitted by Matt on June 7, 2011.
Subject: Organic Chemistry
Subject Topic(s): Conformational Analysis and Steric Strain
Question Type: Concept (explain why this is so..)
Keyword(s): conformational analysis, Newman projections

Follow MendelSet on Twitter, become a fan on Facebook.

Printer-friendly version

MS 900 - Conformers and Cyclohexane Practice

MS 900 - Conformers and Cyclohexane Practice

Login