Ch. 4 - Alkanes and CycloalkanesSee all chapters
All Chapters
Ch. 1 - A Review of General Chemistry
Ch. 2 - Molecular Representations
Ch. 3 - Acids and Bases
Ch. 4 - Alkanes and Cycloalkanes
Ch. 5 - Chirality
Ch. 6 - Thermodynamics and Kinetics
Ch. 7 - Substitution Reactions
Ch. 8 - Elimination Reactions
Ch. 9 - Alkenes and Alkynes
Ch. 10 - Addition Reactions
Ch. 11 - Radical Reactions
Ch. 12 - Alcohols, Ethers, Epoxides and Thiols
Ch. 13 - Alcohols and Carbonyl Compounds
Ch. 14 - Synthetic Techniques
Ch. 15 - Analytical Techniques: IR, NMR, Mass Spect
Ch. 16 - Conjugated Systems
Ch. 17 - Aromaticity
Ch. 18 - Reactions of Aromatics: EAS and Beyond
Ch. 19 - Aldehydes and Ketones: Nucleophilic Addition
Ch. 20 - Carboxylic Acid Derivatives: NAS
Ch. 21 - Enolate Chemistry: Reactions at the Alpha-Carbon
Ch. 22 - Condensation Chemistry
Ch. 23 - Amines
Ch. 24 - Carbohydrates
Ch. 25 - Phenols
Ch. 26 - Amino Acids, Peptides, and Proteins

Sometimes we’ll be asked to actually calculate the amount of energy a Newman Projection “spends” while rotating. 

Important Barrier of Rotation Values

Concept #1: 4 Values You Should Memorize

These are the values we’ll need so we can solve for the unknown interactions in these questions. 

Example #1: The barrier to rotation for the following molecule is 22 kJ/mol . Determine the energy cost associated with the eclipsing interaction between a bromine and hydrogen atom.

Now it’s time to put your knowledge to the test. Remember to draw the eclipsed version to know what the interactions are!

Practice: The barrier to rotation for 1,2 -dibromopropane along the C1—C2 bond is 28 kJ/mol. Determine the energy cost associated with the eclipsing dibromine interaction.

Additional Problems
What is the total strain energy (in kcal/mol) of the least stable (highest energy) conformation possible for 2-methylbutane? All of the choices below are expressed in units of kcal/mol.   A. 0          B. 0.9          C. 1.8           D. 2.7         E. 3.4 F. 4.4        G. 4.6         H. 5.8           I. 7.2           J. None of these
Draw the highest energy eclipsed Newman projection for the molecule shown below and calculate its relative energy. (CH3, CH3 eclipsed = 4.0 kcal/mol; CH 3, H eclipsed = 1.5 kcal/mol; H, H eclipsed 1.0 kcal/mol)
The barrier to rotation of bromoethane is 15 kJ/mol. Based on this information, determine the energy cost associated with the eclipsing interaction between a bromine atom and a hydrogen atom.
Sketch an energy diagram showing a conformational analysis of 2,2,3,3-tetramethylbutane. Use Table 4.6 to determine the energy difference between staggered and eclipsed conformations of this compound.
Consider the following two conformations of 2,3-dimethylbutane. For each of these conformations, use Table 4.6 to determine the total energy cost associated with all torsional strain and steric strain.
Consider the following two conformations of 2,3-dimethylbutane. For each of these conformations, use Table 4.6 to determine the total energy cost associated with all torsional strain and steric strain.
Consider the structures of cis-1,2-dimethylcyclopropane and trans-1,2-dimethylcyclopropane: (a) Which compound would you expect to be more stable? Explain your choice.
Consider the structures of cis-1,2-dimethylcyclopropane and trans-1,2-dimethylcyclopropane: (b) Predict the difference in energy between these two compounds.
What is the total strain energy (in kcal/mol) of the specific conformation of 2, 3–dimethylbutane shown below? All of the choices below are expressed in units of kcal/mol.   A. 0          B. 0.9          C. 1.8          D. 2.7         E. 3.4 F. 3.6       G. 4.4          H. 5.8           I. 7.2          J. None of these
What is the total strain energy (in kcal/mol) of the least stable (highest energy) conformation possible for 2,3-dimethylbutane? All of the choices below are expressed in units of kcal/mol.   A. 0          B. 0.9          C. 1.8           D. 2.7         E. 3.4 F. 3.6        G. 4.4         H. 5.8           I. 7.2           J. None of these
On the template below DRAW the most stable conformation of 2,2,4-trimethylpentane, with respect to the C3-C4 bond. Be sure to show the substituent at each and every position.
Sketch an approximate potential energy diagram for rotation about the carbon–carbon bond in 2,2-dimethylpropane similar to that shown in Figures 3.4 and 3.7. Does the form of the potential energy curve of 2,2-dimethylpropane more closely resemble that of ethane or that of butane? 
True or false? The specific conformation of 2,3-dimethylbutane shown below is the most stable (lowest energy) conformation possible for this molecule.A. True          B. False