Ch. 4 - Alkanes and CycloalkanesWorksheetSee 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
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
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. 3.4F. 4.4        G. 4.6         H. 5.8           I. 7.2           J. None of these