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

We can use an instrument called a calorimeter to determine how much potential energy is stored in molecules. 

The Relationship Between Heat of Combustion and Stability

Concept #1: Understanding Heat of Combustion

Factors that Affect Alkane Stability

Concept #2: Shape and strain make alkanes unstable

Strain is a super general word. So now let’s go more into specifics of the types of strain you need to recognize. 

Types of Strain
  1. Angle Strain

Concept #3: What is angle strain?

The ideal bond angle for sp3 hybirdized carbon is 109.5°, so the more we deviate from that number, the more unstable the angle will be! (Aka cyclopropane sucks). 

  1. Torsional Strain

Concept #4: What is torsional strain?

Torsional strain increases with the number of eclipsing hydrogens in a molecule. Some of these rings are so small they can’t twist to prevent these interactions, which makes them unstable.  

Example #1: Which of the following conformations of cyclohexane would have the lowest heat of combustion?

Additional Problems
In each of the following groups of compounds, identify the one with the largest heat of combustion and the one with the smallest. In which cases can a comparison of heats of combustion be used to assess relative stability?  (a)  Cyclopropane, cyclobutane, cyclopentane 
In each of the following groups of compounds, identify the one with the largest heat of combustion and the one with the smallest. In which cases can a comparison of heats of combustion be used to assess relative stability?  (b)  cis-1,2-Dimethylcyclopentane, methylcyclohexane, 1,1,2,2-tetramethylcyclopropane 
In each of the following groups of compounds, identify the one with the largest heat of combustion and the one with the smallest. In which cases can a comparison of heats of combustion be used to assess relative stability? 
In each of the following groups of compounds, identify the one with the largest heat of combustion and the one with the smallest. In which cases can a comparison of heats of combustion be used to assess relative stability? 
For the pair drawn below, choose the MORE STABLE molecule. Does the  UNSTABLE molecule chosen below have  ANGLE STRAIN? Does the  UNSTABLE molecule chosen below have  TORSIONAL STRAIN? Does the  UNSTABLE molecule chosen below have  STERIC STRAIN?
For the pair of molecules drawn below, choose the letter that corresponds to the MORE STABLE molecule. Does the UNSTABLE molecule chosen above have ANGLE STRAIN? A – Yes B – No Does the UNSTABLE molecule chosen above have TORSIONAL STRAIN? A – Yes B – No Does the UNSTABLE molecule chosen above have STERIC STRAIN? A – Yes B – No
Draw all cycloalkanes that correspond to molecular formula C5H10, and circle the least strained of them.
For the pair of molecules drawn below, choose the letter that corresponds to the  MORE STABLE molecule. Does the  UNSTABLE molecule chosen below have  ANGLE STRAIN? Does the  UNSTABLE molecule chosen below have  TORSIONAL STRAIN? Does the  UNSTABLE molecule chosen below have  STERIC STRAIN?
Identify the more stable stereoisomer in each of the following pairs, and give the reason for your choice: 
One of the following two stereoisomers is 20 kJ/mol (4.9 kcal/mol) less stable than the other. Indicate which isomer is the less stable, and identify the reason for its decreased stability.
Identify which of the following compounds is expected to have  the larger heat of combustion:
What factor is responsible for a greater heat of combustion per CH 2 for cyclopropane than the heat of combustion per CH2 for cyclohexane? a) Cyclohexane has a different hydrogen-to-carbon atom ratio than cyclopropane. b) Cyclohexane is a strained ring relative to cyclopropane. c) Cyclopropane is a strained ring relative to cyclohexane. d) Cyclohexane has more carbon atoms than cyclopropane. 
For each of the following pairs of compounds, identify the compound that would have the higher heat of combustion:
For each of the following pairs of compounds, identify the compound that would have the higher heat of combustion:
For each of the following pairs of compounds, identify the compound that would have the higher heat of combustion:
For each of the following pairs of compounds, identify the compound that would have the higher heat of combustion:
Which value is closest to the internal C-C-C bond angle in cyclohexane? a) 90° b) 100° c) 110° d) 120°
Rank the following compounds in order of increasing stability based on relative ring strain.  
Which compound would you expect to be the more stable: cis-1,2-dimethylcyclopropane or trans-1,2-dimethylcyclopropane? Explain your answer.  
Choose the more stable alkene in each of the following pairs. Explain your reasoning. 
Answer the following questions regarding cyclopropane and propane. a. Provide an approximate pKa value for the C-H bond of propane. b. Using pictures and a minimum amount of words, name and describe the types of strain associated with cyclopropane. c. The Csp3 orbitals are able to relieve some of the strain by adjusting slightly. Draw a molecular orbital representation of cyclopropane. d. Please provide a name for the type of “adjusted” bonds you drew in part c. e. Due to the reorganization of the hybrid orbitals in cyclopropane, the s character of the carbon-hydrogen bonds actually increases. Do you think this will increase or decrease the pKa value of a C-H bond in cyclopropane? Why or why not?
Choose the more stable alkene in each of the following pairs. Explain your reasoning.  (d) ( Z )-Cyclononene or ( E )-cyclononene 
(a) Suggest an explanation for the fact that 1-methylcyclopropene is some 42 kJ/mol (10 kcal/mol) less stable than methylenecyclopropane. 
Rank the following molecule in order of increasing stability (least stable to most stable) A) 4, 1, 3, 2 B) 4, 3, 2, 1 C) 2, 3, 1, 4 D) 3, 2, 4, 1 E) 3, 2, 1, 4
Even though the methyl group occupies an equatorial site, the conformation shown is not the most stable one for methylcyclohexane. Explain. 
Which do you expect to be the more stable conformation of cis-1,3-dimethylcyclobutane, A or B? Why?
What type(s) of strain is/are present in cyclobutane? (a) Steric strain (b) Angle strain (c) Torsional strain (d) Torsional strain and angle strain