Ch. 9 - Alkenes and AlkynesWorksheetSee 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

Not all alkenes were created equal. Like carbocations, alkenes are stabilized through a phenomenon called hyperconjugation

Hyperconjugation allows adjacent -R groups (mostly C-C and C-H σ-bonds) to create shared molecular orbitals with π-bonds, stabilizing the bond.

Concept #1: Understanding trends of alkene stability.

So basically, the more –R groups attached to the double bond, the more stable the double bond will be.

Specifically, when it comes to di-substituted bonds, the order os stability is gem > trans >  cis.

  • trans > cis due to steric hindrance. Groups have less freedom of movement in the cis position.
  • gem > trans due to better hyperconjugation. The full explanation for this trend is beyond the level of this course. Just memorize it.

Rank the following alkenes in order of lowest to highest heat of combustion. 

Example #1: Rank the following alkenes in order of lowest to highest heat of combustion. 

Additional Problems
The heats of reaction were measured for addition of HBr to cis- and trans-2-butene. Use these data to calculate the energy difference between cis- and trans-2-butene. How does this energy difference compare to that based on heats of hydrogenation (Section 6.3) and heats of combustion (Section 5.6)?
Rank the following alkenes according to their increasing stability. (least stable most stable) A. 1, 4, 3, 2 B. 1, 4, 2, 3 C. 2, 3, 1, 4 D. 2, 4, 3, 1 E. 4, 1, 2, 3 F. 3, 2, 4, 1
How many sigma bonds are stabilizing the alkene drawn below by hyperconjugation?
For the following constitutional isomers, rank them from 1-4 according to overall alkene stability, with a 1 under the LEAST STABLE ALKENE and a 4 under the MOST STABLE ALKENE.
Match each alkene with the appropriate heat of combustion: Heats of combustion (kJ/mol): 5293; 4658; 4650; 4638; 4632 Heats of combustion (kcal/mol): 1264.9; 1113.4; 1111.4; 1108.6; 1107.1   (a) 1-Heptene  (b) 2,4,-dimethyl-1-pentene (c) 2,4,-dimethyl-2-pentene (d) (Z)-4,4-dimethyl-2-pentene (e) 2,4,4-trimethyl-2-pentene
Choose the more stable alkene in each of the following pairs. Explain your reasoning.  (a) 1-Methylcyclohexene or 3-methylcyclohexene 
Choose the more stable alkene in each of the following pairs. Explain your reasoning.  (b) Isopropenylcyclopentane or allylcyclopentane 
Choose the more stable alkene in each of the following pairs. Explain your reasoning.  (e) ( Z )-Cyclooctadecene or ( E )-cyclooctadecene 
(b) On the basis of your answer to part (a), compare the expected stability of 3-methylcyclopropene with that of 1-methylcyclopropene and that of methylenecyclopropane. 
Using your knowledge about the stability of alkenes, predict which alkene belongs in the three positions on the following energy diagram. Place the letter of the corresponding alkene in the appropriate box according to its relative heat of hydrogenation.
From the compounds listed below, which has the lowest heat of hydrogenation. a.B b.E c.D d.A e.C   F. All of the above are the same. 
Match the following alkenes with the appropriate heats of hydrogenation: Heats of hydrogenation in kJ/mol (kcal/mol): 151(36.2); 122(29.3); 114(27.3); 111(26.5); 105(25.1). 
Identify the most stable and the least stable alkene.
Identify the most stable and the least stable alkene.    Most stable:            Least stable:           
Rank the following alkenes in order of their stability (least stable on left to most stable on right).    (a) V < III < IV < II < I (b) I < II < IV < III < V (c) III < IV < II < V < I (d) V < IV < II < III < I (e) I < IV < II < III < V
Identify the most stable and the least stable alkene
Place the following molecules in the order of increasing stability:
Which is the most stable isomer of but-2-ene?