Ch. 6 - Thermodynamics and KineticsWorksheetSee 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

Now we’re going to discuss the most important intermediate of all organic chemistry, the carbocation.

Carbocation Stability

Concept #1: Determining Carbocation Stability

Carbocations are stabilized by a phenomenon called hyperconjugation.

Hyperconjugation is the delocalization of charge by the interaction of an empty p-orbital with an adjacent, eclipsed σ-bond. Basically it’s like resonance for single bonds (more complicated than that but don’t worry about it). 

Since this is only possible with -R groups, the more substituted the carbocation, the more stable

Which of the following leaving groups would generate the most stable carbocation?

Example #1: Which of the following leaving groups would generate the most stable carbocation?

Additional Problems
The overall equation for the addition of HCl to alkenes is:    If the transition state for proton transfer from HCl to the alkene (arrow  5 ) resembles a carbocation and this step is rate-determining, what should be the effect of alkene structure on the rate of the overall reaction?  Fastest rate                                    Slowest rate A. H2C=CH2    CH3CH=CHCH3    (CH3)2C=C(CH3)2 B. CH3CH=CHCH3    (CH3)2C=C(CH3)2    H2C=CH2 C. CH3CH=CHCH3    H2C=CH2    (CH3)2C=C(CH3)2 D. (CH3)2C=C(CH3)2    CH3CH=CHCH3    H2C=CH2
Identify which of the following substrates will undergo an SN1 reaction more rapidly. Explain your choice
Select the compound in each of the following pairs that will be converted to the corresponding alkyl bromide more rapidly on being treated with hydrogen bromide. Explain the reason for your choice.  (a) 1-Butanol or 2-butanol   
Select the compound in each of the following pairs that will be converted to the corresponding alkyl bromide more rapidly on being treated with hydrogen bromide. Explain the reason for your choice.  (b) 2-Methyl-1-butanol or 2-butanol 
Select the compound in each of the following pairs that will be converted to the corresponding alkyl bromide more rapidly on being treated with hydrogen bromide. Explain the reason for your choice.  (c) 2-Methyl-2-butanol or 2-butanol 
Select the compound in each of the following pairs that will be converted to the corresponding alkyl bromide more rapidly on being treated with hydrogen bromide. Explain the reason for your choice.  (d) 2-Methylbutane or 2-butanol
Select the compound in each of the following pairs that will be converted to the corresponding alkyl bromide more rapidly on being treated with hydrogen bromide. Explain the reason for your choice. (f) 1-Methylcyclopentanol or trans -2-methylcyclopentanol 
Select the compound in each of the following pairs that will be converted to the corresponding alkyl bromide more rapidly on being treated with hydrogen bromide. Explain the reason for your choice.  (g) 1-Cyclopentylethanol or 1-ethylcyclopentanol 
Rank the following molecules in order of increasing relative rate of SN1 solvolysis with methanol and heat (slowest to fastest reacting). A) 5 < 4 < 3 < 2 < 1 B) 1 < 2 < 5 < 4 < 3 C) 2 < 3 < 4 < 1 < 5 D) 3 < 2 < 4 < 5 < 1 E) 2 < 3 < 4 < 5 < 1
The following statements are true. Choose from among the following three possibilities and in the space provided, write the letter of the one or more phenomena that best explain the true statement. A. The inductive effect     B. Hyperconjugation     C. Resonance delocalization of a charged species
Which carbocation is the most stable?
The driving force for carbocation rearrangements is that the less stable 2° carbocation is converted into a more stable 3° carbocation ( True or False )
Circle the correct answer  
For each of the following reactions identify the arrow-pushing pattern that is being utilized:
For each of the following reactions identify the arrow-pushing pattern that is being utilized:
For the following carbocations, rank them from 1-3 according to overall carbocation stability, with a 1 under the LEAST STABLE CARBOCATION and a 3 under the MOST STABLE CARBOCATION.
Which SN1 reaction of each pair would you expect to take place more rapidly? Explain your answer.  
Rank the three carbocations shown in terms of increasing stability:
Rank the three carbocations shown in terms of increasing stability:
When 2-methylbutene is reacted with hydrochloric acid, where does the carbocation form and why? 1. The carbocation forms at the tertiary position because of steric hindrance. 2. The carbocation forms at the secondary position because of hyperconjugation from neighboring carbons. 3. The carbocation forms at the primary position because nature says so. 4. The carbocation forms at the tertiary position because of hyperconjugation from neighboring carbons.
Which alcohol is dehydrated fastest in concentrated H2SO4?  
Predict whether each of the following carbocations will rearrange. If so, draw the expected rearrangement using curved arrows.  
Predict whether each of the following carbocations will rearrange. If so, draw the expected rearrangement using curved arrows.
Predict whether each of the following carbocations will rearrange. If so, draw the expected rearrangement using curved arrows.
Predict whether each of the following carbocations will rearrange. If so, draw the expected rearrangement using curved arrows.  
Which two compounds ionize with loss of bromide ion to form the same carbocation? a. 1 and 2 b. 2 and 3 c. 1 and 4 d. 3 and 4
Explain the following observations: When tert-butyl bromide is treated with sodium methoxide in a mixture of methanol and water, the rate of formation of tert-butyl alcohol and tert-butyl methyl ether does not change appreciably as the concentration of sodium methoxide is increased. However, increasing the concentration of sodium methoxide causes a marked increase in the rate at which tert-butyl bromide disappears from the mixture.  
When tert-butyl bromide undergoes SN1 hydrolysis, adding a “common ion” (e.g., NaBr) to the aqueous solution has no effect on the rate. On the other hand, when (C6H5)2CHBr undergoes SN1 hydrolysis, adding NaBr retards the reaction. Given that the (C6H5)2CH+ cation is known to be much more stable than the (CH3)3C+ cation (and we shall see why in Section 15.12A), provide an explanation for the different behavior of the two compounds.  
Rank the following carbocations in order of stability?
Two stereoisomers of 1-bromo-4-methylcyclohexane are formed when trans-4-methylcyclohexanol reacts with hydrogen bromide. Write structural formulas of: (b) The carbocation intermediate in this reaction
Rank the following carbocations in order of stability: 
Which one of the following is the least stable?
Which one of the following carbocations is most stable?
Which of the following carbocations is the most stable?  
Which carbocation is the least stable? 
Which of the following would provide the most stable carbocation after loss of Cl-?(i) Chloromethane(ii) Chloroethane(iii) 2-choloropropane(iv) tert-Butyl chloride
This carbocation rearranges. In the box to the right, draw the carbocation that is the result of the rearrangement. Include any nonzero formal charges in your drawing.