Ch. 21 - Conjugated Systems I: Stability and Addition Reactions

Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition

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Mullins 1st Edition

Ch. 21 - Conjugated Systems I: Stability and Addition Reactions

Problem 4

Chapter 20, Problem 4

Rank the following alkenes in order of stability (1 = most stable; 5 = least stable). Explain your order.

Verified step by step guidance

Step 1: Understand the concept of alkene stability. Alkene stability is influenced by the degree of substitution of the double bond. More substituted alkenes are generally more stable due to hyperconjugation and inductive effects.

Step 2: Analyze the structures provided. Count the number of alkyl groups attached to the carbon atoms of the double bond in each alkene. These groups contribute to the substitution level of the double bond.

Step 3: Rank the alkenes based on substitution. For example, a tetrasubstituted alkene (four alkyl groups attached to the double bond) is more stable than a trisubstituted alkene, which is more stable than a disubstituted alkene, and so on.

Step 4: Consider additional factors such as steric hindrance and conjugation. Conjugation with adjacent π systems (like benzene rings or other double bonds) can increase stability, while steric hindrance can decrease it.

Step 5: Assign the stability ranking based on the above analysis. The alkene with the highest substitution and favorable conjugation will be ranked as the most stable (1), and the alkene with the lowest substitution or unfavorable steric effects will be ranked as the least stable (5).

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Alkene Stability

The stability of alkenes is primarily influenced by the degree of substitution at the double bond. More substituted alkenes (those with more alkyl groups attached to the carbon atoms of the double bond) are generally more stable due to hyperconjugation and the inductive effect, which help to stabilize the double bond. Therefore, the order of stability typically follows the trend: tetrasubstituted > trisubstituted > disubstituted > monosubstituted > unsubstituted.

Hyperconjugation

Hyperconjugation is a stabilizing interaction that occurs when the electrons in a sigma bond (usually C-H or C-C) interact with an adjacent empty or partially filled p-orbital or a pi-bond. In alkenes, this effect is particularly significant in more substituted alkenes, where the presence of additional alkyl groups allows for greater overlap and stabilization of the double bond, thus enhancing the overall stability of the alkene.

Inductive Effect

The inductive effect refers to the electron-donating or withdrawing effects of substituents through sigma bonds. Alkyl groups are electron-donating, which can help stabilize the positive charge that may develop in the transition state of reactions involving alkenes. This effect contributes to the increased stability of more substituted alkenes, as the electron-donating nature of the substituents reduces the overall electron deficiency at the double bond.

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Related Practice

Textbook Question

Draw all possible resonance structures for the reactive intermediates shown.

(c)

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Textbook Question

Draw all possible resonance structures for the reactive intermediates shown.

(b)

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Textbook Question

Rank the following carbocations in order of stability (1 = most stable; 5 = least stable ). Explain your order.

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Textbook Question

Predict the product and provide a mechanism for the reaction of 1-methylcyclohexene with HBr.

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Textbook Question

To this point, hydrogenation has always been an exothermic process. Using the numbers from Figure 21.6, calculate ∆H^o_hydr for each step of the reduction of benzene.

∆H₁ + ∆H₂ + ∆H₃ = ∆H_tot = ―49.5 kcal /mol (―208 kJ/mol)

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Textbook Question

Hydroboration, an electrophilic addition reaction like those studied in Section 21.4, only gives 1,2-addition to buta-1,3-diene, regardless of temperature. Why?

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