Textbook Question
Predict the dehydrohalogenation product(s) that result when the following alkyl halides are heated in alcoholic KOH. When more than one product is formed, predict the major and minor products.
(d)
(e)
9. Alkenes and Alkynes
Dehydrohalogenation
2:34 minutesProblem 50d
Textbook Question
Predict the products formed by sodium hydroxide-promoted dehydrohalogenation of the following compounds. In each case, predict which will be the major product.
d. cis-1-bromo-2-methylcyclohexane
Verified step by step guidance1
Identify the reaction type: This is a dehydrohalogenation reaction, which is an elimination reaction (E2 mechanism) promoted by a strong base like sodium hydroxide (NaOH). The reaction involves the removal of a hydrogen atom (β-hydrogen) and a halogen atom (bromine in this case) to form a double bond.
Determine the β-hydrogens: In cis-1-bromo-2-methylcyclohexane, the bromine atom is attached to carbon-1. The β-hydrogens are located on the carbons adjacent to carbon-1 (i.e., carbon-2 and carbon-6). Analyze the stereochemistry of the molecule to identify which β-hydrogens are anti-periplanar to the bromine atom, as this is a requirement for the E2 mechanism.
Apply Zaitsev's rule: Zaitsev's rule states that the major product of an elimination reaction is the more substituted alkene. In this case, consider the possible alkenes that can form by removing β-hydrogens from carbon-2 or carbon-6. The more substituted alkene will be the major product.
Draw the possible products: Removing a β-hydrogen from carbon-2 will form one alkene, while removing a β-hydrogen from carbon-6 will form another. Draw the structures of these alkenes, ensuring that the double bond is placed between carbon-1 and the β-carbon from which the hydrogen was removed.
Determine the major product: Compare the substitution patterns of the alkenes formed. The alkene with more alkyl groups attached to the double-bonded carbons will be the major product, as it is more stable due to hyperconjugation and inductive effects.
Verified video answer for a similar problem:This video solution was recommended by our tutors as helpful for the problem above
Video duration:
2mWas this helpful?
Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Dehydrohalogenation
Dehydrohalogenation is an elimination reaction where a hydrogen halide (HX) is removed from a molecule, typically resulting in the formation of a double bond. In the context of organic chemistry, this reaction is often facilitated by strong bases like sodium hydroxide (NaOH). The reaction can lead to the formation of alkenes, and the regioselectivity of the product can depend on the structure of the starting material.
Recommended video:
Guided course
03:02
The dehydrohalogenation mechanism.
Zaitsev's Rule
Zaitsev's Rule states that in elimination reactions, the more substituted alkene is generally the major product. This is because more substituted alkenes are typically more stable due to hyperconjugation and the inductive effect. Understanding this rule is crucial when predicting the major product of dehydrohalogenation, especially when multiple elimination pathways are possible.
Recommended video:
Guided course
01:18Defining Zaitsev’s Rule
Stereochemistry of Alkenes
Stereochemistry refers to the spatial arrangement of atoms in molecules and is particularly important in the formation of alkenes. In the case of cis-1-bromo-2-methylcyclohexane, the stereochemical configuration can influence the outcome of the elimination reaction. The formation of trans or cis alkenes can affect stability and reactivity, making it essential to consider stereochemical factors when predicting products.
Recommended video:
1:38Polymer Stereochemistry Concept 1
3:02mWatch next
Master The dehydrohalogenation mechanism. with a bite sized video explanation from Johnny
Start learningRelated Videos
Related Practice