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Ch. 9 - Alkenes II: Oxidation and Reduction
Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition
Mullins1st EditionOrganic Chemistry: A Learner Centered ApproachISBN: 9780137566471Not the one you use?Change textbook
All textbooksMullins 1st EditionCh. 9 - Alkenes II: Oxidation and ReductionProblem 11
Chapter 8, Problem 11

Provide arrow-pushing mechanisms for Assessments 9.10(b) and 9.10(c) that rationalize the regioselective and stereospecific formation of each halohydrin.
(b) Chemical reaction diagram showing the addition of bromine and water to an alkene, illustrating halohydrin formation.
(c) Chemical structure showing bromine and water reacting with an alkene, illustrating halohydrin formation.

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Step 1: Begin by identifying the starting alkene structure for each reaction in Assessments 9.10(b) and 9.10(c). Recall that halohydrin formation involves the addition of a halogen (e.g., Br₂ or Cl₂) and water (H₂O) across the double bond of the alkene.
Step 2: Draw the first step of the mechanism, which involves the electrophilic attack of the halogen molecule (e.g., Br₂) on the π-electrons of the alkene. This forms a cyclic halonium ion intermediate. Use curved arrows to show the movement of electrons: one arrow from the π-bond of the alkene to the halogen molecule and another arrow from the halogen molecule back to itself to form the halonium ion.
Step 3: Next, show the nucleophilic attack of water (H₂O) on the more substituted carbon of the halonium ion. This regioselectivity occurs because the more substituted carbon can better stabilize the partial positive charge in the transition state. Use a curved arrow to depict the lone pair of electrons on water attacking the carbon.
Step 4: Illustrate the deprotonation of the oxonium ion (formed after water adds to the halonium ion). A base (often another water molecule) abstracts a proton (H⁺) from the oxonium ion, leaving behind the neutral halohydrin product. Use curved arrows to show the movement of electrons during this step.
Step 5: Finally, rationalize the stereospecificity of the product. The halohydrin forms with anti-addition stereochemistry because the nucleophilic attack of water occurs from the side opposite to the halogen in the halonium ion. This anti-addition ensures that the halogen and hydroxyl group are on opposite sides of the resulting molecule.

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Key Concepts

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

Arrow-Pushing Mechanism

Arrow-pushing mechanisms are diagrams used in organic chemistry to illustrate the movement of electrons during chemical reactions. They show how nucleophiles and electrophiles interact, indicating the formation and breaking of bonds. Understanding these mechanisms is crucial for predicting the products of reactions and rationalizing regioselectivity and stereospecificity.
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Regioselectivity

Regioselectivity refers to the preference of a chemical reaction to yield one structural isomer over others when multiple possibilities exist. In the context of halohydrin formation, regioselectivity is influenced by factors such as the stability of carbocation intermediates and the nature of the nucleophile. Recognizing these factors helps in predicting which isomer will be formed in a given reaction.
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Stereospecificity

Stereospecificity is the property of a reaction where the configuration of the reactant determines the configuration of the product. In halohydrin formation, stereospecificity is important because it dictates whether the resulting halohydrin will have a specific stereochemical arrangement. This concept is essential for understanding how different reaction pathways can lead to distinct stereoisomers.
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Related Practice
Textbook Question

Suggest an alkene that could be used to make each of the following halohydrins.

(a)

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Textbook Question
(••) Predict the product(s) that would result when the alkenes are allowed to react under the following conditions: (iii) Br₂, H₂O(h)
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Textbook Question

Suggest an alkene that could be used to make each of the following halohydrins.

(c)

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Textbook Question

Predict the product(s) of each of the following reactions, making sure to indicate the relative stereochemical outcome. Indicate any racemic mixtures by drawing both enantiomers.

(a)

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Textbook Question

In contrast to the addition of Br₂ the addition of HBr [Section 8.3] is not stereospecific. Why?

Textbook Question

Explain why water attacks the carbon of the bromonium ion as opposed to the bromonium ion itself in the second step of halohydrin formation.

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