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Ch.6 - Alkyl Halides; Nucleophilic Substitution
Wade - Organic Chemistry 9th Edition
Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook
All textbooksWade 9th EditionCh.6 - Alkyl Halides; Nucleophilic SubstitutionProblem 19c,d,e
Chapter 6, Problem 19c,d,e

For each pair of compounds, state which compound is the better SN2 substrate.
c. 2-bromobutane or isopropyl bromide
d. 1-chloro-2,2-dimethylbutane or 2-chlorobutane
e. 1-iodobutane or 2-iodopropane

Verified step by step guidance
1
Step 1: Recall the key factors that influence the SN2 reaction. The SN2 mechanism involves a single-step nucleophilic substitution where the nucleophile attacks the electrophilic carbon from the opposite side of the leaving group. The rate of the reaction depends on steric hindrance, the quality of the leaving group, and the structure of the substrate.
Step 2: Analyze the steric hindrance for each pair of compounds. In SN2 reactions, less steric hindrance around the electrophilic carbon makes the substrate more reactive. For example, primary alkyl halides are more reactive than secondary alkyl halides, and secondary alkyl halides are more reactive than tertiary alkyl halides.
Step 3: Compare the substrates in part (c): 2-bromobutane (a secondary alkyl halide) and isopropyl bromide (also a secondary alkyl halide). Consider the steric hindrance around the electrophilic carbon in each compound to determine which is the better SN2 substrate.
Step 4: Compare the substrates in part (d): 1-chloro-2,2-dimethylbutane (a primary alkyl halide with bulky substituents) and 2-chlorobutane (a secondary alkyl halide). Evaluate the steric hindrance caused by the bulky substituents in the primary alkyl halide and compare it to the steric hindrance in the secondary alkyl halide.
Step 5: Compare the substrates in part (e): 1-iodobutane (a primary alkyl halide) and 2-iodopropane (a secondary alkyl halide). Consider both the steric hindrance and the leaving group (iodine, which is an excellent leaving group) to determine which compound is the better SN2 substrate.

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Key Concepts

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

SN2 Mechanism

The SN2 (substitution nucleophilic bimolecular) mechanism involves a single concerted step where a nucleophile attacks an electrophile, leading to the simultaneous displacement of a leaving group. This mechanism is characterized by a backside attack, which results in inversion of configuration at the carbon center. The rate of the reaction depends on the concentration of both the nucleophile and the substrate.
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Steric Hindrance

Steric hindrance refers to the crowding around a reactive center that can impede the approach of a nucleophile. In SN2 reactions, substrates with less steric hindrance (such as primary alkyl halides) are more favorable because they allow easier access for the nucleophile. Conversely, tertiary substrates are less favorable due to increased steric bulk, which hinders the nucleophile's attack.
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Leaving Group Ability

The ability of a leaving group to depart from a substrate is crucial in determining the reactivity in SN2 reactions. Good leaving groups, such as iodide or bromide, stabilize the negative charge after leaving, facilitating the reaction. In contrast, poor leaving groups, like fluoride or hydroxide, hinder the reaction, making substrates with better leaving groups more favorable for SN2 substitution.
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Related Practice
Textbook Question

Rank the following compounds in decreasing order of their reactivity toward the SN2 reaction with sodium ethoxide (Na+ –OCH2CH3) in ethanol.

methyl chloride

tert-butyl iodide 

neopentyl bromide

isopropyl bromide

methyl iodide

ethyl chloride

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Textbook Question

For each pair of compounds, state which compound is the better SN2 substrate.

a. 2-methyl-1-iodopropane or tert-butyl iodide.

b. cyclohexyl bromide or 1-bromo-1-methylcyclohexane

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Textbook Question

Draw a perspective structure or a Fischer projection for the products of the following SN2 reactions.

(a) trans-1-bromo-3-methylcyclopentane + KOH

(b) (R)-2-bromopentane + KCN

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Textbook Question

When diethyl ether (CH3CH2OCH2CH3) is treated with concentrated HBr, the initial products are CH3CH2Br and CH3CH2OH. Propose a mechanism to account for this reaction.

Textbook Question

Draw a perspective structure or a Fischer projection for the products of the following SN2 reactions.

(e)

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Textbook Question

Draw a perspective structure or a Fischer projection for the products of the following SN2 reactions.

(c)

(d)

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