From Strawberry Fields to the Chemistry Classroom

By Simone N. Walker, Quyen T. Doan, William J. Heuett, Deana Jaber

Abstract

This case study investigates the use of methyl bromide as a pesticide and methyl iodide as an alternative pesticide in strawberry fields in California. By 2005, methyl bromide should have been completely phased out under the Clean Air Act and the Montreal Protocol on Substances that Deplete the Ozone Layer, but the United States requested exemptions and the deadline was renewed to 2015. In 2010, methyl iodide was introduced as a substitute. However, because of the similarities of the chemical reactions of the two pesticides, similar problems surfaced. The case uses a dialogue between “Kelly,” a junior in college studying organic chemistry, and “Jessica,” an environmental scientist at the National Institutes of Health, to introduce students to substitution reactions (SN1 and SN2) of alkyl halides and the factors that affect these reactions. Prerequisite knowledge for the case includes an introduction to organic chemistry, structure and bonding of organic compounds (alkenes and alkynes), and stereochemistry. The case is designed for use in introductory organic chemistry and biochemistry courses but could also be used in biology courses.

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Date Posted

05/07/2021

Overview

Objectives

Recognize the different types of alkyl halides (primary, secondary, or tertiary).
Differentiate between SN1 and SN2 reactions of alkyl halides.
Predict the final product of the reaction including its stereochemistry.
Draw the mechanisms of SN1 and SN2 reactions.
Apply the concept of carbocation rearrangement in respective mechanisms, SN1 reactions.
Determine the relative stability of carbocation intermediates.
Identify and recognize the factors that influence SN1 and SN2 reactions such as the strength of the nucleophile (good or poor) and the solvent used in the reaction (protic or aprotic solvents).

Keywords

Organic chemistry; alkyl halides; substitution reactions; nucleophilic substitution; nucleophile; pesticides; methyl bromide; methyl iodide; SN1; SN2; strawberries; farming

Subject Headings

Biochemistry

Environmental Science

Organic Chemistry

EDUCATIONAL LEVEL

Undergraduate lower division, Undergraduate upper division

FORMAT

PDF

TOPICAL AREAS

Policy issues, Regulatory issues

LANGUAGE

English

TYPE/METHODS

Analysis (Issues), Dilemma/Decision, Directed

Teaching Notes & Answer Key

Teaching Notes

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Teaching notes are intended to help teachers select and adopt a case. They typically include a summary of the case, teaching objectives, information about the intended audience, details about how the case may be taught, and a list of references and resources.

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Answer Key

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Materials & Media

Supplemental Materials

The file below includes additional handouts (flowchart, mechanism template, nucleophile chart, and solvent chart) that the instructor may use to facilitate peer-to-peer dialogue as students work through the case.

nucleophilic_sub_sup.pdf (~1.4MB)

Videos

SN1 vs. SN2: Summary
This video summarizes how to determine if a reaction will be SN1 or SN2 depending on substrate, nucleophile, and solvent. Running time: 13:53 min. Produced by Khan Academy, 2017.