A student’s first exposure to solving “puzzles” in Organic Chemistry “flies under the radar screen” somewhere around week 7-8 when SN1, SN2, E1 and E2 reactions are introduced. Albeit not apparent at the time, these puzzles are rather simple, and have to do with compounds containing 2° leaving groups, i.e. the ones that can go “either way.” Superior nucleophiles in dipiolar aprotic solvents react via SN2; strong bases in any solvent react via E2 (depending upon which book you read). Superior nucleophiles in polar protic solvents react via SN1 competing with E1. Solving the puzzle of the thing that goes “either way” involves knowing where the other factors in the “stew” push the outcome of the reaction.
The alkene functionality is a fundamental building block in synthetic organic chemistry. It is practically ubiquitous, and can be found in a variety of natural products such as beta carotene, the precursor to Vitamin A. Alkenes are known to form under conditions of E1 and E2 elimination. Although an integral part of Organic Chemistry, elimination reactions are not always synthetically appropriate given the delicate functionality often present in complex organic molecules. The more versatile Wittig reaction through which they may be synthesized lies in chemistry pioneered by Nobel Laureate George Wittig (University of Heidelberg).
When basicity and/or steric effects outweigh nucleophilicity, and a good leaving group (LG) is present, we observe elimination chemistry leading to olefins (alkenes) instead of substitution. The E2 reaction (elimination bimolecular) takes place via a concerted mechanism, and involves an antiperiplanar alignment of orbitals – proton and LG lined up across from one another however in the same plane. Continue reading
One of the first things new students in Organic Chemistry are astounded by once the class begins is the absence of mathematics. A sigh of relief is breathed as they are introduced to qualitative topics such as nomenclature and alkanes. Soon a feeling of comfort sets in. Stereoisomerism is introduced and then wham! Nucleophilic substitution (SN1 / SN2 reaction) and elimination appear to come from nowhere.
With each new student I meet I deliver the same message. The most challenging exam of the first half of the course will be SN1, SN2, E1, E2. The seemingly new concept of chemical reactions coupled with reaction mechanisms influenced by reagents, solvent and temperature all too frequently lead to confusion and even intimidation. This doesn’t have to happen. Continue reading