7.1: An Overview of Elimination Reactions
- Page ID
- 451148
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Eliminations are one of the four basic reaction types in organic chemistry (along with additions, substitutions and rearrangements). The simple eliminations that will be examined in this section all occur on sp3-hybrid carbon atoms with the following structural features:
- An sp3-hybrid carbon bearing a potential Leaving Group (abbreviated as C-LG).
- A C-H bond on an sp3-hybrid carbon alpha to the carbon bearing the leaving group. Recall that an "alpha" carbon means the carbon next to the functional group in question (an alkyl halide, for example).
- A Base (abbreviated as B).
The products of these types of eliminations are the conjugate acid of the base, the leaving group and, most importantly for an organic chemist, an alkene.
These eliminations to form alkenes will also help illustrate a number of important mechanistic ideas for polar reaction mechanisms, including:
- Acid/base chemistry,
- Reactions between nucleophiles and electrophiles, and most importantly
- The use of electron movement arrows to show the flow of electrons during the bond making and breaking process
Note that during the course of the reaction, the following bond making and bond breaking events must occur:
- The alpha C-H bond is broken (and a new bond is formed between this proton and the base).
- The carbon-leaving group bond is broken (and becomes a new lone pair on the leaving group).
- A new pi bond is formed between the two carbon atoms using the electrons formerly part of the alpha C-H bond.
Because the pi bond cannot form without both of the other two steps occurring, it turns out there are three possible mechanistic ways of thinking about the order of breaking both the C-H and C-LG bonds:
- Break the C-H bond at the same time as the C-LG bond. This is known as an E2 reaction mechanism.
- Break the C-H bond before breaking the C-LG bond. This is known as an E1CB (CB stands for Conjugate Base) reaction mechanism.
- Break the C-H bond after breaking the C-LG bond. This is known as an E1 reaction mechanism.
Each of these three possibilities leads to a different reaction mechanism that will be examined in more depth in the following sections.