chapter7 - Substitution and Elimination Reactions. 7.1....

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1 Substitution and Elimination Reactions. 7.1. Definitions. In an acid–base reaction such as CH 3 CO 2 H + NH 3 CH 3 CO 2 + NH 4 + , the N acts as a nucleophile (Greek for “loving the nucleus), the H acts as an electrophile (“loves electrons”), and the O that accepts the pair of electrons acts as a leaving group . The acid–base reaction is the simplest model for a substitution reaction, which is a reaction in which a σ bond between atom 1 and atom 2 is replaced by a σ bond between atom 1 and atom 3. Substitution reactions are incredibly important in organic chemistry, and the most important of these involve substitutions at C. For example: This substitution reaction, discovered in 1849, involves the nucleophilic O making a new bond to the electrophilic C, and the bond between the electrophilic C and the leaving group I breaking. Any Brønsted base can also act as a nucleophile, and any nucleophile can also act as a Brønsted base, but some compounds are particularly good bases and particularly poor nucleophiles, whereas some are particularly poor bases and particularly good nucleophiles. Any Brønsted or Lewis acid can also act as an electrophile, but there are many electrophiles that are neither Brønsted nor Lewis acids (as in the example above). A haloalkane, e.g. CH3CH2Br, can in principle undergo either of two polar reactions when it encounters a lone pair nucleophile, e.g. MeO–. First, MeO– might replace Br– at the electrophilic C atom, forming a new C–O bond and giving an ether as the product. This is substitution, because the C–Br σ bond is replaced with a C–O σ bond. Second, MeO– might attack a H atom that is adjacent to the electrophilic C atom, giving MeOH, Br–, and an alkene as products. The electrons in the C–H bond move to form the π bond, and the electrons in the C–X bond leave with X–. This is elimination, because a new π bond is formed, and because the elements of the organic starting material are now divided between more than one product. Elimination requires that the substrate have a C–X bond and adjacent C–H bonds, while substitution requires only that the substrate have a C–X bond.
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2 A reaction involves the formation and cleavage of bonds. A mechanism is a story we tell about the changes in the arrangment of the electrons in the starting materials that led to products. When multiple bonds are made or broken, they are usually not made and broken all at one time. A mechanism describes the order in which the different bonds are made and broken and which electrons moved to break and form particular bonds. A mechanism can also help us generate hypotheses about the rate and stereochemical results of a reaction that we can then use to test whether our idea about how the reaction occurred is correct. We will see soon that there are two mechanisms by which nucleophilic
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chapter7 - Substitution and Elimination Reactions. 7.1....

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