Organic Chemistry: Concepts and Applications - Headley Allan D. 2020
Nucleophilic Substitution Reactions at Acyl Carbons
16.1 Introduction
In the previous chapter, we examined substitution reactions that involve the substitution at sp3 carbons. In this chapter, we will examine substitution reactions that take place at sp2 hybridized carbons, and specifically acyl carbons. You will recall that the acyl group is one that has a carbon—oxygen double bond. For substitution reactions that take place at sp2 hybridized carbons, one atom (or group of atoms) bonded to the acyl carbon in the reactant is substituted for another atom (or group at atoms) in the product. Let us examine the structure of compounds that have sp2 carbons and will undergo these types of substitution reactions. These reactions typically occur at sp2 carbons that are bonded to an electronegative oxygen atom with a double bond in the form of a carbonyl carbon and also contain a good leaving group. In the last sections of the chapter, we will examine substitution reactions that take place with molecules that have other atoms, such as a sulfur, instead of a carbon that is bonded to an oxygen. Shown in Reaction (16-1) is the generic substitution reaction for molecules capable of undergoing an acyl substitution.
(16-1)
Note that the acyl carbon is double bonded to a very electronegative oxygen atom, which makes the covalent bond a polar covalent bond with the bonding electrons more closely associated with the electronegative oxygen, rather than the carbon. As a result, the carbon is partially positive, making it electrophilic and can react with a nucleophile. As we will see in this chapter, the leaving groups are similar to the ones that we have seen before; likewise, the nucleophiles are much the same as those covered in the previous chapter.
Problem 16.1
Analyze the following reactions carefully and predict the substitution products. (Note that if there is an acidic medium for the reaction, it could convert a poor leaving group, such as ─NH2 and ─OH to a much better leaving group, such as ─NH3+ ─OH2+, respectively).
