Nucleophilic substitution reactions, haloalkanes leaving group

In Chapter 7, we learned that amines are moderate nucleophiles (Table 7.2) due to the presence of a lone pair of electrons on the nitrogen atom. Therefore, they should undergo nucleophilic substitution reactions with haloalkanes and other compounds containing a good leaving group (Section 7.5). 

Predictions about the mechanism for a particular nucleophilic substitution reaction must be based on considerations of the structure of the haloalkane, the nucleophile, the leaving group, and the solvent. Following are five nucleophilic substitution reactions and an analysis of the factors that favor an S l or Sj 2 mechanism for each and the products that result from the mechanism used. Note that in the following examples, we ignore competing elimination because it has not been discussed yet. 

Substitutions involving haloalkanes involve a type of substition called Nucleophilic substitution, in which the substituent Y is a nucleophile. A nucleophile is an electron pair donor. The nucleophile replaces the halogen, an electrophile, which becomes a leaving group. The leaving group is an electron pair acceptor. Nuclephilic substition reactions are abbreviated as Sn reactions. 

Predictions about the mechanism for a particular nucleophilic the structure of the haloalkane, the nucleophile, the leaving substitution reaction must be based on considerations of group, and the solvent. 

At one extreme, bond breaking and bond forming occur simulfaneously. Thus, departure of the leaving group is assisted by the incoming nucleophile. This mechanism is designated Sf 2, where S stands for Substitution, N for Nucleophilic, and 2 for a bimolecular reaction. This type of substitution reaction is dassilied as bimolecular because both the haloalkane and nucleophile are involved in the rate-determining step. 

In Summary Nucleophilic substitution is a fairly general reaction for primary and secondary haloalkanes. The halide functions as the leaving group, and several types of nucleophilic atoms enter into the process. 

Bimolecular nucleophilic substitution is a one-step transformation The nucleophile attacks the haloalkane, with simultaneous expulsion of the leaving group. Bond making takes place at the same time as bond breaking. Because the two events occur in concert, we caU this process a concerted reaction. 

The transfonnation is called nucleophilic aromatic substitution. The key to its success is the presence of one or more strongly electron-withdrawing groups on the benzene ring located ortho or para to the leaving group. Such substituents stabilize an intermediate anion by resonance. In contrast with the Sn2 reaction of haloalkanes, substitution in these reactions takes place by a two-step mechanism, an addition-elimination sequence similar to the mechanism of substitution of carboxylic acid derivatives (Sections 19-7 and 20-2). 

In Chapter 9, we introduced the basic principles of nucleophilic substitution and elimination reactions. We focused almost entirely upon the reactions of haloalkanes and alcohols. In this chapter, we will expand upon these reactions and consider a much wider range of nucleophiles, leaving groups, substrates, solvents, and their effects on nucleophilic substitution and elimination reactions. We will ask ... 

The net result is a substitution reaction in which the stoichiometry resembles that of an Sj 2 substitution reaction of haloalkanes. However, an Sj 2 reaction occurs in a single step in which the nucleophile bonds to the carbon atom as the leaving group leaves. Nucleophilic acyl substitution occurs in two steps, and the rate-determining step is usually nucleophilic attack at the carbonyl carbon atom to form a tetrahedral intermediate. The loss of the leaving group occurs in a second, faster step. 

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