Alkyl halides, from nucleophilic substitution

Section 19 12 Nitnles which can be prepared from primary and secondary alkyl halides by nucleophilic substitution with cyanide ion can be converted to car boxyhc acids by hydrolysis... 

Because nitriles can be prepaied from alkyl halides by nucleophilic substitution with cyanide ion, the overall process RX RC=N RCH2NH2 leads to primary fflnines that have one more carbon atom than the starting alkyl halide. 

To check this possibility we performed experiments with different concentrations of NaBr in the NaY zeolite. Table 2 presents the results. It can be seen that upon increasing the amount of NaBr impregnated on NaY, there is preference to formation of the cyclobutyl bromide over allylcarbinyl bromide, indicating that the relative position between the bromide ions and bicyclobutonium governs the product distribution. Hence, zeolites may act as solid solvent, favoring ionization of alkyl halides and nucleophilic substitution reactions. In contrast to liquid solvents, where solvation is mostly uniform, the zeolite surface seems to provide unsymmetrical solvation of the cations, leading to product distribution that is different from solution. 

Basic, electron-rich reagents are called nucleophilic reagents (from the Greek, nucleus-loving). The typical reaction of alkyl halides is nucleophilic substitution ... 

Alkylation of Ammonia Salts of primary amines can be prepared from ammonia and alkyl halides by nucleophilic substitution reactions. Subsequent treatment of the resulting aminium salts with a base gives primary amines ... 

Alkyl halides undergo nucleophilic substitution reactions, in which a nucieophiie dispiaces the haiide leaving group from the aikyi haiide substrate. 

Nitriles are commonly available from aldoximes (aldehydes) and amides (carboxylic acids) by dehydration, from alkyl halides by nucleophilic substitution, and from primary amines by oxidation. It is therefore not surprising that these... 

Alkyl Halides and Nucleophilic Substitution Chapter 7 from Study Guide/Solutions Manual to accompany Organic... 

I Primary alkyl halides S 2 substitution occurs if a good nucleophile is used, 2 elimination occurs if a strong base is used, and ElcB elimination occurs if the leaving group is two carbons away from a carbonyl group. 

Although a substantial number of reactions are described in the text, they belong to a relatively modest number of mechanistic types. The preparation of alkyl halides from alcohols and HX, the cleavage of ethers, and the preparation of amines from alkyl halides and ammonia (and many other reactions) all, for example, occur by a nucleophilic substitution mechanism. The following is a brief review of the main mechanistic pathways discussed in the text. 

In the elimination (a dehydrohalogenation), the reagent (BT) reacts as a base, abstracting a proton from the alkyl halide. Most nucleophiles are also basic and can engage in either substitution or elimination, depending on the alkyl halide and the reaction conditions. 

Other substitution reactions we ve seen include some of the reactions used for preparing alkyl halides from alcohols. We said in Section 10.7, for example, that alkyl halides can be prepared by treating alcohols with HX—reactions now recognizable as nucleophilic substitutions of halide on the protonated alcohols. Tertiary alcohols react by an S>jl pathway involving unimolecular dissociation of the protonated alcohol to yield a carbo-cation, whereas primary alcohols react by an 8 2 pathway involving direct bimolecular displacement of H2O from the protonated alcohol (Figure 11.23). 

Aromatic rings are usually not good enough nucleophiles to react with alkyl halides. If Lewis acids are added to improve the electron sink, then electrophilic aromatic substitution occurs via the carbocation if it is more stable than secondary (an SnI viewed from the electrophile). With Lewis acids and methyl, primary, or secondary alkyl halides, electrophilic aromatic substitution usually occurs via direct displacement of the leaving group (an Sn2 viewed from the electrophile). 

Alkyl halides undergo competitive substitution and elimination reactions. The ratio of products derived from substitution and elimination depends on the nature of the alkyl halide, the base/nucleophile, the solvent and the temperature. SN2 reactions are normally in competition with E2 reactions, while SN1 reactions are normally in competition with El reactions. 

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