Reactions of alkyl halides
Nucleophilic substitution
Primary and secondary alkyl halides can be converted to a large variety of different functional groups such as alcohols, ethers, thiols, thioethers, esters, amines, azides, alkyl iodides, and alkyl chlorides. Larger carbon skeletons can be created by the reaction of alkyl halides with nitrile, acetylide, and enolate ions. Substitution reactions of tertiary alkyl halides are more diffi-cult and less likely to give good yields.
Eliminations
Elimination reactions of alkyl halides allow the synthesis of alkenes and are best carried out using a strong base in a protic solvent with heating. Pri-mary, secondary, and tertiary alkyl halides can all be used. However, the use of a bulky base is advisable for primary alkyl halides. The most substituted alkene is favored if there are several possible alkene products.
Nucleophilic substitution
The nucleophilic substitution of alkyl halides is one of the most powerful methods of obtaining a wide variety of different functional groups. Therefore, it is possible to convert a variety of primary and secondary alkyl halides to alcohols, ethers, thiols, thioethers, esters, amines, and azides (Fig. 1). Alkyl iodides and alkyl chlorides can also be synthesized from other alkyl halides.
It is also possible to construct larger carbon skeletons using alkyl halides. A sim- ple example is the reaction of an alkyl halide with a cyanide ion (Fig. 2). This is an important reaction since the nitrile product can be hydrolyzed to give a carboxylic acid.
The reaction of an acetylide ion with a primary alkyl halide allows the synthe-sis of disubstituted alkynes (Fig. 3a). The enolate ions of esters or ketones can also be alkylated with alkyl halides to create larger carbon skeletons. The most successful nucleophilic substitutions are with primary alkyl halides. With secondary and tertiary alkyl halides, the elimination reaction may compete, especially when the nucleophile is a strong base. The substitution of ter-tiary alkyl halides is best done in a protic solvent with weakly basic nucleophiles. However, yields may be poor.
Eliminations
Elimination reactions of alkyl halides (dehydrohalogenations) are a useful method of synthesizing alkenes. For best results, a strong base (e.g. NaOEt) should be used in a protic solvent (EtOH) with a secondary or tertiary alkyl halide. The reaction proceeds by an E2 mechanism. Heating increases the chances of elimination over substitution.
For primary alkyl halides, a strong, bulky base (e.g. NaOBut) should be used. The bulk hinders the possibility of the SN2 substitution and encourages elimina-tion by the E2 mechanism. The advantage of the E2 mechanism is that it is higher yielding than the E1 mechanism and is also stereospecific. The geometry of the product obtained is determined by the antiperiplanar geometry of the transition state. For example, the elimination in Fig. 4 gives the E-isomer and none of the Z-isomer.
If the elimination occurs by the E1 mechanism, the reaction is more likely to compete with the SN1 reaction and a mixture of substitution and elimination products is likely.
The E2 elimination requires the presence of a β-proton. If there are several options available, a mixture of alkenes will be obtained, but the favored alkene will be the most substituted (and most stable) one (Zaitsev’s rule; Fig. 5).
The transition state for the reaction resembles the product more than the reac-tant and so the factors which stabilize the product also stabilize the transition state and make that particular route more likely. However, the opposite preference is found when potassium tert-butoxide is used as base, and the less substituted alkene is favored.