Carbonium ions from alkyl halides

Carbonium ions can be generated at a variety of oxidation levels. The alkyl carbocation can be generated from alkyl halides by reaction with a Lewis acid (RCl + AICI3) or by protonation of alcohols or alkenes. The reaction of an alkyl halide and aluminium trichloride with an aromatic ring is known as the Friedel-Crafts alkylation. The order of stability of a carbocation is tertiary > secondary > primary. Since many alkylation processes are slower than rearrangements, a secondary or tertiary carbocation may be formed before aromatic substitution occurs. Alkylation of benzene with 1-chloropropane in the presence of aluminium trichloride at 35 °C for 5 hours gave a 2 3 mixture of n- and isopropylbenzene (Scheme 4.5). Since the alkylbenzenes such as toluene and the xylenes (dimethylbenzenes) are more electron rich than benzene itself, it is difficult to prevent polysubsiitution and consequently mixtures of polyalkylated benzenes may be obtained. On the other hand, nitro compounds are sufficiently deactivated for the reaction to be unsuccessful. 

Crafts process is to generate carbonium ions from the alkyl or acyl halides. It would be expected, then, that a number of other combinations of starting materials and reagents which lead to carbonium ions should be capable of effecting acylation or alkylation. Indeed we find that olefins (p. 35), alcohols (p. 36), ethers (p. 36), and esters (p. 37) can be used as starting materials for aromatic alkylation reactions in the presence of such catalysts as boron trifluoride, sulfuric acid, or anhydrous hydrogen fluoride.69 Acylations can be carried out with acids (p. 37),64 acid halides (p. 230), and acid anhydrides (p. 37). The Fries reaction65 (in which phenolic esters are converted to hydroxy aromatic ketones by means of aluminum chloride) appears to be an example of a typical acylation reaction in which the ester itself acts as the source of an acyl carbonium ion ... 

A carbonium ion is formed by proton-transfer from the complex acid to the olefin. The polymerisation is initiated by the carbonium ion, and the growing end of the polymer consists of an ion-pair. For reactions in alkyl halide solvents the situation was less clear. Early experiments [8] suggested that the addition of water had little or no effect. This prompted Pepper [8] to suggest that the alkyl halide solvent itself was acting as co-catalyst ... 

The rate of alkylation in the above sequence is actually dependent on the rate of dissociation of the alkyl halide and the nucleophilicity of the cyanide complex (12). The most stable carbonium ion derived from the alkyl halides in the above sequence—i.e., having the highest rate of dissociation—is the triphenylmethyl-carbonium ion, which showed widely different rates of reaction with various cyanide complexes having widely different nucleophilicities toward crabon (18) ... 

Electrophiles for aromatic substitution include the halonium ion, the nitronium ion and the carbonium ion. The latter may be generated from alkyl and acyl halides using Lewis acid catalysis in the Friedel-Crafts reactions. 

What we really want as standards for stability of carbonium ions are, of course, the kinds of compounds they are generated from alcohols at this particular point or, later, alkyl halides (Chap. 14). However, the relative stabilities of most ordinary neutral molecules closely parallel the relative stabilities of the alkanes, so that the relative order of stabilities that we have arrived at is certainly valid whatever the source of the carbonium ions. To take an extreme example, the difference in stability between methyl and /m-butyl cations relative to the alkanes, as we have just calculated it, is 71 kcal. Relative to other standards, the difference in stability is alcohols, 57 kcal chlorides, 74 kcal bromides, 78 kcal and iodides, 76 kcal. 

In certain cases, there is no free carbonium ion involved. Instead, the alkyl group is transferred—without a pair of electrons—directly to the aromatic ring from the polar complex, I, between AICI3 and the alkyl halide ... 

On the other hand, there is additional evidence (of a kind we cannot go into here) that makes it very likely that there is a second mechanism for Friedel-Crafts alkylation. In this mechanism, the electrophile is not an alkyl carbonium ion, but an acid-base complex of alkyl halide and Lewis acid, from which the alkyl group is transferred in one step from halogen to the aromatic ring. 

The rate-determining step of an SnI reaction is the formation of a carbonium ion. Judging from our previous experience, therefore, we expect the reactivity of an alkyl halide 10 depend chiefly upon how stable a carhomuni ion it can form. 

If a carbonium ion were involved, we should expect the halide to be derived from the more stable, and therefore the more highly branched, alkyl group. It is also pertinent that anisole is not cleaved by dry hydrogen chloride or hydrogen bromide in carbon tetrachloride solution unless a small amount of pyridine, dimethylaniline, or aniline is added.18 Furthermore, the presence of these bases increases the rate of cleavage of ethers in acetic acid solution. All these experiments imply that more than two molecules participate in the formation of the transition complex leading to etherification or cleavage (see p. 83). 

Reduction of alkyl halides. LiAlH, is satisfactory for reduction of primary and secondary halides or tosylates to the hydrocarbons, but in the case of a tertiary halide the product is predominantly the olefin. Alkyl halides and tosylates, even tertiary, are reduced in good to high yield by sodium borohydride in 65% aqueous diglyme. When a relatively stable carbonium ion incapable of elimination is formed, yields are high, but yields are still satisfactory when elimination is possible. The reaction is very slow in the absence of water. A homogeneous solution required for kinetic studies is prepared from 80% (volume) aqueous diglyme, which can be made 1.80 M in the reagent. 

---