Aromatic and Aralkyl Hydrocarbons

3 Aromatic and Aralkyl Hydrocarbons An aromatic ring in a molecule stabilizes the molecular ion peak (rule 4, Section 2.7), which is usually sufficiently large that accurate intensity measurements can be made on the M + 1 and M + 2 peaks. 

A prominent peak (often the base peak) at m/z 91 (C6H5CH2+) is indicative of an alkyl-substituted ben- 

It has been proposed that, in most cases, the ion of mass 91 is a tropylium rather than a benzylic cation. This explains the ready loss of a methyl group from xylenes, although toluene does not easily lose a methyl group. The incipient molecular radical ion of xylene rearranges to the methylcycloheptatriene radical ion, which then cleaves to the tropylium ion (QH/). 

The frequently observed peak at m/z 65 results from elimination of a neutral acetylene molecule from the tropylium ion. 

Hydrogen migration with elimination of a neutral alkene molecule accounts for the peak at m/z 92 observed when the alkyl group is longer than C2. 

A characteristic cluster of ions resulting from an a cleavage and hydrogen migration in monoalkyl benzenes appears at mtz 77 (C6H5+), 78 (C6H6+), and 79 (C6H7+). 

Alkylated polypheny Is and alkylated polycyclic aromatic hydrocarbons exhibit the same /3 cleavage as alkylbenzene compounds. 

---

A few alkyl- and aralkyl-substituted aromatic hydrocarbons find limited use in perfumery. Examples include /i-cymene [99-87-6], which is a component of many essential oils and when pure has a weak, citrus odor, as well as diphenylmethane [101-81-5] which has an odor like geranium ... 

In the preparation of hydrocarbons an alkyl, aryl, or aralkyl halide is mixed with an excess of an aromatic hydrocarbon, and anhydrous aluminum chloride is added in small portions. The reagents must be free from moisture. If the reactants are not liquid or the reaction is very vigorous, a solvent is used. Benzene, nitrobenzene, carbon disulfide, or o-dichlorobenzene may be employed as solvents. When polyhalides are used, more than one molecule of the hydrocarbon may react. For example, benzene with chloroform yields triphenylmethane, and benzene with carbon tetrachloride gives triphenylmethyl chloride ... 

The halogen derivatives of aromatic hydrocarbons are divided into two classes namely, (a) aryl halides and (b) arylalkyl halides or aralkyl halides. 

Aralkyl and di-aryl ethers may be chlorosulphonylated by the method described for hydrocarbons, below. The sulphonyl chlorides of aromatic ethers are usually obtained as oils, and should be converted into the sulphonamides without purification. 

---