In the empirical formula,
for naphtha lene, there are two more carbon atoms than hydrogen atoms, and from a consideration of its chemical properties, its products on dis ruption and from various modes of synthesizing it, naphthalene is best represented by formula I., or conventionally by formula associated with a hydrogen atom each, numbered as shown. Although such a formula would appear to consist merely of two benzene rings, neither•ring is in fact benzenoid in charac ter until the unsaturation of the other has been destroyed (see below). Positions I, 4, 5, 8 are termed a-positions; 2, 3, 6, 7 are j3-positions. Among di-derivatives, as in the benzene series, the I :2-position is ortho, 1:3-meta, and i :4-Para; of the heteronu cleal di-derivatives, the only important names are those given to the 2:6- and the 1:8-, which are called respectively amphi- and peri-positions. Peri-substituted compounds react in many respects as though they were true ortho-compounds. For instance, naph thalene-I :8-dicarboxylic acid (naphthalic acid) easily forms an anhydride; the corresponding diketone (acenaphthene-quinone) readily condenses with ortho-diamines; :8-naphthylene-diamine combines with suitable organic substances to form another ring; while sultams I. (from peri-amino-sulpho-derivatives), sultones
II. (from peri-hydroxy-sulpho-derivatives), and lactones III. (from peri-hydroxy-carboxy-derivatives) may be produced by the elimination of water. There are ten possible disubstitution products if the entering groups are the same, but if different, 14. The number of trisubstituted products is much greater.
When naphthalene is nitrated, aminated, chlorinated, bromi nated or iodinated, the entering group invariably replaces the hydrogen atom in the a-position. On sulphonation, however, both a- and f3-positions may be occupied by the entering group, the amount of each monosulphonic acid depending on the tempera ture and duration of the reaction, and also to a lesser extent on the strength and quantity of the acid used. All other monosub stituted .3-derivatives must be prepared by indirect processes. Even with one group already in the nucleus, in either the a- or the L3-position, further substitution tends to take place at another a-position, though there are exceptions, again more especially on sulphonation.