588 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS In organic solvents NOCI, N2Oa, N204 and NOBF4 have been used as preparative ni- trosation reagents (23, 25). Use of N•Oa (2) or NzO4 (28) in carbon tetrachloride or acetic acid gives high yields of nitrosamides. With secondary amines NzO4 in methylene chloride reacts cleanly either as a nitrosating agent at 0 ø or as a nitrating agent at -80 ø (29). Since NO2 is in equilibrium with N204 (49), statements in the literature on effects of either substance must be critically viewed. Nitrosation of amides occurs faster in two-phase systems composed of an organic solvent and aqueous HNOz solution at pHi or in methylene chloride extracts of 2 M aqueous HNO2 than in water alone at pHi (50). Nitrosation of amines dissolved in methylene chloride in contact with solid sodium nitrite occurs by a reaction which in- volves the solvent (51). Recent studies have shown that secondary amines react with NzOa and NzO4 gases dissolved in aqueous alkaline solutions (pH 6-14) at a rate greater than in acidified ni- trite (25-27). Although both nitrogen oxides might be expected to undergo rapid hydrolysis at pH 5 to yield unreactive NOz- and NOa-, amines of widely different reactivity compete effectively with water and OH- for dissolved NzOa and N204. Nitric oxide (NO) alone is inactive but is oxidized by oxygen to NOz and thus to the reactive nitrosating agents NzOa and N204 (25, 27, 36). Rapid nitrosation by NO under anaerobic conditions occurs in the presence of iodine or Ag(I), Cu(I), Cu(II), Zn(II), Fe(III) or Co(II) salts (19, 27). b. Organic Species. N-Nitrosamines themselves act as nitrosating agents. Aromatic nitrosamines, such as nitrosodiphenylamine, transnitrosate secondary amines under neutral conditions in organic solvents (52, 53) probably by a free radical mechanism (53). The process is more rapid in acidic aqueous solution and occurs by a heterolytic mechanism (53, 54). The slower transnitrosation between aliphatic secondary amines requires more extreme conditions or catalysis by nucleophilic agents, such as thiocy- anate and halide ions (23, 55, 56). Nitrosation of morpholine by aromatic and aliphatic C-nitro compounds in tetrahy- drofuran at 70øC has recently been reported (57). Further work is required to ascertain whether nitrosation occurs by direct reaction of amine with the C-nitro function or is caused by agents derived from inorganic nitrite present as a synthetic contaminant or decomposition product. Primary and secondary nitroalkanes decompose to nitrite in dilute alkaline solutions (58). 2. Nitrogen Compounds a. Primary Amines. The well-known deamination of primary aliphatic amines with ni- trite in cold aqueous acid yields a variety of products (22). The rapid reaction proceeds through unstable primary nitrosamine and diazonium ion intermediates. The latter reacts with nucleophiles present to form substitution, elimination and rearrangement products. RNH2 + NO•-,H + • R--•--N=O I H R•I=N--OH Nuc N + alcohols, alkenes ( R--N• + H•O (lO)

NITROSAMINE CHEMISTRY 589 Secondary amines and subsequent nitrosamines formed by reaction of the diazonium ion with the primary amine starting material (eq 11) have been isolated (22). This reac- tion occurs in low yield because the amine is largely protonated and unreactive under the strongly acidic and low temperature conditions commonly used. R--N•N + + RNH2 --N., NO%,H + ) R.,NH ) R.,NNO (11) Diamines with a second primary amine function appropriately located for intramolecular reaction with the diazonium ion form secondary nitrosamines at high temperatures or long reaction times as illustrated by the following examples (59). H2N NH2 + 2NO• ) HCI salt moist 160øC, 2 hr 22% yield 0.025 M 0.050 M in citrate-phosphate buffer, pH 3.4 100øC, 1 hr 22øC, 6 days 1.6% 0.39% Under similar conditions n-butylamine, a monoamine, gave much lower yields of N- nitrosodibutylamine. Higher levels of stable o•-alkoxynitrosamines are produced from the reaction of primary amines with aldehydes in the presence of alcohols and nitrite under mildly acidic conditions (60, 61). RNH= + CH20 + R'OH + NOj, H + R--N--CH=OR' I NO (12) However, mixtures of primary amines and aldehydes without alcohol do not react with nitrite at pH 3 (62). b. Secondary Amines. Nitrosamines formed directly from secondary amines are stable. In moderately acidic aqueous nitrite solutions N2Oa, formed from two molecules of HNO2 (eq 6) is the nitrosating agent. The rate-determining step in the reaction is electrophilic attack by N=Oa on the free electron pair of the unprotonated amine (eq 13). Rate equation 14 describes the kinetics. R=NH + NzOa -- RzNNO + HNOz (13) rate = k[R2NH] [HNO=] z (14) Thus, two factors determine the effect ofpH on the rate of nitrosation: (i) extent of conversion of NO2- to HNO2 and thus to NzOa (favored by lower pH) (ii) concentration of unprotonated amine (favored by higher pH).