Monochloramine, often called simply chloramine, is the chemical compound with the fomula NH2Cl. Together with dichloramine (NHCl2) and nitrogen trichloride (NCl3), it is one of the three chloramines of ammonia. It is an colorless liquid at its melting point of −66 °C (−87 °F), but it is usually handled as a dilute aqueous solution, in which form it is sometimes used as a disinfectant. Chloramine is too unstable to have its boiling point measured.
3D model (JSmol)
CompTox Dashboard (EPA)
|Molar mass||51.476 g mol−1|
|Melting point||−66 °C (−87 °F; 207 K)|
|GHS Signal word||Danger|
|H290, H314, H315, H319, H335, H372, H412|
|P234, P260, P261, P264, P270, P271, P273, P280, P301+330+331, P302+352, P303+361+353, P304+340, P305+351+338, P310, P312, P314, P321, P332+313, P337+313, P362, P363, P390, P403+233, P404, P405|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Drinking water disinfection
NH2Cl is commonly used in low concentrations as a secondary disinfectant in municipal water distribution systems as an alternative to chlorination. This application is increasing. Chlorine (referred to in water treatment as free chlorine) is being displaced by chloramine—to be specific monochloramine—which is much more stable and does not dissipate as rapidly as free chlorine. NH2Cl also has a much lower, but still active, tendency than free chlorine to convert organic materials into chlorocarbons such as chloroform and carbon tetrachloride. Such compounds have been identified as carcinogens and in 1979 the United States Environmental Protection Agency began regulating their levels in U.S. drinking water.
Adding chloramine to the water supply may increase exposure to lead in drinking water, especially in areas with older housing; this exposure can result in increased lead levels in the bloodstream, which may pose a significant health risk.
Swimming pool disinfection
In swimming pools, chloramines are formed by the reaction of free chlorine with amine groups present in organic substances, mainly bio-derived. Chloramines, compared to free chlorine, are both less effective as a sanitizer and, if not managed correctly, more irritating to the eyes of swimmers. Chloramines are responsible for the distinctive "chlorine" smell of swimming pools. Some pool test kits designed for use by homeowners do not distinguish free chlorine and chloramines, which can be misleading and lead to non-optimal levels of chloramines in the pool water. There is also evidence that exposure to chloramine can contribute to respiratory problems, including asthma, among swimmers. Respiratory problems related to chloramine exposure are common and prevalent among competitive swimmers.
US EPA drinking water quality standards limit chloramine concentration for public water systems to 4 parts per million (ppm) based on a running annual average of all samples in the distribution system. In order to meet EPA-regulated limits on halogenated disinfection by-products, many utilities are switching from chlorination to chloramination. While chloramination produces fewer regulated total halogenated disinfection by-products, it can produce greater concentrations of unregulated iodinated disinfection byproducts and N-nitrosodimethylamine. Both iodinated disinfection by-products and N-nitrosodimethylamine have been shown to be genotoxic.
Synthesis and chemical reactions
NH2Cl is a highly unstable compound in concentrated form. Pure NH2Cl decomposes violently above −40 °C (−40 °F). Gaseous chloramine at low pressures and low concentrations of chloramine in aqueous solution are thermally slightly more stable. Chloramine is readily soluble in water and ether, but less soluble in chloroform and carbon tetrachloride.
- NH3 + NaOCl → NH2Cl + NaOH
This reaction is also the first step of the Olin Raschig process for hydrazine synthesis. The reaction has to be carried out in a slightly alkaline medium (pH 8.5–11). The acting chlorinating agent in this reaction is hypochlorous acid (HOCl), which has to be generated by protonation of hypochlorite, and then reacts in a nucleophilic substitution of the hydroxyl against the amino group. The reaction occurs quickest at around pH 8. At higher pH values the concentration of hypochlorous acid is lower, at lower pH values ammonia is protonated to form ammonium ions NH+
4, which do not react further.
- 2 NH3 + Cl2 ⇌ NH2Cl + NH4Cl
- 2 NH2F + CaCl2 → 2 NH2Cl + CaF2
- RR′NCl + H2O ⇌ RR′NH + HOCl
The quantitative hydrolysis constant (K value) is used to express the bactericidal power of chloramines, which depends on their generating hypochlorous acid in water. It is expressed by the equation below, and is generally in the range 10−4 to 10−10 (2.8×10−10 for monochloramine):
- 3 NH2Cl → N2 + NH4Cl + 2 HCl
However, only a few percent of a 0.1 M chloramine solution in water decomposes according to the formula in several weeks. At pH values above 11, the following reaction with hydroxide ions slowly occurs:
- 3 NH2Cl + 3 OH− → NH3 + N2 + 3 Cl− + 3 H2O
- 2 NH2Cl + H+ ⇌ NHCl2 + NH+
- 3 NHCl2 + H+ ⇌ 2 NCl3 + NH+
At low pH values, nitrogen trichloride dominates and at pH 3–5 dichloramine dominates. These equilibria are disturbed by the irreversible decomposition of both compounds:
- NHCl2 + NCl3 + 2 H2O → N2 + 3 HCl + 2 HOCl
- NH2Cl + 2 H+ + 2 e− → NH+
4 + Cl−
Reactions of chloramine include radical, nucleophilic, and electrophilic substitution of chlorine, electrophilic substitution of hydrogen, and oxidative additions.
Chloramine can, like hypochlorous acid, donate positively charged chlorine in reactions with nucleophiles (Nu−):
- Nu− + NH3Cl+ → NuCl + NH3
Examples of chlorination reactions include transformations to dichloramine and nitrogen trichloride in acidic medium, as described in the decomposition section.
Chloramine may also aminate nucleophiles (electrophilic amination):
- Nu− + NH2Cl → NuNH2 + Cl−
The amination of ammonia with chloramine to form hydrazine is an example of this mechanism seen in the Olin Raschig process:
- NH2Cl + NH3 + NaOH → N2H4 + NaCl + H2O
Chloramine electrophilically aminates itself in neutral and alkaline media to start its decomposition:
- 2 NH2Cl → N2H3Cl + HCl
The chlorohydrazine (N2H3Cl) formed during self-decomposition is unstable and decomposes itself, which leads to the net decomposition reaction:
- 3 NH2Cl → N2 + NH4Cl + 2 HCl
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