edited bv -,
DARRELL H. BEACH
chemical of the month Chloramine Harry H. Sisler University 01 Florida Gainesville, FL 32611
Chloramine, NHzC1, is a derivative of ammonia in which one hydrogen atom is replaced by a chlorine atom. Notwithstanding its unstable character-liquid or solid chloramine is violently explosive-it is a readily and conveniently used intermediate in a number of industrial processes, where it is handled in the gas phase or in the form of dilute solutions. The discovery of a process for preparing chloramine by the gasphase reaction of ammonia and chlorine has made this chemical readily available for chemical synthesis both in the laboratory and in industry. Chloramine may be prepared in dilute, aqueous solution by reaction of ammonia with sodium hypochlorite NHx,,, + OC1-I,,)
-
NHzCl(,,) + OH-I,,I
a t temperatures near O°C, yields from which are nearly quantitative at NHJIOCI- mole ratios of three to one. This reaction is the first step in the well-known Raschig synthesis of hydrazine (1,2).A much more convenient and useful synthesis of chloramine, however, is that referred to above, involving the reaction of chlorine with ammonia in the gas phase. Clnl,) + 2NHa1,)
-
NHzClir)+ NHaCIl,)
The ammonium chloride is removed by a porous plug or other mechanical means. By proper control of flow rates, reactor geometry, mole ratios, and reactor component temperatures, yields near quantitative can he achieved ( 3 ) .Large excesses of ammonia are not required for high yields provided other parameters such as rapid flow rates out of the reaction zone are attained. Yields of 90% are obtainable at NHjIClz mole ratios of 2.111 (4). The chloramine obtained mixed with excess ammonia and nitrogen may be passed directly into the system with which it is to react. Solutions of chloramine in various solvents, free of ammonia, can he obtained by bubbling the effluent from the gas phase reactor into the desired solvent and then passing the resulting solution through a column of anhydrous copper(I1) sulfate to remove the ammonia. Anhydrous chloramine freezes a t -66°C. The liquid is highly explosive and distillation is not recommended. Like the related compound ammonia, chloramine consists of pyramidal molecules
I H
The bond angles for ClNH are 102' and the one for HNH is 106°. The molecule is less polar than the NHa molecule since the polarities of the two N6--H6+ bonds and the polarity of the unshared electron pair (sp3) are opposed by the polarity of the N6+-C16- bond. Chloramine is an extremely versatile molecule in its chemical reactions. Its best-known reactions are those in which it acts as an aminating reagent toward electron donor species (Lewis bases) in accordance with the following general equation: B:
1002
+ NHGI
-
B:NHzf
+ C1
Journal of Chemical Education
The Culver Academies Culver, Indiana 4651 1
The preponderance of the evidence in those eases where kinetics of reactions of chloramine in this class have been studied favors the assumption of a himolecular displacement mechanism (SN2),although in a few instances the evidence points in other directions. When the Lewis hase is ammonia the product is the well-known rocket fuel hydrazine: NH3 + NH2Cl NHaNHzf + C1NH3NH2++ NH8 NHzNHz+ NHI+ This reaction was first carried out in aqueous OH- solutions. Because of strong association through hydrogen bonding, the hydrazine is difficult to separate from the water. However, the difficulty of removing water from the hydrazine can be avoided by running the reaction in liquid ammonia or other nonaqueous solvent (5). If the Lewis hase is an amine such as dimethylamine, a suhstituted hydrazine is obtained (6).
--
-
2(CH&NH + NH2Cl (CHahNNH2+ [(CH3hNHz]Cl 1,l-dimethylhydrazine is a major component of a rocket fuel (aerozine: 50% NH2NH2 and 50% (CHs)zNNHz) used in the space program and in the Titan 111ballistic missile). The fact that the above reaction makes possible the synthesis of 1,ldimethylhydrazine without the use of the highly carcinogenic N-nitrosodimethylamine, from which it is formed by reduction with hydrogen, accounts for some of the interest in chloramine. With tertiary amines very high yields of 1,1,1trisuhstituted hydrazinium chlorides are obtained (7)
-
h e s to give triazanium saks (8). NHzNH2+ NHzCl [H2NNHzNHdCl Triazanium chloride is unstable except at very low temperatures, so under ordinary conditions the reaction of chloramine with hydrazine yields nitrogen and ammonium chloride. With 1,l-disuhstituted hydrazines, however, the disubstituted triazanium salts obtained are stable, crystalline solids, as, e.g.
[
'"1
(CH3)%NNH2 + NH&I -+ ( C H h N \NH.
a
By-products of this reaction include (CH&NN=CHz and (CH3)zNN=NN(CH&, the formation of both of which involves the intermediate formation of dimethyldiazine:
-
(CHZI~NNH + ~NHZCI ( C H ~ & N + N H & C ~ These substituted triazanium salts, because of their nitrogen content, have received considerable attention as possible components of solid rocket fuels. Other reactions with electrondonor species include the following where R is an alkyl or aryl Ref. No. group R,P: + NH,CI --c [RIPNH,]CI (9)
-
(CsHr).PC1+ 2 NH, + NH,CI [(CsH5),P(NH2),]CI+ NH,C1
where n
=
3, 4
In some instances dimethylchloramine, (CH&NCI, is more effective than chloramine in this type of reaction. Chloramine can also act as a chlorinating agent. When a mixture of a gaseous olefin and chloramine is exposed to ultraviolet radiation or high temperature (-450°C), hydrogen atoms on the olefin are replaced by chlorine (21). For example,
Similar reactions occur with some saturated hydrocarbons. For example,
In all reactions of chloramine with molecules having the linkage
These reactions are believed to occur by means of a free-radical mechanism in which the neutral amino radical abstracts a hydrogen atom from the hydrocarbon. UY or
+
NH2CI d -NH2 C1
-
heat
amination occurs on the phosphorus atom rather than the nitrogen atom as exemplified by the preceding example. It has been found also that in the reaction of equimolar mixtures of chloramine, a tertiary phosphine, and the corresponding tertiary amine, the chloramine reacts completely with the phosphine rather than the amine. This indicates that the reaction is kinetically controlled by the more nucleophilic phosphorus atom rather than thermodynamically controlled by the more basic nitrogen atom. This principle is true even for the following extreme case (13,14):
In some instances, the initial amination product undergoes condensation, this is true in the chloramination of stibines (15).
RH + -NH* R' + NH:, % + C1- RCI R' NHsC1- RC1+ 'NH2
+
In aqueous solution there is an equilibrium between chloramine, dichloramine, and nitrogen trichloride which is pH-controlled.
At pH = 3 or less the predominant species is nitrogen trichloride. In the range pH = 3-5, dichloramine predominates. Above pH = 8 the principal species is chloramine. At high hydroxide concentrations, chloramine is converted to hypochlorite ion NH2C1+ OH- e NHa
The acid ionization constant for NHzCl is very low (10-'3to 10-17) but the equilibrium NHzCl
Hydroxylamines are formed by the reaction with alkoxide ions (16): RO-
+ NH2Cl
-
RONHz
Re/. No. 2(CeH&PH + NHzCl- (CsHdzPP(CsHd2 + NH&I (17) 5CeH5PHz + 5NHC1- [CsHsP]s 5NHGI (17) NHzCI (CHa)zAsAs(CHd2 + NHaCl (18) 2(CH&AsH 5CHsAsHz 5NH2CI [CHsAsIa + 5 N H O (18) ZCsHsSH N H D C8H5SSC,H5 + NHqC1 (19) ZCeHsSeH NHzCl CsHsSeSeCsHs + NH&1 (20)
+
+
+
---
+
+ Hz0 e H30t + NHCI-
probably contributes to some chloramination reactions. The chloramine molecule is also not verv basic. as is indicated hv a Kb value of 10-l5 compared with a~~ vake of 1.8 X 10-5 f& ammonia. In terms of common conventions, chloramine in most of its reactions behaves as an oxidizing agent. The following standard oxidation potentials have been obtained for aqueous media
+ CI-
Another interesting reaction of chloramine involves the oxidative coupling of compounds containing X-H where X is a nonmetal. These reactions have been shown in a number of instances to provide good synthetic methods for obtaining substances otherwise difficult to prepare. Several examples follow:
+
+ OCI-
-1.48 V
Basic Solution: CI -
1
NHzCL 0.81
V
1
In summary, chloramine is an unusually versatile synthetic intermediate which, in spite of its thermodynamic instability, can, in dilute solutions or in the gas phase at moderate pressures, be utilized without major hazard. Literature Cited !I) Raschig, F.,Aei .4O,4586 (1907). 121 Coleman,G. H.. and Johnson. H. L.."lnorganicSynlheser," I. 59 (1959).
Volume 60
Number 11
November 1983
1003
131 Siiler. H. H.. Neth. F. T..lliaeo.B. S..and Y8ncv.D.. J . Amw Cliem S o c . 76.1906
78,3874 (1956). 17) Omietsnski, ti.. and Sisler. H. H.. J. Amer Chem Snc., 78,1211 (19561. 18) Utuary, K.,Sisler, H. H , and Kltzmante1.P.. Monalrh. Chrm., 100,401 119691. 19) Side?, H. H., Sarkis, A , Ahuja. H. S., Dmgo, R. L a n d Smith. N, i..,J. Amri Cl~em. S a c . 81,2982 119691. 110) Sides H. H., Ahujs. H. S a n d Smith, N. L.,inorg. Chem., l,8d 11962).
1004
Journal of Chemical Education
ill1 Sialer. H . H . . s n d S l r r l r~ n n . c .,Incrie O O l i,~ lOiiRi ,,~Chem , ,. S .,a-... 1121 Hart, W. A , and Siiler, H. H..inoig. Chrm . 3 , 125 (19641, (13) Nielsen, R. P..Vincent,J. F.. and Sirlcr, H. H., Inom Chem., 2,760 (1963). (14) Sialer, H. H., and Weiis, J..inor~.Chrm ,4, 1514 (1966). (15) M r K e n n w R. L., and Sirler, H . H.,lnory Chem.. 6, 1178 (19671. 116) Theilacker. W.,and Ebke, K..An#eu. Chem., 68, 303 (1956). 117) Hi~hsmith.R. 8 ,and Sirler, H. H.,inorg Chrm.. 7. 1740 11968). ~~~
118) 119) 120) (211
~
Kranniih,L. K.,snd Sislar. H. H., inorg Chpm.8, 1032 (1969). Sirlei, H. H., K0tia.N. K.. sndHighsmith, R. E., J. OPE.Chem.. 35. 1742 ll9701 Sisler, H. H.. and Kot1a.N. K . J . Or#. Chem ,35,1700 (1971). Prskssh. H . and Sisler. H. H., J. Oig Chem.,%,3lll 119701.
