edlted by DARRELL H BEACH
chemical of the month
The Culver Acadernles Cuiver lndtana 4651 1
Frank Cardulla Niles North High School, Skokie, iL 60077
Deceptively simple in structure, the hydrazine molecule possesses several interesting and useful properties rarely found in so simple a species. These properties include a N-N bond capable of undergoing highly energetic reactions, four replacable hydrogens, and two free electron pairs. Manipulating the bond, substituting various species for the hydrogens, and reacting the electrons results in a rich and varied chemistry with many practical applications. History The first preparation of hydrazine, in the form of its sulfate salt, occurred in 1887, when Curtius synthesized the compound from diazoacetic ester. In 1893, Thiele suggested that hydrazine could be prepared by oxidizing ammonia with hypochlorite, a process first demonstrated in 1906 by F.Raschig. Variations on this process constitute the chief commercial methods of manufacture to this day. Hydrazine found its first large-scale use during World War 11, when Germany found it a suitable fuel for the ME-163 fighter plane. In 1953, the Olin Chemical Co. began United States production a t their Lake Charles, Louisiana, plant. Initially capable of producing 2040 metric tons annually, total noncommunist capacity today exceeds 35,000 metric tons distributed among ten plants in five different countries. Structure Both cuprrimrntnl widence and thewetical calculation.. indica~ithat l1ydrnrine is a non~lanarmt~ltvulewith one SH. group rotated 90" from the cisbr trans position.
Hydrazine undergoes some self-ionization, less than water, but more than ammonia. ~ N Z H* I NtH:
+ NzH:
K=
+ NH;
K=
2NH3 + NHI
The respective names of N2H: and NzHi are hydrazinium ion and hydrazide ion. Neutralization in this solvent system is represented by the equation NzH:
+ NtH;
== 2NzH4
Hydrazine forms a monohydrate, NzH4.H20, which apparently exists in aqueous solution as well as in the solid state. This is sun~orted bv the anomalous behavior of aoueous so.. lutions containin. G4'? by weight of hydrazinr, w h i ~11 corremmds to a 1:l mdc ratiu. SuA iolutions. for exnnioir. . . exhibit maximum density and viscosity. ~
~
Chemlcal Properties The -2 oxidation state of the nitrogen atom in hydrazine represents an intermediate value, suggesting that hydrazine could function as both an oxidizing and a reducing agent. Eo values indicate that in hasic medium hydrazine is, indeed, a powerful reducing agent. NtH4
+4 O H -
Nz
+ 4Hz0 + 4e-
E o = +1.16V
The reducing ability is greatly lessened in acidic medium, where hydrazine exists primarily in the form of hydrazinium ion.
H
T ..
H-N-N-H
A
H
The N-N bond length is 0.145 nm, and all N-NLHbond angles are 112". Valence bond theory accounts for this structure by suggesting sp3 hybridization of each nitrogen with one tetrahedral position being occupied by an unshared air of electrons, which accounts for both the basic and nucleophilic character of the molecule. The measured dipole moment of 1.85 Debves s u.~. ~ o rthis t s asvmmetrical arraneement of atoms. Physical Properties Hydrazine is a colorless, fumingliquid in its pure state with an ammoniacal odor. The relatively high melting point (Z°C), and boiling point (113.5°C) suggest considerable hydrogen bonding. This supposition is further supported by the unusually high molar heat of vaporization of 45.3 kJ,more than three times the 14.64 kJ exhibited by isoelectronic ethane. Predictably, hydrazine is miscible with water and soluble in other polar solvents such as alcohols, amines, and ammonia.
This second fact allows one t o stabilize hydrazine solutions against air oxidation by acidification. Hydrazine has a strong oxidizing ability in acidic solution, but the rate of this reaction is generally slow. Mixtures of hydrazine and air at 1atm pressure are explosive over a wide range (4.7-100'70) by volume. The upper limit reflects the endothermic enthalpy of formation of +50.63 kJ per mole. Whereas a temperature of approximately 250°C is required before appreciable decomposition occurs, the reaction can be catalyzed by many metals such as cobalt, molyhdenum, copper, iridium, as well as their oxides. Iron oxides also catalyze the decomposition, necessitating that hydrazine,
The structure, propeliies, and uses of a variety of chemicaisare highlighted in this featurewhich is aimed at increasing the use of descriptive chemistry.
Volume 60 Number 6 June 1983
505
especially in high concentrations, be handled in scrupulously clean systems. Decompositions can occur to form various products
3NzH4,,,-
4NH3,,,
+ Nz,,
LW = -156.9 kJ
The first reaction, a simple decomposition to the elements, is rarely observed. Hydrazine is a somewhat weaker Arrhenius base than ammonia, N2H4
+ HOH * N2H: + O H
Kb = 8.5 X 10-7
and can accept a second proton. N2H:
Kb = 8.9 x 10-'6
The low value for the second dissociation constant effectively precludes the existence of the N2Hi+ ion in aqueous solution due to its extensive hydrolysis to produce highly acidic solutions.
+ HOH
-
NzH:
+ HaO+
The N2Hp ion may exist in the solid state or in concentrated acid solutions, although even this has been quest~oned. Salts of N2H: with oxidizing anions such as NO: and C10; are potentially explosive, and they have actually been studled with this application in mind. Hydrazine monohydrochloride, NzHcHCl, and monohydrohromide, N z H ~ H Bare ~ , available commercially, and have been used as solder fluxes. Other salts have also been utilized for this same purpose and have the advantage of leaving no residue a t the joint, a result of the combined cleaning effect of the acid and the reducing effect of the hydrazine. The nonvolatihty of hydrazine salts make them a convenient source of nonaqueous hydrazine, since the hazards of vapor toxicity and the attendant expense of handling anhydrous liquid hydrazine are avoided. In a manner analogous to the reaction of anhydrous ammonia with metah to form amides, anhydrous hydrazine forms metalhc hydrazides. These compounds can also he synthesized from the amides, metal alkyls, and metal hydrides. Some examples are 2Na
+ 2N2H4
-
2NaNzH8
+ H2
'I'heseal~alimerd I~ydr;rzidv:art cuiremcly ~ n ~ t a land ~ l rmay rxwlude uwun contact with air or u o m ht:~tinr.Thc alkaline e a k h hydiazides, which have the general formula M(NzH2) are somewhat more stable. If there is one property of hydrazine which stands out above the others and has led to the most useful and practical applications, it is hydrazine's exceptional ability to act as a reducing agent. Reduction can yield a varietv. of wroducts. In aqueous . sdution, nitn8gt.n and i a t e r are most ct,nimun, idtlr,ugh other pruducts ?re pms~hlv.wpwidlly hydrantic acid. HZ,.and amniunis. N H . 'l'he latter nrcduct is wnivwhnt iururisinc. because it actidly contain; nitrogen in a higher oxidation state. Standard reduction uotentials wlace hvdrazine in basic solution somewhere ahovesulfite, bui belowhypophosphite. In acidic solution hvdrazine falls between Sn2+ and Ti", The reaction of hjdrazine with oxygen to form nitrogen and water is used in rocket propellants, in fuel cells, and to remove dissolved oxygen from boilers and hot water systems. (See Uses.) Hydrazine reacts with halogens. NzH4 + 2x2
506
-
Nz
+ 4HX
at,reams. ........ . ~
~
Onc ur the, musi usclul chrniical propertie, uf hydrazine is its ,111iIity r ~ i l u wmt~tals.Thi5 !t,,iturc ha+ been a p ~ ~ l iin ~ d the elea.rrdeii P I R ~ ~ I oI Zi Xi. ('I), 1:~.,md ('r 011 111(1td c,r ~ l , ~ s ~ i c surfaces as well a s t h e preparation of silver mirro& and noble-metal catalysts. Mercury compounds can he removed and recovered from waste waters by reduction and separation. Hvdrazine and its derivatives are canahle of reducing a variety of organic compounds and have some advantages oier wressure hvdroeenations. In some cases, reduction with hv" drazine is stereospecific, yielding, for example, only cis isomers. In addition, this wrocess does not demand hiah wressures. One simple example of an organic reduction is the conversion of a ketone to an alkane. A
+ HOH * NzHAt + OH-
N2Ha'
This reaction can he utilized to remove halogens from waste
(X = F, C1, Br, I)
Journal of Chemical Education
Rd=O
+ NzH,
-
RzCHz + Nz
(unbalanced)
These reactions are eenerallv carried out in basic solution a t elevated temperatures of 1 8 0 - 2 1 0 ~ ~ . The four rewlaceable hvdroaens and two unshared electron pairs of hydrazine allow;t toform numerous alkyl and aryl derivatives, ranging from monosubstituted RNHNH2 molecules to pentasubstituted species, R3N+NR2X-. For example, monomethylhydrazine, CH3NHNH2, and unsymmetrical dimethylhydrazine, (CH&NNHz, are produced in commercial quantities for use as fuels in the US space program. (See Uses.) Manufacture
Although there have been several proposed methods for the manufacture of hvdrazine. the onlv commerciallv feasible
.
.
,
,.
.
the leadin:: prwe,, ia 3 procedure called the I
