Reactions of Nitrogen(II) Oxide

15

Reactions of Nitrogen(II)

Oxide

RUSSELL S. DRAGO

Downloaded by COLUMBIA UNIV on August 5, 2012 | http://pubs.acs.org Publication Date: January 1, 1962 | doi: 10.1021/ba-1962-0036.ch015

Chemistry Department, University of Illinois, Urbana, Ill.

The structure, properties, and familiar reactions of nitrogen(II) oxide are reviewed briefly. The dis cussion is mainly concerned with reactions of basic molecules with NO that give rise to products con taining the N O group. The reactions of nitric oxide with sulfite ion, ethyl alcohol, amines, and oximes are discussed. These reactions are cor related through a generalized reaction scheme that involves formation of the reactive free radical intermediate BNO, where Β is a base. In addi tion, reactions that involve base attack on nitric oxide but lead to other products are reviewed. These include nitric oxide reactions with phos phorus and sulfur donors and with organic nitrites. 2

2

lyjuch of the chemistry of nitrogen (II) oxide, NO, is treated in standard inorganic reference texts or in review articles (1, 24). After briefly reviewing this material, our main concern will be with a newly recognized reaction type (12, 13) and the application of this concept to the correlation of some interesting reactions. The various classes of reactions that are commonly treated can be rationalized by a consideration of the structure of nitric oxide which is represented in molecular orbital terminology as: KKa2s* a*2s* σ2ρ * [τ2ρ χ

υ

= w2p *]

= τ2ρ γ\τΓ2ρ * ζ

υ

e

1

The odd electron is in a molecular orbital consisting of both the nitrogen and oxy gen atoms and hence is delocalized over these atoms. The following three general categories of reactions are recognized for NO: 1. Reactions with other radicals which involve pairing of the odd, π electron to form a polar covalent bond. Examples include: 2NO + C l — 2NOC1 2

and Hg CF I + N O — > 3

h

CF NO + 3

I

2

In some instances an ionic nitrosyl cation is believed to exist in products of this type-i.e., in NO+C10 -. 4

143

In FREE RADICALS in Inorganic Chemistry; COLBURN, C.; Advances in Chemistry; American Chemical Society: Washington, DC, 1962.

144

ADVANCES IN CHEMISTRY SERIES

2. Reactions which involve addition of an electron to produce N O " . This species is produced when nitric oxide is bubbled into a solution of potassium in liquid ammonia (15). Although on the basis of molecular orbital theory a paraK + NO

> KNO NH,(1)

magnetic species should result from the addition of one electron to the π* orbital, the product is diamagnetic. This could result from a breakdown in the sequence of molecular orbitals obtained for homonuclear diatomics when applied to NO~ or to a more complex structure for KNO than one containing monomeric N O " . 3. Reactions in which nitric oxide behaves as a ligand toward transition metal ions. In many of these complexes, a lone pair of electrons on NO forms a weak sigma bond with the metal ion and the odd electron in the nitric oxide mole cule pairs with an electron of the metal ion in d y, d , or dy to form a pi bond. The infrared spectra of several of these complexes have been reported (20). Downloaded by COLUMBIA UNIV on August 5, 2012 | http://pubs.acs.org Publication Date: January 1, 1962 | doi: 10.1021/ba-1962-0036.ch015

X

xz

Z

Since the various aspects of the above reactions have been reviewed in detail, the main concern of this article is with reactions of nitrogen (II) oxide that do not fall into the above categories. Lewis Acid Behavior of NO There is considerable evidence that nitric oxide is a Lewis acid. In this type of interaction electron density is accepted into the π-antibonding molecular orX

bital. The structure of the adduct can be represented as Β | Ν — Ο |. The follow ing reactions may involve a species BNO, as a transition state or intermediate. Alkyl Nitrites. An exchange reaction (19) between N O and E t 0 N O pro ceeds through a BNO intermediate or transition state. The exchange is firstorder in nitric oxide andfirst-orderin alkyl nitrite. Two possible mechanisms were proposed (19), both of which involve NO behaving as a Lewis acid: 14

RO N O + 16

14

R 0

N O -*· [Ο — N 1 4

1δ

15

ΝΟ] -> R O N O + N O 14

16

Ο RO N O + 16

14

NO •

RO -

1 6

RO N O + N O

N

14

\ 14

15

N

In the latter mechanism it was proposed (19) that the intermediate rearranges to yield products. Both mechanisms support the proposal that NO is a Lewis acid. Sulfite Ion. The reaction of NO with sulfite ion and several of the reactions discussed subsequently can be explained by a reaction sequence in which a base attacks NO to form BNO in thefirststep, followed by a rapid reaction of the BNO radical with a second molecule of NO. In several of these reactions the dimer B N 0 is formed, although NO does not dimerize to a measurable extent at room temperature (17). The reaction of sulfite ion with NO is an example of this reaction type. When nitric oxide is bubbled into a cooled (0° C.) basic aqueous solution of potassium sulfite, a white solid which has the empirical formula K S 0 N 0 precipitates (10). 2

2

2

3

2

2

S0 ~ + 2NO 3

2

S0 N 0 3

2

2

2


DRAGO

145

Reactions of Nitrogen(ll) Oxide

A single-crystal x-ray examination (7) of the solid potassium salt indicates that the anion contains the N 0 group and has the structure: 2

2

O—S-N

ο

An infrared study indicates (11) that electrons are completely delocalized in the N 0 group. This derealization is described in valence bond theory as a hybrid structure of the primary structures: 2

2

IQ—S-N Downloaded by COLUMBIA UNIV on August 5, 2012 | http://pubs.acs.org Publication Date: January 1, 1962 | doi: 10.1021/ba-1962-0036.ch015

10^

|Q_s=N

^N=(^

I

10^

10—S—Ν _ ιΟ ^N—01

II

^N=0

10— S—Ν _ ^N—01

III

IV

Resonance forms for the sulfite ion have not been included. There has been discussion concerning whether the compound should be considered a sulfonated hyponitiite (16) or a nitrosated hydroxylamine sulfonate (9). The compound can be prepared by nitrosating a sulfonated hydroxylamine (9). There is little to be gained by this dispute, for the structure is best described as in the above diagrams or in molecular orbital terminology. The reaction of sulfite ion and nitric oxide is best considered as a Lewis acid-base reaction in which nitric oxide behaves as the acid and sulfite ion as the donor. A reaction sequence can be formulated with the following equations: S0 "2 + NO — [ S O 3 N O - ]

(BNO)

2

3

[S0 NO~ ] + NO

S0 N 0 -

2

3

3

2

2

2

In the absence of adequate kinetic data, these proposed reaction sequences are speculative in nature and will not be dignified by the term "reaction mechanism." The sequences proposed will be found valuable in both correlating many nitric oxide réaction» and in suggesting additional experiments. Primary, Secondary, and Tertiary Amines. It has been demonstrated (12, 13) that nitric oxide behaves as a Lewis acid toward a large number of primary and secondary amines. The reactions with primary and secondary amines can be generalized by the following scheme: R N H + NO — [R NHNO] (BNO) 2

2

[R NHNO ] + NO -*· [R NHN 0 ] 2

2

2

2

[R NHN 0 ] + R N H - ^ R N H R N N 0 2

2

2

2

+

2

2

2

2

For primary amines R N H is substituted for R NH, RNH for R N , and RNH + for R NH + in the above scheme. The reaction is carried out by bubbling nitric oxide into a cold (—78°) ether solution of the amine. Pure solid products precipi tate, many of which can be reprecipitated from chloroform solution with ether. Very high yields (70 to 80%) are obtained for many products by a high pressure procedure (12). The diethylamine product has a slight dissociation pressure of amine and NO at room temperature. It will slowly disappear if allowed to stand overnight on 2

2

2

2

2


3

146

ADVANCES IN CHEMISTRY SERIES

a desk top, but will keep for weeks at room temperature in a sealed container and indefinitely if stored at —78°. The products are diamagnetic and their infrared and NMR spectra (12, 13) support the formulation R NH + R N N 0 ~ for secondary amines, and R N H + R N H N 0 for primary amines. The sodium salt derivatives of the anion, R N N 0 ~ , can be prepared by the following reaction: 2

3

2

2

2

2

2

2

2

+

2

2

2

2

2

An analogous reaction can be effected for primary amine products. The simi larities in the infrared spectra of the anions in the sodium and alkylammonium salts support the structures proposed for these products. The stabilities and properties of the products obtained from various amines have been described (12). When hindered amines are employed, no product is obtained, presumably because of a steric effect. By analogy with the structure of S 0 N 0 ~ , the following struc ture is proposed for the anion. 3


2

R N H + EtOH + N a + R N N 0 "

+

2

2

2

R NH + + R N N 0 - + N a + OEt~ 2

2

_

2

2

2

C2H5

If this is the correct structure, the two oxygens are cis to one another and should be capable of forming neutral chelate complexes with metal ions, similar to those formed by acetylacetonate. These considerations led to the study of some metallic derivatives of the ligand E t N N 0 " (21). Neutral complexes form with several metal ions and appear to be solvent-stabilized, since they form readily in solution but decompose upon removal of the solvent. Elemental analysis and molecular weight data on a stable copper (II) complex indicate that the Et NN 0 ~ ion behaves as a bidentate ligand (21). Comparison of the infrared spectra of the copper, sodium, potassium, and calcium salts as well as that of an alkylated product, E t N N 0 E t (26), further supports chelation by the anion (21). The mechanism of the reaction between nitric oxide and the amines is not known at present. In a reported (14) kinetic study low concentrations of NO were utilized. Reaction of NO with trace impurities in the solvent gave rise to a species whose spectrum was mistakenly assumed to be that of nitric oxide. The three-step scheme presented above is favored, with the first step probably being rate-controlling. Both piperazine and Ν,Ν'-dimethylethylenediamine form salts with nitrogen (II) oxide (22). The product of the piperazine reaction can be formulated as either 2

2

2

2

2

2

2

2

2

HÎN^^NN 0 2

or

2

HÎN^ ^NH^ 0 N N 2

I

2

N

NN 0 2

2

II

A sodium salt derivative was prepared, with the empirical formula Na Q N N^ ^)NN Q.2H Q, supporting structure II. Attempts to prepare a sodium salt for the product of the N,NMimethylethylenediamine reaction were not successful, so analogy with the formulation of the piperazine product could not be confirmed. Nitric oxide underwent reaction with trimethylamine to pro duce the very unstable compound ( C H ) N N 0 (22). Oxygen Donors. The phase diagram of the binary system dimethyl ethernitric oxide indicates (5) the formation of the very unstable addition compound, 2

2

2

2

2

3

3

2

2


DRAGO

147

Reactions of Nitrogen(ll) Oxide

( C H ) O N 0 . The acid H N 0 , the existence of which has been suggested (-25), is another example of a donor combined with N 0 . Salts of this acid have been isolated (2). Alcohol. Traube Reaction. The Traube reaction (23, 28, 29) involves the conversion of ethyl alcohol, in the presence of added base, to methylenediisonitramine by nitric oxide. The structure of the product has been established by a single-crystal x-ray study. 3

2

2

2

2

2

3

2


° "

N

^

C

H

2

I Ο

" "

N

\

2

Ρ =

N-0

The first step in the reaction is reported to involve the conversion of ethyl alcohol to acetaldehyde: EtOH + 2NO

CH CHO + H 0 + N 0 3

2

2

The subsequent steps can now be fitted into the BNO scheme. As in an aldol condensation reaction, the aldehyde is converted to a carbanion which acts as the donor; in this case toward NO, forming BNO. The second step involves formation of B N 0 . The following sequence of reactions is proposed: 2

2

ICH2CHO"" + 2NO-*OHC C H OHCÇHN 0; 2

2

CH (N202)*2 42

2

2

N 0 '"- ^> 2

OHCCH(N 02)"2 2

2

2

Q

H

>

HCOCT

The hydroxide required for the last step is formed in the alcohol oxidation and formate is recovered as one of the products. Oximes. The reaction of p-benzoquinone dioxime, HON=

Reactions of Nitrogen(II) Oxide

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