Oximes hydroxylamines

R. Mellon (19) but using anhydrous reagents. He obtained an oil that reacted exothermically with hydroxylamine (oxime of m.p. 135°) and that isomerized to 2-oxy-4-methylthiazole (14) upon heating with diluted hydrochloric acid. The thiazolic nature of oxymethylthiazole was clearly demonstrated by its reduction by zinc powder distillation into 4-methylthiazole (23), the first free thiazole ever described. 

When modified fibres of type 5 are treated with hydroxylamine, oxime groups are also easily formed. The interaction with a protein affords a sandwich polymer22. Fibres modified in this way have enhances dyeability. When copolymer fibres are treated with diamine solutions or in acid medium with Fe+3 salts, intermolecular chemical bonds are formed, which results in a considerable increase of the temperature of zero strength and of the heat resistance of fibres. These conversions are shown in Scheme 2. 

Attempts were made in my laboratory to determine carbonyl groups with various carbonyl reagents, e.g., hydroxylamine, semicarbazide or dinitrophenylhydrazine. With hydroxylamine, oximes were formed to an extent that was equivalent to the difference between NaOEt and NaOH consumption. Errors due to binding of hydroxylammonium ions, which would show up in nitrogen determinations as well, were prevented either by methylation of the acidic groups with diazomethane or by ion exchange with dilute sodium hydroxide oximes are stable towards dilute alkali. However, only half the quantity of carbonyl groups reacted with semicarbazide or with dinitrophenylhydrazine. 

However, there are also important features that very significantly differentiate between hydroxylamines, oximes and hydroxamic acids the C=N double bond in 2 and the acetyl group in 3. 

Three fundamental properties that play key roles in determining covalent and/or nonco-valent interactions are the electrostatic potential V(r), the ionization energy 7 (sometimes written IE) and the polarizability a. All three can be obtained experimentally. It is primarily in terms of these properties that we will examine the inter- and intramolecular interactions of hydroxylamines, oximes and hydroxamic acids. Accordingly we shall first briefly discuss F(r), I and a. 

Hydroxylamines, oximes and hydroxamic acids all have adjacent nitrogen and oxygen atoms, as part of their characteristic N—OH group. Since both of these atoms commonly have significant lone pairs, a major determinant of these molecules conformations is the need to minimize the repulsion between these lone pairs. This will be illustrated by the example of hydroxylamine, H2N—OH. 

Features of hydroxylamines, oximes and hydroxamic acids V. COMPARISON OF PROTOTYPICAL EXAMPLES... 

We shall now compare some properties of prototypical examples of hydroxylamines, oximes and hydroxamic acids. These will be structures 1, 2 and 3 with methyl groups at the remaining positions, i.e. dimethylhydroxylamine (6), acetoxime (7) and acetohydroxamic acid (8). 

The resulting estimated average polarizabilities for the hydroxylamines, oximes and hydroxamic acids that we have discussed are given in Table 8. (It should be noted that Miller s approach does not distinguish between oxime isomers.) As anticipated from the correlation between average polarizability and volume (equation 10), the a in Table 8 increase with molecular size. 

In this chapter, we have looked at some of the intrinsic features of hydroxylamine, oxime and hydroxamic acid molecules. The insights obtained, particularly concerning the electrostatic potentials on their molecular surfaces, should provide a useful basis for proceeding to their gas phase and crystal structures and properties. 

In Table 2 are listed the hydroxylamines, oximes and hydroxamic acids for which we have determined the gas phase structures. We tried to select a representative group in each category. There are two types of oximes, as indicated, aldoximes and ketoximes. Due to restricted rotation around the C=N double bond, these can exist in two isomeric forms (except when R = H for an aldoxime and R = R" for a ketoxime). We have investigated both isomers in nearly every instance. For aldoximes, they are generally labeled syn when the H and OH are on the same side of the double bond and anti when on opposite sides. Note that the ketoximes in Table 2 contain one pair of isomers in which the >C=NOH group is not bonded to two carbons instead one bond is to a chlorine. One of these isomers wiU be of interest in Section B.D in the context of hydrogen bonding vi lone pair—lone pair repulsion. 

The NOH group is the structural element that links hydroxylamines, oximes and hydroxamic acids. We will accordingly begin by examining its key features, namely the N—O and O—H bond lengths and the N—O—H angle, for all three families of molecules taken together. 

Before ending this chapter, we would like to draw attention to another type of electrostatic intermolecular interaction that could be quite relevant to hydroxylamines, oximes and hydroxamic acids. We refer to a-hole bonding. This is a highly-directional, nonco-valent interaction between a region of positive electrostatic potential (or cr-hole) on an outer portion of a Group V, VI or VII covalently-bonded atom and a negative site on... 

It is certainly to be anticipated that attractive interactions can take place between positive a-holes on appropriate Group V, VI and VII atoms and the nitrogen and oxygen lone pairs in hydroxylamines, oximes and hydroxamic acids. It is important to be aware of such interactions that may already be present, as well as to make use of them in designing new systems. 

Organic thermochemistry of hydroxylamines, oximes, and hydroxamic acids 55... 

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