Nitramines, primary

The mixing of Group 1A radicals in a molecule may have effects other than those expected from additivity alone. For example, nitrates are acid sensitive and are also sensitive to strongly basic groups. Thus a molecule is immediately suspect if it contains a nitrate group and either (1) a primary nitramine, primary aliphatic nitro, phenolic OH, carboxyl or other acid group, or... 

Primary Versus Secondary Nitramines. Primary nitramines, RN(H)N02 are found experimentally to be strong N2O generators compared to most secondary nitramines upon fast thermolysis [29]. It is possible that a rearrangement to the aci form (3) is favored at higher temperatures such... 

MonoalkyUiydroxylarnine hydrochlorides react with preservation of the hydroxylamine stmcture (10). Primary nitramines combine in such a way as to keep the nitramine stmcture intact. 

Oxidizers. The characteristics of the oxidizer affect the baUistic and mechanical properties of a composite propellant as well as the processibihty. Oxidizers are selected to provide the best combination of available oxygen, high density, low heat of formation, and maximum gas volume in reaction with binders. Increases in oxidizer content increase the density, the adiabatic flame temperature, and the specific impulse of a propellant up to a maximum. The most commonly used inorganic oxidizer in both composite and nitroceUulose-based rocket propellant is ammonium perchlorate. The primary combustion products of an ammonium perchlorate propellant and a polymeric binder containing C, H, and O are CO2, H2, O2, and HCl. Ammonium nitrate has been used in slow burning propellants, and where a smokeless exhaust is requited. Nitramines such as RDX and HMX have also been used where maximum energy is essential. 

Where direct nitration results in degradation, a primary amine can be acylated, nitrated directly to form a secondary niiramine, which is then hydrolyzed to form the primary nitramine as exemplified in the following reaction sequence ... 

This synthesis of N-nitromorpholine is representative of a rather general reaction for the preparation of both primary and secondary nitramines.3 It represents the simplest process for obtaining both types of compounds. The reaction is unique in that a nitration is carried out under neutral or alkaline conditions. Acetone cyanohydrin nitrate may also be used for the nitration of many active methylene compounds.8... 

Eremenko and co-workers used nitryl fluoride for the deamination of amines at subambient temperatures in acetonitrile. The same reaction occurs with primary nitramines and their alkali metal salts bis-nitramines react to give the corresponding bis-nitrate esters. 

The reaction of primary aliphatic amines and nitramines with nitronium salts also leads to deamination and the formation of alkyl nitrates. 

Treatment of primary nitramines with absolute nitric acid yields the corresponding nitrate ester and nitrous oxide. 

There are four important groups of A-nitro compounds which are relevant to energetic materials synthesis. These are primary nitramines, secondary nitramines, secondary nitramides (including A-nitroureas and A, A -dinitroureas) and nitrimines. The synthesis and incorporation of these A-nitro functionalities into organic compounds is the focus of this chapter. 

Primary nitramines have acidic protons and are able to undergo condensation reactions to form functionalized nitramines. These reactions are discussed in Section 5.13 because the products have potential application as energetic polymer precursors or find use for the synthesis of other explosives. 

The chemical properties of primary and secondary nitramines are important in relation to their use as explosives. Primary nitramines contain acidic hydrogen in the form of —N//NO2 and, consequently, in the presence of moisture, primary nitramines corrode metals and form metal salts, some of which are primary explosives. This is one reason why powerful explosives like methyinitramine (1) have not found practical use. Ethylenedinitramine (EDNA) (2) suffers from similar problems but its high brisance (VOD 8240 m/s, d = 1.66 g/cm ) and low sensitivity to impact have seen it used for some applications. 

The direct nitration of a primary amine to a nitramine with nitric acid or mixtures containing nitric acid is not possible due to the instability of the tautomeric isonitramine in strongly acidic solution (Equation 5.1). Secondary amines are far more stable under strongly acidic conditions and some of these can undergo electrophilic nitration with nitric acid in a dehydrating medium like acetic anhydride. 

Unlike the direct nitration of amines under acidic conditions, nucleophilic nitration is an excellent route to both primary and secondary nitramines. In these reactions the amine or the conjugate base of the amine is used to attack a source of NO2. This source may be a nitrogen oxide, nitronium salt, cyanohydrin nitrate, alkyl nitrate ester or any other similar source of nitronium ion. 

Despite the moderate to good yields obtained for a range of primary and secondary nitramines, the above methods have not found wide use. Their use in organic synthesis is severely limited by the incompatibility of many functional groups in the presence of strong bases. This is particularly relevant to the synthesis of explosive materials, where nitrate ester and C-nitro functionality are incompatible with strong bases. 

Ordinarily, alkyl nitrate esters will not nitrate amines under neutral conditions. However, Schmitt, Bedford and Bottaro have reported the use of some novel electron-deficient nitrate esters for the direct At-nitration of secondary amines. The most useful of these is 2-(trifluoromethyl)-2-propyl nitrate, which nitrates a range of aliphatic secondary amines to the corresponding nitramines in good to excellent yields. Nitrosamine formation is insignificant in these reactions. 2-(Trifluoromethyl)-2-propyl nitrate cannot be used for the nitration of primary amines, or secondary amines containing ethylenediamine functionality like that in piperazine. Its use is limited with highly hindered amines or amines of diminished nucleophilicity due to inductive or steric effects. 

The reaction of dinitrogen pentoxide with primary aliphatic nitramines and amines leads to deamination and the formation of a nitrate ester as the major product. Consequently, dinitrogen pentoxide cannot be used for the synthesis of primary nitramines. In contrast, both primary and secondary arylamines undergo efficient A-nitration with dinitrogen pentoxide in chlorinated solvents. ... 

The reaction of nitronium salts with primary amines is not usually a feasible route to primary nitramines, except in the case of some electron-deficient arylamines . Picramide... 

The efficient At-nitration of secondary amines has been achieved by transfer nitration with 4-chloro-5-methoxy-2-nitropyridazin-3-one, a reagent prepared from the nitration of the parent 4-chloro-5-methoxypyridazin-3-one with copper nitrate trihydrate in acetic anhydride. Reactions have been conducted in methylene chloride, ethyl acetate, acetonitrile and diethyl ether where yields of secondary nitramine are generally high. Homopiperazine is selectively nitrated to At-nitrohomopiperazine or At, At -dinitrohomopiperazine depending on the reaction stoichiometry. At-Nitration of primary amines or aromatic secondary amines is not achievable with this reagent. 

The direct Af-nitration of Ai-alkylamides is the most important of the synthetic routes to secondary nitramides. Hydrolysis of these secondary nitramides is an important but indirect route to primary nitramines, a reaction discussed in Section 5.10. 

Although a large number of secondary nitramides have been prepared they have not found wide use as explosives because of their facile hydrolysis to acidic primary nitramines in the presence of water. Research has focused on the synthesis of cyclic and bicyclic A-nitroureas and A, A -dinitroureas because of their high performance. 

The instability of primary nitramines in acidic solution means that the nitration of the parent amine with nitric acid or its mixtures is not a feasible route to these compounds. The hydrolysis of secondary nitramides is probably the single most important route to primary nitramines. Accordingly, primary nitramines are often prepared by an indirect four step route (1) acylation of a primary amine to an amide, (2) A-nitration to a secondary nitramide, (3) hydrolysis or ammonolysis with aqueous base and (4) subsequent acidification to release the free nitramine (Equation 5.17). Substrates used in these reactions include sulfonamides, carbamates (urethanes), ureas and carboxylic acid amides like acetamides and formamides etc. The nitration of amides and related compounds has been discussed in Section 5.5. 

A particularly useful synthesis of primary nitramines involves the nitration of the appropriate carbamate ester followed by ammonolysis with gaseous ammonia in diethyl ether. The ammonium salt of the nitramine precipitates in pure form and is carefully acidified to give the free nitramine. The corresponding carbamate esters are readily synthesized from the action of chlorocarboxylic acid esters on alkylamines in the presence of alkali hydroxides. 

The dehydration of the nitrate salts of some primary and secondary amines can yield the corresponding nitramine. Dimethylnitramine has been prepared in 65 % yield from the dehydration of dimethylamine nitrate in acetic anhydride to which 4 mole % of anhydrous zinc chloride has been added." The same reaction in the absence of chloride ion only generates a 5 % yield of dimethylnitramine." Some arylnitramines derived from weakly basic amines have been prepared via the addition of the amine nitrate salts to acetic anhydride. " ... 

Primary nitramines have been formed from the reaction of aliphatic isocyanates with nitro-nium tetrafluoroborate in ethyl acetate or acetonitrile, followed by hydrolysis. °... 

Metathesis reactions between iV-chloramines and silver nitrite in alkaline solution are reported to give the silver salt of the corresponding primary nitramine. The method is of little synthetic value. ... 

Primary nitramines contain an acidic proton which enables them to behave as nucleophiles and undergo addition and condensation reactions. These reactions are extremely useful in two respects. Firstly, these reactions convert primary nitramino functionality into secondary nitramino functionality, which is no longer acidic and much more chemically stable. Secondly, these addition and condensation reactions can be used to prepare functionalized derivatives of polynitramines which can be used to synthesize energetic polymers and other explosive compounds. 

The anions of primary nitramines, like other nucleophiles, can undergo Michael 1,4-addition reactions with a range of a,-unsaturated substrates to form secondary nitramines of varying molecular complexity (Equation 5.18). Kissinger and Schwartz prepared a number of secondary nitramines from the condensation of primary nitramines with a,/3-unsaturated ketones, esters, amides and cyanides. In a standard experiment a solution of the primary nitramine and... 

Primary nitramines react with amines in the presence of an aldehyde to form 1,3-amino-nitramines in a reaction analogous to the Mannich condensation. In these reactions the amine and aldehyde component combine to form an intermediate imine which is then attacked by the nitramine nucleophile. 

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