Post by Anders Hoveland on Feb 1, 2011 12:03:45 GMT -8
Many of the readers with an interest in chemistry have wondered about molecules with two nitro groups on the same carbon, since this arrangement could potentially lead to more powerful explosives than putting a single nitrate group a carbon. Unfortunately, there has been a void of information about these types of molecules. Many readers doubt that these compounds would be stable enough to exist, or suspect that they would be too dangerously sensitive to consider. Molecules incorporating the gem-dinitro group exist however, and they show excellent potential for more powerful explosives that are also less sensitive to detonation. The biggest concern with these types compounds
Thermal stability of gem-dinitro compounds
Gem-dinitro refers to two nitro groups on the same carbon atom. Gem-dinitro groups show good potential for incorporation into the structures of new explosive molecules and propellants. Molecules containing either mono- or di-nitro alkanes are generally much less sensitive than nitrate esters, while still being quite energetic when detonated.
There are two primary reasons that nitro groups are not often incorporated into typical explosive molecules. The first is that, in many cases, it is much more complicated to introduce more than one nitro group onto the same molecule. While aromatic rings are easily nitrated into corresponding di- and tri-nitro compounds, most other molecules are much more difficult to nitrate to nitro compounds. Substitution reactions, in which a bromoalkane reacts with nitrite ions, give satisfactory yields for single substitution, but the yields greatly decrease for di- and tri- nitro substitution. The most energetic mono-nitro alkane is nitromethane, which has a significantly lower density relative to other common explosives. Simple nitrations with mixed acids generally fail to produce nitro alkanes. This is because of the Meyer reaction, in which (R)2CH(NO2) groups disproportionate under acidic conditions, oxidizing the carbon to leave either a ketone or carboxyl group. Molecules in which the carbon bonded to the nitro group is also bonded to three other carbon atoms are not vulnerable to this type of disproportionation. An example of such a molecule would be (CH3)3C(NO2).
The second reason is that most, but certainly not all, molecules which contain the gem-dinitro group are not thermally stable, despite usually being fairly insensitive to impact.
The examples of stable gem-nitro molecules seem to have one thing in common. In all cases, elimination of HNO2 and resultant formation of an unsaturated C=C bond, is not possible. In other words, the molecules lack an (R)2CHC(NO2)2CH(R)2 segment, or if such a segment does exist, the carbon-carbon bonds are under a high degree of strain.
Dinitropropanes that do not have a hydrogen atom on the same carbon as the dinitro group require a higher temperature for thermal decomposition than those that have such a hydrogen. P. S. DeCarli, D.S. Ross, Robert Shaw, E. L. Lee, H. D.Stromberg. For example, the solid compound 2,2-dinitro propane is thermally unstable when warmed. At 75degC, it partially decomposes, losing two thirds of its weight after two days. There are, however, several conditions under which gem-dinitro compounds can be thermally stable. In constrast, dinitromethane shows little thermal instability at room temperature, and the pure liquid shows no sign of decomposition after being stored for several months at 0degC. Dinitromethane is, however, significantly less thermally stable than mono-nitro alkanes.
Adding a fluorine atom to the gem-dinitro group, with a structure –CF(NO2)2, greatly lends stability to the gem-ditro group. An example of this is the energetic plasticizer bis(2-fluoro-2,2-dinitroethyl) formal (FEFO), which has excellent thermal stability. FEFO decomposes first at 150 ° C by rearrangement of the nitro group in the loss of nitric oxide and nitrite. Nitrogen dioxide is also formed at 170 ° C.
Other examples of gem-dinitro compounds without any thermal stability problems include 1,1-diamino-2,2-dinitroethylene (DADNE) and 1,3,3-trinitoazetidine (TNAZ). In the first case, the amino groups act as electron donors to the nitro groups through the carbon-carbon double bond. The molecule is effectively aromatic, which is indicated by the yellowish color of the pure compound. An extra electron is a gem-dinitro group, whether as an anion such as in the salt potassium dinitromethanate (K+ O2NCH=NO2- ), or present in an aromatic compound, greatly adds stability to the compound, both thermally and in terms of resistance to detonation. In the case of TNAZ, the high bond strain from the square ring configuration creates a high energy barrier for a hydrogen atom to ionize off and leave a double carbon-carbon bond as the extra electron reduces one of the nitro groups to a nitrite anion. In such instances, the -CH2—C(NO2)2— segment of the molecule, eliminate eliminates nitrous acid (HONO), leaving behind –CH=C(NO2)—. Formation of an unsaturated bond is much more difficult when there is a high degree of bond strain.
Thermal stability of gem-dinitro compounds
Gem-dinitro refers to two nitro groups on the same carbon atom. Gem-dinitro groups show good potential for incorporation into the structures of new explosive molecules and propellants. Molecules containing either mono- or di-nitro alkanes are generally much less sensitive than nitrate esters, while still being quite energetic when detonated.
There are two primary reasons that nitro groups are not often incorporated into typical explosive molecules. The first is that, in many cases, it is much more complicated to introduce more than one nitro group onto the same molecule. While aromatic rings are easily nitrated into corresponding di- and tri-nitro compounds, most other molecules are much more difficult to nitrate to nitro compounds. Substitution reactions, in which a bromoalkane reacts with nitrite ions, give satisfactory yields for single substitution, but the yields greatly decrease for di- and tri- nitro substitution. The most energetic mono-nitro alkane is nitromethane, which has a significantly lower density relative to other common explosives. Simple nitrations with mixed acids generally fail to produce nitro alkanes. This is because of the Meyer reaction, in which (R)2CH(NO2) groups disproportionate under acidic conditions, oxidizing the carbon to leave either a ketone or carboxyl group. Molecules in which the carbon bonded to the nitro group is also bonded to three other carbon atoms are not vulnerable to this type of disproportionation. An example of such a molecule would be (CH3)3C(NO2).
The second reason is that most, but certainly not all, molecules which contain the gem-dinitro group are not thermally stable, despite usually being fairly insensitive to impact.
The examples of stable gem-nitro molecules seem to have one thing in common. In all cases, elimination of HNO2 and resultant formation of an unsaturated C=C bond, is not possible. In other words, the molecules lack an (R)2CHC(NO2)2CH(R)2 segment, or if such a segment does exist, the carbon-carbon bonds are under a high degree of strain.
Dinitropropanes that do not have a hydrogen atom on the same carbon as the dinitro group require a higher temperature for thermal decomposition than those that have such a hydrogen. P. S. DeCarli, D.S. Ross, Robert Shaw, E. L. Lee, H. D.Stromberg. For example, the solid compound 2,2-dinitro propane is thermally unstable when warmed. At 75degC, it partially decomposes, losing two thirds of its weight after two days. There are, however, several conditions under which gem-dinitro compounds can be thermally stable. In constrast, dinitromethane shows little thermal instability at room temperature, and the pure liquid shows no sign of decomposition after being stored for several months at 0degC. Dinitromethane is, however, significantly less thermally stable than mono-nitro alkanes.
Adding a fluorine atom to the gem-dinitro group, with a structure –CF(NO2)2, greatly lends stability to the gem-ditro group. An example of this is the energetic plasticizer bis(2-fluoro-2,2-dinitroethyl) formal (FEFO), which has excellent thermal stability. FEFO decomposes first at 150 ° C by rearrangement of the nitro group in the loss of nitric oxide and nitrite. Nitrogen dioxide is also formed at 170 ° C.
Other examples of gem-dinitro compounds without any thermal stability problems include 1,1-diamino-2,2-dinitroethylene (DADNE) and 1,3,3-trinitoazetidine (TNAZ). In the first case, the amino groups act as electron donors to the nitro groups through the carbon-carbon double bond. The molecule is effectively aromatic, which is indicated by the yellowish color of the pure compound. An extra electron is a gem-dinitro group, whether as an anion such as in the salt potassium dinitromethanate (K+ O2NCH=NO2- ), or present in an aromatic compound, greatly adds stability to the compound, both thermally and in terms of resistance to detonation. In the case of TNAZ, the high bond strain from the square ring configuration creates a high energy barrier for a hydrogen atom to ionize off and leave a double carbon-carbon bond as the extra electron reduces one of the nitro groups to a nitrite anion. In such instances, the -CH2—C(NO2)2— segment of the molecule, eliminate eliminates nitrous acid (HONO), leaving behind –CH=C(NO2)—. Formation of an unsaturated bond is much more difficult when there is a high degree of bond strain.