Nitro Alkanes and Nitro Arenes
Structure
The nitrogen is trigonal planar with a bond angles of 120°, there are two resonance forms so implying that the two oxygens are equivalent;
Electronic Effects
These are strongly electron withdrawing both inductively, -I, and mesomerically, by resonance, -R. This means that both the C-N s bond and the p system are strongly polarised, C d+-NO2d-.
Due to the -I inductive effect of the s bond the pKa values of nitro group containing compounds can also be affected;
Ph-NO2, this substituent is ortho, para directing due to the -R resonance effect of the p bond. The -I will also still operate but is less obvious here.
Spectroscopy
I.R.
The n max for the N=O stretch is 1500-1600 cm-1, compared to the C=O stretch at 1650-1800 cm-1.
NMR
For a CH protons, adjacent to the group, the chemical shift, d, = 4.3, due to electron withdrawing effect;
U.V .
The nitro group causes a pronounced shift of l max to longer wavelengths when conjugated to unsaturated p systems, a bathochromic shift. This is why nitro compounds are often yellow.
Synthesis of Nitro-Compounds
Aliphatic nitro compounds are synthesised by;
Gas Phase Nitration of Alkanes
This commercial free radical process involves the NO2 radical;
Electrophilic Nitration of Enolate Anions
The use of the enolate active methylenes forms a stable product due to the chelation of the counter ion;
Nitrate SN2 displacement of alkyl halides
Two products, a nitro compound and a nitrite ester, are produced due to the sodium nitrate acting as an ambident nucleophile, either a N or O nucleophile;
The use of silver nitrate produces only the nitro compound as it is not an ambident nucleophile.
Oxidation with Peracids
Nitro compounds can be produced by oxidised of amine by peracids;
Aromatic Nitro Compounds
These are synthesised by electrophilic aromatic substitution with NO2+ ions;
Reactions
As the nitro group is strongly electron withdrawing and shows affinity with the C=O group. Thus addition across the N=O is possible and reduction is easy.
a Anions
a anions are easily formed with base and stabilised by resonance as nitronate anions, c.f. C=O enolate formation;
The protons on nitro methane, MeNO2, have a pKa of 10.2, c.f. MeCOCH2CO2Et pKa 11. So in order to remove the a protons on nitro alkane an appropriate base in required.
As the nitronate ion is delocalised it is a soft nucleophile and show usual reactivities of stabilised, soft, carbanions.
Alkylation
Henry Reaction
This reaction is analogous to the Aldol reaction;
If R' is aryl the mechanism of elimination is;
If R' is alkyl the mechanism of elimination is;
Michael Additions
The Michael addition is a conjugate addition as the double bond is the soft centre of the ester, the carbonyl carbon being the hard centre. It proceeds by the following mechanism;
Ambident Nucleophiles
Nitronate anions themselves can act as ambident nucleophiles with either attack from the C, a soft nucleophile, or from the O, hard nucleophile. These will attack soft or hard electrophiles respectively;
This ambident behaviour can be seen in the Nef reaction;
O-alkylation is Possible with a hard alkylating agent like Meerwen's Bact, this is a Me+ source;
The ambident nature of the nitro group makes it a very versatile reagent.
Reduction
General
In principle the reduction of nitro compounds should follow the path;
The reduction of nitroso groups is generally more easily achieved but the nitro group can be reduced in a number of different ways;
Photochemically
Metal/H+ Reactions
Metals, such as Fe, Zn, Sn can be used with H+ to reduce the nitro group by a sequence of single electron transfer (SET)/protonation reactions;
The mechanism for the Zn/H+ reaction is;
H2/Pd Surface
Reactions H2/Pd or Pt can be used by heterogeneous H- transfer on the metal surface and then reaction with the nitro compound;
Sulphur Reagents
NaSH or Na2S x or (NH4)2S2 can be used and possibly precede by SET reactions;
Metal hydrides
Reagents like LiAIH4, but not usually NaBH4, reduce the nitro group by hydride, H-, transfer. The product will depend on the nature of the reducing agent, but the end point is ultimately an amine.
Summary
?N SP2 hybridised and strongly electron withdrawing
?H acidic
?Anion reacts with E+, RBr RCHO, Henry reaction etc.
?Soft anion soft undergoes Michael additions
?Group reduced by:
?Metal/H+ (electron source)
?H-, (NaBH4 or LiAlH4)
H2/Pd or Ni (R)
Nitroso Compounds
Structure
The nitrogen is trigonal planar with a bond angle of a125°. Nitroso compounds tend to dimerise in the following way;
Electronic Effects
Nitroso groups are strongly electron withdrawing, like the nitro group, but the situation is complicated by the dimerisation reaction.
Spectroscopy
I.R.
The n max for the N=O stretch in the monomer is 1560 cm-1, in the dimer the n max for the N-O stretch is 1200 cm-1.
NMR
For a CH protons, adjacent to the group, the chemical shift, d, = 4.0, due to electron withdrawing effect.
Synthesis
Aromatics
The synthesis of aromatic nitroso compounds can achieved by nitrosation with NO+, form HNO2, on strongly activated, electron rich, aromatics like ArNH2 and ArOH;
They can also be produced from nitro compounds by reduction to the hydroxylamine then followed by oxidation;
The nitroso compound comes out of aqueous solution, due to its non hydrophilic nature, and so avoids further oxidation, this is a very general method and can be used for a verity of compounds. The direct reduction from NO2-R to NO-R is not a generally feasible process though a photochemical method is known, see nitro group section.
Aliphatics
The synthesis of aliphatic nitroso compounds can achieved by nitrosation of active, acidic, methylene compounds under acidic conditions, H+/NOCl, N2O3 or N2O4;
Under basic conditions, C5H11ONO/NaOEt;
Under neutral conditions enol silyl ether /NOCl;
Another method of synthesis is by Markovnikov addition of NOX (X=Cl, NO2, NO3) to olefins.
Tertiary Aliphatics
To synthesis nitroso compounds with tertiary alkyls another method has to be employed;
Reactions
Addition and Condensation
As the nitrosyl group is strongly electron withdrawing and more similar to the C=O than the NO2 group There is polarisation of the N=O bond and so behaves as a weak C=O. It undergoes addition of nucleophiles and condensation with primary amines and the anions of active methylene compounds, e.g malonates, b ketoesters;
The best method for preparation of secondary hydroxylamines is by addition of a Grignard reagent to a nitroso group and carrying out an acidic work up;
The production of a nitrone can be seen in the following reaction;
The hetero Diels-Alder reaction is also possible;
Reduction
Reduction occurs as for NO2 groups with metals, metal hydrides, hydrogen/ catalyst.
Oxidation
Oxidation is readily brought about with peracids inter alia, this is not possible for nitro groups.
Radical Addition-Spin Trapping
Monomeric nitroso compounds are used to detect transient free radicals, mainly carbon centred radicals, by 'trapping' them as stable nitroxide radicals. These can be detected and assayed in an EPR spectrometer, this is the electron equivalent of the NMR spectrometer. EPR stands for Electron Paramagnetic Resonance. This allows the intermediacy of such otherwise undetectable transients to be proven and is therefore a valuable mechanistic probe.
If the following thermal decomposition of benzoyl peroxide, a radical initiation reaction, is to be monitored;
The phenyl radical would be undetectable by EPR but by addition of a tertiary nitrosyl the more stable nitroxide is produced which is observable;
Summary
?N SP2 hybridised and electron withdrawing
?ON group like carbonyls undergoes addition with RMgX, condenses with RNH2 and 'active methllene', e.g malonate anion
?When a-H present, readily isomerises to oxime
?Very rapid addition of radicals R across N=O bond to give stable nitroxide radicals