Nitro Alkanes and Nitro Arenes
Structure0
The nitrogen is trigonal planar with a bond angles of 120°, there are two resonance form s so implying that the two oxygens are equivalent; 0
Electronic Effects0
These are strongly electron withdrawing both inductively, -I, and m esomerically, by resonance, -R. This m eans that both the C-N s bond and the p system are strongly polarised, C d+-NO2d-. 0
Due to the -I inductive effect of the s bond the pKa values of nitro group containing
com pounds can also be affected; 0
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. 0
Spectroscopy0
I.R.
The n max for the N=O stretch is 1500-1600 cm-1, com pared to the C=O stretch at 1650-1800 cm-1. 0
NMR
For a CH protons, adjacent to the group, the chemical shift, d, = 4.3, due to electron withdrawing effect; 0
U.V .
The nitro group causes a pronounced shift of l max to longer wavelengths when conjugated to unsaturated p system s, a bathochromic shift. This is why nitro compounds are often yellow.
Synthesis of Nitro-Compounds0
Aliphatic nitro com pounds are synthesised by; 0
Gas Phase Nitration of Alkanes
This commercial free radical process involves the NO2 radical; 0
Electrophilic Nitration of Enolate Anions
The use of the enolate active m ethylenes forms a stable product due to the chelation of the counter ion; 0
Nitrate SN2 displacement of alkyl halides0
Two products, a nitro com pound and a nitrite ester, are produced due to the sodium nitrate acting as an am bident nucleophile, either a N or O nucleophile; 0
The use of silver nitrate produces only the nitro com pound as it is not an am bident nucleophile. 0
Oxidation with Peracids
Nitro compounds can be produced by oxidised of amine by peracids; 0
Aromatic Nitro Compounds
These are synthesised by electrophilic aromatic substitution with NO2+ ions; 0
Reactions0
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. 0
a Anions
a anions are easily formed with base and stabilised by resonance as nitronate anions, c.f. C=O enolate form ation; 0
The protons on nitro m ethane, MeNO2, have a pKa of 10.2, c.f. MeCOCH2CO2Et pKa 11. So in order to rem ove the a protons on nitro alkane an appropriate base in required. 0
As the nitronate ion is delocalised it is a soft nucleophile and show usual reactivities of stabilised, soft, carbanions. 0
Alkylation0
Henry Reaction
This reaction is analogous to the Aldol reaction; 0
If R' is aryl the m echanism of elimination is; 0
If R' is alkyl the m echanism of elimination is; 0
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 m echanism;
Am bident Nucleophiles
Nitronate anions them selves can act as am bident nucleophiles with either attack from the C, a soft nucleophile, or from the O, hard nucleophile. These will attack soft or hard electrophiles respectively; 0
This am bident behaviour can be seen in the Nef reaction; 0
O-alkylation is Possible with a hard alkylating agent like Meerwen's Bact, this is a Me+ source; 0
The ambident nature of the nitro group makes it a very versatile reagent. 0
Reduction
General
polarisedIn principle the reduction of nitro compounds should follow the path; 0
The reduction of nitroso groups is generally m ore easily achieved but the nitro group can be reduced in a number of different ways; 0
Photochemically0
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; 0

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