INTRODUCTION

INTRODUCTION

Introduce one or more nitro groups (-NO2) into a reacting molecule :Nitroaromatic / nitroparaffinic compound when nitro group attached to carbonNitrate ester when nitro group attached to oxygenNitramine when nitro group attached to nitrogen

Industrial importance :SolventsDyestuffsPharmaceuticals,ExplosivesIntermediates for the preparation of other compound

NITRATING AGENTS

Several reagents Nitric Acid (NO2) : fuming, concentrated, and aqueousMixtures of Nitric Acid : nitric acid with sulfuric acid, acetic anhydride, acetic acid, phosphoric acid, chloroformNitrogen Pentoxide (N2O5)Nitrogen Teroxide (N2O4)

Nitric acid as a powerful nitrating agentMixed acid of HNO3 and H2SO4 has nitryl ion, NO2+ and the Vant Hoff i factor is 4Have 3 different ultraviolet-absorption spectra :In dilute aqueous solution is nitrate ion NO3- In an inert weakly polar solvent is unionized HNO3 In mixed acid of sulphuric acid and esters is unionized HNO3

Nitric acid in sulphuric acidPositively charged that can migrates from the anode to the cathodeSeveral crystalline nitryl salts have been isolated and characterized : NO2+ NO3- , NO2+ ClO4- , NO2+ HS2O7 -

The effect of water present in sulphuric acid and the per cent of nitric acid molecules that are ionized to form nitryl ions

AROMATIC NITRATION

Aromatic compound to be nitrated contains a substituentHas a profound effect on the electron densities around the various carbon atomsOrtho and para predominateMeta predominate

A substituent influences the electron density in two important waysInductive effect (-I or +I) associated with the dipole momentExamples : NMe3+, -NO2, -COOEt, -halogen Mesomeric effect (-M or +M)Examples : -O-, alkyl Effects of +IAll the position in the ring to be more reactiveOrtho and para more reactive than meta

Effects of -I Reduces the reactivity of all positionsThe effect being greater in the ortho and para positionExamplesNitration of toluene produces predominantly ortho and para nitrotolueneNitration of nitrobenzene produces predominantly meta dinitrobenzene

Effects of MSusbtituent can decrease the electron density in the ringDeactivate all the positions, the meta being less deactivated than the ortho and paraEffects of +MSusbtituent can increase the electron density in the ringActivate all the positions, the effect being more pronounced at the ortho and para position

Groups of +I and +M facilitate susbstitution and be ortho para directingGroups of I and M make substitution more difficult and be meta directingTwo effects are in opposition will be more difficult to predict, example : -I and +M in phenyl acetate, anisole, acetanilide-I and +M in halogen

Several factors effect on the ratio of ortho and para+I I +M M effectsSize of the substituent

Polar effects I and M

KINETICS AND MECHANISM OF AROMATIC NITRATION

The kinetics of nitration reaction depend upon the reaction mediumThe rate proportional to the concentration of added nitric acid and organic substrate

The effect of water on the rate of nitrationRises sharply with increasing sulphuric acidFalls off at the higher acid concentrationDue to the increase in concentration of nitryl ion

Nitration in organic solvent depend upon the aromatic compound being nitratedNitrobenzene or ethyl benzoate nitrated at a rate which is proportional to the concentration of the substrateCompound which are more reactive than benzene react at a rate which is independent of the concentration of substrate

Nitrous acid (HNO2) has an inhibiting and catalytic effect on aromatic nitrationThe inhibiting is in the nitration of compounds having no activating groupsNitrous acid forms the nitrosyl ion, NO+ Decrease the concentration of nitrl ionsReduce the reaction rate

The catalytic effect is in reactive substratesExamples are anisole and dimethylalanineDue to formation of nitroso compound which is oxides to the nitro compound

2 conditions are necessary for catalyst by nitrous acid :Substrate sufficiently reactiveConcentration of nitryl ions is very low

Oxynitration occurs between benzene, mercuric nitrate which yields dinitrophenol and picric acid

NITRATION OF PARAFFINIC HYDROCARBON

Gas phase reaction to form nitroparaffinParaffin are quite inert to such reagentsSusceptible to attack by certain atoms and free radicalsCarried out in vapor phase T=350-450 oC (free radical reaction)Several reagents are HNO3 70 % strength or less and N2O4

Nitration of isopentane above produces only mononitroparaffinsNo significant amount of polynitro compoundCleavage of the carbon skeleton occursNo rearrangement of the carbon skeletonCarried out by :

Several condition occurred in the nitration1. Optimum temperature

2. The addition of oxygen increases the yield based on nitric acid and oxidation of butane

Oxygen increases the yield of nitromethane and nitroethane and decrease the yield of nitrobutane

Nitrogen dioxide also reacts with paraffin to yield nitroparaffinT=325 oC, t=1.9 min, propane:NO2 = 4.2, conversion=16.6%, yield based on moles of hydrocarbon=51%With addition of oxygen, T=285 oC, t=3 min, O2:NO2 =0.75, conversion=29%, yield=71%

Bromine has a beneficial effect on both yields and conversions to nitroparaffin using nitric acidT = 423 oC, t = 1.5 sec, C3H8 : NO2 = 8.2, C3H8 : HNO3 = 9.9, H2O : HNO3 =15, Br2 : HNO3 = 0.0015, conversion = 47.7%, the yield based on propane 55.5%

Highly branched hydrocarbon undergo less fission during nitration than do their less-branched isomersSubstitution is favored when highly branched structures are nitrated

Temperature coefficients for hydrogen substitution are in the order primary > secondary > tertiaryThe rate of substitution is in the reverse order at low temperatureNitration of paraffin's step :

Liquid phase nitration of olefinNitrating agent is nitrogen dioxide N2O4 Air is added to oxidize any nitric oxide to the dioxide

Liquid phase nitration of olefinNitrating agent is nitrogen dioxide N2O4 Air is added to oxidize any nitric oxide to the dioxide Temperature = -10 to 25 oC, time = 1-2 hr, total yield = 65-85%

Liquid phase nitration are less importanceLow yields, lower conversions, and the occurrence of unwanted side reactionsThe principal is replacement of hydrogen atoms by nitro groupsEase of formation of products follows the order tertiary > secondary > primary nitroparaffins

Example of liquid phase nitration is nitrocyclohexaneProduced by nitration of cyclohexane and oxidation and adipic acidAccelerated by the addition of nitrogen dioxideTemperatures 100-200 oC and pressures of 2-10 atm

Nitration of acetyleneReaction of acetlyne with nitric acid yields tetranitromethaneTetranitromethane useful in increasing cetane number of diesel fuels and military explosiveThe total reaction as follows (at 50 oC)