Nitration 5a4t67

Nitration

Introduction • Introduction of one or more nitro groups (-NO2) into a reacting molecule. Nitro aromatic or Nitro paraffinic compound: When nitro group attached to carbon. Nitrate ester: When nitro group attached to oxygen. Nitramine: When nitro group attached to nitrogen. We shall consider only those nitrations in which nitro group replaces hydrogen atom, since these reactions are technically important

Applications of Nitration products As • Solvents • Dyestuffs • Pharmaceuticals • Explosives They also serve as useful intermediates for the preparation of other compounds, particularly amines which are prepared by the reduction of the corresponding nitro compound.

Nitrating Agents • Fuming, concentrated, and aqueous nitric acid • Mixtures of nitric acid with sulfuric acid, acetic acid, acetic anhydride, phosphoric acid, and chloroform. • Nitrogen pentoxide , N2O5 • Nitrogen tetroxide, N2O4

In order to make an intelligent choice of nitrating system for particular nitration, it is desirable to know what species are present in the various systems and to

The Nitryl ion NO

+ 2

Mixed acid: • The system nitric acid-sulfuric acid is the most important nitrating medium from a practical standpoint. • Nitric acid exists in strong sulfuric acid as the Nitryl ion, NO2+ • The Van’t Hoff i factor ( the number of particles generated by one molecule of solute) of nitric acid in sulfuric acid is found to be 4.

Aromatic Nitration • Nitryl ion is an electrophilic reactant. • Carbon atom of aromatic ring contains strong electron density. • Nitro group can attached to ortho, meta or para positions depending upon the electron density. • The amount of these isomeric product will depend upon the substituent. • Certain substituent cause the electron density to be greater at ortho and para position than meta position, hence they yield nitration products in which ortho and para isomers predominate. • Other substituent cause the electron density to be greater at meta position rather than ortho and meta, hence they are called meta directing.

The isomer distribution arising from the nitration of various monosubstituted benzenes is shown as

Naphthalene Series • Two different mononitro derivatives, are generally formed • The alpha and beta compounds, also known as 1nitronaphthalene and 2-nitronaphthalene. • Upon nitration , the first nitro group enters almost into the alpha or 1 position; • a second nitro group enters into position 5 or 8.

Theory of Aromatic Substitution • According to the theory , a substituent influences the electron density in following two important ways

1.Inductive effect (-I,+I) 2.Mesomeric effect (-M, +M)

Inductive effect -I • Effect produces when substituent • attracts the electrons causing all the position in the ring to be less reactive than benzene. • The effect being greater on ortho • and para leaving meta more reactive. • • Making them meta directing group. • Groups which produce –I effect in • order of decreasing strength are: -NMe3, -NO2, - Halogen

+I Effect produces when substituent repel the electrons causing all the position in the ring to be more reactive than in substituted benzene. The effect make ortho and para positions more reactive than meta. Making them ortho / para directing group. Groups which produce +I effect in order of decreasing strength are: -O, alkyl.

Effect of side chain reduces the effect of substituent

Mesomeric Effect -M • Effect produces when substituent is an electron with drawing group.• • Substituent which shows a –M effect deactivate all the positions. • • Meta position being less deactivated then ortho and para • making them meta directing compounds. • E.g. -NO2, Carbonyl group • (C=O), -C≡N, -COOH, -SO3H etc.

+M

Effect produces when substituent is from an electron donating group. Substituent which shows a +M effect activate all the positions. Effect being more pronounced on ortho and para positions making it ortho / para directing compounds.

E.g. -OH, -OR, -SH, -SR, -NH2, -NR2 etc.

Mesomeric effects are stronger than inductive effects

Mesomeric effect

INDUCTIVE EFFECT Vs RESONANCE EFFECT

• In most of the cases, resonance effect is stronger and outweighs inductive effect.  • For example, the -OH and -NH2 groups withdraw electrons by inductive effect (-I). However they also release electrons by delocalization of lone pairs ( +R effect). Since the resonance effect is more stronger than inductive effect the net result is electron releasing to rest of the molecule. This is clearly observed in phenol and aniline, which are more reacting than benzene towards electrophilic substitution reactions. • Whereas the inductive effect is stronger than the resonance effect in case of halogen atoms. These are electronegative and hence exhibit -I effect. However at the same time they also release electrons by delocalization (+R effect) of lone pair.  • .

Kinetics of Aromatic Nitration • Kinetics of the nitration reactions depend upon the reacting mixture. • Nitration in a mixed acid (mixture of nitric and sulfuric acid): • Compounds which are nitrated conveniently measurable rate in this system are those which have strong –I and –M effects such as nitrobenzene and ethyl benzoate. • Rate of these nitration is proportional to the concentration of the added nitric acid and of organic substrate.

• Nitration in organic solvents (Mixture of nitromethane or acetic acid with nitric acid): • Kinetics of the process depend upon the aromatic compound being nitrated. • Compound which posses strong deactivating group are nitrated at the rate which is proportional to concentration of substrate. • Compound which are more reactive than benzene such as toluene react at the rate which is independent of substrate.

Kinetics of Aromatic Nitration • Nitration in aqueous nitric acid • highly reactive substrate shows zero order kinetics and less reactive compound show first order kinetics

• Effect of nitrous acid on nitration: • Causes inhabiting effect in the nitration of compound having no activating group and thus the reaction should carried out in strong acid or mixed acid. • Causes catalytic effect in the nitration of compound having reactive group and thus can be nitrated in a weak nitric acid.

• Oxynitration: • Reaction occur between benzene and 50 percent nitric acid containing 0.2 molar mercuric nitrate. • Yield up to 85 percent dinitrophenol and picric acid.

Nitration of paraffinic compounds • Gas phase reaction • Unlike aromatic compounds the paraffinic compounds are quite inert to nitrating agent. • Parrafins can be attacked by certain atoms and free radicals. • The nitration of these compounds is carried out commercially in vapour phase at temperature of 350-450 degree centigrade. • It is a free radical reaction. • Nitric acid of 70 percent strength or less is generally used. • Variety of product are formed for example by the nitration of 2methylpentane which yields nitromethane, nitroethane, 2-nitropropane, 2nitrobutane, 1-nitroisobutane, 1-nitro-3-methylbutane, 2-nitro-3methylbutane. • The reaction is carried out by ing the reactant through the reaction chamber in a flow system. Products are condensed and distilled. • There is optimum temperature at which highest yield is obtained.

• Oxygen increase the yield of nitromethane and nitroethane and decrease the yield of nitrobutane. Addition of oxygen also lowers the optimum temperature and improves conversion and yield. • Nitrogen dioxide also reacts with paraffins to yield nitroparaffins at 325 0C • Bromine has also a beneficial effect on yield and conversions. • Substitution is favorable when highly branched hydrocarbons are nitrated. • • • •

Liquid phase nitration Less important because of low yield , low conversions and side reactions. Replacement of hydrogen by nitro group. No reaction possible involving replacement of alkyl group by nitro groups

Nitration of Acetylene • The reaction of acetylene with nitric acid yields tetranitromethane. • Tetranitromethane is useful compound that is used for increasing the cetane number of diesel fuel and also used in military explosive. • The reaction occur in two steps. • In first step acetylene is allowed to react with highly concentrated nitric acid at 500C contaning mercury nitrate in a reactor provided with cooling coils and thermostat. Solution of trinitromethane(nitroform) in 85% HNO3 containing NO2 results. • In second step sulfuric acid is added to the system and upon heating nitroform is converted into tetranitromethane (TNM)

THERMODYNAMICS OF NITRATION • Nitration reaction is highly exothermic. • A study of the thermal properties of nitrating acids is essential for an adequate understanding of this unit process • The nitration reaction must be controlled by systematic cooling designed to withdraw the energy evolved • When all the energy set free by an exothermic reaction is forced to appear as heat, the quantity of it lost to the cooling mechanism equal the decrease in enthalpy Q = -∆H

Engineering factors for nitration • 1. 2. 3. 4.

TEMPERATURE Temperature effect the following factors in aromatic nitrations Kinetic rate constant for various chemical steps increase with temperature. Solubilities in the acid phase of both nitrated and unnitrated aromatics probably increase with temperature Viscosities decrease and diffusivity coefficients increase with temperature. interfacial area also changes with temperature. Equilibrium constants changes with change in temperature

continued • AGITATION increase degree of agitation tend to promote transfer of reactants in the two phase system, as a result, increased agitation generally causes increased rate of reaction. • COMPOSITION Composition of the acid and organic phase effect the concentration of reactants. in addition, concentration effect, to some extent, the mutual Solubilities of two phases. • RATIO Ratio of acid to organic phase is important relative to the type of emulsion formed

Process Equipments For Technical Nitration  Batch Nitration  Continuous Nitration

Batch Nitration • Nitration is usually done in closed cast iron or steel vessels. Modern practice is to use mild carbon steel. • Nitrator consists of a cylinderical vessel containing some kind of cooling surface, a means of agitation, feed inlets and product outlet lines. • They are also equipped with a large diameter quick dumping line for emergency use if the reaction gets out of control. • The contents of the nitrator are dumped rapidly into a large volume of water

Batch Nitration • Cooling is generally accomplished by coils of tubes through which either cold water or brine for cooling may be circulated or hot water and steam for heating. • For control of temperature in nitrations, a wall jacket is usually not sufficient enough except in the case of vessels of very small capacity. • Advantages of coils: • High coolant velocity is possible • More compact so can be installed anywhere in the tank.

• Disadvantages of coils: • Fouling and scaling problem. Cleaning is no easy.

Batch Nitration • A common accessory for the nitrator is a suction line in the vapour space above the liquid charge to remove the acid fumes and oxides of nitrogen which may be liberated. • Two factors which are of prime importance in the design of nitrators are • Degree of agitations • Control of temperature

Continuous Nitration • The actual nitration reactions in a continuous process are carried out in the same type of vessel as used for batch nitration, with the exception that an overflow pipe or weir arrangement is provided for the continuous withdrawal of product and that continuous feed of reactants is provided. • Automization is there is continuous processes.

Nitrators  Schmid Nitrator  Biazzi Nitrator

Schmid Nitrator • The material to be nitrated is fed into the top of the nitrator and is immediately drawn down through the sleeve and thoroughly mixed with the spent acid and reacting material. • In the bottom of the nitrator fresh mixed acid is fed in and mixed with the other reactant by means of agitator and baffles provided. • The reacting material then upwards with high velocity through the tubes surrounded by refrigerated brine. Product and spent acid are withdrawn continuously from the nitrator through the overflow line.

Biazzi Nitrator • In this apparatus the turbine type agitator provides intensive agitation. A vortex is formed in the center about the agitator shaft. • The reactants fed from the top are immediately drawn into the vortex thoroughly mixed and circulated down through the center of the bank of cooling coils. • The high velocity imparted to the nitrator contents makes for efficient mixing and heat transfer. Due to throwing of cold body on hot body flashing and evaporation takes place so you have to provide suction line for vapours.

Mixed Acid Composition From technical standpoint of using mixed nitric and sulfuric acid, there are two primary conditions that must be met. these are 1. The amount of 100% nitric acid present in nitration must be enough to satisfy the stoichiometric requirements of nitration reaction. it is usually present in excess in order to maintain reasonably fast overall reaction. 2. The amount of100% sulfuric acid with its associate so3 must be sufficient to promote reaction

Controlling quantities • D.V.S (dehydrating value of sulfuric acid) D.V.S is the ratio of H2SO4 to H2O present at the end of reaction. • Nitric ratio nitric ratio is the ratio of the weights of 100% nitric acid to weight of material being nitrated.

D.V.S and stability of nitrator charge • Increase in D.V.S favors high stability of nitrator charge, while decrease in D.V.S results in lowering stability. • Increasing D.V.S tends to derive the nitration or esterification farther towards completion, whereas too low D.V.S would permit accumulation of incompletely nitrated materials, with increase dilution and it would be favorable to oxidation. • D.V.S. ratio is always on the high, safer side, kind of automatic safety factor.

Preparation of nitro paraffin's

Preparation Nitrobenzene

Preparation of αnitro naphthalene