Physical Properties of Benzene1. Aromatic hydrocarbons like benzene are colorless and have characteristic odor.2. Benzene is toxic and carcinogenic in nature.3. It is a non-polar molecule and exists in the form of colorless liquid and highly inflammable in nature.4. That is the reason, the bottle of benzene are marketed with the warning of toxic and flammable liquid.5. Because of the high percentage of carbon atom compare to alkanes, Benzene burns with sooty flame and less denser than water.6. The density of benzene is 0.8765 g/cm3and melts at 278.7 K. The boiling point of benzene is 353.3 K temperature.Chemical Properties of Benzene Benzene undergoes substitution reactions in spite of the high degree of unsaturation. This behavior of benzene is called as aromaticity or aromatic character.Aromaticity of benzene can be easily explained on the basis of resonance structure of benzene. During additional reactions of benzene, it will lose its aromaticity, hence its preferred to undergo substitution instead of additional reaction. In benzene there are three pi bonds located in hexagonal ring in alternate manner. These pi bonds get delocalized in ring and make molecule stable. The carbon atoms in benzene are sp2hybridized and each carbon atom has one unhybridized p-orbital. These six unhybridized p-orbitals get delocalized above and below the plane of ring.

Since six pi electrons are delocalized over whole ring, therefore the cyclically conjugated double bonds represents by a circle and the carbon-carbon bond length becomes equal.This structure of benzene is called as resonance hybrid of benzene and generally used to represents the benzenemolecule.

Thus due to resonance and high electron density, benzene mainly undergoes electrophilic substitution reaction. It can show additional and oxidation reactions also in the presence of strong reagents. Some common chemical properties of benzene are as follows.Electrophilic substitution reactionThe most common substitution with benzene is electrophilic substitution reaction which is a multi-step reaction. The catalysts and co-reagents react to generate a strong electrophilic species in initial step of the substitution. Electrophile interacts with benzene with to form a cyclohexadienyl cation which is known as Wheland complex or the s complex or the arenium ion. In second step, base involves in reaction and reacts with s complex to form substituted product through deprotonation.

Arenium ion is a stable intermediate due to delocalization of positive charge on ring.

Electrophilic substitution reactions with electrophile, products and catalyst as follows.

Since the intermediate formed during the substitution reaction is not aromatic in nature, therefore reaction will continue until the aromaticity has been regained.

Nitration1. A nitro group can be introduced into benzene by using a nitrating mixture to form nitro benzene.2. The nitrating mixture is a mixture of concentrated nitric acid and concentrated sulfuric acid.3. Here sulfuric acid acts as catalyst and responsible for the formation of electrophile that isnitronium ion (NO2+).4. When benzene is treated with this nitrating mixture at a temperature below 50C, it forms nitrobenzene. Its an example of electrophilic substitution reaction of benzene and completed through the formation of arenium ion as an intermediate.Since sulfuric acid is a strong acid than nitric acid, it gets protonated the nitric acid which causes the loss of a water molecule and form electrophile, nitronium ion. In the absence of sulfuric acid, it is not possible to protonate the nitric acid due to its acidic properties.

Nitration MechanismThe reaction of benzene with concentrated nitric acid and sulfuric acid give nitro benzene. This reaction is known as nitration of benzene. It follows electrophilic substitution mechanism and completed in three steps. The presence of concentrated sulfuric acid activates the nitric acid to form a stronger electrophile; nitronium ion (NO2+). Since this is the reaction between two acids, therefore one acts asBronsted acid and another as Bronsted base. Out of these two acids, sulfuric acid is a stronger one, hence acts as Bronsted acid and protonated nitric acid. The protonation of nitric acid results loss of water molecule and form nitronium ion.

In the second step, electrophile attacks on benzene ring to form intermediate and lose the aromaticity.

Further this intermediate reacts with base that is bisulphate ion (HSO4-ion) produce in first step. Base gets deprotonate the intermediate to form nitrobenzene and sulphuric acid which acts as a catalyst for reaction.

Environmental FateBenzene is mainly found in crude oil, gasoline and cigarette smoke.Because of various industrial processes like burning coal, tobacco smoke, gasoline leaks, it enters in air, water, and soil. Many natural sources like crude oil seeps, volcanoes and forest fires are also responsible for expose of benzene. Benzene is degradable substance up to a level by volatilization, bio-degradation under aerobic conditions or photo oxidation with hydroxyl radicals. In metropolitan areas, it found around0.58 ppb in airsample and less than5 ppb in sediment sample. While the level of benzene found in surface water samples is around 100 g/L. The degradation of benzene in air, water and soil results the formation of other aromatic compounds like nitro benzene, nitro phenols, dihydroxy benzene etc.

Continuous reactorsAn alternative to a batch process is to feed the reactants continuously into the reactor at one point, allow the reaction to take place and withdraw the products at another point. There must be an equal flow rate of reactants and products. Whilecontinuous reactorsare rarely used in the laboratory, a water-softener can be regarded as an example of a continuous process. Hard water from the mains is passed through a tube containing an ion-exchange resin. Reaction occurs down the tube and soft water pours out at the exit.

Continuous reactors are normally installed when large quantities of a chemical are being produced. It is important that the reactor can operate for several months without a shutdown.The residence time in the reactor is controlled by the feed rate of reactants to the reactor. For example, if a reactor has a volume of 20 m3and the feed rate of reactants is 40 m3h-1the residence time is 20 m3/40 m3h-1= 0.5 h. It is simple to control accurately the flow rate of reactants. The volume is fixed and therefore the residence time in the reactor is also well controlled.The product tends to be of a more consistent quality from a continuous reactor because the reaction parameters (e.g. residence time, temperature and pressure) are better controlled than in batch operations.They also produce less waste and require much lower storage of both raw materials and products resulting in a more efficient operation. Capital costs per tonne of product produced are consequently lower. The main disadvantage is their lack of flexibility as once the reactor has been built it is only in rare cases that it can be used to perform a different chemical reaction.Types of continuous reactorsIndustry uses several types of continuous reactors.(a)Tubular reactorsIn a tubular reactor, fluids (gases and/or liquids) flow through it at high velocities. As the reactants flow, for example along a heated pipe, they are converted to products (Figure 4). At these high velocities, the products are unable to diffuse back and there is little or no back mixing. The conditions are referred to as plug flow. This reduces the occurrence of side reactions and increases the yield of the desired product.With a constant flow rate, the conditions at any one point remain constant with time and changes in time of the reaction are measured in terms of the position along the length of the tube.The reaction rate is faster at the pipe inlet because the concentration of reactants is at its highest and the reaction rate reduces as the reactants flow through the pipe due to the decrease in concentration of the reactant.

Tubular reactors are used, for example, in thesteam crackingof ethane, propane and butaneand naphtha to produce alkenes.(b) Fixed bed reactorsA heterogeneous catalyst is used frequently in industry where gases flow through a solid catalyst (which is often in the form of small pellets toincrease the surface area). It is often described as a fixed bed of catalyst (Figure 5).Among the examples of their use are themanufacture of sulfuric acid(the Contact Process, with vanadium(V) oxide as catalyst), themanufacture of nitric acidand the manufacture of ammonia(the Haber Process, with iron as the catalyst).

A further example of a fixed bed reactor is incatalytic reforming of naphthato produce branched chain alkanes, cycloalkanes and aromatic hydrocarbons using usually platinum or a platinum-rhenium alloy on an alumina support.(c) Fluid bed reactorsA fluid bed reactor is sometimes used whereby the catalyst particles, which are very fine, sit on a distributor plate. When the gaseous reactants pass through the distributor plate, the particles are carried with the gases forming a fluid (Figure 6). This ensures very good mixing of the reactants with the catalyst, with very high contact between the gaseous molecules and the catalyst and a good heat transfer. This results in a rapid reaction and a uniform mixture, reducing the variability of the process conditions.One example of the use of fluid bed reactors is in theoxychlorination of ethene to chloroethene(vinyl chloride), the feedstock for the polymer poly(chloroethene) (PVC). The catalyst is copper(II) chloride and potassium chloride deposited on the surface of alumina. This support is so fine, it acts as a fluid when gases pass through it.

(d) Continuous stirred tank reactors, CSTRIn a CSTR, one or more reactants, for example in solution or as a slurry, are introduced into a reactor equipped with an impeller (stirrer) and the products are removed continuously. The impeller stirs the reagents vigorously to ensure good mixing so that there is a uniform composition throughout. The composition at the outlet is the same as in the bulk in the reactor. These are exactly the opposite conditions to those in a tubular flow reactor where there is virtually no mixing of the reactants and the products.

A steady state must be reached where the flow rate into the reactor equals the flow rate out, for otherwise the tank would empty or overflow. The residence time is calculated by dividing the volume of the tank by the average volumetric flow rate. For example, if the flow of reactants is 10 m3h-1and the tank volume is 1 m3, the residence time is 1/10 h, i.e. 6 minutes.

A variation of the CSTR is the loop reactor which is relatively simple and cheap to construct (Figure 11). In the diagram only one loop is shown. However, the residence time in the reactor is adjusted by altering the length or number of the loops in the reactor.Loop reactorsare used, for example, in themanufacture of poly(ethene)and themanufacture of poly(propene). Ethene (or propene) and the catalyst are mixed, under pressure, with a diluent, usually a hydrocarbon. A slurry is produced which is heated and circulated around the loops. Particles of the polymer gather at the bottom of one of the loop legs and, with some hydrocarbon diluent, are continuously released from the system. The diluent evaporates, leaving the solid polymer, and is then cooled to reform a liquid and passed back into the loop system, thus recirculating the hydrocarbon.Nitration reactions are among the basic reactions used in chemical synthesis, and have remained indispensable for the synthesis of pharmaceuticals, agricultural chemicals, pigments, explosives and precursors for polymers. The majority of nitrations give off considerable amounts of heat. The highly exothermic nature of these reactions sometimes with explosive potential along with the acidic corrosivity of the nitrating agent, makes nitration processes potentially very hazardous. Marked warming can also cause large numbers of secondary, consecutive and decomposition reactions to accompany nitration processes. The occasional result is the formation of unwanted by products such as higher nitrated compounds or oxidation products. As a consequence, exothermic nitrations exhibit restrictions with respect to yield and purity of target products. Nitrations of aromatic compounds are usually electrophilic substitution reactions which require the acid-catalyzed formation of nitronium ions (NO2+) as reactive species, typically realized by employing a mixture of sulfuric acid and nitric acid. The purpose of using sulfuric acid is not only to donate protons to the nitric acid, thus forming nitronium ions, but also to bind water that is formed during the reaction. The use of microreactors for performing aromatic nitration reactions has been described by several authors. The main drivers in most cases were to find routes to overcome restrictions in heat and mass transfer resulting in improved process performance and safety. For example, the nitration of benzene and Other aromatic compounds is often strongly limited by the mass transfer performance within the reactor that is used. In particular in the case of biphasic nitration reactions, a good mass transfer performance is essential to suppress the formation of unwanted by-products such as higher nitrated compounds (e.g. dinitro and trinitro compounds) or oxidation products. Therefore, the use Of microreactors offers a good possibility to overcome common restrictions in mass transport and thus achieve higher yields and selectivities in nitration reactions. Burns and Ramshaw (12) were among the first to describe the use of microreactors for the isothermal nitration Of aromatic compounds (Scheme 4.5). They chose the nitration of benzene as a first test reaction to study the concept Of enhancing diffusion in a capillary slug flow micro reactor applied for the reaction Of two immiscible liquid phases (in this case benzene and aqueous nitrating acid (H2S04 + HNO3)). A high sulfuric acid concentration was used to ensure fast nitration kinetics and promote a mass transfer-limited regime. The reaction was performed in stainless-steel capillaries Of different width (127 and 254 um) at temperatures between 60 and 90C (in later studies, PTFE capillary microreactors were used to avoid corrosion problems within the setup). Relative high conversion rates achieving up to 50% nitrobenzene were obtained for residence times Of only a seconds: 94% conversion was obtained in 24s while maintaining low by-product levels. As expected, the narrower capillary reactor yielded significantly higher conversion than the broader reactor due to smaller diffusion lengths 1214. An enhancement of reaction rate was also observed when higher flow rates were applied, leading to increased mixing. Results for the benzene nitration indicated reaction rates in the range 18 min-1 that can be provided from a capillary slug-flow reactor depending on the process conditions applied. Consequently, residence times for complete conversion were estimated to be in the range 10-60s.

Additional CFD calculations indicated that the enhancement of mass transfer is a result of an internal circulation flow within the plugs (Figure 4.1). As a consequence, mixing inside the plug is also enhanced. yielding decreasing amounts of sequential by-products.

This is actually the mass transfer added process because this is increased microreactor that is why we used it.DecanterA decanter is used to separate the organic and acid phases.The term 'decant' is usually associated with wine. Decanting is also a chemical laboratory process used to separate mixtures.Answer:Decanting is a process to separatemixtures. Decanting is just allowing a mixture ofsolidandliquidor twoimmiscibleliquids to settle and separate by gravity. This process can be slow and tedious without the aid of a centrifuge. Once the mixture components have separated, the lighter liquid is poured off leaving the heavier liquid or solid behind. Typically, a small amount of the lighter liquid is left behind. In laboratory conditions, small volumes of mixtures are decantedin test tubes. If time is not a concern, the test tube is kept at a 45 angle in a test tube rack. This allows the heavier particles to slide down the side of the test tube while allowing the lighter liquid a path to rise to the top. If the test tube were held vertically, the heavier mixture component could block the test tube and not allow the lighter liquid to pass as it rises.NitrobenzeneC6H5NO2,verypoisonous,flammable,paleyellow,liquidaromaticcompoundwithanodorlikethatofbitteralmonds.Itissometimescalledoilofmirbaneornitrobenzol.Nitrobenzenemeltsat5.85C;,boilsat210.9C;,isonlyslightlysolubleinwater,butisverysolubleinethanol,ether,andbenzene.Plug Flow Hydrogenation Reactor Plug Flow Reactor employed in industrial application where high exothermic or explosive energy involved in carrying the chemical reaction. It ensures safe heat transfer between the instrument and the surrounding. It is commonly used to ensure static mixing of the components.

Works effectively under condition

Constant density Balanced conditions A single reaction Plug flow Features

Uniform distribution Short residence time Advanced technique Smooth appearance Quality design Ensure safe heat transfer Static mixing of components Flow Capacity of Tubular Reactors

Very wide range of flows are possible, subject to pump range and pressure drop across reactor. Gas and liquid flow rate determines: the "pattern" of flow Gas/liquid mixing and contact with catalyst Conversion 'Ease of scale up Flows rates widely selected to favour that "trickle bed" mode.

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