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.
12 | Page