BUNA - S or Styrene-Butadiene Rubber

Buna – S or styrene-butadiene rubber is made by emulsion co-polymerization of butadiene and styrene, usually in the ratio of 3:1 in presence of a free radical catalyst at about 5⁰C for 12-15 hours.

A material that is very like natural rubber is produced by radical co-polymerization of styrene and butadiene. A one-electron oxidizing agent acts as an initiator and a thiol (RSH) is used to start the polymerization process. The ratio of butadiene: styrene in the mixture is about 3:1, so there are no long runs of one monomer in the product. Butadiene is used as a starter unit. An allylic radical is produced as a first radical which is stabilized by conjugation with the remaining alkene in the old butadiene molecule. This radical is now added to another butadiene or to styrene. As a result, a benzylic radical is produced with the more stable trans double bond. Finally, a random co-polymer is produced which contains about 3:1 butadiene to styrene with mostly E -alkenes. Buna – S or styrene butadiene rubber is used in tyres and other applications where a tough and flexible rubber is needed.

Manufacture of Buna-S or Styrene-Butadiene Rubber

The production of starting materials are---

Butadiene: The main starting material is a gas at ordinary temperature. It is manufactured either from ethyl alcohol or obtained as a petroleum product.

Styrene: Styrene is manufactured by Daw process. In this process benzene mostly free from sulphur (thiophene) is dried azeotropically, and ethylene gas is passed through it at 85⁰C-90⁰C and 5 psi pressure in presence of AlCl₃ acting as a catalyst.

C₂H₄ + C₆H₆ ---> C₆H₅―CH=CH₂

Production process: Buna-S or styrene-butadiene rubber is manufactured by emulsion co-polymerization of butadiene and styrene. Two process such as hot process and cold process have been developed, depending on the reaction temperature. At the present time about 75% of the total production is carried out by the cold process, because of the formation of better and good quality by lowering the reaction temperature.

In the cold process, which gives better results at lower temperature, reducing agents are used. Free radical is obtained from the reducing agents such as thiol (RSH), benzoyl peroxide or sodium persulphate etc. The free radical helps the partial polymerization. The free radical is thus obtained react with the monomers and remain incorporated in the final product. The composition is butadiene 72%, styrene 28%, tertiary dodecyl mercaptan 0.17%, potassium resin soap 4.5%, sodium hydroxide 0.2%, ferrous sulphate hydrate (FeSO₄ ∙ 7H₂O) 0.14%, cumene hydroperoxide 0.15%, sodium phenyl naphthyl amine 1.25% and water 200%.

Tertiary dodecyl mercaptan is used as chain modifier and N-phenyl-2-naphthyl amine is used as antioxidant. The emulsion co-polymerization of butadiene and styrene in presence of a free radical catalyst takes place at about 5⁰C for 12-15 hours. Since much heat is liberated during the process, soap in water emulsion is added to control the temperature of the reaction. A latex type of rubber is thus obtained. After the 90% copolymerization is completed, the latex and the monomers run in a blow down pressure vessel and treated with an inhibitor, in order to arrest the reaction. The latex is then made free from the monomers. Modifiers, such as alkyl hydrosulphides control the size of polymer molecules. The monomer free latex is thus obtained is treated with an anti-oxidant and then it is coagulated from the emulsion by NaCl and H₂SO₄ solution (1.8-2 P^H). Unreacted butadiene and styrene are recovered and reused. Both 1,2 and 1,4 addition of styrene to butadiene occur, the later predominating.

In cold process, the chain length of the copolymer is less. Hence rubber is more elastic, less hard and more resistant. Styrene-butadiene rubber is more resistant to abrasion and weathering than natural rubber. Its toughness may be further reduced by adding some extenders, such as petroleum-based oils.  

 Super Phosphate

The three elements of vital importance for plant growth are nitrogen, potassium and phosphorus, the super phosphate being the principal source of the last element, that is phosphorous.

Super Phosphate of Lime

The artificially prepared mono calcium phosphate Ca(H₂PO₄)₂ is known as super phosphate of lime. Commercial super phosphate is a mixture of mono calcium phosphate and crystalline calcium sulfate, that is---

Ca(H₂PO₄)₂ + CaSO₄ ∙ 2H₂O

Super phosphate of lime contains about 16% P₂O₅. The soluble calcium phosphate prepared by treating rock phosphate with H₂SO₄ is called super phosphate of lime.

Ca₃(PO₄)₂ + 2H₂SO₄ + 4H₂O ---> Ca(H₂PO₄)₂ + 2CaSO₄ ∙ 2H₂O

Manufacture of Super Phosphate of Lime

Raw materials: Rock phosphate of the composition 3Ca₃(PO₄)₂, CaF₂ is the starting material for manufacturing both forms of superphosphate. Strong H₂SO₄ (93-98) % is used in most plants.

Preparation process: Normal super phosphate is manufactured by mixing equal quantities of powered phosphate rock and chamber acid (specific gravity 1.45-1.60) into a cast iron mixture provided with a stirring mechanism. The mass stirred about 5 minutes and then it is allowed to remain for 1 day. Now the temperature rises to about 100⁰C-110⁰C as the reaction is exothermic. A mixture of fumes consists of HF (from CaF₂), SiF₄ (from CaF₂ and silica) and CO₂ (from lime stone) are evolved. These gases make the material porous.

As the reaction proceeds, the mixture stiffens and ultimately sets to a solid mass. After someday, it becomes perfectly dry. The gases are washed by spraying water in two successive towers. The resulting HF solution is then neutralized either by Na₂CO₃ or by NaF and finally treated with washed sand to form hexafluoro silicic acid. The latter is further neutralized by Na₂CO₃ to form sodium silico fluoride (Na₂SiF₆) or with magnesium to form magnesium silico fluoride (MgSiF₆).

Reaction: The main reaction involved in the formation of super phosphate of lime is---

Ca₁₀(PO₄)₆F₂ +7H₂SO₄ + 3H₂O ----> 2CaH₄(PO₄)₂∙ H₂O + 7CaSO₄ + 2HF

[Ca₃(PO₄)₂, CaF₂]                       [Ca(H₂PO₄)₂]

Na₂SiF₆ or MgSiF₆ are useful by products. Super phosphate of lime is used principally as a fertilizer. It is water soluble fertilizer.

Triple Super Phosphate

Triple super phosphate or concentrated super phosphate contain about (44-47)% P₂O₅ which is nearly three times as in normal super phosphate.

Manufacture of Triple Super Phosphate

Raw materials: 78% H₃PO₄ (content 52-54% P₂O₅), powdered rock phosphate (35% P₂O₅).

Reaction: The reaction involved in the formation of triple super phosphate is---

CaF₂,3Ca₃(PO₄)₂+14H₃PO₄ = 10Ca(H₂PO₄)₂+2HF

Preparation process: crushed rock phosphate is mixed with requisite quantity of phosphoric acid (H₃PO₄) of proper strength (strength is about 78%) in cast iron mixer, lined internally with lead, protected by acid proof bricks at 60⁰C–70⁰C. The mass is discharged with 20% moisture to a wet storage site, where the mass is allowed to age for about 30 days, and thereby the mixture is slowly proceeded to completion. The acid after the aging is disintegrated, dried at 200⁰C, sized and packed.

Type of rock and acid used in the manufactured process of triple super phosphate, and on the granular or non-granular nature of the product decided the properties of triple super phosphate.

Flow chart for the preparation of triple super phosphate

FLOW CHART FOR THE PREPARATION OF TRIPLE SUPER PHOSPHATE

 Nitrogen Fixation

Nitrogen fixation is a process by which nitrogen from atmosphere convert into ammonia by nature using bacteria such as Rhizobium, which is present in the nodules in the roots of legumes such as beans, peas etc. The process occurs at 0.8 atm pressure of N₂ and at ambient temperature in Rhizobium bacteria and also some other independent bacteria. For the industrial synthesis of ammonia requires high temperature and pressure, and also requires iron oxide as catalyst and percentage of yield is only 15% to 20%, whereas the process of nitrogen fixation occurs at ordinary temperature and pressure. The process of nitrogen fixation is catalyzed by the metalloenzyme nitrogenase present in the bacteria. The reaction is as follows---

N₂+8H⁺+16MgATP+8e-->2NH₃+H₂+16MgADP + 16PO₄^3-

The ammonia produced in the process of nitrogen fixation is required for cell growth. The electrons required in this process, are supplied to metalloenzyme nitrogenase by reduced forms of ferredoxin and flavodoxins. Vanadium or iron or Fe-V or Fe-Fe or Fe-Mo may be present as a metal centres in the enzyme nitrogenase. The enzyme nitrogenase which is responsible for nitrogen fixation process contain two proteins. One protein is an iron protein (Fe-protein), whose molecular weight is about 60000, this protein contains an Fe₄S₄ cluster. Another protein is an iron molybdenum protein (Fe-Mo), whose molecular weight is about 220000, a tetramer containing two Mo atoms, and 30 to 32 Fe atoms together with sulphur. Mg-ATP (and MgADP) binds to the reduced form of the Fe-protein about 20 A⁰ from the Fe-S cluster. The Fe-proteins possible linked to the FeMo protein by salt bridge, transfer an electron for every two molecules of MgATP hydrolyzed. When enough electrons accumulate by FeMo protein (eight electrons are required for N₂ conversion to 2NH₃ by the enzyme nitrogenase because the reaction also produced H₂) these are transfer to N₂ with proton transfer from water.

Schematic representation of nitrogen fixation

The FeMo-cofactor site of enzyme nitrogenase

Two cuboidal fragments present in the cofactor, one is Fe₄S₃ fragment and other is Fe₃MoS₃ fragment. These fragments are linked to each other by sulphide bridges. The cluster is bound to the protein by a cysteine at one iron atom at the end and a histidine at the molybdenum atom at another end. This is a six coordinated molybdenum and the coordination number of molybdenum is fulfilled by the homocitrate ion, which is present at the extreme right side. During the enzyme turnover there is a little change in the coordination of Mo. At the interface of the two cube fragments the iron atoms have open coordination sites. These iron atoms may be served as the site for binding dinitrogen instead of Mo.

Another interesting point of this nitrogen fixation process is that formation of nitrates from lightning discharges. By using assimilatory nitrate and nitrate reductases, the nitrates are converted to ammonia.

The enzyme nitrate reductase is also containing molybdenum.

No attempts have been proved successful for the production of ammonia in industrial scale at ordinary temperature and pressure. It is still an unanswered question how the enzyme manages to carry out the reaction for the production of ammonia from atmospheric nitrogen by nitrogen fixation process at ambient temperature and less than 1 atm pressure of N₂. 

 ESSENTIAL AMINO ACIDS

Amino acids, as their name implies are compounds that contains both an amino group and a carboxylic acid group. The general structure of amino acid is---

R―CH(NH₂) ―COOH

Where, R is the side chain, it varies from amino acid to amino acid. In this structure of amino acid, the ―NH₂ group is present at α position, located next to ―COOH group. So, it is the general structure of α-amino acids.

The general structure of β-amino acids is---

R―CH(NH₂) ―CH₂―COOH

Where, the ―NH₂ is present at β position.

The general structure of γ-amino acids is---

R―CH(NH₂) ―CH₂―CH₂―COOH

Where, the ―NH₂ group is present at γ position.

The amino acids are particularly important because they are the monomeric units of biologically important polymers called peptides. Proteins are simply large peptides. Peptides and proteins serve many important roles in biological system, e.g., all enzymes are proteins. Hydrolysis of protein by acids, alkalis or enzymes, yield a mixture of amino acids. Mixture of amino acids obtained by the hydrolysis of proteins are mostly α-amino acids.

Twenty α-amino acids, however are known by widely accepted traditional names. These are the amino acids that occurs commonly as constituents of most proteins. The amino acids can be grouped according to the nature of their side chains.

Amino acids with side chain containing H or aliphatic hydrocarbon

Amino acids with side chains containing aromatic groups

Amino acids with side chains containing ―SH, ―SCH₃, or ―OH groups

Amino acids with side chains containing carboxylic acid or amide groups

Amino acids with basic side chains

In addition to the above twenty α-amino acids there are two other α-amino acids

The conversion of cysteine to cystine required additional comment. The ―SH group of cysteine makes a thiol. One property of thiol is that they can be converted to disulphide by mild oxidizing agents.

Configuration of natural amino acids

There are twenty commonly naturally occurring amino acids, called essential amino acids. One is achiral (glycine), all the others have a stereogenic centre. Stereochemical studies of these naturally occurring amino acids have shown that all have the same configuration about the carbon atom carrying the alpha amino group and this configuration is the same as that in L (-) glyceraldehyde. All have the ‘S’ configuration, except (-) cysteine and (-) cystine, which because of the high priority of the sulfur atom gets the stereo descriptor R.

Zwitter ionic structures of amino acids---

An amino acid contains ―NH₂ group, a basic one and ―COOH group, an acidic one. Thus, amino acid exists as Zwitter ion, i.e., a dipolar ion. The Zwitter ionic form of an amino acid is as follows---

NH₃⁺―CH(R)―COO^-

Evidences for the formation of Zwitter ionic structure of amino acid---

1. High melting point of amino acids in comparison to amines or carboxylic acids.

Ph―CO―NH―CH₂―COOH this is N-benzoyl glycine (hippuric acid), its melting is 190⁰C.

The melting point of glycine (H₂N―CH₂―COOH) is 262⁰C.

2. Amino acids are insoluble in polar aprotic solvent such as ether. Most amines and carboxylic acids on the other hand, dissolve in ether. Although the water solubility of the different amino acids varies, all are highly soluble in water.

3. The dipole moment of the amino acids is very large, much larger than those of similar sized molecules with only an amine or a carboxylic acid group.

Dipole moment of propanoic acid (CH₃―CH₂―COOH) is 1.7 D (µ = 1.7 D).

Dipole moment of n-butyl amine (CH₃―CH₂―CH₂―CH₂―NH₂) is 1.4 D (µ = 1.4 D).

Whereas, dipole moment of glycine (H₂N―CH₂―COOH) is 14 D (µ = 14D).

Obviously, a large dipole moment is expected for the molecules that contain a great deal of separated charge.

 SMOG

By burning the fossil fuels like coal or petrol in industries or in homes or in automobiles poisonous gases like SO₂ or NO₂ are produced. These poisonous gases are mixed with the droplets of fog suspended in the air near the surface of earth. A colloidal dispersion is formed, when the droplets of fog which contains poisonous gases like SO₂ and NO₂ gets condensed on the solid particles of smoke or dust present in air. This colloidal dispersion is called smog. During the winter season smog usually present in the lower atmosphere is taken by men and animals during breathing and damages their health.

Photochemical Smog or Los Angeles smog

The poisonous gases like oxides of nitrogen (mainly NO₂), SO₂, CO, CO₂, or unburnt hydrocarbons (mainly R-H) released by homes, industries, or by automobiles sector. In the poisonous gases NO₂ is present, which is cleavage by photochemically by ultraviolet (UV) radiations present in the sunlight, and produced atomic oxygen (O).

NO₂ + U.V light ------> NO + O

The produced atomic oxygen (O) reacts with hydrocarbons (R-H), and forms a variety of free hydrocarbon radicals, ozone (O₃) and PAN (Peroxy Acetyl Nitrate).

Formation of ozone and hydrocarbon radicals--

O₂ + O + M -----> O₃ + M (M = Catalyst = N₂)

R-H + O -----> R• (hydrocarbon radical) + •OH

R• + O₂ -----> RO₂• (peroxy radical)

RO₂• + R-H ---> RO₂H (organic peroxide) + R•

RO₂• + O₂ ------> RO• (alkoxy radical) + O₃

Formation of PAN (Peroxy Acetyl Nitrate) ---

From ketone---

R―CO―R + hv -----> R• + RCO• (acetyl radical)

RCO• + O₂ -->R―CO―O―O• (peroxy acetyl radical)

R―CO―O―O•+NO₂-->R―CO―O―O―NO₂ (PAN)

From aldehyde---

R―CO―H + •OH -----> R―CO• + H₂O

R―CO• + O₂ -----> R―CO―O―O•

R―CO―O―O•+NO₂-->R―CO―O―O―NO₂ (PAN)

If PAN i.e., Peroxy Acetyl Nitrate is present, in the concentration of 0.1 ppm (parts per million) in atmosphere, it is harmful to human or animals. When PAN is mixed with fog and gets condensed on dust particles or smoke present in the atmosphere, a colloidal dispersion is formed, which is called photochemical smog.

PAN mixed with fog + smoke or dust particle-> photochemical smog.

Due to presence of heavy automobile traffic in Los Angeles, a city of United States of America a huge number of poisonous gases (mainly NO₂) released in the atmosphere, which produced a huge amount of photochemical smog. So photochemical smog is also called Los Angeles smog.

Sulphurus Smog

This smog is basically a mixture of liquid particles of H₂SO₄, SO₂ and other particles. If these particles and gases in the air are accompanied by stagnant air, humidity, cloudy skies, temperature contrast and dense fog, then sulphurus smog is formed. The smell of sulphurus smog is different from photochemical smog.

Particulate Smog

Particulate smog is the sum of small dust particles, the diameter of the particles is 0.1 µm. Combustion of fossil fuels in automobiles, dust emitted from thermal power plants create this type of smog. The difference between photochemical smog and particulate smog is that, particulate smog does not contain ozone gas.

Harmful Effects of Smog

Various types of harmful effects which are creates by smog are---

Smog produces asthma, bronchitis, and heart problem for all the human beings who breath in it.

Irritation in eyes, nose, throat, and skin is creates by smog.

Natural growth and development of human and animals is affected by smog.

In the presence of photochemical smog, the leaves of the plants develop a metallic sheen, which is harmful for plant.

In the presence of photochemical smog rubber loses its elasticity, and becomes inflexible and brittle.

The visibility is reduced by smog, so hindrance is produced in the air and in road traffic.