Category: Chemistry Notes PPT

Chemical bonds are forces that keep atoms joined together. Chemical bonds are classified into covalent bonds, coordinate bonds, ionic bonds and hydrogen bonds. Covalent bonds are those bonds that are formed by sharing of electrons between two atoms. It is also known as a molecular bond. Here in this article, we are going to discuss sigma and pi bonds which are covalent bonds only but are formed by different types of overlapping between orbitals. Here we will discuss pi bonds in detail and will have a short look at sigma bonds and the difference between pi and sigma bonds. 

What are Pi Bonds? 

The covalent bond which is formed by lateral overlapping of the half-filled atomic orbitals (p – orbitals) of atoms is called a pi bond. It is denoted by. We find pi bonds in alkenes and alkynes. The electrons which take part in the formation of pi covalent bonds are called pi – electrons.  The formation of pi bond is given below between the two orbitals 

 

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Characteristics of Pi Bond 

• Pi bonds are formed by sideways overlapping of two parallelly oriented pi orbitals of adjacent atoms. 

• In pi bonds, overlapping takes place at the side of the two lobes of p – orbitals so the extent of overlapping is less than the sigma bond. Hence, pi bonds are weaker than sigma bonds. 

• In pi bonds, the electron density is concentrated in the region perpendicular to the bond axis. 

• The molecular orbital of the pi bond is oriented above and below the plane containing the nuclear axis. 

• All atoms of the molecule must be in the same plane if the pi bond is formed in the molecule. 

• In general, double covalent bonds consist of one pi and one sigma bond while triple covalent bonds consist of one sigma and two pi – bonds. Here you need to note that if only one covalent is present between atoms then it will always be a sigma covalent bond. 
  

Formation of Pi Bond in Oxygen Molecule 

Oxygen molecules are formed by joining two oxygen atoms covalently. Each oxygen atom has a total of 8 electrons. By writing its electronic configuration we can see each oxygen atom has two p – orbitals which have only one electron in them in the valence shell. 

The electronic configuration of oxygen atom –

 

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In the above figure, you can see 2py, 2pz orbitals of the valence shell of the oxygen atom are singly occupied. When two oxygen atoms approach each other to form an oxygen molecule, one set of singly occupied p – orbitals get head-on overlapped axially and form a sigma bond. While the other set of singly occupied p – orbitals get sideways overlapped and form a pi bond. Thus, in an oxygen molecule, one sigma and one pi bond are formed. 

What is Sigma Bond? 

The strongest covalent bond which is formed by the head-on overlapping of the atomic orbitals is called the sigma bond. It is denoted by. We find sigma bonds in alkanes, alkenes, alkynes. Formation of sigma bond is given below between the orbitals- 

 

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As you can see above figure sigma bond is formed by the following three types of overlapping between orbitals –

• S – S Overlapping – s- orbital is spherical in shape and whenever they reach a point of maximum attraction, they overlap and form a sigma bond. This can be understood clearly by taking an example of a hydrogen molecule. The hydrogen molecule is formed by two hydrogen atoms. Each hydrogen atom contains one s – orbital in the valence shell which is singly occupied. When two hydrogen atoms approach each other to form a hydrogen molecule, these two singly occupied s – orbitals of valence shells combine with each other and form a molecular orbital or sigma bond. 

Electronic configuration of H atom –

 

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Formation of sigma bond in hydrogen molecule –

 

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• S – P Overlapping – Sigma bond is formed by head-on overlapping of s – orbital and p – orbital. It can be understood by the formation of hydrogen fluoride molecules. In the formation of hydrogen fluoride molecule, singly occupied 1s – orbital of the valence shell of hydrogen get head-on overlapped with singly occupied p – orbital of the valence shell of fluorine and forms a sigma bond. 

Electronic configuration of H atom –

 

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Electronic configuration of F atom –

 

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Formation of sigma bond in HF molecule –

 

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• P – P Overlapping – Overlapping of p – orbitals form both pi bond and sigma bond. When p – orbitals are overlapped sidewise or lateral, that time pi bond is formed as discussed initially. When the bonds are formed by the head-on overlapping of p – orbitals are called sigma bonds. It can be illustrated by the formation of fluorine molecules. For the formation of fluorine molecules two singly occupied 2pz orbitals of the valence shell of fluorine atom get overlap and form molecular orbitals or sigma bonds. 

Electronic configuration of F atom –

 

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Formation of sigma bond in fluorine molecule –

 

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Difference Between Pi and Sigma Bonds

 

This ends our coverage on the topic “Pi bonds”. We hope you enjoyed learning and were able to grasp the concepts. If you are looking for solutions to NCERT Textbook problems based on this topic, then log on to website or download Learning App. By doing so, you will be able to access free PDFs of NCERT Solutions as well as Revision notes, Mock Tests and much more.

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Acrylate polymers are made from acrylate monomers and are a type of polymer. Transparency, break resistance, and flexibility are all characteristics of these plastics. Acrylics and polyacrylates are other names for them. As an adhesive, acrylate polymer is extensively used in cosmetics such as nail polish.

Polyacrylates are a type of polymer that is flexible, robust, and rubbery. Their glass transition temperature is significantly lower than that of ambient air. They’re noted for their excellent transparency, impact toughness, and elasticity, as well as their moderate heat resistance up to 450 K in dry heat. They are also weatherable and ozone resistant due to the absence of double bonds in the backbone.

Polyacrylate Polymer

For increased temperature applications (430-450) K), acrylic elastomers are generally a less expensive option to fluorocarbon polymers (FKM), silicones (VMQ), and fluoro silicone (FVMQ). Hoses, seals, gaskets, and dampers that must perform under long-term exposure to high temperatures and hydrocarbon oils are commonly employed in the automotive sector.

Polyacrylate polymers are also employed in solvent-borne coatings and printing inks that require quick drying times. These chemicals function well and can be used in a variety of applications, including automotive lacquers and industrial coatings. They are commonly administered with a spray gun, aerosol spray, or dipping and are designed as one- or two-part systems. The bulk of these goods include dangerous solvents, necessitating strict handling precautions. Furthermore, due to environmental concerns about emissions, most organic solvents are restricted.

Acrylics are utilised in pressure-sensitive adhesive formulations in addition to paints, inks, and coatings. They can be made with a wide range of adhesion qualities, from extremely low tack (barely tacky) to extremely strong tack that bonds to surfaces permanently.

Sodium Polyacrylate 

Sodium polyacrylate, often known as waterlock, is a sodium salt of polyacrylic acid with the chemical formula [CH2CH(CO2Na)]n that finds use in a variety of consumer goods. In water, this super-absorbent polymer (SAP) may absorb 100 to 1000 times its mass. An anionic polyelectrolyte having negatively charged carboxylic groups in the main chain, sodium polyacrylate is an anionic polyelectrolyte. A chemical polymer made up of chains of acrylate molecules is sodium polyacrylate.

It has sodium in it, which allows it to absorb vast amounts of water. An anionic polyelectrolyte, sodium polyacrylate is also known as sodium polyacrylate. Because of the ionic interactions between the molecules, it creates a thick and transparent solution when dissolved in water. Sodium polyacrylate has a number of mechanical advantages. Mechanical stability, great heat resistance, and excellent hydration are just a few of the benefits.

Sodium Polyacrylate Uses

As an absorbent substance, sodium polyacrylate is utilised in paper diapers and maximum absorbency garments. It’s also used in ice packs to turn the water used as a cooling ingredient into a gel, reducing spillage in the event that the ice pack leaks. Sodium polyacrylate uses has also been investigated for a variety of applications, including water nanofiltration to absorb water and concentrate liquids with microorganisms foods, including bread, juice, and ice cream.

It’s also utilised in eco-engineering to increase moisture availability in the soil by acting as a water-retaining agent in rocky slopes. This improves the soil’s water retention capacity and infiltration capacity in sandy soil. The table below contains categories and lists of various sodium polyacrylate-based products and applications.

1. Sequestering Agents

As a chelating agent, sodium polyacrylate is extensively used in detergents.

First, chelating agents are employed in detergents because they have the potential to neutralise heavy metals found in dirt, water, and other things in clothing. When sodium polyacrylate is added to detergent, it makes it more effective in cleaning garments.

2. Thickening Agents

Diapers, hair gels, and soaps all contain sodium polyacrylate, which can absorb and retain water molecules. Because it enhances the viscosity of water-based compounds, sodium polyacrylate is called a thickening agent. Sodium polyacrylate absorbs urine water in diapers, increasing their capacity to hold liquid and reducing rashes.

3. Coatings

To limit the quantity of moisture around wires, sodium polyacrylate can be used as a coating for electrical cables. Water and moisture near wires can interfere with electrical signal transmission. This could result in a fire hazard. Sodium polyacrylate can absorb water and prevent it from surrounding or entering wires due to its efficient absorption and swelling capability.

4. Agriculture 

Sodium polyacrylate is used in agriculture to assist plants to retain moisture in the soil. It can be used as a water reservoir for plants, and florists frequently utilise it to keep flowers fresh. Furthermore, the United States Department of Agriculture has approved the use of sodium polyacrylate in the production of domestic fruits and vegetables.

5. Drug Delivery Applications

Microencapsulation using sodium polyacrylate can be utilised to distribute compounds like probiotics. Because probiotic viability drops significantly throughout the gastrointestinal tract due to severe acid conditions, delivering probiotics to the digestive system might be problematic. Although Alginate (Alg) is the most often utilised native microcapsule matrix, research comparing different encapsulation methods shows that mixing Alg with sodium polyacrylate produces better outcomes.

6. Removal of Metal Ions from the Environment

Metal ions can be absorbed and recovered using sodium polyacrylate and other super-absorbent polymers, or SAPs. Because of their high toxicity, bioaccumulation, and non-degradability, heavy metals are extremely dangerous pollutants that can harm aquatic habitats and humans. Heavy metals can be produced by activities such as mining and petroleum refining, necessitating a simple and environmentally sustainable procedure to absorb these hazardous metals in order to avoid fatal consequences.

Did You Know?

The skin is not irritated by sodium polyacrylate. It is made up of big polymers that are incapable of penetrating the skin. However, sodium polyacrylate is occasionally combined with acrylic acid, which is a byproduct of the manufacturing process. Acrylic acid, which is a byproduct of the production of sodium polyacrylate, can create a rash when it comes into contact with the skin. The absorbent substance in paper diapers should be less than 300 PPM. In addition, if sodium polyacrylate is utilised as a powder, it should not be breathed. If sodium polyacrylate is spilt in an area with water, it can make the ground highly slippery.

Polytetrafluoroethylene (PTFE) is a tough, solid, nonflammable and waxy synthetic resin formed by tetrafluoroethylene polymerization. It is known by such trademarks as Fluon, Teflon, Polyflon, and Hostaflon, PTFE material is distinguished by its high melting point, resistance, and slippery surface to attack by almost all the chemicals. These properties have made it more familiar to the consumers as the coating on nonstick cookware; it is fabricated into industrial products, including pipeliners, bearings, and parts for pumps and valves.

About Polytetrafluoroethylene

PTFE material was first discovered serendipitously by Roy Plunkett in 1938, an American chemist for E.I. du Pont de Nemours & Company (now – DuPont Company), who found that a gaseous tetrafluoroethylene refrigerant tank had been polymerized to a white powder. It was also applied as a corrosion-resistant coating during World War-II to protect the metal equipment that can be used in the handling of radioactive material for the Manhattan Project. Due to the difficulty faced in devising methods for handling the high-melting, slippery stuff, PTFE saw no industrial use for more than a decade after the war. DuPont then, in 1960, released its trademarked Teflon-coated nonstick cookware.

Tetrafluoroethylene (C2F4) is an odourless and colourless gas, which is made by heating chlorodifluoromethane (CHClF2) at a temperature range of 600–700 °C. Chlorodifluoromethane, in turn, can be obtained by reacting the hydrogen fluoride (H.F.) with chloroform (CHCl3). Tetrafluoroethylene monomers (which are small, single-unit molecules) are either emulsified or suspended in water and then polymerized (that are linked into giant multiple-unit molecules) under high pressure in the free-radical initiator presence. The polymer contains a chain of carbon atoms having two fluorine atoms that are bonded to each carbon:

The fluorine atoms that surround the carbon chain, such as a protective sheath, create a relatively dense and chemically inert molecule with very strong bonds of carbon-fluorine. The polymer can be inert to most chemicals, which does not melt at less than 327 °C, and contains the lowest coefficient of friction of any well-known solid. These properties enable the usage of bearings and bushings that require no lubricant, as the liners for equipment. They can be used in the transportation and storage of organic solvents and strong acids, as the electrical insulation under the conditions of high-temperature, and in its most familiar application, as a cooking surface, which does not need oil or fat uses.

PTFE product’s Fabrication is not easy due to the material not flowing readily even above its melting point. At the same time, the moulded parts are made by heating and compressing fine powders mixed with volatile lubricants. To provide a permanent coating, metallic surfaces are dipped or coated with an aqueous dispersion of PTFE particles. PTFE dispersions are also woven into fibres.

Production

PTFE material can be produced by free-radical polymerization of tetrafluoroethylene. The net equation is given as follows:

n F2C=CF2 → −(F2C−CF2)n−

Since tetrafluoroethylene may explosively decompose into carbon and tetrafluoromethane, special polymerization apparatus might be needed to avoid hot spots that could induce this dangerous side reaction. Typically, this process is initiated with persulfate, which hemolyzed in generating sulfate radicals:

[O3SO−OSO3]2− ⇌ 2 SO4−

The resulting polymer can be terminated with the ester groups of sulfate, which can be hydrolyzed to form O.H. end-groups.

Since PTFE is poorly soluble in almost every solvent, the polymerization is conducted as in water emulsion. Also, this process produces a suspension of polymer particles. In an alternate way, the polymerization can be conducted using a surfactant like PFOS.

Applications

The primary application of PTFE, consuming up to 50% of production, is for the insulation of wiring in computer applications (for example, coaxial cables, hookup wire) and aerospace. This particular application exploits the fact that PTFE has outstanding dielectric properties, especially at higher radio frequencies, by making it suitable for use as an excellent insulator in cables and connector assemblies and in the printed circuit boards, which are used at microwave frequencies. This makes the choice of material, combined with its high melting temperatures, as a high-performance substitute for the weaker and lower-melting-point polyethylene, commonly used in low-cost applications.

In industrial applications, due to its low friction, PTFE is used for plain bearings, gears, slide plates, gears, gaskets, seals, bushings and other sliding parts applications where nylon and acetal outperform.

Safety

PTFE Pyrolysis can be detectable at 200 °C, and it evolves many fluorocarbon gases and a sublimate. An animal study was conducted in 1955, and it concluded that it is unlikely that these products would be produced at temperatures below 250 °C in quantities affecting health. Products such as nonstick coated cookware have had their PFOA removed since 2013, and before this, products that contain PFOA were not found to be the major sources of exposure.

Ecotoxicity

Sodium trifluoroacetate, including the similar compound chlorodifluoroacetate, can both be generated when PTFE undergoes thermolysis process and produces longer chain polyfluoro- and/or poly chloroform- (C3-C14) carboxylic acids as well, which can be equally persistent. A few of these products have recently been linked with possible adverse environmental and health impacts and are being phased out of the U.S. market.

Potassium cyanide is a chemical compound with the formula KCN. This colorless crystalline salt, which will be similar in appearance to sugar, and is highly soluble in water. Most KCN is used in gold mining, organic synthesis, and electroplating. Smaller applications in our daily lives include making jewelry for chemical gilding and buffing.

Chemistry is really important and efficient for the research and study of living organisms because it helps students and scientists to understand the life processes of every living thing on earth at the molecular level. At any molecular level, every process of life takes place due to the involvement of various minor or major chemical reactions.

Thus, it is important for the students to learn their chapters well and understand all the chemistry concepts by practicing with a maximum number of past years’ question papers and sample question papers available on the website. This will help them to understand the time management skill and learn the marking schemes that carry maximum marks and plan which question needs what type of answers. Break down larger portions into smaller effective points and write them down in a separate notebook so it will help you in revising before the exams. Make note of the important questions that keep repeating in the recent past year question papers and give more weightage to those questions and prepare a little extra because it might repeat in the current year also. If you have any doubts about the equations and chemical formulations that are taught during the classes then try to spend some extra time in the lab and get to understand all the concepts by trying out the experiments and practicing them really well. This will definitely help you write your formulas and equations really well.

More About Potassium Cyanide

A compound named potassium cyanide is a colorless crystalline salt, similar to sugar particles in its appearance. Its general formula is KCN and IUPAC’s name is Potassium Cyanide. Potassium Cyanide is a very poisonous inorganic salt. Potassium Cyanide is a highly toxic substance in nature and exposure to this element can be very much lethal for humans. Apart from all these circumstances, it is considered to be highly soluble in water. Mostly it is used in the gold mining industries for the extraction of gold and silver ores. It is also used in various other industries for electroplating, fumigation, chemical gilding, and buffing.

Structure of Potassium Cyanide

Chemical Formula is KCN.

The molecular formula is KCN.

The molar mass of potassium cyanide is 65.12 g/mol.

Potassium cyanide is a compound formed of potassium( K⁺ cation ) and Cyanide (CN^– anion). In which potassium is positively charged and cyanide is negatively charged ions. Carbon has a triple bond with the nitrogen ion. It is similar to the structure of NaCl crystalline solid.

Properties of Potassium Cyanide

Potassium Cyanide is a white crystalline colorless solid.

Its density is 1.52 g/ml.

Its melting point is 634.5’c

Potassium cyanide is highly soluble in water.

It decomposes slowly in the air and rapidly after heating.

Potassium Cyanide, when reacted with hydrogen peroxide, forms a less harmful cyanide derivative.

KCN+ H₂O₂ -> KOCN+ H₂O

It is a toxic substance. It tastes bitter and it exhibits burning sensations.

Preparation of Potassium Cyanide

Potassium Cyanide can be prepared by various methods in industries as per their use. The most commonly used methods are:

When hydrogen cyanide is reacted with an aqueous solution of potassium hydroxide:-

HCN + KOH -> KCN + H₂O 

When Formamide is reacted with KOH:-

HCONH₂ + KOH -> KCN + 2H₂O

Uses of Potassium Cyanide

Potassium Cyanide and Sodium Cyanide are widely used for the production of nitriles and carboxylic acids.

Potassium Cyanide was produced for the decomposition of potassium ferrocyanide. It was produced before the invention of the Castner process.

In the colloidal process, KCN is used as a photographic fixer.

It is used in the gold mining procedure, where KCN forms water-soluble Potassium gold cyanide and Potassium Hydroxide in the presence of oxygen.

4Au+ 8KCN +O₂ +2 H₂O -> 4KAu(CN)₂ + 4KOH

KCN is used in mining industries of gold and other metals.

Potassium cyanide is used for the preparation of plastic and another organic synthesis.

It is used for electroplating and fumigation.

Potassium Cyanide is also used in warehouses.

It is also used as an insecticide by farmers.

KCN

Potassium cyanide is a highly toxic inorganic substance with the general formula KCN. It is generally prepared by the reaction of hydrogen cyanide (HCN) and potassium hydroxide (KOH). This is made up of a solid base (KOH) and a weak acid (HCN).  This is used for the extraction of gold, silver, and other metals. It is one of the forms of cyanide prepared for the decomposition of potassium ferrocyanide before the invention of the Castner process.

Potassium Cyanide is harmful to health. If someone inhales this element then they may face severe health problems which can even lead to death. A person may also suffer from headaches, vomiting, dizziness, and seizures which can be harmful to human health.

Potassium cyanide is a salt prepared with the help of hydrogen cyanide or a reagent formamide.

When dealing with reactions which are reversible, it is necessary to rule out the direction of any reaction at a given point. For instance, when ammonia is produced commercially by mixing nitrogen and hydrogen, the whole process needs to be optimised for efficient yield. Therefore, prediction direction of reaction is immensely vital.

It is known that direction of a chemical reaction can be figured out by the reaction quotient and equilibrium constant. So, in the following section, you will get to know about equilibrium constant, prediction of direction of reaction and reaction quotient.

Let’s start!

Equilibrium Constant

All reversible reactions have a direction, but some irreversible reactions also exist if they favour the yield of reactants or products. Suppose some amount of colourless dinitrogen tetroxide is added in a reversible reaction of production of nitrogen dioxide from dinitrogen tetroxide. After a certain point in time, you would observe that the gas will change to yellowish-orange colour and will get darker gradually until it becomes constant.

Initially, the concentration of NO2 in the container is 0 mole. As N2O4 gets transformed into NO2, the concentration of NO2 rises to a specific level and then remains fixed.

In the same manner, N2O4 concentration decreases until it approaches equilibrium. When both NO2 and N2O4 concentrations stay constant, the reaction is said to have reached equilibrium. But you must remember that even if a reaction is constant at the state of equilibrium, the reaction still occurs. Therefore, it is also referred to as dynamic equilibrium.

In order to calculate equilibrium constant Kc, consider a balanced reversible equation aA + bB ⇋ cC + dD. If the molar concentration of each species is known, Kc can be evaluated by the following equation:

Kc = [C]c [D]d / [A]a [B]b

Next, let us proceed to what is reaction quotient?

Reaction Quotient

The measure of amounts of reactants and products involved in a reaction at a specific time is known as the reaction quotient Q.

Suppose a reversible reaction aA + bB ⇋ cC + dD, where all these variables are the stoichiometric coefficients of a balanced reaction, Q can be evaluated by the below-given equation:

Q = [C]c [D]d / [A]a [B]b

The above equation may seem familiar to you as the concept of Q is closely associated with equilibrium constant K. The reaction quotient Q can be evaluated for both cases whether a reaction is in equilibrium or not, but K is based on equilibrium concentrations.

Reaction quotient magnitude determines that what is there is a reaction container. But what does it mean? Consider a reaction that holds only starting substances, and the product concentrations are zero. As the numerator is zero, reaction quotient is also zero. If a reaction contains only products, [A] = [B] = 0 in denominator of the equation, Q becomes infinitely huge.

Maximum number of times, some or the other mixture of products and reactants will be there, but you can keep it in mind that extremely small Q values show that mostly reactants are there. On the other hand, when extremely Q values are there, it shows that products are mostly present in the reaction container.

Prediction of the Direction of a Reaction

The direction of a chemical reaction is explained through an experiment example below, which shows the production of ammonia when hydrogen and nitrogen undergo a reaction.

Take a look!

At the temperature of 350° Celsius hydrogen gas (H2) and nitrogen gas (N2) will undergo a reaction to yield ammonia gas (NH3). This equilibrium reaction can be expressed by the below-mentioned chemical equation:

N2 (g) + 3H2 (g) ⇋ 2NH3 (g)

For instance, consider an experiment where 1.00 moles of nitrogen and 1.00 moles of hydrogen is added to a 1.00 Litre sealed container and heated at a temperature of 350° Celsius. After that, the yield of ammonia is monitored, and consumption of hydrogen and nitrogen with time by measuring concentration of each element in the container.

The observations of the experiment are represented in the graph and table below:

As expected, it is seen that concentration of nitrogen and hydrogen (reactants) decreases, whereas the concentration of ammonia (product) increases, till equilibrium when time t = 4 is reached. At this point, concentration of each chemical substance does not change. So, what will happen to the mass-action expression (Q or reaction quotient) value as the whole reaction is moving towards equilibrium in a forward direction?

Calculation of the mass-action expression value at each time is done and represented in the following table and graph:

Since the reaction is moving towards equilibrium in forward direction, the mass-action expression Q value is increasing.

At the state of equilibrium around time = 4, the reaction quotient Q value is constant and is similar to the equilibrium constant K value for the above reaction.

At the point of equilibrium: Q = 6.09 = K

The other way to illustrate this is that as long as the reaction quotient Q value is lesser than equilibrium constant K value, the reaction favours forward direction:

If Q < K, reaction moves in forward direction.

Now consider that at time = 6, 1.00 moles of ammonia are added to this mixture at more than 350 degree Celsius, then concentration of each substance is monitored. The observations are given in the following graph and table:

The Le Chatelier’s Principle predicts that if more ammonia is added to a system which was at equilibrium, it will carry the reaction in reverse direction, by consuming some amount of added ammonia to yield more nitrogen and more hydrogen till the time equilibrium state is reached somewhere near time t = 10

Now again, the reaction quotient value can be calculated and graphically represented at each time:

You can see that Q value is increasing as soon as more ammonia (product) is put to the mixture at equilibrium.

As this reverse reaction proceeds, by consuming ammonia and producing nitrogen and hydrogen, reaction quotient Q value becomes high but reduces until when equilibrium constant is equal to reaction constant.

As long as reaction quotient value is more than equilibrium constant, the reaction will move in the reverse direction until the time equilibrium is created.

If Q > K, reaction moves in reverse direction.

So, concerning the above example, it can be summarised that when:

• Q = K, the reaction is in equilibrium, and there is no net reaction in any direction.

• Q > K, the reaction moves in reverse direction or reactants’ direction, which is from right to left.

• Q < K, the reaction moves in forward direction or products’ direction, which is from left to right.

Predicting reaction direction in Chemistry is an important concept which you need to understand well as a lot of questions come from this portion in exams. If you want to know more about reaction quotient and equilibrium constant, install the app today.

We know that alkyl iodides are comparatively less stable than alkyl chlorides, fluorides and bromides. Alkyl iodides are often used as synthetic intermediates because of their advantages over the alkyl bromides. However, they are not used in the preparation of alkyl halides since they are costlier than the other halogens. We know that a compound with weaker bonds tends to hydrolyse faster. 

Since it is observed that alkyl iodide hydrolyses faster, it is assumed that the strength of the C-X bond in the alkyl iodides has a lesser influence on the degree of the polarisation of the bond and more on the rate. Hence, if the difference between the energies of the starting and end product is higher, the faster would be the rate. This is because the activation energy is lower. In today’s lesson, we will learn about the preparation of nitroalkanes from alkyl halides and aluminium and iodine reaction with alkyl halides.

Synthesis of Alkyl Iodide

You would know about the photochemical iodination of the alkanes with iodine, but it has almost no significance. However, the iodination of the carbonyl compounds along with their enol derivatives is more readily derived. For the activated methylene groups like malonates, the iodination process is derived under the phase transfer catalysis by using K₂CO₃ as the base and I₂ as a halogen source. Let us look at how alkyl iodide is synthesized.

1. Alkyl halides can be prepared by the addition of iodine-iodine to alkenes. When the elemental iodine is added across the double bonds, it yields vicinal di-iodo compounds. However, this method of preparation is not much in use since its reverse reaction is thermodynamically favourable.

2. Alkyl iodides are readily prepared by SN² halide exchange according to the conditions of Finklestein reaction. Though halide exchange is a reversible reaction of an alkyl chloride or bromide, a solution of sodium iodide immersed in acetone at reflux condition affects the conversion to alkyl iodide. This is because of the shift of the equilibrium positions that are caused due to the precipitation of sodium chloride, which is a by-product and is less soluble in acetone when compared to sodium iodide.

3. Due to the SN² nature of halide substitution, the secondary and tertiary halides tend to react slower with the iodide ion. They generally need various conditions like iron or zinc halide catalysis. Alkyl chlorides, fluorides and bromides can be converted to iodides by heating them with excessive HI[_{aq}], with or without the phase transfer catalysis.

4. To convert alkyl bromides to alkyl iodides, the poor solubility of potassium or sodium iodides is overcome in different methods, including using dipolar aprotic solvents like adding crown ether for solubilising the metal counterion and applying phase transfer catalysis.

5. The tertiary alkyl nitro compounds are converted to their corresponding iodides by reacting them with trimethylsilyl iodide. However, this reaction is restricted only to the tertiary systems since the primary and secondary nitroalkanes would yield nitriles and oximes.

Alkyl Iodide Aluminium and Iodine Reaction

Let us now look at the aluminium and iodine reaction of an alkyl iodide. Alkyl iodides tend to undergo elimination reactions with bases or nucleophiles. This results in loss of hydrogen iodide from the molecule and produces an alkene. There are two majorly occurring mechanisms,E₁ and E₂.

The most effective and preferred mechanism is E₂ for the synthesis of alkenes from alkyl iodide. The E₂ mechanism can be used for all forms of alkyl iodide, which are primary, secondary and tertiary. The E₁ reaction, on the other hand, is not synthetically useful since it occurs similar to SN¹ reactions. However, tertiary alkyl iodide and a few secondary alkyl iodides can react through this mechanism. 

The E₂ mechanism process is one-stage and involves both the alkyl iodide and the nucleophile. This is a second-order reaction and depends on the concentration of both the reactants. The E₁ mechanism, on the other hand, involves a two-stage process. It includes loss of halide and forms a carbocation, followed by the loss of the susceptible proton for forming an alkene. The first stage is the rate-determining step which involves loss of the halide ion, which makes the reaction a first-order reaction.

The carbocation intermediate which is formed is stabilized by the substituent alkyl groups. In the mono-molecular substitution SN¹ reaction, first, the dissociation of the C-X bond in the alkyl halide takes place with the formation of a carbonium ion. Then a rapid reaction with the nucleophilic agent is followed.