Category: Chemistry Notes PPT

Catenation meaning or the catenation in chemistry is defined as a chemical linkage into the chains of atoms of a similar element, by only occurring among the atoms of an element, which contains a valence of at least two and that produces the relatively strong bonds with itself. This property is significant among the silicon and sulfur atoms, predominant among the carbon atoms, and slightly present among the atoms of nitrogen, germanium, tellurium, and selenium.

Occurrence

Carbon

Catenation takes place most readily with the carbon atoms, which produces the covalent bonds with the other carbon atoms to form structures and longer chains. This is the main reason for the presence of a huge number of organic compounds in nature. Carbon is well-known for its catenation properties, with organic chemistry importantly being the study of catenated carbon structures (also referred to as catenae). In biochemistry, carbon chains combine any of the different other elements, such as oxygen, biometals, and hydrogen, onto the backbone of carbon.

But, by no means carbon is described as the only element that is capable of forming such catenae, and the many other main-group elements are capable of producing an expansive range of catenae, including boron, hydrogen, phosphorus, sulfur, and silicon.

The element’s ability to catenate is essentially determined by its bond energy to itself, which decreases as more dispersed orbitals (those with a higher azimuthal quantum number) overlap to form the bond. As a consequence, the carbon element, which has the least or minimal diffuse valence shell p-orbital, will form the longer p-p sigma bound chains of atoms compared to the heavier elements, which have higher valence shell orbitals.

The ability to catenate is influenced further by a combination of electronic and steric influences, such as the element’s electronegativity, the molecular orbital n, and the ability to form different forms of covalent bonds. For the carbon atom, the sigma overlap between the adjacent atoms is strong enough that perfectly stable chains are formed. With the other elements, this was once thought to be not easier despite plenty of evidence to the contrary.

Hydrogen

The structure of the water theories involves the 3-dimensional networks of both chains and rings, and tetrahedra, which are linked via hydrogen bonding. A polycatenated network, with the rings produced from metal-templated hemispheres, which are linked by the hydrogen bonds, was reported in 2008.

In organic chemistry, hydrogen bonding is well known to facilitate the formation of chain structures. For example, 4-tricyclene C10H16O represents catenated hydrogen bonding between the hydroxyl groups by leading to the production of helical chains; crystalline isophthalic acid – C8H6O4 is built up from the molecules, which are connected by the hydrogen bonds, forming infinite chains.

Whereas in the unusual conditions, a one-dimensional series of hydrogen molecules confined within a single wall, the carbon nanotube can be expected to become metallic at relatively low pressure, at 163.5 GPa. This is up to 40% of the ~400 GPa thought to be needed to metalize ordinary hydrogen, a pressure that can be difficult to access experimentally.

Silicon

Silicon forms the sigma bonds to other silicon atoms (where the disilane is given as the parent of this compounds’ class). But, it is not easy to prepare and isolate SinH2n+2 (which is analogous to the saturated alkane hydrocarbons) with n greater than up to 8, as their thermal stability decreases with the increases in the number of silicon atoms. Silanes, which are higher in molecular weight compared to the disilane, decompose to hydrogen and polymeric polysilicon hydride. However, with the suitable pair of the organic substituents on each and every silicon in the place of hydrogen, it is also possible to prepare polysilanes (at times, the erroneous ones are referred to as polysilanes) that can be defined as the analogues of alkanes. These particular long-chain compounds contain surprising electronic properties – of high electrical conductivity, for example, arising from the sigma delocalization of electrons present in the chain.

Even the pi bonds of silicon-silicon are possible. But, these bonds are less stable compared to the carbon analogues. Disilane is quite reactive than ethane. Disilynes and disilene are quite rare, unlike alkynes and alkenes. Examples of disciplines long thought to be too unstable to be isolated, which were reported in 2004.

Example of Catenation

Most of the common examples of elements or catenation that exhibit catenation is given as follows:

Catenation takes place most readily in carbon by forming the covalent bonds to produce longer structures and chains with the other carbon atoms. This is the reason the huge number of organic compounds are found in nature. In organic chemistry, carbon is the best-known element for its catenation properties, with the analysis of catenated carbon structure.

By no means carbon is the only element capable of forming such catenae; however, and many other main group elements are capable of producing a wide range of catenae, including sulfur, boron, and silicon.

Carbon isn’t the only element capable of generating catenae; silicon, sulfur, and boron are just a few examples.

In Group 4, there is a Catenation Property.

Catenation is a trait shared by all members of the carbon family, or group 4. Catenation is most likely to occur in the family’s first member.

Catenation tendencies are as follows:

Si > Ge > Sn > Pb > C > Si > Ge > Sn > Pb > C > Si > Ge > S

The proclivity for catenation reduces as one progresses through life.

Atoms and Nuclei

In the 20th century, research on the material world changed to the atomic structure, which is the essence of the material world. In 1897, J.J.J. Thomson discovered the electron by showing that atoms have more elementary particles. Fourteen years later, Rutherford discovered that most of the mass of an atom lies in small nuclei with a radius  100,000 times less than atoms. A ray, on the other hand, turns out to be composed of photons, which correspond to particles in a wave. These discoveries gave birth to new concepts. When these concepts and discoveries are combined, new ideas emerge. The result is a quantum theory. This theory provides a good interpretation of the phenomena in the atomic and subatomic world. In this microscopic world, distances are measured in nanometers and phantom meters.

The electrons in the atoms are held together by the electromagnetic force of the nucleus of the atom. At this level, we need a quantum-mechanical approach to understand the energy states of the electrons in an atom. But I don’t have time to discuss this in detail.

 

Quantum Numbers and Atomic Orbitals

Quantum mechanics of the structure of an atom is a mathematical approach that describes the behavior of electrons in atoms. The first is expressed as a wave function, each of which is characterized by a series of numbers. Each set of numbers represents a state, also known as an orbital state. Quantum Numbers and Atomic Orbitals is a page that provides additional information on this
topic. The film shows the relationship between the periodic table of the elements and the shape and concept of atomic orbitals. These concepts are necessary to understand bonding. 

For example, bonds formed between carbon atoms in diamond, silicon, graphite, etc. 

Electronic Configuration

The electronic configuration of an element or atom describes the energy state of the electrons in it. Pauli’s exclusion principle and Hund’s law are some of the theories concerned with the assignment of electron configurations. An electron in a hybrid orbital atom can have multiple orbital properties and have similarities. That is, atomic orbitals can be combined into hybrid orbitals. These hybrid orbitals are especially useful when discussing chemical bonding. For carbon, the hybrid orbitals are made up of the 2s, 2p0, 2p + and 2p orbitals. Orbital 4 separating the symbols s and p is called an sp³ hybrid orbital because orbitals 1s and 3p are used. The shape and orientation of this orbit should have been explained in the lecture, but here we need a diagram. The diamond bond is perfectly explained by the sp³ hybrid orbital. 

Benzene and Graphite Bonds 

Benzene bonds should be discussed in detail in organic chemistry courses. Simply put, the orbital used to form a sigma bond is an sp² hybrid orbital formed as a combination of 2s, 2p + and 2p orbitals. Also, when 2p0 orbitals overlap, a pi bond is formed. 

Resonance, Benzene and Graphite 

Again, the structure of benzene is a great example of the concept of resonance. If someone claims that the 3 double and 3 single bonds of benzene alternate along the ring, you could start with a single or double bond. Because the six bonds are about the same length, the substructure is a benzene structure. Therefore, the combination of the two structures is used to represent the structure of benzene, and the approach is called resonance. That is, the electrons in the double bond are delocalized throughout the ring. The description of benzene bonds applies to one sheet of graphite. In graphite, electrons are delocalized in two planes. So it’s no surprise that graphite is an excellent sheet conductor. 

Buckminster Fullerene 

The graphite structure is the result of the expansion of pi electrons in the plane. Because all the rings in graphite are made up of 6 carbon atoms, the sheet is flat. If the hybrid orbital is somewhat flexible, it is easy to see that a five-membered ring is also possible. However, the formation of pentacyclic rings results in distortions of the planar structure that are not normally considered.

Change in State Definition

In the field of physics, the matter can exist in distinct forms or states in the universe. However, there are only the primary four fundamental states of matter solid, liquid, gas, and plasma. We observe these matters every day around us in our lives. For instance, water exists in various states around us, such as it exists as water vapour in the gaseous state, ice in the solid-state, and water in the liquid state. In addition to that, there are several other intermediate states of matter such as liquid crystal, glass, quark-gluon plasma etc. that can only exist due to harsh conditions such as increased temperature, high pressure, extreme density and high energy. The different states of process exhibit different or change in the properties. Going further let us study the change in state definition and different states of process in depth.   

States of Matter

As we already know, the four fundamental states of matter are solid, liquid, gas, and plasma. Solids exhibit tightly packed constituent particles or atoms or molecules. The movement of the particle is restricted due to a strong force between them that only enables them to vibrate. This movement results in the solids having a specific volume and specific shape. It is impossible to change its structure. They only change structure when an external force is applied, for instance, breaking or cutting. Liquid has a fluid tendency that is mostly incompressible. They retain a specific volume; however, they take the shape of their container. If the external pressure and temperature remain constant, then the size won’t change. The existence of liquids depends upon the maximum temperature called critical temperature. 

In addition to that, gases are compressible fluids. Unlike solids and liquids, gases can expand to fill their container and take the shape of the box. The gas molecules have adequate kinetic energy to minimize the effect of intermolecular forces. Also, the distance between the adjacent molecules of a gas is way higher than the size of the molecule. Gases do not have a fixed volume or shape. Similar to gas plasma do not have a fixed volume or shape. The main difference is that plasmas are strongly conducive to electricity and can produce currents, magnetic fields, electromagnetic forces etc. Let us look at the change in state definition and an example of Change in State in Chemical reaction.

Change in State

Matter can transition from one state to another under certain conditions. The main requirement is the application of temperature and pressure. Let us look at the change in states for the three most common matter solid, liquid, and gas.

• Solid to Liquid or Liquid to Solid:- Solids can change their state to liquid state when the temperature is increased. On increasing the temperature, the kinetic energy of the particles also increases. And with that, as the energy rises, the particles start vibrating with much higher frequency than before. Hence, the attractive forces between particles start decreasing. That results in the particles detaching themselves from their fixed positions and starts moving in a free manner. Subsequently the solid goes through a phase transition and becomes a liquid. This phenomenon is referred to as melting, and the temperature at which this process occurs is called the melting point. Similarly, when the temperature is reduced in case of liquids (example below zero degree celsius for water). Hence, liquids go through a phase transition to become solids, and the process is referred to as freezing. The temperature at which this process occurs is called the freezing point.

• Liquid to Gas or Gas to Liquid:- when the temperature is increased for liquids (heat) their particles start to move at increased speeds. The energy supplied due to the temperature enables the particles of liquid to overcome the force of attraction. It ultimately leads them to go through a phase change and transition into a gas (vapour). The process is called evaporation, and this temperature is referred to as the boiling point. For instance, when we boil the water at high temperatures, it starts evaporating from the container. Similarly, gases can change their phase into the liquid state by the process of condensation.

• Gas to Solid or Solid to Gas:- The process that leads the transition of solid to a gas is known as sublimation. One typical example of this phenomenon is dry ice or solid carbon dioxide. At room temperatures, it turns into a gas. When a gas becomes a solid by skipping the liquid state, then that process is called deposition. Let us look at the Example of Change in State in Chemical Reaction.

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                                            (Change in states of matter)

In Chemistry, we define chemical equilibrium as a state in which the rate of the forward reaction is equal to the rate of the backward reaction. In other words, we can say it refers to the state of a system in which the concentration of the reactant and the concentration of the products do not change with time. After that, the system will not display any further change in their properties and it becomes constant. Here we will study what is chemical equilibrium, what is equilibrium in Chemistry, and various factors affecting chemical equilibrium.

Equilibrium Meaning in Chemistry

Chemical equilibrium definition refers to the state of a system where the concentration of the reactant and the concentration of the products do not change with respect to time and the system does not display any further change in properties.

Chemical equilibrium is said to be achieved by the system when the rate of the forward reaction is equal to the rate of the reverse reaction. When there is no further change in the concentrations of the reactants and the products due to the equal rates of the forward and reverse reactions, at the time point of time the system is said to be in a dynamic state of equilibrium.

The graph with the concentration on the y-axis and time on the x-axis is plotted and it is shown in the above diagram. Once the concentration of both the reactants and as well as the products stops showing any change, in that state chemical equilibrium is said to be achieved.

Define Equilibrium 

Let’s understand this with an example. Consider hydrogen and iodine gas. These gases react to form hydrogen iodide. Here the reaction is given below:

H2(g)  +  I2(g)  ⇌  2HI(g) 

Reaction A: Forward reaction H2(g)  +  I2(g) → 2HI(g) 

Reaction B: Reverse reaction 2HI(g) → H2(g)  +  I2(g) 

Initially, only the forward reaction occurs because HI was not present. As soon as some HI is formed, it begins to decompose back into H2 and I2. After that, the rate of the forward reaction decreases while the rate of the reverse reaction keeps on increasing. In the long run, the rate of the combination of  H2 and I2 to produce HI becomes equal to the rate of decomposition of HI into H2 and I2. When the rates of the forward and rate of reverse reactions become equal to one another, then the reaction has achieved its state of balance. 

The above diagram shows the equilibrium in reaction.

Chemical equilibrium can be attained in any of the cases 

1. when the reaction begins with all reactants and also no products

2. when all products and no reactants

3. some of both are available.

The figure below shows changes in the concentration of H2, I2, and HI for two different reactions. In the reaction represented by the graph on the left side of  (A), the reaction begins with only H2 and I2 present. There is no HI initially. As the reaction proceeds towards equilibrium, the concentrations of the  H2 and I2 gradually start decreasing, while the concentration of the HI gradually starts to increase.  When the curve levels out and the concentrations become constant, that time equilibrium state has been reached. At equilibrium, concentrations of all substances are constant.

As we can see in reaction B, the process begins with only HI and there were no  H2 and I2 present. In this situation, the concentration of HI gradually decreases while the concentrations of H2 and I2 gradually increase until an equilibrium state is reached again.

As we have noticed that in both cases, the relative position of equilibrium is the same, which is shown by the relative concentrations of reactants and products.

The concentration of HI at equilibrium is significantly higher as compared to the concentrations of  H2 and I2. This is true only when the reaction began with all reactants or all products. The position of equilibrium is a property of the particular reversible reaction and it does not depend upon how equilibrium has been achieved.

The above diagram represents the equilibrium between reactants and products is achieved regardless of whether the reaction starts with the reactants or products.

Types of Chemical Equilibrium

There are two types of chemical equilibrium:

Homogeneous Chemical Equilibrium

In this type of reaction, the reactants and the products of chemical equilibrium are all in the same phase

It is also divided into two types: (i) Reactions having the number of molecules of the products is equal to the number of molecules of the reactants. For example,

H2(g)  +  I2(g)  ⇌  2HI(g) 

N2(g)  +  O2(g)  ⇌  2NO(g) 

(ii) Reactions having the number of molecules of the products is not equal to the total number of reactant molecules. For example,

2SO2(g)  +  O2(g)  ⇌  2SO3(g) 

COCl2(g)  ⇌  CO(g)  +  Cl2(g) 

Heterogeneous Chemical Equilibrium

In this type of reaction, the reactants, as well as the products of chemical equilibrium, are present in different phases. Examples are given below:

CO2(g)  +  C(s)  ⇌  2CO(g) 

CaCO3(s)  ⇌  CaO(s)  +  CO2(g) 

Conditions for Equilibrium

Chemical equilibrium is a dynamic process. The forward and reverse reactions continue to occur even after the equilibrium state has been achieved. However, the rates of the reactions are the same here, and there is no change in the relative concentrations of reactants and products for a reaction that is at equilibrium. Following are the conditions and properties of a system at equilibrium.

• The system must be closed, which means no substances can enter or leave the system.

• Equilibrium is
  a dynamic process. Even though we don’t necessarily see the reactions, both forward and reverse reactions are taking place.

• The forward and reverse rates of reactions must be equal.

• The number of reactants and products need not be equal. However, after equilibrium is achieved, the amounts of reactants and products will always be constant.

Factors Affecting Chemical Equilibrium

According to Le-Chatelier’s principle, if there is any change in the factors that affect the equilibrium conditions, then the system will counteract or reduce the effect of the overall transformation. This principle is applied to both chemical and physical equilibrium.

There are several factors that affect equilibrium conditions like temperature, pressure, and concentration of the system that affect equilibrium.

Change in pressure occurs due to the change in the volume. If there is a change in pressure it can also affect the gaseous reaction because the total number of gaseous reactants and products are now different. In the heterogeneous chemical equilibrium, according to the principle of Le Chatelier, if there is a change of pressure in both liquids and solids it can be ignored because the volume is independent of the pressure.

The effect of temperature on chemical equilibrium depends upon the sign of ΔH of the reaction and it also follows Le-Chatelier’s Principle.

When the temperature increases, the equilibrium constant of an exothermic reaction decreases.

In the case of an endothermic reaction the equilibrium constant increases with an increase in temperature.

Together with the equilibrium constant, the rate of reaction is also affected by the change in temperature. In the case of exothermic reactions according to Le Chatelier’s principle, the equilibrium shifts towards the reactant side when there is increased temperature. In the case of endothermic reactions the equilibrium shifts towards the product side with an increase in temperature. 

A catalyst does not affect the chemical equilibrium. It only speeds up a reaction. The same amount of reactants and products will be present at equilibrium in a catalysed in a non-catalysed reaction. The presence of a catalyst only facilitates when the reaction proceeds through a lower-energy transition state of reactants to products.

When an inert gas like argon is added to a constant volume it does not take part in the reaction so the equilibrium remains in an undisturbed state. If the gas added is a reactant or product that is involved in the reaction then the reaction quotient will change.

Examples of Chemical Equilibrium

In chemical reactions, the reactants are converted into products by the forward reaction and the products can be converted into the reactants by the backward reaction. They are two states, reactants, and products both are present in different compositions.

After some time when the reaction starts, the rate of the forward and the backward reactions may become equal. After this, the number of reactants converted will be formed again by the reverse reaction so that the concentration of reactants and products do not change anymore. Hence, the reactants and products will be in chemical equilibrium.

Importance of Chemical Equilibrium

It is useful in many industrial processes like,

• It is used in the preparation of ammonia with the help of Haber’s process. Here nitrogen gas combines with hydrogen gas to form ammonia. The yield of ammonia happens more at low temperature, high pressure, and in the presence of iron as a catalyst.

• It is used in the preparation of sulphuric acid by contact process: In this process, the fundamental reaction is the oxidation of sulphur dioxide into sulphur trioxide. This involves chemical equilibrium.

Conclusion

Now we have understood what equilibrium is in Chemistry. We have discussed some very important concepts and principles related to the studies of reaction rates and chemical equilibria. The equilibrium is a state in Chemistry in which there is no net change in the concentrations of reactants and as well as the products. As we know there is no apparent change at equilibrium, this does not mean that all chemical reactions have ceased.

During a chemical reaction, the emission of light (of any wavelength) is normal. However, if the reaction may also give off a sufficient quantity of heat. We say this type of reaction is chemiluminescence.

The word “chemiluminescence” has two words, i.e.,  chemi and luminescence. Here, chemo means chemical reaction and luminescence means something that gives off light.

So, chemiluminescence is also known as chemiluminescence. This is the process of emission of light as a result of the chemical reaction. 

Therefore, given reactants A and B, with an exciting intermediate ◊, we have:

                                     [A] + [B] → [◊] → [Products] + light

We will understand what is chemiluminescence with illustrative chemiluminescence examples in detail.

Chemiluminescence Definition

We define chemiluminescence as the light emission as a result of the chemical reaction. 

During the product formation, light isn’t necessarily the only form of energy released by a chemiluminescent reaction. Besides, heat may also release, making the reaction exothermic.

How Chemiluminescence Works?

During a chemical reaction, the reactant atoms/molecules/ions collide with each other. These particles interact with each other to form what we call a transition state. 

From the transition state, product formation occurs. 

The transition state is the state at which enthalpy stays maximum. However, the products usually have less energy than the reactants. 

Further, a chemical reaction occurs when there is an increase in the stability/decrease in the energy of the molecules. 

Thus chemical reactions that release energy as heat, the vibrational state of the product remains excited. The energy disperses through the product, makes it warmer, in short, the release of heat. The same happens in chemiluminescence. 

Now, we will understand what chemiluminescence is.

What is Chemiluminescence?

The process of chemiluminescence is the same we discussed in the “how chemiluminescence works,” section.

In this process, electrons become excited. The excited state is the transition/intermediate state. 

When excited electrons come back to the ground state, the energy is released in the form of chunks of energy called photons. 

The decay to the ground state can occur through a quick release of light, like fluorescence or a forbidden transition (likewise phosphorescence).

In theoretical terms, each molecule that participates in a reaction releases one photon of light. 

In reality, the production is much lower.  For instance, non-enzymatic reactions possess around 1% quantum efficiency. 

On adding a catalyst, a great increase in the brightness or luminescence can be seen in many reactions.

Chemiluminescence Examples

Example 1:

An H2 (hydrogen) atom in its ground state has a single electron. A single electron is in a shell, i.e.,  n = 1. Since each shell has its own energy level.

When the hydrogen atom absorbs a quantum (quantized) amount of energy, it reaches a higher energy level (shell n = 2).

When hydrogen reaches an excited state or a high-energy state. We make an asterisk (*) aside the molecule to indicate this.

The electron retraces to its original position, i.e., the ground state (shell n = 1). 

In the process, a packet of energy (a photon) releases in the form of electromagnetic radiation. 

The wavelength of the light emitted depends on the amount of energy. 

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Furthermore, if the wavelength is within the range of visible light, the electron transition is perceived as the light of a particular colour. Therefore, the wavelength of light determines the colour.

Example 2:

A reaction between hydrogen (H2) and oxygen (O2) to form water (H2O) is an example of a chemical change. In this reaction, the H-H bond in H2and the O-O bonds in O2 break.

Here, new H-O bonds form to make H2O. 

For the most part, when chemicals undergo a change in this manner, they either give off heat (exothermic) or absorb heat (endothermic). Hence H2 plus O2 reaction is exothermic.

We say that a few very interesting kinds of chemical reactions occur in which the energy produced is given off not as heat but as light. 

These reactions are what we term chemiluminescent. In living organisms, we call it bioluminescent.

Examples of Chemiluminescence

C8H7N3O2 (luminol) when reacts with  H2O2 (hydrogen peroxide)  → gives → 3-APA (vibronic excited state) → 3-APA (decays to a lower energy level) + light (release)

Here, 3-APA is 3-Aminopthalalate.

Point To Note:

No difference is there in the chemical formula of the transition state. Wherefore, only the energy level of the electrons. 

This happens because iron is one of the metal ions that catalyzes the reaction. Chemists use the luminol reaction to detect blood. 

Iron from hemoglobin results in the chemical mixture glows brightly.

Another good example of chemiluminescence is the reaction occurring in glow sticks. 

The color of the glow stick occurs from a fluorescent dye called a fluorophore, which absorbs the light from chemiluminescence and releases it as another color.

Point To Note:

Chemiluminescence not only occurs in liquids; however, in gases as well. 

For instance, a gas-phase reaction between vaporized phosphorus and oxygen results in the green glow of white phosphorus in the damp air.

Citric acid is an organic compound of the chemical formula [C_{6}H_{8}O_{7}]. Hence it contains elements carbon, oxygen, and hydrogen. It is a white-colored solid and also a weak organic acid. Naturally, it is found in citrus fruits such as lemons, limes, etc. The citric acid cycle occurs in the metabolism of all aerobic organisms. Citric acid is an intermediate in the citric acid cycle in biochemistry. The molecule of citric acid has six atoms of carbon, seven oxygen atoms, and eight hydrogen atoms. It has a planar structure and three carboxylic acid groups (COOH) and a hydroxyl group (OH). The extended formula of it is [CH_{2}COOH-COHCOOH-CH_{2}COOH].

Explanation of Citric Acid Cycle 

The citric acid cycle (CAC) is also called the TCA cycle (tricarboxylic acid cycle) or the Krebs cycle. In this cycle, the reaction involved is helpful to release the stored energy through the method of oxidation of acetyl-CoA which is derived from proteins, carbohydrates, and fats. However, this series of reactions is called the tricarboxylic acid (TCA) cycle, for the three carboxyl groups on its first two intermediates, or the Krebs cycle, after its discoverer, Hans Krebs. Hence this series of chemical reactions is important for all aerobic organisms to produce energy through the oxidation of acetate derived from fats, carbohydrates, and proteins into carbon dioxide ([CO_{2}]).

The citric acid cycle occurs in the mitochondria and provides large amounts of energy in aerobic conditions by donating electrons to three NADH and one FADH (flavin adenine dinucleotide), which in order to create the proton gradient donate electrons to the chain of electron transport. 

The citric acid cycle pathway is considered as a  major and also main metabolic pathway that connects the metabolism of carbohydrates, fat, and protein.  In the following article, simple citric acid cycle reactions are explained.

Citric Acid Cycle Reactions

The citric acid cycle is an eight-step series of chemical reactions. These reactions include hydration reaction, redox reaction, dehydration reaction, and decarboxylation reactions. Adenosine triphosphate or Guanosine triphosphate is formed in each step of the citric cycle and also three molecules of NADH and one FADH2 molecule that helps in further steps. The eight reactions of the citric acid cycle with structures of each step of citric acid cycle reactions are given below.

• Reaction 1: Citrate synthase- In the first reaction of the citric acid cycle, the enzyme citrate synthase catalyzes the reaction. For the formation of the citric acid in the first step of the reaction, oxaloacetate is joined with acetyl-CoA. A water molecule attacks the acetyl once the two molecules are joined and leads to the release of coenzyme A from the complex.

  

• Reaction 3: Isocitrate Dehydrogenase- In the third reaction of the citric acid cycle, two events occur. In the first reaction, the generation of NADH from NAD takes place. The oxidation of the oxygen-hydrogen group is catalyzed by enzyme isocitrate dehydrogenase at the fourth position of isocitrate to get an intermediate which then has a carbon dioxide ([CO_{2}]) molecule removed from it to yield alpha-ketoglutarate. The reaction is given below.

• Reaction 4: Alpha-ketoglutarate dehydrogenase- Alpha-ketoglutarate loses a carbon dioxide molecule in the fourth reaction of the cycle and coenzyme A is added in its place. With the help of NAD decarboxylation occurs, which is converted to NADH. The reaction is catalyzed by the enzyme alpha-ketoglutarate dehydrogenase. The molecule of the reaction formed at last is called succinyl-CoA.

• Reaction 5: Succinyl-CoA Synthetase- A molecule of guanosine triphosphate (GTP) is synthesized in the fifth step of the reactions where it is catalyzed by the enzyme succinyl-CoA synthetase. With the addition of a free phosphate group to a GDP molecule, the GTP synthesis occurs. A free group of phosphate first attacks the succinyl-CoA molecule and releases the CoA. It is transferred to the GDP to form GTP after the phosphate is attached to the molecule. The resulting product is the molecule succinate. The reaction is as follows.

• Reaction 6: Succinate Dehydrogenase- The enzyme succinate dehydrogenase catalyzes the reaction where it removes two hydrogens from succinate in the sixth reaction of the citric acid cycle. A molecule of FAD that is a coenzyme similar to NAD is reduced to [FADH_{2}] in the reaction as it takes the hydrogens from succinate. The product of this reaction is fumarate. The reaction is given below. 

           

            

 

Do You Know?

The citric acid cycle was discovered by the chemist of Germany named Hans Adolf Krebs. He discovered this cycle in 1937 and marked a milestone in biochemistry. The Nobel Prize was given to him for his contribution to Physiology or Medicine in 1953.

Conclusion

The citric acid cycle is an important catabolic pathway of oxidizing acetyl-CoA into [CO_{2}] and generating ATP. The complex, as well as simple citric acid cycle reactions of the cycle, are carried out by eight enzymes that completely oxidize acetate. We got the information on the citric acid cycle through this article in detail.

What is Clemmensen Reduction?

The Clemmensen reaction was first reported by Clemmensen of Park Davis in 1913. The aldehydes and ketones reacted with zinc amalgam (Zn/Hg alloy) in concentrated hydrochloric acid (HCL) in Clemmensen Reduction, resulting in aldehyde or ketone hydrocarbon formation. The Clemmensen reduction uses zinc and mercury in presence of strong acid.

The general reaction is:

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The reaction mentioned above is particularly effective in aryl-alkyl ketones reduction formed in Friedel-Crafts acylation. The reduction of Clemmensen is most widely used to transform acyl benzene (from acylation by Friedel-Crafts) to alkylbenzene but it also works well with other acid-insensitive ketones or aldehydes. The two-step sequence of Friedel-Crafts acylation is followed by Clemmensen reduction. It constitutes a classical strategy for the primary alkylation of arenes.

Clemmensen Reduction Mechanism

It allows the deoxygenation of aldehydes or ketones to form the corresponding hydrocarbon. The stratum must be stable to strong acid. The Clemmensen Reduction works well with other acid-insensitive ketones or aldehydes. With an excess of amalgamated zinc (mercury treated zinc, Zn (Hg) and concentrated hydrochloric acid (HCl), the carbonyl compound is heated. On the surface of the zinc, the reduction happens. There are two proposals for the Clemmensen Reduction Mechanism:

• Carbanionic Mechanism: In the Carbanionic mechanism, zinc attacks directly to the protonated carbon.

• Carbenoid Mechanism: It is a radical process and reduces the happenings on the metal surface of zinc. The reaction of the carbenoid mechanism takes place at the surface of the zinc catalyst.

The equation below follows the intermediacy of zinc carbenoids to justify the Clemmensen Reduction mechanism. The Clemmensen Reduction enables aldehydes or ketones to be deoxygenated to obtain the corresponding hydrocarbon. The substrate must be a strong acid that is stable. The reduction of the Clemmensen is complementary to the reduction of the Wolff-Kishner, which is operated under very simple conditions. To explain the Clemmensen Reduction process, the following equation employs the intermediacy of zinc carbenoids.

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Wolff-Kishner reduction reaction is similar to Clemmensen Reduction Reaction. In the Wolff-Kishner reduction reaction, carbonyl compounds are heated with hydrazine and potassium hydroxide in presence of boiling solvents like ethylene glycol or diethylene glycol to form alkanes. In this, carbonyl compounds react with hydrazine to form hydrazone. On heating under the normal conditions, these form alkanes with the evolution of Nitrogen gas (N₂). 

The Clemmensen reduction reaction and Wolff Kishner reduction reaction differ in a few conditions. In Clemmensen Reaction, the conversion of ketones or aldehydes into alkanes takes place, whereas, in the case of Wolff-Kishner Reaction, the conversion of carbonyl groups into methylene groups takes place. These conversions are processed by reducing functional groups. Both the reactions require specific reaction conditions and the catalyst for the successful progression of the reaction.

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Applications of Clemmensen Reduction

1. Alkane from alkenyl chloride (halide) can be prepared from any organic compound which can be transformed into alkenyl halide.

2. The reaction is widely used to convert the carbonyl group into a methyl group.

3. Preparation of polycyclic aromatics and aromatics containing unbranched side hydrocarbon chain.

4. The reaction helps to reduce the aliphatic and mixed aliphatic-aromatic carbonyl compounds.

5. The reduction of Clemmensen is most widely used to transform acyl benzene (from acylation by Friedel-Crafts) to alkylbenzene.

Toluene can be Formed with the Help of Clemmensen Reduction as Explained Below:

In this, Benzaldehyde undergoes Clemmensen’s Reduction in the presence of zinc amalgam and concentrated hydrochloric acid to form Toluene. 

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