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. And also will discuss what is sigma and pi bond, the difference between the pi bond and sigma bond.

What is Sigma Bond?

The strongest covalent bond which is formed by the head-on overlapping atomic orbitals is called the sigma bond. It is denoted by σ. We find sigma bonds in Alkanes, Alkenes, Alkynes. Formation of sigma bond is given below between the orbitals:

Properties

• Sigma molecular orbitals form it up.

• It is made up of atomic orbitals that overlap head-to-head.

• Around the bond axis, it is cylindrically symmetrical.

• Because of the maximal overlap, it is a significantly stronger bond.

• It is created by the linear overlap of the s-s, s-p, and p-p orbitals.

What is Pi Bond?

The covalent bond which is formed by lateral overlapping of the half-filled atomic orbitals of atoms is called pi bond. It is denoted by π. We find pi bonds in alkenes and alkynes. Formation of pi bond is given below between the two orbitals:

Properties

• Pi molecular orbitals make it up.

• It is generated by the atomic orbitals overlapping side by side.

• Above and below the two nuclei, there are two sections of the electronic cloud.

• Because there is less overlap, it is a weaker tie.

• It’s made up of parallel p-p orbital overlap that’s coplanar.

Why is Sigma Bond Stronger than the Pi Bond?

The sigma bond is stronger than the Pi bond, according to quantum mechanics, since the orbital paths of Pi bonds are parallel, resulting in less overlap between the p-orbitals. Two atomic orbitals come into contact with each other through two areas of overlap, forming Pi bonds.

Difference Between Pi and Sigma Bonds

Key Points

With so many differences, it’s clear that these two bonds are crucial. Although the sigma and pi bond differences are complimentary, they are not convertible. Still, there are a few more key points to consider:

• It is a proven truth that if two atoms form a single bond, the bond will be a sigma bond.

• It is a truth that if two bonds are present, one must be sigma and the other a pi bond.

• Also, if there are three bonds, one of them will be a sigma bond, while the other two will be pi bonds.

• The overlapping of atomic orbitals is the main difference between these bonds. Pi bonds are created by the lateral overlap of two atomic orbitals, whereas sigma bonds are formed by the head-to-head overlapping of atomic orbitals.

• Sigma bonds are extremely strong covalent bonds, and sigma electrons are the electrons that participate in their creation.

• S-s overlap, s-p overlap, and p-p overlap are the three most prevalent overlap conditions for forming sigma bonds.

• The parallel orientation of the two p orbitals in adjacent atoms with proper sideways overlap results in the creation of a pi bond.

The lowest unit of matter is an atom, which is made up of three subatomic particles: protons, neutrons, and electrons. The central nucleus is occupied by protons and neutrons, while electrons orbit the nucleus in specific orbits. During the nineteenth and twentieth centuries, these three subatomic particles were identified. Molecules are formed when atoms come together to create molecules, which interact to make matter (solid, liquid or gas).

In this article, We will discuss proton and neutrons and their discovery i.e. how proton/neutron was discovered and properties of protons and neutrons etc.

Discovery of Protons  

The presence of positively charged particles in an atom had been first observed in 1886 by E. Goldstein based on the concept that atoms are electrically neutral i.e., it has the same number of positive and negative charges. He performed a series of experiments and observed that when high voltage electricity passed through a cathode tube fitted with a perforated cathode (pierced disk) containing gas at low pressure a new type of ray was produced from the positive electrode (anode) which moves towards the cathode. These new rays he termed as canal rays, positive rays, or anode rays.  

In 1909, Rutherford discovered protons in his famous gold foil experiment. He bombarded alpha particles on an ultrathin gold foil. Rutherford thought that a hydrogen nucleus must be the fundamental building block of all nuclei, and also possibly a new fundamental particle as well since nothing was known from the nucleus that was lighter. Based on Wilhelm Wien’s theory, who in 1898 discovered the proton in streams of ionized gas, Rutherford postulated the hydrogen nucleus to be a new particle in 1920, which he called proton. Rutherford named it the proton, from the Greek word “protos,” meaning “first.” 

What is a Proton?

“The fundamental particle of an atom, which is denoted by symbol p or p+. It has a positive electric charge of +1e elementary charge and a mass slightly less than that of a neutron”

The particles that are positively charged are called protons. A proton is usually represented as p its charge is “+1.” The number of protons in the nucleus of an atom is equal to the atomic number (Z) of the atom.

Mathematically it can be written as, 

Number of Protons = Atomic Number

For example, the atomic number of Krypton (Kr) atoms is 36. Hence, the nucleus of the Krypton atom contains 36 protons. 

Who Discovered Protons?

Goldstein observed positive rays in the anode ray experiment in 1886. In 1909, Rutherford discovered protons in his gold foil experiment.

How was Proton Discovered?

In a gold foil experiment, Rutherford bombarded alpha particles on an ultrathin gold foil and then detected the scattered alpha particles on a zinc sulphide (ZnS) screen. According to Rutherford’s observation,

Most of the alpha particles were not deflected; they passed through the foil. Some alpha particles get deflected at a small angle. Very few particles bounced back (1 in 20,000).

Based on these observations, Rutherford proposed the following: 

• Most of the atom’s mass and its entire positive charge are confined in a small core, called a nucleus. The positively charged particle is called a proton.

• Most of the volume of an atom is empty space.

• The number of negatively charged electrons dispersed outside the nucleus is the same as the number of positively charged electrons in the nucleus. It explains the overall electrical neutrality of an atom.

Properties of Protons

Protons are also called Positive Rays or Anode Rays. Let us look at the properties of protons here.

• They are positively charged.

• They travel in straight lines and can cast shadows on the object placed in their path.

• These positive rays are deflected by electric as well as magnetic fields. 

• Mass of the proton is found to be 1.672 x 10^-24 g.

• The charge on the proton is +1.602 x 10^-19 coulombs. 

• The volume of a proton is given by 4/3 πr³ (1.5 x 10^-38 cm³)
  

What is a Neutron?

Neutron can be Defined as “A subatomic particle of an atom denoted by n or n0. It has no net electric charge and a mass slightly greater than that of a proton.”

For his novel observation, Chadwick was awarded the Nobel Prize in 1935. It is to be noted here that except for hydrogen all atoms contain neutrons. Hydrogen atoms contain only a proton and an electron. 

Who Discovered Neutrons?

In his experiment, Chadwick bombarded beryllium atoms with high-energy alpha particles. He observed that some new particles are emitted which carry no charge, and the mass of this particle is the same as that of protons. A neutron is usually represented as “n” and its charge is zero.

The total number of protons and neutrons present in an atom indicates the mass number of that atom. 

Mathematically it can be written as, 

Mass Number = (Number of Protons) + (Number of Neutrons)

Or

Number of Neutrons = Mass Number – Atomic Number or number of protons

In the case of krypton, 

Mass number = 83.80

Protons = 36

83.80 = 36 + (Number of Neutrons)

Number of Neutrons = 83.80 – 36 = 47.8 or 48

How were Neutrons Discovered?

• James Chadwick used a polonium source to fire alpha radiation at a beryllium sheet. As a result, uncharged, penetrating radiation was produced.

• This radiation was incident on paraffin wax, which is a hydrocarbon with a high hydrogen concentration.

• With the use of an ionisation chamber, the protons ejected from the paraffin wax (when impacted by the uncharged radiation) were seen.

• Chadwick studied the interaction between the uncharged radiation and the atoms of numerous gases and measured the range of the freed protons.

• He came to the conclusion that the unusually penetrating radiation was made up of uncharged particles with the mass of a proton (approximately). Neutrons were later given to these particles.

Properties of Neutrons

• These are neutral particles.

• The mass of neutrons is equal to that of protons (the Mass of the neutron is 1.675 x 10^-24 g).

• The specific charge of a neutron is zero.

• The density of the neutron is 1.5 x 10¹⁴ g/cc.
  

What are Electrons?

The electron is defined as a subatomic particle having a negative one elementary electric charge. Electrons are said to be the first generation of the lepton particle family. It is because they have zero known components or substructures, and because of this, they are considered elementary particles.

Discovery of Electrons

In 1897, J.J. Thomson discovered electrons by working on a cathode ray tube. Thomson demonstrated that cathode rays were negatively charged by passing high voltage electricity through a cathode tube containing a gas at low pressure. He observed a new type of ray was produced from a negative electrode (cathode) which moves towards the anode. These new rays of particles were called cathode rays (as they come out of cathode). The key characteristics of cathode rays are as follows:

• They travel in a straight line.

• They carry mass and possess kinetic energy.

• The mass and charge of the cathode ray particles are independent of the nature of the gas taken in the discharge tube.

An electron is usually represented as “e” and its charge is “-1”.  An electron can be defined as:

“The fundamental particle of an atom, which has a negative one elementary charge and it is denoted by e−. It has mass approximately 1/1836 that of the proton.”

Atoms do not carry any specific electrical charge. Therefore, a balance between the protons and the electrons is necessary for which atoms contain equal numbers of protons and electrons. 

Mathematically it can be written as, 

Number of Electrons = Number of Protons = Atomic Number

For example, the nucleus of an atom of krypton has 36 protons in it. The balance between protons and electrons is maintained when a krypton atom has 36 electrons.

Properties of Electrons

• The specific charge (e/m) of electrons was found by Thomson as 1.76 x 108 coulomb/gram. The specific charge of electrons decreases with an increase in velocity. It is due to an increase in velocity which otherwise increases the mass of electrons. 

• The radius of the electron is found to be 10^-15 cm.

• The density of electrons was found to be 2.17 x 10¹⁷ g/cc.

• Charge on one mole of the electron is 96500 coulombs or 1 faraday. 
  

Discovery of Electrons, Protons and Neutrons

The above article is very knowledge full and interesting as it deals with the discovery of protons and neutrons. The properties of neutron and proton are also discussed. Along with this discovery of electrons is also mentioned.

• The migration of a salt to the surface of a porous material, where it forms a coating, is known as efflorescence (which means “to flower out” in French). The most important step entails dissolving an internally held salt in water or, on rare occasions, another solvent. The salt is now kept in solution in the bath, which migrates to the surface and evaporates, leaving a salt coating.

• Water is the invader in “primary efflorescence,” and the salt was already present internally; “secondary efflorescence,” on the other hand, is a phase in which the salt is initially present externally and then brought inside in solution.

• Efflorescences can be present in both natural and man-made environments. It can often indicate internal structural weakness (migration/degradation of component materials) on porous construction materials, but it can also present a cosmetic outer issue (primary efflorescence causing staining). As seen in the spalling of brick, efflorescence can clog the pores of porous materials, causing internal water pressure to destroy the materials.

• In this article, we will study mineral efflorescence, masonry efflorescence, brick efflorescence,  white efflorescence, anti efflorescence and efflorescence treatment in detail.

Efflorescence Treatment

1. Cleaning the wall with high-pressure water has long been a popular method for removing efflorescence. However, since the salts are water-soluble, there’s a chance they’ll seep back into the wall and reappear as crystals later. As a result, the first line of protection should be to clean the walls with a stiff brush to eliminate the rest of the white. Water pressure washing can then be used to remove any remaining salt from the walls.

2. When calcium carbonate or calcium sulphate is the source of efflorescence, it is more difficult to remove. It sticks to the wall very firmly and is difficult to scrape with a brush. In this scenario, an acid-based treatment is recommended.

Masonry Efflorescence

Brick Efflorescence

In efflorescence, magnesium sulphate, calcium sulphate, sodium sulphate and carbonate (and sometimes chloride and nitrates) are commonly found. These salts have been connected to the brick itself, construction sand, base soil, groundwater, construction water, and loose earth left in contact with brickwork. Bricks containing more than 0.05 per cent magnesium sulphate should not be used in building. Sand should have a soluble salt content of less than 0.1 per cent (chloride and sulphate combined).

Efflorescence in Cement

As water percolates into poorly compacted concrete, cracks, or poorly constructed joints, the lime compounds in the concrete leach out, resulting in efflorescence (the formation of salt deposits on the concrete surface). Calcium hydroxide Ca(OH)₂, one of the hydration products that is slightly soluble in water, migrates to the concrete surface through the capillary mechanism, causing this. 

The solid Ca (OH)₂ reacts with atmospheric carbon dioxide CO₂ to form calcium carbonate CaCO₃, a white layer on the concrete surface, after evaporation.

Efflorescence on Walls

White marks on internal walls can be caused by efflorescence on plaster, which can occur behind paint and wallpaper.

These white, fluffy salts are “crunchy” to the touch, and the crystals that form underneath wallpaper or paint are strong enough to force these coatings off the plaster, or “pop” the plaster. This type of sulphate crystal formation can occur in any building, regardless of age, where water enters the structure. Evaporation would cause the water to leave the wall, leaving the salts behind. While the salts can be rubbed off, they often reappear and cause more harm to the décor.

Efflorescence Plaster

Efflorescence on plaster surfaces is caused by the presence of salts in lime, cement, sand, bricks, and often even water used in building. The soluble salts dissolved by moisture are drawn to the surface through pores after the plasterwork is completed and fully dry. These soluble salts absorb moisture from the air and store it in patches as a white crystalline material when it dries. The surface is marred by unsightly efflorescence patches. This weakness allows the structure to deteriorate over time. It is usually of a transient type, as it vanishes in rainy weather and reappears in dry weather.

Efflorescence on Stone

Efflorescence will cease in all but the most severe cases as crystallised salts block capillaries in the stone. The white deposits will slowly wear off with use and exposure until the process has stopped. Water and a stiff-bristled (non-metal) brush will always suffice for clients who are in a hurry to get rid of it. The haze will resume if the efflorescing process is not stopped or the factors that cause it are still present, and it will need to be cleaned again.

Examples of Efflorescent Salt

1. By the process of homogeneous nucleation, a 5 molar concentration aqueous droplet of NaCl can spontaneously crystallise at 45 per cent relative humidity (298 K) to form a NaCl cube. The initial water is freed into the gaseous state.

2. Gypsum (CaSO₄.2H₂O) is a hydrate solid that will give up its water to the gas phase and form anhydrite in a sufficiently dry environment (CaSO₄).

3. When exposed to sunlight, copper(II) sulphate (bluestone) (CaSO₄.5H₂O) is a blue crystalline solid that steadily loses water of crystallisation from its surface, resulting in a white layer of anhydrous copper(II) sulphate.

4. When exposed to sunlight, sodium carbonate decahydrate (Na₂CO₃.10H₂O) can lose water.

Did You Know?

Three key factors contribute to the formation of efflorescence on concrete and brick masonry walls. The following are the conditions:

Soluble salts could be present in concrete and brick masonry walls, and the salts could be found in masonry brick, mortar, adjacent dirt, and backing material.

Water must be present in the concrete and brick masonry walls, and it must come into
contact with soluble salt in order to dissolve it.

The pore structure of concrete and brick masonry walls must allow soluble salt to migrate to the surface, where water can evaporate and leave the salt.

An atom can attract shared electrons in a covalent bond. It is seen as the higher the electronegativity value, the more strongly the elements attract the shared electron. Electronegativity predictably varies in the periodic table. It increases from bottom to top in groups and increases from left to right across periods. Hence, fluorine becomes the most electronegative element, and francium becomes one of the least electronegative element. 

Talking about the electronegative trends, they are not so smooth among the transition and inner transition metals. But the electronegativity trend is fairly regular for main group elements. 

Ionic Bond

It is defined as when atoms with an electronegativity difference of greater than 2 units are joined together, the bond between them is named as an ionic bond. Here, the less electronegative elements have a positive charge and the more electronegative element has a negative charge.

For example, We will include Sodium and Chlorine for further understandability of ionic bonds.

Sodium has an electronegativity of .93 whereas Chlorine has an electronegativity of 3.16. When Sodium and Chlorine form an ionic bond, where Chlorine takes away the electron from Sodium cation Na+ and Chloride anion Cl-. Chloride and Sodium ions attract each other very strongly due to the opposite charges, therefore form a crystal lattice.

Covalent Bond

When atoms with an electronegativity difference of fewer than two units are joined, the bond is said to be a covalent bond. Here, electrons are shared by both atoms. When the same atom shares electrons in a covalent bond, this results in no electronegativity difference between them. Hence, there is a symmetrical distribution of electrons between bonded atoms. These bonds are nonpolar covalent bonds. For this let us assume as an example of when two chlorine atoms are joined together, the electron spends as much time close to one chlorine as it spends with another. This results in a nonpolar molecule.

When the electronegativity difference is between 0 and 2, the more electronegative element attracts the shared more strongly. They are not strong enough to remove the electrons completely to form an ionic compound. The electrons are shared in an unsymmetrical distribution of electrons between the bonded atoms. These bonds are called polar covalent bonds. There are two things included further in a covalent bond. These are discussed further:

1. Partial Negative Charge, D– : The more electronegative atom has a partial negative charge. It is because the electrons spend time closer to the atom.

2. Partial Positive Charge, D+ : The less electronegative atom has a partial positive charge. It is because the electrons are not completely pulled away from the atom. 

Let us take an example of a hydrogen chloride molecule to understand it even further. In the hydrogen chloride molecule, chlorine is electronegative than hydrogen by 0.96 units. Hence, the shared electron spends more time close to the chlorine atom making the chlorine molecule slightly negative, whereas the hydrogen end of the molecule is slightly positive. This results in a polar molecule.

(Image to be added soon)

Patterns of Electronegativity In The Periodic Table

The distance of electrons from the nucleus remains constant in a periodic table row but talking about a periodic table column. The distance is not relatively constant. Coulomb’s law is the force decided between charges. It is given as below: 

    F = k ( Q1Q1) / x2

In this expression, Q represents a charge, k represents constant and r is the distance between the charges. The statement concluded from the equation is that the distance between the charges increases, the force decrease. This is also called quadratic change. This change results in an increase in electronegativity from bottom to top in a column in the periodic table. Elements at the topmost of the column have higher electronegativities than elements at the bottom of a given column. 

The overall electromagnetic trend in the periodic table is diagonal from the lower-left corner to the upper right corner. Since there are exceptions in every concept, here as well we find this trend not applicable to all elements. We need to memorize some of the elements and their trend. The sequence is followed by F, O, Cl, N, Br, I, S, C, H, and lastly metals.

Electronegativity Across a Period

The positively charged protons attract the negatively charged electrons. As the number of protons increases, the electronegativity will increase. Looking at this explanation, the electronegativity increase from left to right in a row in the periodic table. 

As we go down a group, electronegativity decreases. The attraction that a pair feels for a particular nucleus depends upon the following factors:

1. Number of protons in the nucleus

2. Amount of screening by inner electrons

3. Distance from the nucleus

Before diving deep into the explanation of the Electrophilic Substitution of Benzene, let us first have a quick look at what is electrophilic substitution. In the world of chemistry, the chemical species which accept the electron pair and therefore create the bond with the nucleophiles are regarded as the electrophile. Now, when the hydrogen atom is displaced from a functional group, a molecule’s moiety causes its main chemical reactions, in a compound, this reaction is known as electrophilic substitution reaction. 

In simpler terms, you can say that when the hydrogen atom is replaced by an electrophile, then such reaction is an electrophilic substitution reaction, and one such reaction is the electrophilic substitution of benzene.

Aromatic compounds are typical of electrophilic aromatic substitution reactions and are important ways of adding functional benzene ring groups. An electrophilic aliphatic substitution reaction is the other primary form of electrophilic replacement reaction.

Generally, electrophilic substitution reactions proceed through a three-step process involving the following steps.

1. The appearance of an electrophile.

2. The appearance of a carbocation (which is intermediate).

3. The elimination from the intermediate of a proton charge.

An Overview of the Electrophilic Substitution of Benzene

Now, let us have a quick look at benzene. Benzene is a highly flammable chemical that has a sweet odour, it is either a colourless or light yellow liquid, and is found in a liquid from room temperature, which evaporates instantaneously into the air, and has the molecular formula C6H6. 

Now, coming to the electrophilic substitution of benzene. Here, the hydrogen atom that is found in benzene gets substituted by the electrophile, because that is exactly what electrophilic reactions are, it displaces or substitutes the hydrogen atom. Here it substitutes the hydrogen atom of benzene. There is no fixed pattern of these reactions, it occurs randomly. Also, the aromaticity of benzene is not disrupted in the reaction.

In several of the reactions of compounds containing benzene rings – the arenas, electrophilic substitution occurs. Aromatic nitration, aromatic halogenation, aromatic sulfonation, and Friedel-Crafts reaction alkylation and acylation are some of the most important electrophilic aromatic substitutions.

What is the Electrophilic Substitution Reaction of Benzene?

As per the chemical reactivity of benzene compared to that of alkenes in the preference order to addition reactions, substitution reactions occur. These reactions are generally referred to as electrophilic aromatic substitution because the reagents and conditions used in these reactions are electrophilic. The catalysts and co-reagents are used to produce the powerful electrophilic species required to perform the initial substitution step.

Experiments have shown that substituents on a benzene ring may have a profound effect on reactivity. As determined by molecular dipole moments, this activation or deactivation of the benzene ring against electrophilic substitution can be associated with the electron-donating or electron-withdrawing effect of the substituents.

The second element that becomes important in substituted benzene reactions concerns the position at which electrophilic substitution takes place.

General Mechanism of the Electrophilic Substitution Reaction of Benzene?

A two-step process-the addition of the electrophile, followed by deprotonation-is the mechanism of electrophilic aromatic substitution.

A significant feature of this process is that if we know the product since it is the atom or group that replaces the H+, we can define the electrophile. Conversely, we can predict the product’s structure if we know the electrophile. The catalyst’s job is to bond with the leaving group and make it a better group to leave. 

A more thorough study includes substitution reactions of compounds having an antagonistic orientation of substituents. The symmetry of the molecule would again simplify the decision if the substituents are similar. If a substituent has a pair of non-bonding electrons usable for adjacent charge stabilization, the product deciding power would usually be exercised.

Three steps are involved in the electrophilic substitution reaction mechanism.

Step 1: Electrophile Generation

In the generation of electrophiles from the chlorination, alkylation, and acylation of an aromatic ring, anhydrous aluminium chloride is a very helpful Lewis acid. Electrophile production takes place due to the presence of Lewis acid. The electron pair from the attacking reagent is accepted by the Lewis acid. The resulting electrophiles are Cl+, R+, and RC+O respectively (from the combination of anhydrous aluminium chloride and the attacking reagent).

Step 2: Formation of carbocation

The electrophile, forming a sigma complex or an arenium ion, attacks the aromatic ring. One of the hybridized carbons in this ion of uranium is sp3. This arenium ion, in a resonance structure, finds stability. The sigma complex or the arenium ion loses its aromatic character since the delocalization of electrons stops at the sp3 hybridized carbon.

Step 3: Deprotonation

Deprotonation is the third step of electrophilic substitution. Deprotonation is the reaction’s driving force, making it energetically possible to proceed. This step’s activation energy is much lower, and the reaction happens very rapidly.

Examples of Electrophilic Substitution Reaction

Some examples of electrophilic aromatic substitution include nitration and halogenation of benzene. The electrophiles are nitronium ion (NO2+) and sulphur trioxide (SO3), and they react with benzene individually to provide nitrobenzene and benzene sulfonic acid, respectively.

1. Benzene Sulfonation

Benzene sulfonation is a method of fuming sulphuric acid (H2SO4 + SO3) to heat benzene to create benzene-sulfonic acid. In nature, the reaction is reversible.

2. Benzene Nitration

Via the protonation of nitric acid by sulfuric acid, the source of the nitronium ion induces the loss of a water molecule and the creation of a nitronium ion.

3. Benzene Halogenation

In the presence of Lewis acid, such as FeCl3, FeBr3, Benzene reacts with halogens to form aryl halides. This reaction is known as benzene halogenation.

4. Sulfuric Acid Activation of Nitric Acid

The first step in benzene nitration is to activate HNO3 with sulfuric acid to create a nitronium ion, a stronger electrophile.

Enantiomers are molecules that exist in two forms that are mirror images of one another but cannot be superimposed. Enantiomers are also known as enantiomorphs. Since the object and its mirror image are similar, an object with a plane of symmetry cannot be an enantiomer.

Enantiomers are chemically similar in any other way. The direction in which enantiomers rotate polarised light when dissolved in solution, either Dextro (d or +) or Levo (l or -), is what distinguishes them as optical isomers. When two enantiomers are present in equal proportions, they form a racemic mixture, which does not rotate polarized light because the optical activity of each enantiomer cancels out the optical activity of the other.

As we already discussed enantiomers definition now will study what are enantiomers and enantiomers examples in detail.

Physical Properties of Enantiomorph

1. Physical properties such as melting point, boiling point, infrared absorptions, and NMR spectra are usually similar between enantiomers.

2. However, although the enantiomer’s melting point and other properties would be identical to those of the other enantiomer, the melting point of a mixture of the two enantiomers varies.

3. This is due to the fact that intermolecular interactions between opposite enantiomers between R and S enantiomers can vary from those between like enantiomers between two molecules with both R and S stereochemistry.

4. Chiroptical techniques, the most popular of which is optical rotation, are the only physical techniques that can differentiate between a compound’s two enantiomers.

5. The sign and magnitude of the torsional angles, as well as the bond lengths and angles, determine the chiroptical properties of a molecule, with the sign of the torsional angles being the only distinction between enantiomers.

Enantiomorph Structure 

Consider how chirality is formed when a tetrahedrally coordinated atom is bound to four separate substituents, as shown below.

• Stereoisomers, which are non-superimposable mirror copies of one another, were first introduced as enantiomers.

• Any molecule that cannot be superimposable on its mirror image and hence exists as a pair of enantiomers is said to be chiral. Any molecule that can be superimposable on its mirror image, on the other hand, is achiral.

• Two enantiomers are possible if a molecule contains a single atom that is tetrahedrally bound to four separate substituents.

• It is important, however, that the four substituents are distinct from one another because if any two of them are the same, the structure would become superimposable on its mirror image, and therefore achiral. A stereogenic core, or simply a stereocenter, is an atom that is bound to four separate atoms.

• In contrast to chirality, which is a property of the molecule as a whole that cannot be localised around one atom or a group of atoms, a stereocenter is a property of the molecule as a whole that can be localised around one atom or a group of atoms.

• The existence of a stereocenter is not needed for chirality in a molecule; rather, it is the most common cause of chirality.

Enantiomers Examples

Dextro lactic acid and laevo lactic acid, whose chemical structures are shown below, are an example of a pair of enantiomers.

Given below Is the Enantiomers Examples:

• S- and R-methyl chlorophenoxy propionic acid are the names of these isomers (often abbreviated to MCPP and referred to as mecoprop). This compound is thought to be a combination of S- and R-enantiomers, with the R- enantiomer having herbicidal properties. As a result, this substance is often used as a herbicide.

• It’s worth noting that, unlike cis and trans isomers, almost all pairs of enantiomers share physical properties including solubility and melting point. They are suspected, however, to rotate light in opposite directions (both the enantiomers of a compound must be optically active).

Did You Know?

Chiral recognition is the process of distinguishing between a chiral molecule’s two enantiomers. It is difficult to distinguish enantiomers from one another since the physical properties that are commonly used to distinguish molecular species are similar. Physical variations can only be found by encounters with a discriminating secondary species.

1. Chirality is the structural basis of enantiomerism.

2. Enantiomers are molecules that exist in two forms that are mirror images of one another but cannot be superimposed.

3. Enantiomers are chemically similar in any other way. The direction in which enantiomers rotate polarised light when dissolved in solution determines whether they are Dextro (d or +) or Levo (l or -) rotatory, hence the term optical isomers.

4. Since the optical activity of each enantiomer is cancelled by the other, a racemic mixture is formed when two enantiomers are present in equal proportions and do not rotate polarised light.