Isomerism (structural and stereo) bonding

Isomerism occurs when two or more chemical compounds with dissimilar molecular formulas have a common molecular formula. The term is derived from the Greek terms isos, meaning “equal,” and meros, meaning “portion,” and was created in 1830 by Swedish physicist Jacob Berzelius.

Isomerism occurs when the atoms in a molecular formula are organized differently, resulting in isomers with distinct physical and chemical properties.

Isomerism is classified into structural isomerism and stereoisomerism, each of which has subtypes.

Structure Isomorphism: Homologous Series

Isomers differ structurally in the manner and order in which their constituent atoms are bonded. The term “constitutional isomerism” is commonly used to allude to this.

Three types of structural isomerism exist: positional isomerism, functional group isomerism, and chain isomerism.

Isomers In Environments: Homologous series

Positional isomerism, alternatively referred to as position isomerism, refers to isomers with the same functional groups but are arranged differently along the same carbon chain.

C6H4Br2 has three isomers: 1,2-dibromobenzene, 1,3-dibromobenzene, and 1,4-dibromobenzene. The bromine atoms are positioned differently on the cyclic structure between these isomers.

Dibromobenzene for example, the molecule C3H8O has two isomers: 1-propanol, sometimes referred to as n-propyl alcohol, and 2-propanol, usually referred to as isopropyl alcohol. Between these isomers, the position of the hydroxyl group on the carbon chain varies.

Functional Group isomers

When atoms form various functional groups, this phenomenon is referred to as functional group isomerism.

C2H6O is composed of two isomers: dimethyl ether and ethanol or ethyl alcohol, including an ether group (–O–) and a hydroxyl group (–OH).

Chain isomers

Chain of isomers Isomerisms differ in how their carbon chains are organized, which might be branched or straight.

For instance, n-pentane, 2-methyl butane or isopentane, and 2,2-dimethylpropane or neopentane are three isomers of C5H12.

n-Pentane, 2-methyl butane, and 2,2-dimethylpropane all exhibit chain isomerism.

Stereoisomerism

Stereoisomers are isomers that share the same atoms and bonds but have distinct spatial orientations. Stereoisomers are these isomers, which get their name from the Greek word stereos, which means “solid.”

Stereoisomerism is classified into two types: conformational isomers and configurational isomerism, with the latter divided into optical and geometric isomerism.

Conformational Isomerism

Stereoisomers may be changed into one another by conformational isomerism by rotating around one or more single bonds, the bonds. These rotations provide non-superimposable atomic configurations in space. And, theoretically, the number of possible conformations for a molecule is limitless, ranging from the most stable lowest energy structure to the least stable highest energy structure. Conformers are the names given to these isomers.

When looking at ethane, C2H4, from one end along the carbon-carbon link using the Newman projection, the hydrogen atoms of one methyl group may be in one of the following conformations relative to the hydrogen atoms of the other methyl group.

When the hydrogen atoms of one methyl group are buried behind those of another, the angle between the carbon-hydrogen bonds on the front and rear carbons, referred to as the dihedral angle, may be 0, 120, 240, or 360 degrees. This is the most unstable and energy-dense state.

According to the staggered conformation, the angle between the hydrogen atoms of one methyl group and the hydrogen atoms of the other methyl group may be 60, 180, or 360 degrees. This is the least energetic and hence the most stable configuration.

The skew conformation is an intermediate condition.

Conformations of Ethane and Newman projections

Due to the overlap of electron pairs in the carbon-hydrogen bonds of the two methyl groups, ethane conformers are stable:

• They are as dissimilar as possible in terms of confirmation.
• They are as close to one another as feasible in the shrouded conformations.

Between these two conformations, the potential energy barrier is rather low, about 2.8 kcal/mole (11.7 kJ/mole). At room temperature, the molecules have a kinetic energy of 15-20 kcal/mole (62.7-83.6 kJ/mole), which is more than enough to spin freely around the carbon-carbon bond. As a consequence, no particular shape of ethane can be recognized.

The theoretical energy barrier for rotation around double carbon-carbon bonds is around 63 kcal/mole (264 kJ/mole), which corresponds to the energy required to break the bond. (For further information on geometric isomerism, see geometric isomerism.) This quantity is about three times the kinetic energy of the molecules at room temperature, preventing them from freely spinning. At temperatures more than 300 °C, molecules may generate enough thermal energy to break the connection, allowing them to freely spin around the remaining link. This allows for the reorganization of the trans-isomer into the cis-isomer and vice versa.

Configurational Isomerism

In configurational isomerism, interconversion between stereoisomers does not occur spontaneously at room temperature because it involves bond breaking and new bond creation rather than rotations around single bonds.

There are two forms of configuration isomerism: optical isomerism and geometric isomerism.

Compounds having one or more chirality centers display optical isomerism. Chiral centers are tetrahedral atoms with four unique ligands. As the chiral center, a carbon, phosphorus, sulfur, or nitrogen atom may be used.

It’s worth mentioning that the term chirality derives from the Greek word cheiros, which literally translates as “hand.”

Because optical isomers lack a center of symmetry or a plane of symmetry, they cannot be superimposed. The Greek terms enántios, which means “opposite,” and meros, which means “part,” describe enantiomers.

With two exceptions, two enantiomers have physical and chemical properties that are equal to those of other isomers.

Thus, optical isomerism refers to the revolving orientation of a polarized light plane.

When a solution of one enantiomer rotates the plane of polarized light clockwise, the enantiomer (+) is revealed. On the other hand, a solution of the other enantiomer spins the plane of polarized light in the other direction by the same angle, and so the enantiomer is designated as such (-).

Two enantiomers may be recognized while remaining indistinguishable in a chiral environment, such as the active site of chiral enzymes.

For a molecule with n chiral centers, the maximum number of stereoisomers is 2n.

Geometrically Shaped isomerases: Homologous series

Compounds with strong double bonds between carbon atoms, carbon atoms, and nitrogen atoms, or nitrogen atoms and nitrogen atoms. Cyclic compounds are rigid owing to their ring structure.

Stilbene (C14H12) has two isomers due to the presence of a carbon-carbon double bond. In one isomer, referred to as the cis isomer, the identical groups are on the same side of the double bond, but in the other, referred to as the trans isomer, they are on different sides.

The cis-trans isomers trans-stilbene and cis-stilbene are two examples.

Trans and cis are derived from the Latin words trans, meaning “across,” and cis, meaning “on this side of.”

Cis-trans isomerism occurs in cyclic carbon compounds that have an odd number of carbon atoms substituted in opposite positions, i.e., para-substituted, and is unaffected by chiral centers. The cycloalkane 1,4-dimethyl cyclohexane has two stereoisomers: cis-1,4-dimethyl cyclohexane and trans-1,4-dimethyl cyclohexane.

Two instances of geometric isomerism are trans-1,4-dimethyl cyclohexane and cis-1,4-dimethyl cyclohexane.

Stereoisomerism is impossible if one of the atoms that cannot freely rotate contains two identical groups. Why? To flip between the trans and cis isomers, the groups attached to non-spinable atoms must be switched. When two groups are identical, the switch produces the same molecule.

Geometric isomers are diastereomers or diastereoisomer stereoisomers that do not have mirror images. Among the other diastereomers are meso compounds and non-enantiomeric optical isomers.

Conclusion: homologous series

We have seen that there are structural isomers, which have the same chemical formula but vary in their bonding arrangement of the atoms. They have the same chemical formula and atomic arrangement. The only thing that separates them is the way the molecule’s groups are arranged.