There are many organic compounds which have different chemical properties but same molecular formula as they have same different structural formula. For example, the molecular formula of ethanol (C2H5OH) and methoxymethane (CH3OCH3) is same that is C2H6O. Both compounds show different chemical and physical properties due to different structural formula with same molecular formula.
Such type of compounds is referred as isomers and the phenomenon is called as isomerism. Thus, the isomerism can be defined as phenomenon by virtue of which two or more compounds having same molecular formula can be assigned different molecular structures and isomers are molecules with the same molecular formula, but different arrangements of atoms.
Isomers can be broadly classified into two categories -
- Structural isomers or Constitutional isomers
Both types of isomers can be further classified on the basis of their structural difference and orientation in three dimensional space.
1. Structural isomers or Constitutional isomers
Constitutional (or structural) isomers have different molecular structures due to different arrangement of atoms in their molecules. They differ in the order in which the atoms are connected with each other so they contain different functional groups and or bonding patterns like branching.
For example, 1-propanol, 2-propanol and methoxyethane have same molecular formula (C3H8O) but differ in their structure. Because of the differences in their structure, they tend to give different chemical and physical properties.
There are five types of structural isomers
(a) Chain isomerism
These types of isomers are differing in their skeletons of carbon atoms; therefore arise due to the possibility of branching of carbon chain. Chain isomerism is because of the linkage of carbon atoms in molecule. Chain isomerism is also known as nuclear isomerism. Some common examples of chain isomers are as follows.
Two chain isomers with molecular formula
C4H10 : n-Butane and 2-Methylpropane
Three chain isomers of C5H12 : Pentane, 2-Methylbutane and 2,2 -dimethylpropane
(b) Positional isomers
These isomers have different positions of functional groups or multiple bonds. For example; but-2-ene and but-1-ene are positional isomers as they differ in the position of double bond. Similarly the molecular formula with C3H7Br show two position isomers which are differ in the position of bromine atom; 1-Bromopropane and 2-Bromopropane.
Derivatives of benzene also show positional isomerism which is differing in the position of functional group attached on different position on aromatic ring. For example - molecular formula C7H7Cl can show four positional isomers, differ in the position of methyl group (-CH3) and Chloro group (-Cl).
Another example of positional isomers is with molecular formula C5H12O; 1-pentanol, 2-pentanol, 3-pentanol.
(c) Functional isomerism
Isomers with different functional group belongs to different families are functional isomers of each other. For example; the molecular formula of C3H6O can show many functional isomers with different functional groups like; alkene, cycloalkane, ether, aldehyde, ketone, alcohol, epoxide.
Some other examples of functional isomers are as follows:
- Ethanol and methoxymethane
- Methyl propanoate and butanoic acid
- 2-Pentanone and 3-Methylbutanal
- Butanal, Methyl ethyl ketone, 2-Methylpropanal (isobutyraldehyde)
(d) Tautomerism (keto-enol isomers)
This is a special type of functional isomerism, in which isomers exist simultaneously in dynamic equilibrium with each other. It arises due to 1, 3-migration of hydrogen atom from one polyvalent atom to other within the same molecule. The best example of tautomerism is acetylacetone which exists in two forms; keto and enol.
These isomers are present in equilibrium and also called as tautomers. The relative abundance of tautomers depends upon their stability, for example, the enol form of acetylacetone is more stable and hence more abundant compare to keto form.
These isomers differ in structure due to difference in the distribution of carbon atoms about the functional group. Metamerism is common in ether and secondary amines. For example; ethoxyethane and
1-methoxypropane are Metamers of each other.
Similarly secondary amines with C4H11N molecular formula have two metamers;
These compounds have the same constitution and sequence of carbon bonds but differ in their relative positions of atoms or groups in space. Stereoisomers can be classified in two types;
(a) Conformational isomers
They are also called as conformers or rotamers and produced by rotation of carbon-carbon sigma bonds.
They are rapidly interconvertible into each other at room temperature, therefore cannot isolate. For example, Staggered and eclipsed form of ethane which arises due to C2-C3 sigma bond rotation.
Similarly chair and boat conformations of Cyclohexane are also arise due to rotation in carbon-carbon sigma bond.
(b) Configurational isomers
Cannot arise due to rotation of carbon- carbon sigma bond, therefore can be isolated easily. They can be classified again in two types.(i) Geometrical isomerism
These isomers differ in the spatial position around a bond due to restricted rotation. If same groups are placed on same side of double bonded carbon atom, they known as cis-isomer and Z-notation is used to represent the cis-isomer. If same groups are placed on opposite side of double bonded carbon atom, they are called as trans-isomers and shown by E-isomer. Trans and cis-isomers show different physical and chemical properties like melting point, boiling point and other chemical reactions. For example; (E)-2-butene and (Z)-2-butene.
(ii) Optical isomerism
These isomers differ in the interaction with plane polarised light. They are non-superimposable mirror image of each other and also called as Enantiomers. Those optical isomers which are not mirror images, known as diastereomers or diastereoisomers. Enantiomers are used to represent by R and S notation. The Enantiomers of 1-Chloro-ethanol are as follow.
These are isomers which have same molecular formula with different connectivity of atoms, therefore also called as structural isomers. Constitutional isomers have same molecular formula but different way of arrangement of atoms in molecule. For example, ether has carbon-oxygen-carbon bond and an alcohol with C-O-H bond, hence ether and alcohols are Constitutional isomers of each other.
Some examples of Constitutional isomers are as follows, given two molecules have same molecular formula but one has a five-carbon ring with hydroxy group (-OH), two methyl groups (-CH3) and one bromo group (-Br) while another molecule is composed of a six member carbon ring with Methoxy group (-OCH3) and bromo group (-Br).
Other examples of constitutional isomers are with molecular formula C12
O are as follow.
And with molecular formula C16
an show two constitutional isomers.
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Structural isomers are compounds with the same molecular formula but different structures formula due to different manners of linking the atoms. Structural isomers can be five types.
1. Chain isomers
Differ in carbon skeletons, for example; alkanes with formula C7H16 show nine chain position isomers and the structural formula of those isomers are as follow.
- Heptane: CH3CH2CH2CH2CH2CH2CH3
- 2-methylhexane: CH3CH(CH3)CH2CH2CH2CH3
- 3-methylhexane: CH3CH2CH(CH3)CH2CH2CH3
- 2,3-dimethylpentane: CH3CH(CH3)CH(CH3)CH2CH3
- 2,4-dimethylpentane: CH3CH(CH3)CH2CH(CH3)CH3
- 2,2-dimethylpentane: CH3C(CH3)2CH2CH2CH3
- 3,3-dimethylpentane: CH3CH2C(CH3)2CH2CH3
- 3-ethylpentane: CH3CH2CH(C2H5)CH2CH3
- 2,2,3-trimethylbutane: CH3C(CH3)2CH(CH3)CH3
2. Functional isomers
Have differed functional groups. For example; molecular formula C4H8O2 show eight functional isomer; Butanoic acid , 2-Methylpropanoic acid , Propyl formate, Methyl propanoate, Ethyl acetate, 3-Hydroxy-2-butanone, Isopropyl formate and Dioxan.
3. Position isomers
Differ in position of functional group. For example; 2-aminobutanoic acid, 3-aminobutanoic acid and 4-aminoabutanoic acid are differ in position of amino group on parent carbon chain. Similarly Phthalic acids also have three positional isomers; 1,2-benzenedicarboxylic acid, 1,3-benzenedicarboxylic acid and 1,4-benzenedicarboxylic acid.
Involve those molecules which have an unequal distribution of alkyl groups on either side of the functional group. For example in ethers with the molecular formula C4H10O represents methoxypropane (CH3OC3H7 ) and ethoxyethane (C2H5OC2H5).
Also called as keto-enol tautomerism in which a compound shows two interconvertible forms, keto form) and enolic form. These two forms continuously change into each other by the oscillation of a proton and pi-electrons.
Skeletal isomer is another name for chain isomers which differ in connectivity of carbon skeleton but have the same functional groups. For example; 1-Butanol and 2-Methylpropanol has same functional group that is alcoholic group. But 1-butanol consists of a four carbon atom parent chain in its skeleton while another molecule has three carbon atom skeletons in parent chain, hence both are skeleton isomers.
Another example of skeleton isomers is with molecular formula, C5H12, there are three skeleton isomers; pentane, 2-methylbutane and 2,2-dimethylpropane.
Stereoisomers which can be converted into one another by the rotation of a carbon-carbon sigma bond are called as conformational isomers. They are usually represented by using Newman projection. The rotation of carbon- carbon signal bond forms different conformations like eclipsed, gauche and anti form.
In eclipsed form the same groups are located in same line with one another, however in gauche form identical groups located at 60° with respect to each other. The anti form which is also called as staggered form the identical groups are placed at 180° from one another. Hence these conformers can be inter convert into each other by rotation of carbon-carbon single bond. For example, all these three forms of butane are as follow.
Conformers are not considered as a true isomers as they have same structures but with different energy due to the difference in electronic environment in various orientations of molecule. Different conformers of butane at various bond angle of carbon-carbon bond are as follow.
Another best example of conformational isomers is ethane molecule (C2H6). In ethane molecule the rotation of carbon-carbon single bond arise different conformers of ethane. There are three possible conformers of ethane, eclipsed, staggered and skew form. Out of these forms, the staggered form has minimum energy and maximum stability and eclipsed has maximum energy with minimum stability.
The dihedral angle in eclipsed form is minimum that is 0° only, therefore the bonds on both carbon atoms cover each other. Due to less dihedral angle, the bonded hydrogen atoms are very close to each other which make the molecule unstable. In staggered conformer, this repulsion is very less due to high dihedral angle; therefore it is more stable than eclipsed conformation.
The energy barrier between staggered and eclipsed conformer is around 12.6 kj/mol , therefore both conformers can be interconvert at room temperature only. Cyclic compounds can also show conformational isomerism like Cyclohexane (C6H12). Cyclohexane is a planer ring with twelve pairs of eclipsed carbon-hydrogen bonds which can be destabilized by eclipsing interactions or torsion strain. Since all the carbon atoms in Cyclohexane are sp3 hybridized, therefore the bond angle must be 109°45’. But it is a planer hexagon ring so the C−C−C angles must be 120° not 109°45’.
Due to this some angle strain arises in molecule and planer gets distorted which creates two conformers; chair and boat form. In chair Cyclohexane, there are two types of carbon-hydrogen bonds. Six parallel carbon-hydrogen bonds are perpendicular to the surface and called as axial bonds while other set of six bonds are in the plane of the ring and known as equatorial bonds. The flipping of carbon-carbon bond converts axial form to equatorial form. The rotation of carbon-carbon bond forms four conformers of Cyclohexane.
- Chair conformation: In this conformation, all bonds are in staggered form, make molecule most stable puckered conformation.
- Twist boat or skew boat conformation: This conformation exists in between chair and boat conformation and more stable than boat but less stable than chair conformation.
- Half chair conformation: This is highly unstable and strained conformation of Cyclohexane.
- Boat conformation: This is less stable conformation than chair form by 6.8 kcal/mole is due to flagpole interaction of hydrogen atoms.
Stereoisomers which have different three dimensional relationship of bonded group about one or more atoms are known as configurational isomers. These do not readily inter convert at room temperature and can be separated.
Configurational isomers can be further subdivided into two types
- Geometrical or cis-trans isomers
- Optical isomers
Stereoisomers which have the same molecular formula and same connectivity with different orientation of groups or atoms across a double bond are known as geometrical isomers and phenomenon is called as geometrical isomerism. Geometrical isomerism is generally observed in alkenes or substituted cylcoalkanes due to restricted rotation of group around double bond or cyclic ring. Like in alkenes, both double bonded carbon atoms are sp2 hybridized carbons, and consists of sigma and pi bonds. Since the pi bond formed due to side way overlapping of unhybridised p-orbitals, it locates perpendicular to sigma bond and restricted the carbon-carbon free rotation.
If the double bonded carbon atoms bonded with different groups, they may exhibit a pair of configurational isomers called as geometrical isomerism and designated as geometrical isomers. The necessary condition for geometrical stereoisomerism is that each carbon of the double bond must have two different groups. Hence in given structural formulas, example-2 and 3 will show geometrical isomerism as example 1 has same group (A) on one of the double bonded carbon atom.
For the nomenclature of geometrical isomers, there are two possible naming systems. One is E/Z system and another is cis/trans naming. If there are two same groups bonded on both sides of the double bonded carbon atom, then cis and trans is used. Cis notation used for same side and trans notation is used for opposite sides.
In case of different groups bonded on double bonded carbon atom, the E/Z system of nomenclature has to use. Z notation came from German word Zusammen and used when the higher priority groups are orientated on the same side across the double bond. The priority of group determine by using the Cahn-Ingold-Prelog rules. The E notation came from a German word Entgegen and is used when the higher priority groups are orientated on different sides across the double bond.
Hence cis-trans nomenclature use for simple alkene while E/Z notation is used for substituted alkenes.
Unlike optical isomers, geometrical isomers show different physical and chemical properties like melting point and boiling point. The trans-isomers have high melting point due to symmetrical structure and cis- isomers show high boiling point due to strong intermolecular force of attraction between molecules.Cis Trans Isomers
Cis-trans isomers are the substances which have same molecular formula with different arrangement of their atoms in space. There are two main requirements for a molecule to show cis-trans isomerism.
- Presence of a C=C bond or cyclic ring in the molecule
- Different groups bonded on double bonded carbon atoms
There are many organic compounds like alkene, cycloalkanes and bio molecules like glucose, show cis-trans isomerism. In cis-isomer, same groups bonded on carbon atoms are positioned on same side while in trans-isomer, they located on opposite side. The best example of cis-trans isomers is 2-butene. In this alkene each double-bonded carbon atom has two different groups and can exhibited cis- and trans- isomers.
There is no free rotation due to the presence of carbon-carbon double bond as the rotation would require about 66 kcal/mol of energy which is not available under normal conditions.
Another example of cis-trans isomers is cis- and trans-1,2-dimethylcyclopropane
due to restricted rotation around cyclic ring.
Some other examples of cis-trans isomers are as follow.
1. cis-trans Dichloroethene
2. Maleic and fumaric acid
3. Cis and trans isomers of 9-octadenoic acid
4. Cis-trans isomers of1,2-Dibromocyclopentane
Molecules which have the same molecular and structural formulae, but they cannot be superimposed on the other, hence non-superimposable mirror image of each other are known as optical isomers and the phenomenon is called as optical isomerism. These non-superimposable mirror images are called as Enantiomers. There are some essential conditions for optical isomerism.
- The molecule must be asymmetrical in nature.
- There must be no plane of symmetry in molecule.
- There must be at least one chiral carbon atom in molecule. A chiral carbon or asymmetric carbon atom is bonded with four different groups in a molecule.
For example, the mirror image of bromochlorofluoroiodomethane is non-superimposable on each other. If we replace -iodo group with -bromo group, there will be no chiral carbon atom in molecule and mirror images of molecule will be identical to each other. Hence previous molecule will be chiral and optical active.
Optical isomers show similar chemical and physical properties but differ in their optical properties. They rotate the plane of polarization of plane-polarized light in opposite direction and also classified according to that only.
- An enantiomer which can rotate the polarized light clockwise is called as dextrorotatory enantiomer and represents by‘d’ or ‘(+)’ notation.
- Similarly another enantiomer which can rotate the polarized light anticlockwise is known as levorotatory and represents by ‘l’ or ‘(-)’ notation.
- A mixture containing equal concentrations of both enantiomers is optically inactive due to external compensation and called a racemic mixture or racemate.
- Some compounds contain more than one chiral centre but optically inactive due to an internal plane of symmetry are known as meso compound.
- Optical isomerism is a type of configurational isomerism as these isomers differ in their three-dimensional arrangement of atoms or groups bonded to a chiral centre.
- Optical isomers which are not mirror images of each other but consist of chiral carbon atom called as Diastereomers.
For example, 2,3,4-trihydroxybutanal consists of two chiral carbon atoms, therefore can exhibits four stereoisomers (2n, where n is the number of chiral carbon atom). Out of these four forms, there are four diastereomers pairs and two enantiomer pairs.
The nomenclature of optical isomer is assigned as R or S by using the Cahn–Ingold–Prelog (CIP) sequence rules. The first rule is to assign the priorities to the four groups or atoms bonded to the chiral atom. The group with lowest atomic number is given lowest priority and numbered as 4. The group or atom with highest atomic numbered as 1. According to second step the lowest priority (4) group sights down the bond from carbon. Finally, connects atoms from 1 to 2 to 3 by an arrow. If the arrow moves clockwise, the molecule is named as an R enantiomer and counterclockwise movement will be an S enantiomer.
The molecular formula of C5H10 contains two less hydrogen atoms than pentane (C5H12) hence it must either contain a double bond or one ring like in an alkene or cycloalkane. Hence chain, functional and position isomerism is possible.
Various possible isomers are as follow:
- 1,2- Dimethylcyclopropane
Out of these molecules, (a) and (b) are Position isomers and (c), (d), (e) are chain and position isomers of each other.
Isomers of C5H12
C5H12 is the molecular formula of alkane and shows three chain or skeleton isomers; pentane, 2-Methylbutane and 2,2-dimethylpropane.
Isomers of C4H8
C4H8 is a general formula of alkene and show four isomeric alkene, 1-Butene, 2-Butene and 2-Methylpropene. 1-Butene and 2-Butene are position isomers of each other as the position of double bond is differ in both molecule. 2-Methylpropene and 1-butene are chain isomers of each other. 2-Butene can further show geometrical isomerism and exhibits in cis-2-butene and trans-2-butene.
There are five structural isomers of C6H14
- Hexane (CH3CH2CH2CH2CH2CH3): Six carbon atoms in a straight chain.
- 2-Methylpentane (CH3CH(CH3)CH2CH2CH3): One methyl group as side branch on the second carbon atom of a five-carbon parent chain.
- 3-Methylpentane(CH3CH2CH(CH3)CH2CH3): One methyl group as side chain on the third carbon atom with a five-carbon parent chain.
- 2,3-Dimethylbutane (CH3CH(CH3)CH(CH3)CH3): One methyl branch on the second carbon and one methyl group on the third carbon with of a four-carbon parent chain.
- 2,2-Dimethylbutane (CH3C(CH3)2CH2CH3): Two methyl groups as side chain on the second carbon with of a four-carbon parent chain.