Isomerism: Types and Structural Isomerism

• Isomers are two or more organic compounds sharing the same molecular formula and molecular weight, but differing in physical and chemical traits.
• Berzilius named isomerism

Types of Isomerism

• Structural Isomerism
• Stereoisomerism

Structural Isomerism

• Chain Isomerism
• Position Isomerism
• Ring Chain Isomerism
• Functional Isomerism
• Metamerism
• Tautomerism

Chain Isomerism (CI)

• Compounds possess the same molecular formula, functional group, functional group position, multiple bond or substituent, but vary in carbon chain arrangement, leading to distinct parent names.
• Butane (4C) and 2-Methyl propane (3C)
• 1-Butene (4C) and 2-Methyl-1-propene (3C)
• 1-Butanol (4C) and 2-Methyl-1-propanol (3C)
• Pentanoic acid and 3-Methyl butanoic acid and 2,2-Dimethyl propanoic acid

Position Isomerism (PI)

• Compounds share a molecular formula, functional group, and carbon chain, yet differ in functional group, multiple bond, or substituent placements.
• But-1-ene and But-2-ene
• 1-Butanol and 2-Butanol
• 1-Chlorobutane and 2-Chlorobutane

CI and PI Examples

• Butane and Isobutane (C4H10) are chain isomers.
• Pentane, 2-Methyl butane, and 2,2-Dimethylpropane (C5H12) are chain isomers.

C6H14 Isomers

• Hexane
• 2-Methyl pentane
• 3-Methyl pentane
• 2,3-Dimethyl butane
• 2,2-Dimethyl butane

Isomer Relationships

• a-b, a-c, c-d, b-d represent chain isomers.
• b-c and d-e represent position isomers.

C3H6Cl2 Isomers

• 1,1-Dichloropropane
• 1,3-Dichloropropane
• 2,2-Dichloropropane
• 1,2-Dichloropropane
• All four are position isomers because the chlorine atom's position varies across structures.

Aromatic Isomers

• C8H10 has 4 aromatic isomers.
• C6H4X2 and C6H4XY each yield 3 aromatic isomers.
• C6H3X3 results in 3 aromatic isomers, while C6H3XYZ yields 10.

Structures as Chain Isomers

• CH3-CH2-CH=CH2
• structures like CH≡CH

Ring Chain Isomerism (RCI)

• Ring chain Isomerism involves compounds sharing a molecular formula but differing in their mode of linking (open chain or closed chain) of carbon atoms.
• They share a molecular formula, classifying them as ring chain isomers.

Isomer Relationships

• a-b shows functional isomers.
• a-c, b-c represents Ring-chain and functional isomers.

Special Points

• Alkenes with cycloalkanes and alkynes (alkadienes) with cycloalkenes exhibit ring-chain isomerism.
• Ring-chain isomers are also functional isomers.

Functional Isomers

• a,b represents functional isomers.
• a,c and b,c denote ring-chain and functional isomers;
• a,d represents Ring-chain and functional isomers.
• c,d denotes Only chain isomers
• b,d represents Ring-chain and functional isomers.

Functional Isomerism

• Compounds share a molecular formula but differ in functional groups.
• Alcohols and ethers
• Aldehydes and ketones
• Acids and esters
• Cyanides and isocyanides
• Nitro compounds and nitrites
• Keto and enol forms
• Amides and oximes
• Primary, secondary, and tertiary amines
• Alcoholic and phenolic compounds
• Alkyl halides typically do not display functional isomerism.

Metamerism

• Compounds share a molecular formula and a polyvalent functional group, yet differ in the alkyl groups attached to said group.
• Polyvalent functional groups involves more than one valence.

Metamerism Examples

• CH3-O-CH2-CH2-CH3 and CH3-CH2-O-CH2-CH3
• CH3-CH2-NH-CH2-CH3 (N-Ethyl ethanamine) and CH3-NH-CH2-CH2-CH3 (N-Methyl propanamine)
• 2-Hexanone and 3-Hexanone are metamers and position isomers.
• 2-Hexanone and 2-Methyl-3-pentanone represents Metamers, chain and position isomers

Tautomerism/Desmotropism

• Introduced by "Laar," also called desmotropism, meaning "bond turning".
• Tautomers share a molecular formula but have different structural formulas because the active hydrogen moves between two atoms.
• Kryptomerism, allotropism, or dynamic isomerism refers to tautomerism.

Tautomerism Conditions

• Carbonyl compounds need at least one α-H to exhibit tautomerism.
• Tautomers are also functional isomers in dynamic equilibrium, indicated by ⇌ .
• The H-atom shifts along with the pi bond.
• Tautomers exist in dynamic equilibrium.
• The number of electrons and lone pairs remains consistent between tautomers.
• Occurs solely in liquid and gaseous phases, not solids.
• Catalysis possible via acids or bases.
• "Note that tautomers are also functional isomers."
• Example tautomerism reaction: CH3-C=O-CH3 (keto) ⇌ CH2=C-OH-CH3 (enol)

Stereo Isomerism

• Stereoisomers
• Configurational
• Conformational

Configurational

• (non-interconvertible resolvable)
• Geometrical diastereomers
  • (non-mirror image Geo. isomers)
• Optical Isomers
  • Enantiomers
    • (Mirror-image stereoisomers)
  • Optical
    • (non-mirror image optical isomers)

CONFORMATIONAL ISOMERISM definition

• It's when Stereo isomers interconvert with each other easily, especially at room temperature.

CONFIGURATIONAL ISOMERISM definition

• Defined by stereo isomers that do not interconvert at room temperature but possess unique characteristics

Geometrical isomerism detail

• Stereo isomers unable to interconvert at room temperature given restricted rotation are known as geometrical isomers.

Stereoisomers detail

• Stereo isomers that cannot be superimposed due to chirality are known as optical isomers.

Defining Geometrical Isomerism

• Alkenes, oximes, and azo compounds display geometrical isomers due to restricted double bond rotation.
• Cycloalkanes exhibit geometrical isomers stemming from constrained single bond rotation.

Geometrical Isomers in Alkenes

• Restricted rotation about double bond is caused by overlapping of p-orbital.

Geometrical isomer condition

• Alkenes possessing differing atoms or groups on sp² carbons display geometrical isomerism.
• When identical groups occupy the same side, it's termed "cis"; opposite sides indicate "trans".

Physical Properties of Cis-Trans Isomers

• Cis isomers exhibit higher dipole moments due to combined dipoles.
• Cis isomers holds higher boiling points due to greater dipole moments.
• Cis isomers are more soluble in H2O thanks to increased polarity.
• Transisomers melt at higher temperatures influenced by symmetrical arrangements.
• Trans isomers are generally more stable due to reduced van der Waals strain.

Assign E or Z configuration.

• Assign priorities based on atomic number.
• In case of ties, move along the chain until the first difference is found.
• Double/triple bonds are treated as if they were single-bonded to duplicate/triplicate atoms.

Priority Rules

• High priority = HP and Low priority = LP
• Rule I show priority is proportional to atomic number of atom which is directly attached to sp² carbon.
• Rule II demonstrates that if rule-I fails, then consider the next atom.
• Rule III highlight if multiple bond is present then consider them as

Isomerism

• Oximes exhibiting geometrical isomerism need two distinct groups linked to the sp² carbon.

Geometrical isomers in azo compounds

• Cycloalkanes display geometrical isomers arising from restricted rotation about single bonds, evident when two carbons bear differing groups.

Geometric Isomer Calculation

• For R1 ≠ R2, the count of geometric isomers = 2n (n being double bond #).
• With R1 = R2, isomers = 2^(n-1) + 2^(p-1) when n is even, or p = (n + 1) / 2 if odd.

Optical Isomerism

• Some materials can rotate polarized light's plane, called optical activity.

Optical Activity

• Light vibrates in 'ether' particles; ordinary light vibrates in all directions, but polarized light vibrates in one plane.

Polarimeter

• Polarimeters measures rotation angle.

Asymmetric carbon definition

• If carbon bonds to 4 unique atoms/groups it is chiral, marked by *.

bromo chloro iodo methane definition

• Molecule having chiral carbon.

Finding optical isomers

• Total optical isomers = 2^n (n = chiral centers)
• Mirror-image stereoisomers called enantiomers [distinct effects]
• enantiomers rotate PPL to equal extents, opposite-wise

Important!

• (R) & (S) have nothing to do w/ rotation direction by themselves!

Properties of enantiomers

• Optical isomerism is when compounds have multiple chiral carbons

Elements of Symmetry

• Plane of symmetry, Centre of symmetry and n-fold alternating axis of symmetry

A centre of symmetry

• is usually present only in an even numbered ring.

Stereoisomerism

• Two asymmetric carbon atoms in a compound that are the type Cabc-Cabc, and only exist in only three isomeric forms.

Calculating Optical Isomers

• Unsymmetrical molecule exhibit : Optical isomers =2n; No meso forms
• Symmetrical, even numbered carbons: a = 2^n-1; Meso forms = 2^n/2-1
• Symmetrical w/ odd asymmetric carbons: Optical isomers = 2^(n-1) with meso forms=2^((n/2)-1))

Allenes with no asymmetric carbon

• Optically active allenes lack asymmetric carbon, and 1 atom on the corners of a tetrahedron is the only thing they can contain to be optically active.

Alkylidene definition

• Replacement of one double bond in an allene by a ring does not alter the basic geometry of the system and appropriately substituted compounds exist in optically active forms.

Racemic and Meso

• Racemic mixtures have equal enantiomers, inactive due to external compensation
• Meso molecules are inactive via internal compensation only

External compensation

• Occurs when force rotation results and is balanced.

Absolute Configuration (R, S configuration)

• The actual three dimensional arrangement of groups in a molecule containing asymmetric carbon is termed absolute configuration.

Indicate absolute configuration

• RS systems are named by 3 chemists (cahn, ingold, prelog)
• R = latin rectus (right); S = latin sinister (left)
• It is better than the D, L method system

When assigning priority

• Assign priority based on atomic number, and atoms with isotopes are prioritized by mass.

When designating groups

• Designate groups attached to the stereocenter based on atomic weights.
• Double/triple-bonded treated attached two/three atoms
• After assigning top group is written at the top, with the smallest group facing away

If The lowest priority group is not on the dash position

• Step 1: Bring the lowest priority group to dash by even simultaneous exchanges.
• Step 2: Draw an arrow from first priority group to second priority group till third priority group.
• Step 3: If the direction of arrow is clockwise the configuration is R and if anticlockwise it is S.
• Step 4: Draw the Fisher projection formula having equivalent configuration to the wedge-dash formula.

Note It

• IMPORTANT: (R) & (S) designation as nothing to do rotation sign
• Cahn-Ingold rule applies determining if chiral
• For lactic acid with R: R = laevo rotatory designated R-(-)-lactic acid

Special Point : -

• Chiral nitrogen containing tetra alkyl ammonium ion show optical isomerism.
• Chiral nitrogen containig tertiary amine do not show optical isomerism.
• Reason:- Rapid umbrella inversion.
• Chiral C containing carbanion do not show optical isomerism.
• Reason:- Rapid umbrella inversion.
• Substituted Allenes do not have chiral carbons but molecule is chiral, so show optical isomerism. Only those substituted allenes will have optically active in with "each sp² C have different atoms or group".

The different arrangement

• The different arrangement of atoms in space that result from the free rotation arround C-C bond axis are called conformations. The phenomenon is called conformational isomerism.

Conformational definition

• Here 0(60) is dihedral angle, angle between two planes.

Ethane [CH3-CH3] conformational state

• I = III (Eclipsed form): distance betw 2C-H minimum, so high repulsion

Variations of Conformers

• infinite types between eclipsed and staggered [skew forms]

Stability of these Variations

• staggering is more stable then skew.

Dihedral Angle for the Variations

• Dihedral angle in eclipsed form of ethane is 0.
• Dihedral angle in staggered form of ethane is 60.

Energy Changes

• The difference in the energy of various conformers constitutes an energy barrier to rotation.