Organic Chemistry – Basic Principles (Isomerism)

Isomerism

Isomerism: Isomerism is the phenomenon where two or more chemical compounds have the same molecular formula but differ in the arrangement of their atoms in space or in the connectivity of their atoms. These different compounds are called isomers.

Significance: Isomers often exhibit different physical and chemical properties, which is critical in understanding the behavior of organic molecules.

Two Main Types: Isomerism is broadly divided into two main categories:

1. Structural Isomerism (Constitutional Isomerism): Isomers differ in the connectivity of atoms (i.e., the order in which atoms are bonded).
2. Stereoisomerism: Isomers have the same connectivity but differ in the spatial arrangement of their atoms.

Structural Isomerism

Definition: Structural isomers have the same molecular formula but differ in the sequence in which their atoms are bonded.

Types of Structural Isomerism:

1. Chain Isomerism: Isomers differ in the arrangement of the carbon skeleton (straight chain vs. branched chain).

Example: $C_4H_{10}$ can be butane ($CH_3CH_2CH_2CH_3$) or isobutane ($CH_3CH(CH_3)CH_3$).

2. Position Isomerism: Isomers differ in the position of a functional group or a substituent on the same carbon skeleton.

Example: Propanol ($C_3H_8O$). Propan-1-ol ($CH_3CH_2CH_2OH$) and Propan-2-ol ($CH_3CH(OH)CH_3$).

3. Functional Isomerism: Isomers have the same molecular formula but different functional groups.

Example: $C_2H_6O$ can be Ethanol ($CH_3CH_2OH$, an alcohol) or Dimethyl ether ($CH_3OCH_3$, an ether).

4. Metamerism: Occurs in compounds belonging to the same homologous series having the same functional group, but differ in the nature of the alkyl groups attached to the functional group (e.g., ethers, thioethers, secondary and tertiary amines, esters).

Example: $C_4H_{10}O$ (ethers): Diethyl ether ($CH_3CH_2OCH_2CH_3$) and Methyl propyl ether ($CH_3OCH_2CH_2CH_3$).

5. Tautomerism: A special type of functional isomerism where isomers exist in equilibrium with each other, and they readily interconvert, usually by the migration of a proton.

Example: Keto-enol tautomerism in aldehydes and ketones.

Acetone: $CH_3COCH_3$ (keto form) $\rightleftharpoons$ Prop-1-en-2-ol: $CH_2=C(OH)CH_3$ (enol form)

Stereoisomerism

Definition: Stereoisomers have the same molecular formula and the same connectivity of atoms, but differ in the spatial arrangement of their atoms or groups.

Two Main Types:

1. Geometric Isomerism (Cis-Trans Isomerism):
• Occurs in compounds where there is restricted rotation around a bond, such as in alkenes (due to the $C=C$ double bond) or cyclic compounds (due to the ring structure).
• Cis Isomer: Similar groups are on the same side of the double bond or ring.
• Trans Isomer: Similar groups are on opposite sides of the double bond or ring.

Example: But-2-ene ($CH_3CH=CHCH_3$). Cis-but-2-ene and Trans-but-2-ene.

2. Optical Isomerism:
• Occurs in compounds that are chiral, meaning they are non-superimposable on their mirror images.
• Chiral Center: Usually arises from a carbon atom bonded to four different atoms or groups (a stereocenter or asymmetric carbon atom).
• Enantiomers: A pair of optical isomers that are mirror images of each other. They have identical physical properties (except for their interaction with plane-polarized light) but rotate the plane of polarized light in opposite directions.
• Diastereomers: Stereoisomers that are not mirror images of each other.

Example: 2-Chlorobutane ($CH_3CH(Cl)CH_2CH_3$). The second carbon atom is bonded to H, Cl, $CH_3$, and $C_2H_5$, making it a chiral center.

Classification Of Organic Compounds

Organic compounds are classified based on their carbon skeleton and the presence of specific functional groups.

Functional Group

Definition: A functional group is a specific atom or group of atoms within a molecule that determines the characteristic chemical reactions of that molecule. It dictates the class to which the compound belongs.

Classification Based on Functional Groups:

Organic compounds are broadly classified into different families based on the functional groups they contain:

• Hydrocarbons: Compounds containing only carbon and hydrogen.
• Alkanes (single C-C bonds): $C_nH_{2n+2}$
• Alkenes (one $C=C$ bond): $C_nH_{2n}$
• Alkynes (one $C \equiv C$ bond): $C_nH_{2n-2}$
• Aromatic hydrocarbons (contain benzene ring): Benzene, Toluene, etc.
• Organic Compounds Containing Halogens: Haloalkanes/Alkyl halides (R-X), Haloarenes (Ar-X).
• Organic Compounds Containing Oxygen:
• Alcohols (R-OH)
• Phenols (Ar-OH)
• Ethers (R-O-R')
• Aldehydes (R-CHO)
• Ketones (R-CO-R')
• Carboxylic Acids (R-COOH)
• Esters (R-COO-R')
• Organic Compounds Containing Nitrogen:
• Amines (R-$NH_2$, $R_2NH$, $R_3N$)
• Amides (R-CONH$_2$)
• Nitriles (R-C$\equiv$N)
• Nitro compounds (R-$NO_2$)
• Other Functional Groups: Compounds containing sulfur (thiols, sulfides), phosphorus, etc.

Homologous Series

Definition: A homologous series is a group of organic compounds having the same functional group and similar chemical properties, in which the successive members differ by a $CH_2$ group.

Characteristics:

• Members have the same functional group.
• Members have the same general formula.
• Successive members differ by a $CH_2$ group.
• Physical properties show a gradual change (e.g., increasing boiling point, melting point, density) with increasing molecular mass.
• Chemical properties are similar.
• Members can be prepared by similar methods.

Examples:

• Alkanes: $CH_4, C_2H_6, C_3H_8, C_4H_{10}, \dots$ (General formula $C_nH_{2n+2}$)
• Alcohols: $CH_3OH, C_2H_5OH, C_3H_7OH, \dots$ (General formula $C_nH_{2n+1}OH$)
• Aldehydes: $HCHO, CH_3CHO, C_2H_5CHO, \dots$ (General formula $C_nH_{2n+1}CHO$ or $RCHO$)

Classification based on Carbon Skeleton:

• Acyclic (Open Chain) or Aliphatic: Compounds with carbon chains not closed into rings.
• Cyclic (Closed Chain) Compounds:
• Alicyclic Compounds: Cyclic compounds without aromatic character (e.g., cyclopropane, cyclohexane).
• Aromatic Compounds: Cyclic compounds with specific ring structures exhibiting aromaticity (e.g., benzene and its derivatives).

Nomenclature Of Organic Compounds

The IUPAC system provides a systematic and internationally recognized method for naming organic compounds.

The IUPAC System Of Nomenclature

Principles:

• Parent Chain: Identify the longest continuous chain of carbon atoms containing the principal functional group.
• Numbering: Number the parent chain starting from the end nearer to the principal functional group or the first point of difference for substituents.
• Functional Group Suffix: The principal functional group determines the suffix (e.g., -ol for alcohol, -al for aldehyde, -one for ketone, -oic acid for carboxylic acid). The '-e' of the parent alkane name is dropped and the suffix is added.
• Substituents: Other groups attached to the parent chain are named as prefixes and their positions indicated by numbers.
• Alphabetical Order: When multiple substituents are present, they are cited in alphabetical order before the parent name.
• Prefixes for Multiple Identical Groups: Prefixes like di-, tri-, tetra- are used for identical substituents. For complex substituents, bis-, tris-, tetrakis- are used, with the substituent name in parentheses.
• Omit Spaces: Spaces are omitted between the substituent names/numbers and the parent name.

IUPAC Nomenclature Of Alkanes

Steps:

1. Longest Chain: Find the longest continuous chain of carbon atoms.
2. Numbering: Number the chain to give the lowest numbers to the alkyl substituents.
3. Name Substituents: Identify and name the alkyl groups (methyl, ethyl, propyl, etc.).
4. Assemble the Name: Write the position number, then the substituent name, followed by the parent alkane name. Use prefixes (di-, tri-) for multiple identical substituents.

Examples:

• Butane ($CH_3CH_2CH_2CH_3$)
• Isobutane (2-methylpropane): $CH_3CH(CH_3)CH_3$
• Neopentane (2,2-dimethylpropane): $C(CH_3)_4$
• 3-Methylpentane: $CH_3CH_2CH(CH_3)CH_2CH_3$

Nomenclature Of Organic Compounds Having Functional Group(s)

Priority Order of Functional Groups: When multiple functional groups are present, one is chosen as the principal functional group, determining the suffix. The general order of priority is:

Carboxylic acid > Ester > Amide > Aldehyde > Ketone > Alcohol > Amine > Alkene/Alkyne > Haloalkane.

Naming Steps:

1. Identify the principal functional group and the parent chain containing it.
2. Number the chain to give the principal functional group the lowest possible number.
3. Name other substituents and functional groups as prefixes in alphabetical order.

Examples:

• Alcohols: Parent alkane name + -ol (e.g., Ethanol, Propan-1-ol).
• Aldehydes: Parent alkane name + -al (e.g., Ethanal, Propanal).
• Ketones: Parent alkane name + -one (e.g., Propanone, Butan-2-one).
• Carboxylic Acids: Parent alkane name + -oic acid (e.g., Ethanoic acid, Propanoic acid).
• Compounds with Multiple Functional Groups:

3-Hydroxybutanal (Alcohol as substituent, Aldehyde as principal group)

2-Oxopropanoic acid (Ketone as substituent, Carboxylic acid as principal group)

Nomenclature Of Substituted Benzene Compounds

Benzene: The parent compound is benzene ($C_6H_6$).

Monosubstituted Benzene: The substituent name is prefixed to 'benzene' (e.g., Chlorobenzene, Nitrobenzene, Benzaldehyde, Benzoic acid). Common names like Toluene ($C_6H_5CH_3$) and Phenol ($C_6H_5OH$) are also accepted IUPAC names.

Disubstituted Benzene:

• Positions are indicated by numbers (1,2-, 1,3-, 1,4-) or prefixes:
• ortho (o-): 1,2- relative positions.
• meta (m-): 1,3- relative positions.
• para (p-): 1,4- relative positions.
• Example: o-dichlorobenzene, m-xylene, p-nitrotoluene.

Polysubstituted Benzene:

• Numbering starts from the carbon atom attached to the principal functional group or substituent with the highest priority.
• Substituents are listed in alphabetical order with their locants.

Example: 2,4,6-Trinitrotoluene (TNT).

Versatile Nature Of Carbon (Nomenclature from Carbon And Its Compounds)

The vastness of organic chemistry is a direct consequence of carbon's unique bonding capabilities. Systematic nomenclature is essential for navigating this complexity.

Nomenclature Of Carbon Compounds

Systematic Naming: The IUPAC system ensures that every distinct organic compound has a unique name, allowing for clear communication among chemists worldwide.

Foundation: The system is built upon identifying the parent hydrocarbon chain (based on length), assigning numbering based on functional group priority and substituent positions, and then assembling the name using prefixes (for substituents) and suffixes (for principal functional groups).

Reflecting Versatility:

• Catenation: The naming system explicitly accounts for chains, branches, and rings, allowing us to describe the diverse structures formed by carbon's catenation ability.
• Multiple Bonds: Suffixes like '-ene' for double bonds and '-yne' for triple bonds are incorporated into the parent name.
• Functional Groups: A wide array of suffixes and prefixes are used to name the diverse functional groups that carbon can form, each conferring specific properties and reactivity.
• Stereochemistry: Nomenclature also includes ways to describe stereoisomers (cis/trans, R/S), reflecting the 3D arrangement of atoms crucial for many organic molecules.

Conclusion: The IUPAC nomenclature system is intrinsically linked to understanding carbon's versatile bonding nature, providing a framework to identify, classify, and communicate knowledge about the immense world of organic compounds.