Functional Groups, Organic Reaction Types, and Isomerism Study of Isomerism

Introduction to Functional Groups and Chemical Families

It is possible to classify the vast number of organic compounds into a relatively small number of families based on their structures. These molecules are characterized by the presence of certain arrangements of atoms called a functional group. A functional group is defined as the part of a molecule where most of its chemical reactions occur. Effectively, it is the part that determines the chemical properties of a compound. Regardless of the size or complexity of an organic molecule, its chemistry and biology are governed by the functional group it contains. Chemically, a functional group behaves the same way in every part of the molecule it is a part of. Alkanes are noted to not really possess a functional group, as their molecules consist of single $C-C$ and $C-H$ bonds, which are equally present in almost all organic compounds. This lack of a specific functional group explains why alkanes are generally unreactive.

Systematic Classification of Functional Groups

Organic compounds are categorized into families with specific functional group structures and naming conventions. The Alkane family has the functional group $RH$ with a general formula of $RH$; a specific example is ethane ($CH_3CH_3$), known by the IUPAC name Ethane. The Alkene family is characterized by the $R_2C=CR_2$ functional group; an example is ethene ($CH_2=CH_2$), which has the common name Ethylene. The Alkyne family features the $-C≡C-$ functional group with the general formula $RC≡CR$; an example is ethyne ($HC≡CH$), commonly called Acetylene. Haloalkanes or alkyl halides contain a $-C-X$ group (where $X$ is a halogen) and follow the general formula $RX$; an example is chloroethane ($CH_3CH_2Cl$), common name Ethyl chloride.

Oxygen-containing functional groups include Alcohols, Ethers, Aldehydes, Ketones, Carboxylic acids, and Esters. Alcohols have the $-C-OH$ group and general formula $ROH$; ethanol ($CH_3CH_2OH$) is a specific example, also called Ethyl alcohol. Ethers possess the $C-O-C$ group ($ROR$); methoxymethane ($CH_3OCH_3$) is known as Dimethyl ether. Aldehydes feature the $-C-C=O$ with a terminal hydrogen ($RCHO$); ethanal ($CH_3CHO$) is commonly called Acetylaldehyde. Ketones have the $R_2CO$ arrangement; propanone ($CH_3COCH_3$) is commonly known as Acetone. Carboxylic acids contain the $-COOH$ group ($RCOOH$); ethanoic acid ($CH_3COOH$) is commonly referred to as Acetic acid. Esters display the $RCOOR$ structure; methyl ethanoate ($CH_3COOCH_3$) is also known as Methyl acetate.

Nitrogen-containing compounds and other specialized groups include Amines, Amides, and Nitriles. Amines follow the general formula $R_3N$, such as methanamine ($CH_3NH_2$), also called Methyl amine. Amides feature the $RCONR_2$ group; ethanamide ($CH_3CONH_2$) is known as Acetamide. Nitriles contain the $-C≡N-$ group ($RCN$); ethanenitrile ($CH_3CN$) is commonly termed Acetonitrile.

Homologous Series and Structural Representations

Across different functional groups, a series of compounds in which each member differs from the next member by a constant amount ($CH_2$) is called a homologous series. The individual members within such a series are referred to as homologs; examples include methane ($CH_4$), ethane ($C_2H_6$), and propane ($C_3H_8$). In organic symbols, the letter $R$ is used to represent any alkyl group. Common representations include $CH_3$ (Me), $C_2H_5$ (Et), $C_3H_7$ (Pr), $C_6H_5$ (Ph), and $C_6H_5CH_2$ (Bn).

While the molecular formula of a compound shows only the respective numbers of atoms present, the structural formula reveals the actual atom linkage, providing the basis for understanding molecular properties. Organic chemists use several methods to represent structural formulas. The dash structural formula involves drawing chains of atoms in a straight line, showing atom connectivity in two dimensions, though it does not represent the actual shape. The condensed structural formula writes formulas without showing all individual bonds; central atoms are shown with the atoms bonded to them, often using parentheses and subscripts for identical groups (e.g., $2$-propanol as $CH_3CH(OH)CH_3$ or $(CH_3)_2CHOH$). Double and triple bonds in condensed structures are often drawn as they would appear in a Lewis structure.

The bond-line formula, or line-angle formula, is the quickest method as it only shows the carbon skeleton. Bonds are lines, and carbon atoms are assumed at every intersection, start, or end of a line. While nitrogen, oxygen, and halogens are explicitly shown, hydrogen atoms are omitted unless bonded to a drawn atom, with their presence assumed based on carbon's valency. The three-dimensional formula shows the spatial arrangement of atoms. In this system, bonds projecting upward out of the plane are indicated by a solid wedge, bonds lying behind the plane are shown with a hashed wedge, and bonds within the plane are indicated by a simple line.

General Types of Organic Reactions

There are four general types of organic reactions: addition, elimination, substitution, and rearrangements. An addition reaction occurs when two reactants add together to form a single new product with no atoms left over, following the general form $A + B \rightarrow C$. This reaction is characteristic of compounds with multiple bonds. An elimination reaction is essentially the opposite of addition; under specific conditions, a single reactant splits into two products. This method is frequently used in the preparation of compounds with multiple bonds.

In a substitution reaction, one group simply replaces another group in the molecule. An example of this is the reaction between chloromethane and sodium hydroxide to produce methanol and sodium chloride: $CH_3Cl + NaOH \rightarrow CH_3OH + NaCl$. A rearrangement reaction involves the reorganization of bonds and atoms within a single molecule to form an isomer of the original reactant.

Concepts of Isomerism

Isomerism is the phenomenon where two compounds share the same molecular formula but possess different structural formulas. Isomerism is broadly divided into two categories: constitutional isomerism and stereochemical isomerism. Constitutional isomers are defined as isomers that have different connectivity between atoms. Stereochemical isomers, or stereoisomers, have their atoms connected in the same sequence (the same constitution) but differ in how those atoms are arranged in three-dimensional space.

Stereoisomers can be further classified based on rotation about a bond or their reflection in a mirror. Conformational isomers are stereoisomers that are interconvertible by rotation about a single bond. The temporary molecular shapes resulting from the rotation of groups about single sigma ($σ$) bonds are called conformations, and each possible structure is called a conformer. Conformational analysis is the study of the energies associated with a molecule as it undergoes rotation about a single bond.

Conformational Analysis of Ethane and Butane

In conformational analysis, the Newman projection is used, where the observer looks head-on at the $C-C$ bond. The three lines emanating from the center of a circle represent bonds from the front carbon. In ethane, the two unique conformations are staggered and eclipsed. The staggered conformation is more stable and favored because it maximizes the separation of electron pairs in the molecular orbitals of the $C-H$ bonds, minimizing repulsion. The staggered conformation of ethane is $11.8,kJ/mol$ (approximately $2.9,kcal/mol$) lower in energy than the eclipsed conformation, where hydrogens are closer and repulsion is greater.

For butane, rotation about the $C2-C3$ bond reveals six important conformations. There are two distinct staggered conformations: gauche and anti. In the gauche conformation, the torsional angle between the two methyl groups is $60^∘$. In the anti conformation, the methyl groups are at a torsional angle of $180^∘$. The anti conformation is more stable than the gauche conformation by an energy difference of $0.8,kcal/mol$ because the methyl groups are farther apart, resulting in less van der Waals strain. At $25,^∘C$, approximately three times as many butane molecules exist in the anti conformation as in the gauche conformation. The two eclipsed conformations of butane correspond to energy levels $14,kJ/mol$ and $24,kJ/mol$ higher than the anti-conformation minimum.

Steric Strain and Configurational Isomerism

Steric strain, also known as van der Waals strain, is the strain caused when two atoms or groups are forced closer together than their atomic radii allow. Every atom has a van der Waals radius; for a hydrogen atom, this is $120,pm$, and for a methyl group, it is $200,pm$. When non-bonded groups approach within this distance, their surrounding electrons repel each other. In butane, the methyl-methyl eclipsed conformation has significantly more strain than the methyl-hydrogen eclipsed conformation because the larger methyl groups occupy more space and cause greater electron-electron repulsion.

Configurational (geometrical) isomers occur when identical substituents are on a double bond or a ring. When substituents are on the same side, the isomer is designated as cis; when they are on opposite sides, it is designated as trans. Configurational isomers possess different physical properties. Another type of stereoisomerism involves reflection in a mirror. Enantiomers are non-superimposable mirror images, a property found only in chiral molecules. A chiral molecule is one with no plane of symmetry. Chirality typically occurs at an $sp^3$ hybridized carbon carrying four different substituents, known as a chiral center, stereogenic center, or stereocentre. While enantiomers often have similar physical properties like boiling point and solubility, diastereomers—which are non-superimposable, non-mirror image stereoisomers—have different physical properties.