Organic Chemistry Essentials: Isomerism, Reactions & Mechanisms

Posted on Feb 8, 2026 in Chemistry

Isomerism

Isomerism. Organic compounds having the same molecular formula but different structural formulas or different physical and chemical properties are called isomers, and the phenomenon is known as isomerism.

Examples:

  • Chain isomerism
  • Position isomerism

E1 Reactions

E1 reactions. E1 is a unimolecular elimination. It follows first-order kinetics and is a two-step process. It typically requires a weak base. Formation of a carbocation intermediate takes place.

E2 Reactions

E2 reactions. E2 is a bimolecular elimination. It follows second-order kinetics and is a one-step process. It requires a strong base. No carbocation formation takes place.

Saytzeff’s Rule

Saytzeff’s rule. Saytzeff’s rule states that when more than one alkene is formed as a result of an elimination reaction, the more substituted alkene will be the major product.

Ozonolysis

Ozonolysis. Ozonolysis is a chemical reaction in which ozone is passed through an alkene in an inert solvent (for example, CCl4) to form an ozonide. Ozonides are further reacted in the presence of zinc and water to give carbonyl compounds.

Aromatic Compounds

Aromatic compounds. These are carbocyclic compounds that contain alternating single and double bonds between the carbon atoms. They can be further divided into two subtypes: benzenoid aromatic compounds and non-benzenoid aromatic compounds.

Inductive Effect

Inductive effect. When an electron-releasing or an electron-withdrawing species is introduced into a chain of atoms (generally a carbon chain), the corresponding negative or positive charge is relayed through the carbon chain by the atoms belonging to it. This causes a permanent dipole to arise in the molecule and is referred to as the inductive effect.

Define Alkenes

Define alkenes. Alkenes belong to the class of hydrocarbons containing a carbon–carbon double bond. They are also known as unsaturated hydrocarbons.

SN1 Reaction

SN1 reaction. SN1 is a unimolecular nucleophilic substitution. It follows first-order kinetics and is a two-step process. Typical reactivity order: 3° > 2° > 1°.

SN2 Reaction

SN2 reaction. SN2 is a bimolecular nucleophilic substitution. It follows second-order kinetics and is a one-step process. Typical reactivity order: 1° > 2° > 3°.

Aldol Condensation

Aldol condensation. Aldehydes or ketones having at least one α-hydrogen undergo a reaction in the presence of dilute base to form a β-hydroxyaldehyde (an aldol). In the final step, a molecule of water is removed from the aldol to form an α,β-unsaturated carbonyl compound; this elimination step is known as the aldol condensation.

Allylic Rearrangement

Allylic rearrangement. An allylic rearrangement, or allylic shift, is an organic reaction in which the double bond in an allylic chemical compound shifts to the next carbon atom. It is commonly encountered in nucleophilic substitution reactions.

Basicity of Amines

Basicity of amines. Amines are basic in nature due to the presence of a lone pair of electrons on nitrogen, and they can accept a proton. Amines are both Lewis and Brønsted–Lowry bases.

Iodoform (CHI₃)

Iodoform (CHI₃) is a pale yellow crystalline solid with a distinctive, penetrating odor. It was used historically as an antiseptic and disinfectant (for example, in wound dressings like BIPP) because it releases iodine; modern use is limited due to toxicity. Iodoform is also key in the iodoform test for methyl ketones and certain alcohols.

Aldehydes Are More Reactive Than Ketones

Aldehydes are more reactive than ketones in nucleophilic addition reactions primarily due to less steric hindrance (a small H atom versus two bulky R groups) and a weaker electron-donating inductive effect (one electron-donating group versus two), making the carbonyl carbon more electrophilic and accessible for attack by nucleophiles.

Allylic Rearrangement

Allylic rearrangement. An allylic rearrangement, or allylic shift, is an organic reaction in which the double bond in an allylic chemical compound shifts to the next carbon atom. It is encountered in nucleophilic substitution.