A functional group is a specific arrangement of atoms within a molecule that confers characteristic chemical properties. While the carbon skeleton is relatively inert, functional groups are the sites of chemical transformation. Recognizing functional groups is the single most important skill in organic chemistry.

The Major Functional Groups

Priority & Nomenclature (IUPAC)

When naming organic compounds, the principal characteristic group (highest priority functional group) determines the suffix and parent chain. The IUPAC priority order for common groups:

Stereochemistry is the study of the spatial arrangements of atoms in molecules and how these arrangements influence physical, chemical, and biological properties. Two molecules can have identical molecular formulas and connectivity yet be entirely different compounds — because their atoms are arranged differently in three-dimensional space.

Chirality and Stereocenters

A molecule is chiral if it is non-superimposable on its mirror image — like a left hand and a right hand. The most common source of chirality is a stereocenter (or chiral center): a carbon atom bearing four different substituents. The two mirror-image forms are called enantiomers.

R/S Configuration (CIP Rules)

The Cahn-Ingold-Prelog (CIP) system assigns an R or S designation to each stereocenter using priority rules based on atomic number:

Types of Stereoisomers

Maximum Stereocenters Formula

Alkanes (C?H2??2) are fully saturated hydrocarbons. Their C–C and C–H s bonds are strong and largely non-polar, making alkanes relatively unreactive — they undergo radical halogenation and combustion but little else. Alkenes (C?H2?), with their nucleophilic p bonds, are far more reactive and serve as key synthetic starting materials.

Alkane Reactions

Alkene Addition Reactions

The p electrons of alkenes are electron-rich, making them react as nucleophiles toward electrophiles. Addition reactions break the p bond and add atoms across the double bond — the carbon skeleton remains intact but gains new substituents.

Elimination Reactions

Elimination is the reverse of addition — atoms are removed across adjacent carbons to form a p bond. E2 (bimolecular, concerted) and E1 (unimolecular, stepwise) are the two main mechanisms. Zaitsev's rule predicts that the major product has the more substituted (more stable) alkene.

Aromatic compounds possess a cyclic, planar, conjugated p system with 4n+2 p electrons (Hückel's rule) that confers extraordinary thermodynamic stability — the aromatic stabilization energy. Benzene (6 p electrons, n=1) is the prototypical aromatic compound. Unlike alkenes, aromatics undergo substitution (not addition) — to preserve aromaticity.

Hückel's Rule

Electrophilic Aromatic Substitution (EAS)

EAS is the dominant reaction type for benzene and substituted aromatics. An electrophile attacks the electron-rich aromatic ring, forming a arenium ion (Wheland intermediate) that loses a proton to restore aromaticity.

Common EAS Reactions

Directing Effects of Substituents

Nucleophilic substitution reactions involve a nucleophile (electron pair donor) attacking a carbon bearing a leaving group. Two mechanisms operate depending on substrate structure, nucleophile strength, solvent, and temperature. Understanding when each mechanism operates is central to designing efficient synthetic routes.

SN2 — Bimolecular Nucleophilic Substitution

SN1 — Unimolecular Nucleophilic Substitution

Comparison Table

The carbonyl group (C=O) is the most important functional group in organic chemistry. Its polarization — with partial positive charge on carbon and partial negative on oxygen — makes the carbon an electrophile susceptible to nucleophilic attack. The carbonyl group is present in aldehydes, ketones, carboxylic acids, esters, anhydrides, amides, and acyl halides.

Nucleophilic Addition to Aldehydes and Ketones

Nucleophilic Acyl Substitution

In carboxylic acid derivatives (acyl halides, anhydrides, esters, amides), the nucleophile attacks the carbonyl carbon to form a tetrahedral intermediate, then the leaving group departs — overall substitution, not addition, because the C=O is regenerated. Reactivity order: acyl halide > anhydride > ester > amide.

Enols and Enolates

Carbonyl compounds with a-hydrogens can form enols (under acidic conditions) or enolates (under basic conditions) — tautomers in which the a-carbon becomes nucleophilic. Enolates are fundamental in alkylation, aldol reactions, Claisen condensation, and Michael additions.

Acid-base chemistry is woven throughout organic chemistry. Every proton transfer, every nucleophile-electrophile interaction, and every deprotonation of an a-carbon is an acid-base event. Mastery of the pK? scale — and the factors that raise or lower it — is essential for predicting reactivity and designing synthesis.

pK? Reference Table

Factors Affecting Acidity

Four major structural effects modulate the pK? of an organic acid:

The Equilibrium Rule

Structure determination is one of the most important skills in modern chemistry. Spectroscopic techniques use the interaction of electromagnetic radiation (or charged particles) with matter to provide information about molecular connectivity, functional groups, and stereochemistry — without destroying the sample.

Infrared Spectroscopy (IR)

IR spectroscopy measures the absorption of infrared radiation by molecular bonds undergoing stretching and bending vibrations. Each functional group absorbs at a characteristic frequency. The fingerprint region (1500–500 cm?¹) is unique to each molecule; the functional group region (4000–1500 cm?¹) is used for identification.

¹H NMR Spectroscopy

Nuclear magnetic resonance spectroscopy probes the electronic environment of hydrogen (or carbon) nuclei in a magnetic field. NMR provides: chemical shift (d, in ppm) indicating electronic environment; integration (relative number of protons); and splitting pattern (n+1 rule for spin-spin coupling).

Mass Spectrometry (MS)

In mass spectrometry, molecules are ionized (commonly by electron impact, EI, or electrospray, ESI) and the mass-to-charge ratio (m/z) of the resulting ions is measured. The molecular ion peak (M?) gives the molecular weight. Fragmentation patterns reveal structural information. Isotope patterns identify halogens: one bromine gives M and M+2 peaks of nearly equal intensity (7?Br:8¹Br ˜ 1:1).

Organic synthesis is the art and science of building molecular architecture. A skilled organic chemist designs a pathway from simple, available starting materials to a target molecule through a sequence of well-chosen reactions. Retrosynthetic analysis (introduced by E.J. Corey, Nobel Prize 1990) provides a systematic approach by working backward from target to starting material.

Retrosynthetic Analysis

In retrosynthesis, the target molecule is disconnected at strategic bonds (identified by retrosynthetic arrows: ?) to give simpler "synthons" — idealized fragments that correspond to real reagents. The disconnection approach asks: "What bonds were formed in the last step? Which functional group transformation precedes it?"

Named Reactions — Essential Toolkit

Protecting Groups