Functional Groups

Functional groups are the chemically reactive centres of organic molecules, governing reactivity, selectivity, binding affinity, and pharmacokinetic behaviour in equal measure. In medicinal chemistry, the choice and placement of functional groups is the primary determinant of whether a small molecule will bind its target with sufficient potency, survive metabolic clearance, achieve adequate bioavailability, and avoid toxicity. The most prevalent functional groups in approved drugs include amides, amines, hydroxyls, carboxylic acids, halogens (especially fluorine and chlorine), nitriles, esters, and sulfonamides. Each functional group contributes a distinct pharmacological profile characterised by hydrogen-bond donating and accepting capacity, lipophilicity, ionisation state, and metabolic vulnerability. Amide bonds are among the most common structural motifs in pharmaceuticals, with 117 out of the top 200 small-molecule drugs by retail sales in 2023 containing at least one amide bond. Amine groups confer basicity, enabling salt formation and facilitating both ionic and hydrogen-bond interactions with acidic binding-site residues, while hydroxyl groups contribute solubility, hydrogen bonding, and sites for Phase II glucuronidation or sulfation metabolism. Fluorine, present in approximately 20% of marketed drugs, confers increased metabolic stability and lipophilicity, as well as high electronegativity that allows it to modulate pKa, conformational preference, and binding interactions with biological targets. Nitriles act as compact, polar isosteres for carboxylic acids and amides, improving cell permeability without sacrificing target engagement. The strategic deployment of functional groups, guided by structure-activity relationship data and physicochemical property optimisation, remains the central intellectual skill of the practising medicinal chemist.

Every common functional group is both the product and the starting material of a defined suite of named reactions that form the backbone of synthetic organic chemistry and drug synthesis. The amide bond, the functional group most frequently encountered across approved therapeutics, is constructed through activation of carboxylic acids with coupling reagents (HATU, HBTU, DCC) followed by nucleophilic addition of an amine, a process consolidated within the general framework of the Schotten-Baumann reaction for large-scale processes and the Staudinger synthesis for β-lactam amides. The coupling of an amine with a carboxylic acid to form an amide bond is the most popular chemical reaction used for drug discovery, though computational analysis reveals that amines and acids can in principle couple via hundreds of hypothetical yet plausible transformations. The hydroxyl group participates in the Fischer esterification (acid-catalysed condensation of carboxylic acids and alcohols), the Swern oxidation (mild conversion of alcohols to aldehydes or ketones using oxalyl chloride and DMSO), and the Mitsunobu reaction (inversion of a stereocentre through coupling of an alcohol with an acidic pronucleophile under azodicarboxylate/phosphine conditions). Carboxylic acids are generated by the Jones oxidation (chromic acid-mediated conversion of primary alcohols or aldehydes), the Grignard carboxylation (reaction of an organomagnesium bromide with CO₂ followed by acidic workup), and nitrile hydrolysis under acidic or basic conditions, the nitrile itself accessible by the Sandmeyer reaction (diazotisation of anilines and cyanation via copper cyanide). Reductive amination is among the most used reactions in parallel library synthesis for decorating nitrogen pharmacophores. The Beckmann rearrangement converts oximes derived from ketones into amides (or lactams from cyclic ketones), providing an alternative route to this privileged functional group in complex molecule settings. The Buchwald-Hartwig amination installs aryl amines through palladium-catalysed C-N bond formation between aryl halides and primary or secondary amines, enabling the systematic SAR exploration of amine functional groups across aromatic scaffolds.

Photoredox catalysis and electrochemical synthesis have fundamentally expanded the chemistry accessible from common functional groups in drug-like molecules, enabling transformations under mild conditions that complement, and in several key areas surpass, what is achievable by conventional thermal chemistry. Modern developments in photoredox catalysis and electrochemistry have resulted in a renaissance of radical chemistry for (het)arene functionalisation, with radicals exhibiting orthogonal reactivity relative to nucleophiles and electrophiles, meaning functional groups such as alcohols and amines that are often challenging for nucleophilic or electrophilic transformations are well tolerated in radical mechanisms. In the photochemical context, amine functional groups are converted into carbon-centred radicals under visible-light irradiation through single-electron oxidation of the nitrogen lone pair, generating iminium or α-aminoalkyl radicals that participate in deaminative cross-coupling, C-H functionalisation, and remote 1,5-hydrogen atom transfer (HAT) events. The oxidative generation of electrophilic amidyl radicals from amide functional groups, followed by 1,5-H-atom transfer, achieves remote fluorination, chlorination, thioetherification, cyanation, and alkynylation, delivering useful building blocks that can be further elaborated. Carboxylic acid functional groups are exploited in photoredox-catalysed decarboxylation to generate carbon radicals for C-C bond formation, and photoredox-mediated amide synthesis from aldehydes and nitroarenes offers a metal-free route to this most prevalent drug functional group without stoichiometric coupling reagents. Visible-light-induced, metal-free and photocatalyst-free C-H fluorination of heteroarenes has been achieved using N-F fluorinating reagents under blue LED irradiation, enabling late-stage modification of medicinally relevant molecules via site-selective installation of the fluorine functional group. In electrochemistry, anodic oxidation of amine functional groups generates nitrogen-centred radicals and iminium ions that are deployed in Shono-type α-C-H functionalisation of cyclic amines, enantioselective cross-dehydrogenative C-N bond formation, and selective conversion to nitriles. Electrochemical C(sp³)-H fluorination of unactivated C-H bonds using Selectfluor as both fluorine source and mediator has been demonstrated on complex natural products, including sclareolide and protected L-valine, proceeding through a radical chain mechanism initiated by anodic oxidation. Hydroxyl functional groups are deployed in electrochemical deoxyfluorination sequences, first electrochemical or enzymatic oxidation of a C-H bond to an alcohol, followed by fluorination using deoxyfluorinating agents, providing a practical late-stage route for metabolic hotspot blocking in lead optimisation without requiring de novo synthesis of fluorinated analogues. The convergence of photoredox and electrochemistry in photoelectrocatalysis further extends this toolbox: photoelectrochemical iron(III) catalysis enables versatile direct C-H fluoroalkylations of biorelevant heterocycles including xanthines, nucleobases, and nucleosides, proceeding through ligand-to-metal charge transfer-induced formation of fluoroalkyl radicals under paired light and electrochemical activation. Together, these emerging methods permit the systematic modification of functional groups directly within advanced drug candidates, transforming functional groups from static structural features into dynamic synthetic handles for the rapid, targeted exploration of chemical space.