Introduction
A functional group is a specific atom or group of atoms within a molecule responsible for its characteristic chemical behaviour, largely independent of the rest of the (hydrocarbon) structure it is attached to. This chapter surveys the major oxygen- and nitrogen-containing functional groups, giving each the same core treatment: structure, naming, preparation, and characteristic reactions.
1. Alcohols, ethers, and related oxygen compounds
Alcohols (R–OH) are polar due to the electronegative oxygen and capable of hydrogen bonding, giving them notably higher boiling points than hydrocarbons of similar size, and can be classified as primary, secondary, or tertiary depending on how many carbon groups are attached to the carbon bearing the –OH. Ethers (R–O–R′) lack the O–H bond, so they cannot hydrogen-bond with each other (though they can accept hydrogen bonds from other molecules), making them relatively unreactive and useful as solvents.
2. Aldehydes and ketones
Both contain the carbonyl group (C=O), a polarised double bond that makes the carbonyl carbon electrophilic and attractive to nucleophiles. Aldehydes have the carbonyl carbon bonded to at least one hydrogen (making them generally more reactive and more easily oxidised); ketones have it bonded to two carbon groups. Their signature reaction type is nucleophilic addition to the carbonyl carbon (in contrast to the substitution reactions of carboxylic acid derivatives, below), because there is no leaving group attached to displace.
3. Carboxylic acids and their derivatives
Carboxylic acids (R–COOH) are notably acidic (Chapter 9, Chapter 11) because their conjugate base (carboxylate) is resonance-stabilised across both oxygens. Their derivatives — esters, amides, acid chlorides, anhydrides — all contain the same carbonyl carbon but with different groups replacing the –OH, and undergo nucleophilic acyl substitution: a nucleophile adds to the carbonyl carbon, then a leaving group departs, restoring the carbonyl — the general reactivity order among derivatives (acid chlorides most reactive, amides least) follows directly from how good a leaving group each attached group makes.
4. Amines and nitrogen-containing groups
Amines (R–NH₂, R₂NH, R₃N) are basic (the nitrogen lone pair accepts a proton, Chapter 9) and nucleophilic (the same lone pair attacks electrophilic centres, such as carbonyl carbons in acyl substitution). Their basicity is influenced by the same inductive and resonance effects met throughout this Paper: alkyl groups (electron-donating) increase basicity, while aromatic rings (which delocalise the nitrogen lone pair into the ring, reducing its availability) decrease it — aniline is markedly less basic than an aliphatic amine for exactly this reason.
5. Preparation, properties, and conversions
Each functional group can typically be prepared from, and converted into, several others (alcohols oxidise to aldehydes/ketones and further to carboxylic acids; carboxylic acids convert to their various derivatives; nitro groups reduce to amines) — building a mental “conversion map” between functional groups, rather than memorising each preparation and reaction in isolation, is the most efficient way to master this material, since it mirrors how real multi-step organic synthesis problems are actually solved.
Key Takeaways
- Alcohols (hydrogen-bonding, polar) and ethers (non-hydrogen-bonding to themselves) both feature an oxygen but differ sharply in physical properties due to O–H versus C–O–C structure.
- Aldehydes and ketones share the electrophilic carbonyl group and react via nucleophilic addition; carboxylic acid derivatives instead react via nucleophilic acyl substitution because they possess a leaving group.
- Carboxylic acid acidity and amine basicity both trace back to resonance and inductive effects on the relevant conjugate species.
- Functional groups interconvert via well-defined oxidation, reduction, and substitution pathways; mastering these conversions as an interconnected map, rather than isolated facts, is key to solving synthesis problems.