Reagent Selection Map

Use this map when the question is not “what does this reagent do?” but “which reagent should I choose first?”

The map links transformation goals to current reagent passports and chemistry decision guides.

Start from the transformation

Goal	First candidate	Why	Check before using
Reduce aldehyde or ketone	NaBH4	Mild hydride source for common carbonyl reductions.	Functional groups that may also react.
Reduce ester, acid chloride or carboxylic acid derivative	LiAlH4	Stronger hydride reagent for less reactive carbonyl derivatives.	Dry conditions, quench planning and incompatibilities.
Oxidise primary alcohol to aldehyde	PCC	Controlled oxidation under relatively non-aqueous conditions.	Whether over-oxidation or chromium waste is a concern.
Choose SN1/SN2/E1/E2 pathway	Mechanism map	The reagent role depends on substrate, nucleophile/base and solvent.	Leaving group, sterics, heat and beta hydrogens.

Before committing to a reagent, answer:

What functional group must change?
What functional groups must survive?
Is the reagent acting by hydride transfer, oxidation, substitution or base chemistry?
Is water, oxygen or acid/base sensitivity relevant?
Can the work-up destroy remaining reagent without damaging product?

Risk	Typical source	Mitigation in notes
Over-reduction	Using a strong hydride where a mild one was enough.	Compare NaBH4 and LiAlH4.
Over-oxidation	Using aqueous strong oxidants for aldehyde targets.	Check the PCC passport.
Elimination side product	Strong or bulky base with heat and beta hydrogens.	Use the E1/E2 guide.
Rearrangement	Carbocation formation in SN1/E1 conditions.	Use the SN1 note.

Useful next reagent passports:

Pick reagents from the transformation and selectivity risk.

Then open the individual passport before writing conditions or interpreting an exam route.