Most reagent questions are really workup questions. Read the second step before you commit to a product.

Table 1. Common organic chemistry reagents and what they do
ReagentWhat it doesWorks onWatch out for
NaBH4Mild reduction to an alcoholAldehydes, ketonesWill not touch esters or carboxylic acids
LiAlH4Strong reduction to an alcohol or amineEsters, acids, amides, nitrilesViolent with water — quench carefully
DIBAL-HPartial reduction, stops at the aldehydeEsters, nitrilesNeeds low temperature (−78 °C)
PCCOxidation that stops at the aldehyde1° alcoholsAnhydrous — water pushes it to the acid
H2CrO4 (Jones)Full oxidation1° → acid, 2° → ketone3° alcohols do not react
mCPBAEpoxidation; Baeyer–Villiger insertionAlkenes, ketonesMigration follows the more substituted group
OsO4syn dihydroxylationAlkenesToxic — usually catalytic with NMO
O3Ozonolysis — cleaves the double bondAlkenes, alkynesWorkup decides: Me2S → aldehyde, H2O2 → acid
BH3·THF, then H2O2/NaOHHydroboration–oxidationAlkenesAnti-Markovnikov and syn; no rearrangement
Hg(OAc)2/H2O, then NaBH4Oxymercuration–demercurationAlkenesMarkovnikov with no carbocation rearrangement
H2, LindlarPartial reduction to the cis alkeneAlkynesPoisoned catalyst stops at the alkene
Na / NH3(l)Dissolving-metal reduction to the trans alkeneInternal alkynesRadical anion mechanism
SOCl2Swaps OH for ClAlcohols, carboxylic acidsInverts configuration at the carbon
PBr3Swaps OH for Br1° and 2° alcoholsAlso inverts
NBSRadical bromination at the allylic or benzylic positionAlkenes, arenesLow Br2 concentration avoids addition
LDABulky strong base — gives the kinetic enolateKetones, estersUse −78 °C for kinetic control
RMgX (Grignard)Delivers a nucleophilic carbonCarbonyls, epoxides, CO2Destroyed by any acidic proton, including water
R2CuLi (Gilman)1,4-conjugate additionEnones, acyl halidesSofter than a Grignard, so it adds 1,4 not 1,2

Where a reagent has a workup step, the outcome usually depends on it — read the second line of the question before choosing the product.