Choosing between a polar protic and a polar aprotic solvent is one of the most common decisions in synthesis — and one of the most consequential. The wrong class of solvent can slow a reaction dramatically or change its outcome entirely. This guide explains the difference and how to choose.
Polar protic solvents
Protic solvents carry an O–H or N–H bond and can donate hydrogen bonds. Examples include water, methanol and isopropyl alcohol. They stabilise both cations and anions, which favours SN1-type reactions and solvolysis, but they slow SN2 reactions by solvating the nucleophile.
Polar aprotic solvents
Aprotic solvents are polar but cannot donate hydrogen bonds. Key examples include DMSO, DMF, acetonitrile and THF. They solvate cations well but leave anions “naked” and highly reactive — which is why SN2 substitutions, alkylations and many organometallic reactions run far faster in them.
How to choose
- SN2 substitution or a reactive anion? Use a polar aprotic solvent (DMSO, DMF, acetonitrile).
- SN1 / solvolysis or proton transfer needed? Use a polar protic solvent (alcohols, water).
- Organometallics (Grignard, hydrides)? Use anhydrous ethers such as THF — protic solvents will destroy the reagent.
- Easy work-up wanted? Lower-boiling solvents like acetonitrile or THF are easier to remove than DMSO or DMF.
Practical considerations
Beyond reactivity, consider boiling point, water miscibility, toxicity and disposal. Purity matters too — trace water in an “anhydrous” solvent can ruin moisture-sensitive chemistry, so check the specification and request a COA. See our guides to choosing the right solvent and chemical purity grades.
Buy laboratory solvents from Ozon Chemical
Ozon Chemical supplies a full range of polar protic and aprotic solvents — EU-registered, REACH-compliant, with COA/SDS on request and worldwide shipping.
