Solvents: Replacements, Evaporation Rates, Extractions, Drying
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Solvent Replacements
To consider:
MTBE instead of Et2O
EtOAc-MeOH instead of DCM
NMP instead of DMF / DMAc
heptane instead of n-hexane
2MeTHF instead of THF, dioxane (Et2O)
Solvent benchtop evaporation rates
Rough relative vap rates and flow-vap-halftimes (2 mm) at 1 bar. From 25°C to 40°C approx. halves the vap time. For more details: see old lit (1, 2) or new pages (a):
| solvent | rate (a.u.) | halftime @ RT (a.a.) |
| acetone | 1365 | 4 min |
| butyl acetate | ?75 | 50 min |
| butyl butyrate | 38 | ~no vap @ RT |
| CHCl3 | 1000 | 5 min |
| DCM | fast | 1 min |
| EtOAc | 425 | 6 min |
| EtOH | 250 | 20 min |
| hexanes | ?400 | 6 min |
| MeCN | ?280 | 15 min |
| nBuOH | 45 | 2 h |
| toluene | 160 | 25 min |
| water | ?60 | 1 h |
Solvent Drying
How dry do you need it? some reactions are fine with bulk-dried solvents just using a slight excess of one reagent.
Do you need it dry, or do you need it free of non-innocent chemical contaminants? e.g. peroxides in Et2O/THF; methanol in DCM; O2 in solvents under air; etc... "drying" won't fix those. HPLC grade solvents and "dry" solvents (the 100€/L expensive Acroseal variety) may even contain more non-innocent chemical contaminants than ordinary non-dry solvents, due to the drying/storage/purification process. 'Impure' 90% hexane with 10% heptane impurity is better for organometallics, than 99.8% hexane with 0.2% ketones...
Bulk drying and fine drying are usually 2 separate steps. Bulk drying can often be done with Na2SO4, mol sieves, or CaCl2, and decant/filter. For fine drying, by default use mol sieves. The best drying methods per solvent are summarised here. 3A and 4A mol sieves are cheap, easy to activate (just dry in oven at 300°C overnight) and reusable (leave at back of hood in open dish to evaporate solvent overnight then chuck back in the oven: except, do not reuse after DMSO/DMF/NMP). Running solvent through activated alumina columns (solvent system Huc C2) is also a very convenient rapid fine drying process.
Extractions, Solvent Mixing, Polarity
Know extraction efficiency, miscibility, and polarity/eleutropics.