Solvents, Recrystallization, and Melting Point

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Presentation transcript:

Solvents, Recrystallization, and Melting Point

Intermolecular Forces Hydrogen bonding Acceptor/donor Dipole/dipole Dipole/induced dipole Induced dipole/induced dipole Also called London dispersion forces VDWs Forces

General Principles If molecules are about the same mass H-bonding > Dipole-dipole > Induced dipole Induced dipole is weakest interaction, but as surface area increases, cumulative effect can overcome stronger IMF Strategy for predicting relative boiling points

Solvents and Boiling Points   Ether (diethyl ether) Dichloromethane (methylene chloride) acetone methanol Tetrahydrofuran (THF) 35 oC 40 oC 56 oC 65 oC 66 oC hexanes Ethyl acetate (EtOAC) ethanol acetonitrile water 69 oC 77 oC 78 oC 82 oC 100 oC   toluene Pyridine (pyr) Acetic acid Dimethyl formamide (DMF) Dimethylsulfoxide (DMSO) 111 oC 115 oC 118 oC 153 oC 189 oC  

H-Bond Donor, Acceptor, Neither, or Both   Ether (diethyl ether) Dichloromethane (methylene chloride) acetone methanol Tetrahydrofuran (THF) 35 oC 40 oC 56 oC 65 oC 66 oC hexanes Ethyl acetate (EtOAC) ethanol acetonitrile water 69 oC 77 oC 78 oC 82 oC 100 oC   toluene Pyridine (pyr) Acetic acid Dimethyl formamide (DMF) Dimethylsulfoxide (DMSO) 111 oC 115 oC 118 oC 153 oC 189 oC  

Polar Protic, Polar Aprotic, Nonpolar   Ether (diethyl ether) Dichloromethane (methylene chloride) acetone methanol Tetrahydrofuran (THF) 35 oC 40 oC 56 oC 65 oC 66 oC hexanes Ethyl acetate (EtOAC) ethanol acetonitrile water 69 oC 77 oC 78 oC 82 oC 100 oC   toluene Pyridine (pyr) Acetic acid Dimethyl formamide (DMF) Dimethylsulfoxide (DMSO) 111 oC 115 oC 118 oC 153 oC 189 oC  

Solubility “Like dissolves like” Based on IMF

Solubility Rules of Thumb Water soluble if one H-bond donor per 3C Water soluble if ionic and less than 8C Water soluble if small, highly polar

Lab 2a: Purification of Biphenyl by Recrystallization Question: What solvent can be used for an effective recrystallization of biphenyl? How well does recrystallization work as a purification?

Applying Solvents and Intermolecular Forces Purification: separation of target compound from impurities Characterization Identity: Do I have what I think I have? Purity: How pure is the compound?

Recrystallization Purification for solids Useful for Large samples Final purification step Based on differential solubility

Practical Aspects Impure Solid  Dissolve Recrystallize Filter www.whfreeman.com/mohrig4e then watch the Chapter 15 video of recrystallization from mixed solvent

Recrystallization: Theory IMF in solids Temperature dependence Equilibrium process in slow crystal formation

Important Points Choice of solvent Mixed solvents Maintaining hot solvent Safety when boiling Hot filtration Inducing crystallization Washing crystals Too much/too little solvent Oiling out

Melting Point Physical property to characterize substance Identity purity capillary Melt-temp www.whfreeman.com/mohrig4e then watch the Chapter 14 video of packing a capillary tube for melting point

Theory and Application Pure crystals and impure crystals Intermolecular Forces Lattice energy Application Depressed MP Broad MP Mixed MP Eutectic mixture

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