Question
Sat May 31, 2008 By:

better solvent for more ionisation ;water or methyl alcohol and why?

Expert Reply
Sat May 31, 2008

The solvents are grouped into non-polar, polar aprotic, and polar protic solvents and ordered by increasing polarity. The polarity is given as the dielectric constant. The density of nonpolar solvents that are heavier than water is bolded.

Solvent Chemical Formula Boiling point Dielectric constant Density
Non-Polar Solvents
Hexane CH3-CH2-CH2-CH2-CH2-CH3 69 °C 2.0 0.655 g/ml
Benzene C6H6 80 °C 2.3 0.879 g/ml
Toluene C6H5-CH3 111 °C 2.4 0.867 g/ml
Diethyl ether CH3CH2-O-CH2-CH3 35 °C 4.3 0.713 g/ml
Chloroform CHCl3 61 °C 4.8 1.498 g/ml
Ethyl acetate CH3-C(=O)-O-CH2-CH3 77 °C 6.0 0.894 g/ml
Polar Aprotic Solvents
1,4-Dioxane /-CH2-CH2-O-CH2-CH2-O-\ 101 °C 2.3 1.033 g/ml
Tetrahydrofuran (THF) /-CH2-CH2-O-CH2-CH2-\ 66 °C 7.5 0.886 g/ml
Dichloromethane (DCM) CH2Cl2 40 °C 9.1 1.326 g/ml
Acetone CH3-C(=O)-CH3 56 °C 21 0.786 g/ml
Acetonitrile (MeCN) CH3-C≡N 82 °C 37 0.786 g/ml
Dimethylformamide (DMF) H-C(=O)N(CH3)2 153 °C 38 0.944 g/ml
Dimethyl sulfoxide (DMSO) CH3-S(=O)-CH3 189 °C 47 1.092 g/ml
Polar Protic Solvents
Acetic acid CH3-C(=O)OH 118 °C 6.2 1.049 g/ml
n-Butanol CH3-CH2-CH2-CH2-OH 118 °C 18 0.810 g/ml
Isopropanol (IPA) CH3-CH(-OH)-CH3 82 °C 18 0.785 g/ml
n-Propanol CH3-CH2-CH2-OH 97 °C 20 0.803 g/ml
Ethanol CH3-CH2-OH 79 °C 24 0.789 g/ml
Methanol CH3-OH 65 °C 33 0.791 g/ml
Formic acid H-C(=O)OH 100 °C 58 1.21 g/ml
Water H-O-H 100 °C 80 1.000 g/ml

The polarity, dipole moment, polarizability and hydrogen bonding of a solvent determines what type of compounds it is able to dissolve and with what other solvents or liquid compounds it is miscible. As a rule of thumb, polar solvents dissolve polar compounds best and non-polar solvents dissolve non-polar compounds best: "like dissolves like". Strongly polar compounds like sugars (e.g. sucrose) or ionic compounds, like inorganic salts (e.g. table salt) dissolve only in very polar solvents like water, while strongly non-polar compounds like oils or waxes dissolve only in very non-polar organic solvents like hexane. Similarly, water and hexane (or vinegar and vegetable oil) are not miscible with each other and will quickly separate into two layers even after being shaken well.

Solvents with a relative static permittivity greater than 15 can be further divided into protic and aprotic. Protic solvents solvate anions (negatively charged solutes) strongly via hydrogen bonding. Water is a protic solvent. Aprotic solvents such as acetone or dichloromethane tend to have large dipole moments (separation of partial positive and partial negative charges within the same molecule) and solvate positively charged species via their negative dipole.[7] In chemical reactions the use of polar protic solvents favors the SN1 reaction mechanism, while polar aprotic solvents favor the SN2 reaction mechanism.

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