...
uhplcs logo 2023

Comparison Of The Polarity Of Organic Solvents

Comparison Of The Polarity Of Organic Solvents

Table of Contents

Synthetic chemistry is inseparable from solvents, such as reaction media, HPLC column purification as eluent, extraction, etc. TLC spot plates require us to configure the unfolding agent, which then requires us to understand the nature and polarity of the various solvents.

Polarity

Polarity is the degree to which the entire molecular charge is separated. The greater the separation, the greater the polarity. So usually, molecules containing electron-absorbing groups such as N, O, and halogens will be more polar, but it should be noted that chloroform is more polar than dichloromethane because it has extra electron-absorbing chlorine. Still, carbon tetrachloride is less polar than chloroform because it is a symmetrical structure, and the polarity of a compound is determined by the functional groups in the molecule and the molecular structure. The effect of the group on the polarity of the substance should be related to the structure of the substance, and it cannot be said that the group on which the polarity increases or decreases should be analyzed as a whole.

Alkanes are the least polar because the C and H in them are about the same electronegativity and the charge separation is insignificant.

Olefins have double bonds and are more polar than alkanes. Some books say that double bonds are electron-absorbing, so charges are separated. I think this is only explained in terms of the end effect, and theoretically, it could be due to the super conjugation effect formed by the double bond with the surrounding C-H bond causing the electrons to be biased towards the double bond.

A discussion of the relationship between functional group conversion and polarity

736

The details are summarised below.

The following diagram shows the order of polarity of mixed organic solvents (from small to large, the proportion of the mixture is indicated in parentheses) cyclohexane-ethyl acetate (8+2) → chloroform-acetone (95+3) → benzene-acetone (9+1) → benzene-ethyl acetate (8+2) → chloroform-ethyl ether (9+1) → benzene-methanol (95+5) → benzene-ethyl ether (6+4) → cyclohexane-ethyl acetate (1+1) → chloroform-ethyl ether (8+2) → azoxymethane-methanol (99+1) → benzene-methanol (9+1) → chloroform-acetone (83+15) → benzene-ethyl ether (4+6) → benzene-ethyl acetate (1+1) → chloroform-methanol (95+3) → chloroform-acetone (7+3) → benzene-ethyl acetate (3+7) → benzene-ethyl ether (1+9) → ether-methanol (99+1) → ethyl acetate-methanol (99 +1) → benzene – acetone (1+1) → chloroform-methanol (9+1)

Strong polar solvents: methanol > ethanol > isopropanol Medium polar solvents: ethyl cyanide > ethyl acetate > chloroform > methylene chloride > ethyl ether > toluene Non-polar solvents: cyclohexane, petroleum ether, hexane, pentane

Organic solvent polarity table

Compound namePolarityViscosityBoiling pointAbsorption wavelength
i-pentane030
n-pentane00.3336210
Petroleum ether0.010.330~60210
Hexane0.060.3369210
Cyclohexane0.1181210
Isooctane0.10.5399210
Trifluoroacetic acid0.172
Trimethylpentane0.10.4799215
Cyclopentane0.20.4749210
N-heptane0.20.4198200
Butyl chloride10.4678220
Trichloroethylene10.5787273
Carbon terachloride1.60.9777265
Trichlorotrifluoroethane1.90.7148231
i-propyl ether2.40.3768220
Toluene2.40.59111285
p-xylene2.50.65138290
Chlorobenzene2.70.8132
o-dichlorobenzene2.71.33180295
Ethyl ether2.90.2335220
Benzene30.6580280
Isobutyl alcohol34.7108220
Methylene chloride3.40.4440245
Ethylene dichloride3.50.7884228
n-butanol3.72.95117210
n-butyl acetate4126254
n-propanol42.2798210
Methyl isobutyl ketone4.2119330
Tetrahydrofuran4.20.5566220
Ethyl acetate4.300.4577260
i-propanol4.32.3782210
Chloroform4.40.5761245
Methyl ethyl ketone4.50.4380330
Dioxane4.81.54102220
Pyridine5.30.97115305
Acetone5.40.3257330
Nitromethane60.67101330
Acetic acid6.21.28118230
Acetonitrile6.20.3782210
Aniline6.34.4184
Dimethyl formamide6.40.92153270
Methanol6.60.665210
Ethylene glycol6.919.9197210
Dimethyl sulfoxide7.22.24189268
Water10.21100268
463

Commonly used solvent mixes

1) Ethyl acetate/hexane: commonly used in 0 to 30% concentrations. However, removing the solvent completely on a rotary evaporator is sometimes difficult. 

2) Ether/pentane system: concentrations of 0-40% are more commonly used. Very easy to remove on a rotary evaporator. 

Ethanol/hexane or pentane: 5 to 30% is more suitable for strongly polar compounds. 

Dichloromethane/hexane or pentane: 5~30%, may be considered when other solvent mixtures fail. 

3) Pour 1 to 2 mL of the selected solvent system into the unfolding cell and place a large piece of filter paper in the cell. 

4) Spot the compound at the marked baseline. The spotter we used was purchased. In addition, the spotter can also be pulled off a heated Pasteur pipette (you can refer to UROP). When following the reaction as it proceeds, spot the starting reactant, the reaction mixture, and the mixture of the two. 

5) Unfold: Allow the solvent to unfold upwards for approximately 90% of the length of the sheet. 

6) Remove the sheet from the unfolding cell and immediately mark with a pencil where the solvent reaches the front. Calculate the value of Rf from this. 

7) Allow the solvent to evaporate from the sheet.

8) Observe the sheet using a non-destructive technique.

The best non-destructive method is to use an ultraviolet lamp for observation. Place the sheet under the UV lamp and mark all points with UV activity with a pencil. Although this method is not used in 5.301, we will use another commonly used non-destructive method – staining with iodine.

9) destructively observe the sheet.

This method can only be used when the compound has no UV activity. Several very useful stains are provided in 5.301. To use the stain:

  1. Please pick up a dry, thin plate with tweezers and place it into the stain, submerging it from the baseline to the solvent front.
  2. Dry the back of the plate with a paper towel.
  3. Place the plate on a heating plate and observe the changes in the spots.
  4. Remove the plate from the heating plate before the spots become visible and the background color fails to cover the spots. 

10) Modify the choice of the solvent system based on the initial thin-layer chromatography results. If you want to make the Rf larger, make the solvent system more polar; if you want to make the Rf smaller, make the solvent system less polar. If the spot sample on the plate becomes a streak rather than a circle, your sample concentration may be too high. Dilute the sample and perform another thin-plate chromatography; if this still does not work, consider changing the solvent system.

Share:

How Long Does HPLC Take
Uncategorized

How Long Does HPLC Take ?

High-Performance Liquid Chromatography (HPLC), formerly referred to as high-pressure liquid chromatography, is a versatile and powerful technique widely used in analytical chemistry to separate, identify,

Read More »

Can’t Get Enough?

Get More Information and Quotation
for Our HPLC Column Hardware Today !

Subscribe for exclusive offers and updates on new arrivals

Seraphinite AcceleratorOptimized by Seraphinite Accelerator
Turns on site high speed to be attractive for people and search engines.