Question

This is Organic Chemistry Lab, Solvent Extraction.

1. What are the criteria for solubility of organic molecules in water?

2. If methylene chloride was used as the organic solvent in solvent extraction, instead of ether, what would the top layer be in the separation step?

3. What formula was used to calculate interaction of organic compounds with the stationary phase?

4. In the given mixture (liquid-liquid), which compound (be specific) has the weakest interaction with the stationary phase? Why?

Answer 1

1. What are the criteria for solubility of organic molecules in water?

The solubility of organic compounds will be influenced by the following criteria in that order:

i. charged groups, e.g. carboxylate, phosphate

ii. functional groups that donate a hydrogen bond to water, e.g. alcohols, amines

iii. functional groups that accept a hydrogen bond from water, e.g. ketones, aldehydes, ethers

iv. groups that contribute to polarity of the compound, e.g. halides, thiols.

2. If methylene chloride was used as the organic solvent in solvent extraction, instead of ether,

what would the top layer be in the separation step?

The top layer would be aqueous layer as it is less dense than methylene chloride.

3. What formula was used to calculate interaction of organic compounds with the stationary

phase?

The separation of organic compounds is based on the different strengths of interaction of the    

compounds with the stationary phase. The stronger the interaction is, the longer the compound

interacts with the stationary phase, and the more time it takes to migrate through the column or           

on a TLC plate (=longer retention time).

Under a given set of experimental conditions, a particular compound will travel the same

distance along the stationary phase by a specific solvent (or solvent mixture) if all other factors

are kept constant.

The retention time or retention value, Rf value, is defined as the ratio of the distance moved by

the solute (i.e. the organic compound under test) and the distance moved by the the solvent

(known as the solvent front) along the plate, where both distances are measured from the

common origin, that is the point where the sample is initially spotted on the plate.

In other words, every organic substance will have a specific Rf value for every specific solvent

and solvent concentration.

Rf Value = Distance from Baseline travelled by Solute / Distance from Baseline travelled by

Solvent (Solvent Front)

4. In the given mixture (liquid-liquid), which compound (be specific) has the weakest   

interaction with the stationary phase? Why?

No specific mixture is given in the question.

Usually the less polar compounds have less interactions with the stationary phase. The more   

polar compounds have more interactions with the stationary phase and this slows down the

compound elution in a column chromatography.