Question

a) The reaction temperature for this lab is significantly above the boiling point of the solvent used (ethanol boils at 78 o C; the reaction mixture was heated up to 140 o C). Why are we able to conduct reactions above the boiling point of the solvent when using our microwave oven in lab?

b) Explain why hexane is a very poor choice as a solvent for a microwave reaction.

c) After developing a TLC plate, a student looks at the TLC plate under the UV lamp but cannot see any spots. What could the student have possibly done wrong? List three separate possible reasons for not being able to see any spots on a developed TLC plate.

d) After developing a TLC plate, a student looks at the TLC plate under the UV lamp but only sees large, overlapping blobs, instead of small spots. What is the likely problem, and what should the student do to obtain a better TLC?

Answer 1

(1) We are able to conduct reactions at above boiling point of solvent in microwave because it is an enclosed system, the volume of solvent to be used is low compared to conventional reaction, and with microwave temperature can be increased at a faster rate.

(2) Solvents like hexane and other less polar solvents produce no heat as they remain transparent to microwave, ε: indicates the efficiency of converting microwave energy to heat. In case of hexane value of ‘ε’ is negligible.

(3) Reason-1: Some spots need to be stained with specific reagents depends upon the nature of reaction mixture, Reason-2: In UV-Lamp there are options like near UV (315–400 nm) UVB, or middle UV(280–315 nm) UVC, or far UV (180–280 nm) need to try the various options available Reason:3: the spotting sample should have chromophore and auxochrome.

(4) Error-1: The error might be with the sample size spotted on TLC if excess sample is used to spot it spreads to large area and starts merging with the adjacent spots, resulting in a overlapping blobs. Error-2: Solvent system of Mobile phase if the reaction mixture is polar, try with various ratios of solvents like Ethylaceate/hexane, methanol/DCM, chloroform/methanol. Set the mobile phase by optimizing the solvent system.