In this part of the experiment we needed to determine if the unknown (XX) contained the any of the three isomers, para-bromonitrobenzene,ortho-bromonitrobenzene or meta-bromonitrobenzene. The solvent that was chosen to be used was 9:1 Hexane: ethyl acetate solution since it is a polar solution that won’t affect Rf values of the different isomers as much. As mentioned above, in the meta-bromonitrobenzene (M) TLC plate, the unknown (XX) sample spot does not have the same distance from the base line as the reference spot, therefore it does not contain this isomer, however it does contain the ortho-bromonitrobenzene and para-bromonitrobenzene since the Rf values are very similar with minimal error. The ortho-bromonitrobenzene reference and sample spot value were both found to be 0.356 and the para-bromonitrobenzene reference (0.
587) and sample spot (0.609) were very close to each other. The reason there may have been a difference between the two numbers is due to contamination. Using the areas under the peak of the graphs, we determined the absorption of each isomer.
These values were then divided by the sum of the area between the peaks of both graphs to determine the percent absorbance. The percent absorption of the para-bromonitrobenzene was found to be 13299 and the percent absorption of the ortho-bromonitrobenzene was found to be 7319. The para-bromonitrobenzene percent absorption was then used as the y value in the calibration curve equation for para-ortho isomers to solve for the percent mole of each isomer. The values of the para-bromonitrobenzene, ortho-bromonitrobenzene were found to be 65.77% and 34.23%, respectively.
An error that occured in this section was the images taken of the TLC plates were not completely vertical and straight. Since they were not completely vertical, this could have affected the way ImageJ interpreted them, so the values are not fully certain. Errors:One source of error in this lab is human error.
In some cases the line indicating the level of solvent absorption was not drawn fast enough which results in flawed Rf values since it dries very quickly and becomes very difficult to see. One solution to this is to have a line drawn by a device to measure it accurately. Another source of error in this lab is in the experiment B and C where multiple TLC plates were kept in the same small jar at the same time. If the plates touched each other, this would result in cross-contamination and would skew the results of each TLC plate affected.
