Answered: Based on the TLC analysis, did the…

Chemistry

10th Edition

ISBN:9781305957404

Author:Steven S. Zumdahl, Susan A. Zumdahl, Donald J. DeCoste

Publisher:Steven S. Zumdahl, Susan A. Zumdahl, Donald J. DeCoste

Chapter1: Chemical Foundations

Section: Chapter Questions

Problem 1RQ: Define and explain the differences between the following terms. a. law and theory b. theory and...

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SEE PHOTO: Effectiveness of separation. Based on the TLC analysis, did the isolated benzoic acid
appear pure relative to the starting mixture? Did the isolated naphthalene look pure?
How could the purity of the isolated materials be improved?

Expert Solution

Step 1: Thin layer chromatography

Thin layer chromatography or TLC is a separation technique that is used to separate and purify the different components in a mixture. When it comes to separating and purifying compounds, thin-layer chromatography (TLC) uses the same principles as extraction. It involves separating compounds based on their solubility in two different phases, where one is a mobile liquid solvent and the other is a stationary solid with a high surface area. In TLC, the stationary phase is made up of finely divided adsorbent, such as silica (SiO₂) or alumina (Al₂O₃), which is applied in a thin layer (roughly 0.25 mm thick) on a supporting material like a sheet of glass or metal foil. On the other hand, the mobile phase is composed of a volatile organic solvent or a mixture of solvents.

When compounds are moved through a solvent, it is called elution. The solvents used for this process are known as eluting solvents. The strength of an eluting solvent is determined by how strongly it is adsorbed onto the adsorbent. Since typical adsorbents are highly polar, the eluting strength increases with solvent polarity. As a result, low-polarity compounds can be eluted with low-polarity solvents, while higher-polarity compounds require solvents of higher polarity.

The traveling of a component in a TLC can be quantified using the Rf value.

$R subscript f equals fraction numerator d i s tan c e space t r a v e l l e d space b y space t h e space c o m p o u n d over denominator d i s tan c e space t r a v e l l e d space b y space t h e space s o l v e n t space f r o n t end fraction$

In general, the absorptivity of compounds increases with increased polarity. This means that the more polar a compound is, the stronger it binds to the adsorbent. When a compound is strongly bound to the adsorbent, it moves up the TLC plate more slowly. Non-polar compounds move up the plate most quickly (higher R_f value), while polar substances travel up the TLC plate slowly (lower R_f value) or not at all.

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