Question

Possible Quiz Questions 1 What two methods of visualizing spots on a TLC plate will you use in this Give two reasons why would you predict that the elution solvents (hexanes or acetate) would not be visible under UV visualization? week's lab? ethyl 2. Define R 3. Calculate the R of the spot on the following TLC plate. 4. Which compound in each pair is predicted to have the higher Ri? a. CH3CH2CH2CH2OH and CH3CH2CH2CH28r b. CH3CH2CH2CH2OH and CH3CH2CH2CH2CH2CH2CH3 c. CH3COOH and CH3COOCHs 5. Unknown compounds A and B have different Ris. What can you conclude about their identities (Same compound, different compounds, can't tell)? Unknown compounds A and B have the same R on TLC. What can you conclude about their identities (Same compound, different compounds, can't tell)? 6. Ibuprofen has one stereogenic center. What would you observe on TLC if a racemic mixture of both enantiomers were spotted? 7. Drawn below are two TLC plates of a mixture of compounds A and B. One TLC plate was developed in CH2Ch and one was developed in hexane. Which TLC plate corresponds to which developing solvent? Explain.

Answer 1

1.Method used for visualizing spots on TLC (i) Iodine vapour (ii) UV light. Elution solvent(hexane at Ethyl acetate) would not be visible under UV visualization because (i) They can evaporate at temperature. (ii) Both the solvent one UV inactive. \r\n 2 R_r = Distance moved by the substance fream base lime/Distance moved by the solvent fream base lime \r\n (3) R_r = x/y x rightarrow Distance moved by substance y rightarrow moved by solvent. \r\n (4) (a) CH_3 CH_2 CH_2-CH_2-pre (b) CH_3-CH_2 CH_2-CH_2-CH_2-CH_2 CH_3 (C) compound will travel more distance in TLC. so x will be more and R_f will be high. \r\n 5. A and B have different Rf's mean they are different compound. A and B have same Rf'sthen we can't say they are same. \r\n 6.Enontiomery cann't separate in The. Because they have same Rf.(exactly same) \r\n 7. Hexane is non-polar solvent where as CH_2Cl_2 is a polar solvent. In non polar solvent substance will have lower ref than polar solvent. when two substances moved more distance in TLe plate they will separate out.