Question 6: (6 Marks) Sodium borohydride is an excellent reducing agent for converting ketones into alcohols as shown in the conversion of below. (NOTE: don't worry if you have not seen this reaction before, you don't need to know anything about it to answer the questions below) The difference in polarity between the starting material (acetophenone) and the product (1-phenylethanol) means that Thin-Layer Chromatography (TLC) is an effective method to monitor the reaction. O CH3 NaBH4 OH CH3 Acetophenone (acdic workup) 1-phenyl ethanol In a 1:1 Hexane β Ethyl Acetate solvent system Acetophenone has an Rf = 0.5 and 1-phenylethanol has an Rf = 0.3. Please use the TLC plates below to draw an approximate Rf value (i.e. higher or lower) for the solvents systems given in the table below. (HINT: you do NOT need a ruler) Plate A (1 Mark) Acetophenone in 2:1 Hexane β Ethyl Acetate Plate B (1 Mark) Acetophenone in 1:2 Hexane β Ethyl Acetate Plate C (1 Mark) 1-phenylethanol in 1:5 Hexane β Ethyl Acetate Plate D (1 Mark) 1-phenylethanol in 9:1 Hexane β Ethyl Acetate Plate E (1Mark) A Co-spot of the Reaction vs the Starting material in 1:1 Hexane β Ethyl Acetate (assume 100 % conversion to product) Plate A Plate B Plate C Plate D Plate E S C R
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The Rf value is determined by the polarity of the compound and the polarity of the solvent. More polar compounds have lower Rf values because they interact more strongly with the polar stationary phase and move up the plate more slowly. Less polar compounds have Show moreβ¦
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How will you prepare ethyl phenyl ketone from acetophenone?
Preeti K.
(TsOH) as catalyst. . Check your organic textbook for the mechanism addition of alcohol to aldehydes or ketones to form acetals or ketals. The ketone carbonyl, significantly more reactive than the ester carbonyl, will react more rapidly with the ethylene glycol. Formation of the ketal involves an equilibrium in which water is one of the products. The equilibrium can be shifted to favor the product by removing the water as it is formed (Le Chatelier's principle). A Dean-Stark apparatus is used to accomplish this. See glassware appendix for an illustration of this apparatus. Step2 - Grignard addition to the ester. Once the reactive ketone moiety has been protected, the less reactive ester group can be modified. The ester is reduced by a Grignard reaction using phenylmagnesium bromide to produce the corresponding tertiary alcohol (2). Step3 - Deprotection of ketone. Removal of the ketal protecting group by hydrolysis with acid produces keto-alcohol (3). The ketone functionality is restored to the molecule now because the ketone will not react during the last step of the sequence (step 4 below). The mechanism for deprotection is essentially the reverse of that for step 1.
Sri K.
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