This TLC plate was observed from the reduction of benzoin to hydrobenzoate using

Question

This TLC plate was observed from the reduction of benzoin to hydrobenzoate using sodium borohydride as a reducing agent and ethanol as a solvent. It was developed in a solution of 7:3 = hexanes:ethyl acetate.

A) Does the TLC of the product indicate that the desired reduction product was obtained, and do the observed Rf values make sense in terms of the functional groups present in the reactants and products?

B) Is there any reactant in the product mixture? Cite specific Rf values.

C)Does your TLC indicate the presence of benzil? If so indicate Rf of benzoin and benzil. Does the presence of benzil alter the outcome of the reaction in terms of diasteromer obtained?

so/F6 HK | VB N return TLC plote '. Honat fiant OA, OR, A, : 2.9 - 0.51 6. - 24 - 0.57 BI- 0.- 0.11 - bêAZ0). BCrude pvoluct Uisualized na W ight 4.2

Explanation / Answer

Ans: FIrst of all I would like to say that the product of the reaction is Hydroxybenzoin not Hydroxybenzoate because if it would have been Hydroxybenzoate there would have been the presence of ester group in the molecule and the reduction of benzoin does not give an ester group.

Ans a: The desired product is obtained i.e. hydoxybenzoin is obtained from benzoin which indicates by presence of polar spot on the TLC i.e. spot B2. The spot A2 having Rf value of 0.57 is the spot of starting material that is benzoin and the spot A1 having Rf value of 0.79 is the spot of benzil which is non-polar compared to that of benzoin due to the presence of two ketonic groups in the molecule.The spot B2 having an Rf of 0.18 is due to the presence of two hydroxyl groups that are formed in the molecule after reduction of benzoin by sodium borohydride. Compared to benzoin, hydroxybenzoin is polar so the spot is polar to benzoin spot on TLC. Polarity difference is due to presence of two hydroxyl groups in hydroxybenzoin and one hydroxy and one ketone group in benzoin.

Ans b: There is a presence of reactant in the product mixture which is indicated by the presence of spot B1 having Rf value of 0.57. This spot is obtained on TLC due to the reduction of benzil to benzoin along with the reduction of benzoin to hydroxy benzoin.

Ans c: There is the presence of benzil having Rf value of 0.79 and benzoin having Rf value of 0.57. Due to the presence of benzil in reaction the major product is formed along with formation of benzoin. The presence of benzil may not have an impact on the formation of diastereomers formed because sodium borohydride converts benzoin to hydroxybenzoin completely if appropriate mole ration is kept. Not only that it also converts benzil to benzoin. If the mole ratio of sodium borohydride for the above reaction is increased then complete conversion to hydoxybenzoin can be obtained.

Related Questions

Navigate

• Browse (All)
• Browse T
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.