Question

Post-lab Question #1: While both NaBH4 and LiAlH4 can both be used toreduce ketones and aldehydes, the procedure that you followed in lab wouldnot result in the fluorenol product if NaBH4 were simply replaced with LiAlH4. a) Explain why. b) What change to the procedure would have to be made to use LiAlH4 tosuccessfully reduce fluorenone to fluorenol?

Post-lab Question #2: NaBH4 is not capable of reducing esters since estersare less reactive than ketones. Explain structurally why this is the case.

Post-lab Question #3: In lab, NaBH4 was kept in a dessicator to avoidprolonged contact with water in the atmosphere. This is done since waterwill react with NaBH4. Write the balanced equation for the decomposition of NaBH4 by water.

Post-lab Question #4: It is found that a bottle of NaBH4 in a lab contains 70% NaBH4 by mass as a significant portion has reacted with atmosphericwater as described in question 3 above. What is the minimum that astudent should weigh out to fully reduce 3.18 g of fluorenone?

Answer 1

1)

However, the most convenient and least expensive is probably sodium borohydride (NaBH4). This reagent does not require very dry reagents and solvents like the very useful but much more reactive LiAlH4 does. In fact, NaBH4 may be used in aqueous solution. The advantage of NaBH4 is that it is only slowly decomposed by water at neutral or basic pH. LiAlH4 is a stronger reducing agent as compared to NaBH4. For this to be used in the reduction of fluorenone to fluorenol the reaction cannot be carried out in Methanol.

• Carboxylic esters are reduced to give alcohols
• Esters are less reactive towards Nu than aldehydes or ketones.
• They can only be reduced by LiAlH₄ but NOT by the less reactive NaBH₄

The ester group or ketone group is activated toward nucleophilic attack by the following mechanism,

This is the same mechanism by which ketone is activated but in the case of ester the C is less + as compared to a ketone due to the presence of the lone pair on the OR of the ester. SO the nucleiphile needs to be stronger for the attack to take place.

3)

NaBH4 reacts with water to release hydrogen and BH₃ by the following mechanism and also generated OH^-