Question

q1.At the end of this reaction, the procedure asks you to add 10 mL of 2M HCl, what is the purpose of this? Use reaction schemes to illustrate your answer.

q2.As part of the workup procedure for this reaction, you are asked to add 10 % NaOH to the aqueous phases before extracting with ethyl acetate. What is the purpose of this? Use structures help illustrate your answer

-NH₂ H2N -ΝΗ HN- ОН Amberlyst 15 Ethanol, NaBH4 -OH HO- R Unknown Carbonyl Compound (2 equivalents) 2 hydroxyboozylaldehde 2 singlete ructures of Possible Unknown Starting Materials: он ОН

Answer 1

Solution:

Key points:

1. 2 equivalents (11 mmol) of unknown carbonyl compound reacts with 1 equivalent (5.5 mmol) of ethylene diamine and form the 11 mmol of imine compound and 22 mmol of water.  
2. 40.16 mmol of NaBH4 is used as reducing agent to reduce the 11 mmol of imine to give 11 mmol of amine compound. NaBH4 donates a hydride to the imine carbon, generating borane (BH3). Theoretically only 11 mmol (0.25 equivalents) of NaBH4 are necessary. Since borane is weaker reducing agent than NaBH4 excess amount is used.

q1. Addition of 10mL of 2M HCl at the end of the reaction:

The excess NaBH4 used in the reduction reaction is quenched by the addition of 2M HCl solution (20 mmol). The reaction is given below.

NaBH4 + HCl + 3H2O $\rightarrow$ NaCl + H3BO3 + 4H2

q2. Addition of 10% NaOH to the aqueous phase before extracting with ethyl acetate:

As per q1, the reaction was quenched with HCl which is in excess. This will form the protonated salt (HCl salt) of the Product, diamine compound which is soluble in aqueous phase. Hence the diamine compound cannot be extracted to ethyl acetate layer. Further, quenching of excess of NaBH4 produces H3BO3. So 10% NaOH solution is added to neutralize the H3BO3 present in the aqueous phase and also release free amine compound that can be extracted in with ethyl acetate. Structure of diamine salts is given below.

С -