1. The work-up procedure calls for washing the crude 2-chloro-2-methylbutane wit

Question

1. The work-up procedure calls for washing the crude 2-chloro-2-methylbutane with cold sodium bicarbonate solution.

a. What purpose does this wash serve?

b. This washing procedure is accompanied by vigorous gas evolution, which increases the difficulty of handling and requires considerable caution. Alternatively, one might consider using a dilute solution of sodium hydroxide instead of the sodium bicarbonate. Discuss the relative advantages and disadvantages of using these two basic solutions in the work-up.

c. On the basis of your answer to b, why were you instructed to use sodium bicarbonate, even though it is more difficult to handle?

d. What is the purpose of the final wash of the organic layer with saturate sodium chloride solution in the purification process?

Explanation / Answer

bicarb is a relatively moderate base and will neutralize any acid by products ( I presume the chloro derivative was made from the alcohol using HCl in a simple proton induced SN1 reaction.). If one uses the strong base NaOH as the 2-Cl-2 methylbutane is a tertiary halide ..it would undergo elimination of HCl and form the alkene 2-butene.

NaOH soln wash: Easier to handle (no frothing due to gas evolution), but this strong base may hydrolyzesome of the product (can give lower yield).
NaHCO3 soln wash: Converse of above...frothing problem, but too weak a base to hydrolyze product.

Consider an esterification:
RCOOH + R'OH <---> RCOOR' + H2O
Note that this is an equilibrium reaction. It is driven to the right (esterification) by removing water as it is formed, but can be driven to the left (hydrolysis) by the presence of a strong base in the water. Even local concentration of NaOH can effect some hydrolysis.

BTW this frothing is a problem mostly when the neutralization is carried out in the separatory funnel...CO2 pressure tends to 'blow the cork'. To alleviate this, the better method is to neutralize in the (open) reaction vessel, then transfer to the separatory funnel for the water washes.

It is used to remove water from the organic layer. During the experiment some water gets into the organic layer because of the partial miscibility of the organic products in water. Saturated sodium chloride will pull the water from the organic layer following simple solute rules. Sat. NaCl wants to become more dilute and salts have a higher affinity for water than for organic solvents. Therefore removing the bulk of the water in the organic layer.

Related Questions

Navigate

• Browse (All)
• Browse 1
• Subjects

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