Given that the structures of cocaine (on the right side of the equation) and its free base ( on the left side) with an H+ :
a. Would you expect K for the cocaine to be greater or less than 1.0? Explain.
b. Would K for the free base be higher or lower than for cocaine? Explain.
c. If you were going to extract cocaine into dichloromethane (an immiscible organic solvent) from
an aqueous solution, would you make the aqueous solution acidic or basic? Explain why.
a) You would expect K>1 because cocaine is in the presence of an acid, which raises K (low pH = high K value)
b) Base = high pH, so you would expect K to be lower.
c) You would want to make the aqueous solution basic because then more cocaine would enter the aqueous layer (k=aq/non-aq), allowing for more cocaine to be extraced.
To answer these questions, we need to consider the equilibrium reaction between cocaine (written as its protonated form) and its free base form:
Cocaine + H+ ⇌ Cocaine free base
K is the equilibrium constant, which is determined by the concentrations of the reactants and products at equilibrium. K > 1 indicates that the forward reaction (formation of cocaine free base) is favored, while K < 1 indicates that the reverse reaction (reformation of cocaine) is favored. K = 1 indicates that both forward and reverse reactions are occurring to the same extent.
a. We would expect K for cocaine to be less than 1.0. This is because cocaine is a protonated form (Cocaine + H+), and the presence of the positive charge makes it less stable compared to the free base form. Therefore, the reverse reaction (formation of cocaine through protonation) is favored, resulting in a smaller value of K.
b. K for the free base would be higher than for cocaine. This is because the free base form is more stable due to the absence of the positive charge. Therefore, the forward reaction (formation of free base) is favored, resulting in a larger value of K compared to cocaine.
c. If you were to extract cocaine into dichloromethane from an aqueous solution, you would want to make the aqueous solution basic. When the aqueous solution is basic, it will deprotonate cocaine by accepting the H+ ion. This will shift the equilibrium towards the formation of the free base form, which is more soluble in organic solvents like dichloromethane. Therefore, making the aqueous solution basic will help enhance the extraction of cocaine into dichloromethane.