Consider the acid-base reaction that occurs when benzoic acid is extracted with
ID: 854087 • Letter: C
Question
Consider the acid-base reaction that occurs when benzoic acid is extracted with aqueous sodium hydroxide.
Write the equation for this reaction, labeling the acid, base, conjugate acid, and conjugate base.
What is the anticipated relative solubility of benzoic acid and its conjugate base in diethyl ether?
The pKa of benzoic acid is 4.2, whereas that of carbonic acid, H2CO3, is 6.4. On the basis of this information, would aqueous sodium bicar- bonate be sufficiently basic to deprotonate benzoic acid? Explain your reasoning.
The pKa of hydrochloric acid is about ??6. On the basis of this information, would aqueous sodium chloride be sufficiently basic to deprotonate ben- zoic acid? Explain your reasoning.
Why is it unwise to begin addition of the solution of methyl benzoate to the Grignard reagent before the latter has cooled to room temperature and then been placed in an ice-water bath?
Define the term inverse addition as it applies to a Grignard reaction and explain why this is the preferred mode of addition for the reaction between C6H5MgBr and CO2.
Explanation / Answer
a.
Benzoic Acid + NaOH --> Na+ Benzoate- H2O (an acid/base rxn)
Benzoic Acid is the acid, it's conj base is benzoate. NaOH is base, conj acid is water.
b. benzoic acid is MORE soluble in ether, since it is neutral (uncharge) and ether is NON-POLAR. Benzoate (charged/polar) will be LESS soluble in ether.
c.
Since the bicarb is more basic than the benzoic acid it will deprotonate it. It is basic enough to deprotonate because the pKa is larger than the pKa for benzoic acid
d.
NaCl is not basic; it will not deprotonate to an acid to form HCl (think of how acidic HCl is...)
e.
Don't add methyl benzoate to the grignard at RT, because the reaction is highly exothermic and there may be trace water around (what is your solvent for the benzoate?) and that will destroy the grignard. Cooling to O C prevents this side reaction.
Inverse Addn means adding the reactant to the grignard, in this case, and not the other way around. This is the preferred mode to monitor/mediate the exotherm that forms, as well as keep the grignard in excess (less losses due to side rxn with water).
Any questions, comment here..
Best of luck