Return to Contents Chapter 19: Carboxylic Acids Ch 19 contents
Carboxylic Acid Answers
 
Qu1: (i) increasing acidity :
  CH3CH2OH  <  CH3CH2SH  <  CH3CO2H  <  CCl3CO2H  Resonance stabilisation in carboxylates makes carboxylic acids more acidic than alcohols or thiols. 
Electron withdrawing groups, here -Cl, increase the acidity due to further stabilisation of the carboxylate.
Thiols are more acidic than alcohols due to weaker X-H bond and the ability of S to accommodate extra electrons (size).
(ii) increasing pKa  :
CCl3CO2H < CH3CO2H <  CH3CH2SH  <  CH3CH2OH Remember the lower the pKa the stronger the acid, so once you have part (i), this is just be the opposite. 
Tip: When asked about pKa trends, it may be easier to think in terms of acidity then remember to flip the order.
Qu 2: Note the identical nature of the reactions that the aliphatic and aromatic acid undergo:

(a) Thionyl chloride, SOCl2,  is used to prepare acyl chlorides, the base removes the HCl by-product.
(b) LiAlH4 is a hydride reducing agent, acids to primary alcohols.
(c) An alcohol and an carboxylic acid give an ester.
(d) Carboxylic acids react with bases to give carboxylates.
 

Qu 3: First, note that we have an homologous series of C3 to C6 acids we are trying to make. 
Here is a scheme collecting possible syntheses together (based on the more important reactions)

(a) Propanoic acid : need to loose a C from C4. We can do this via ozonolysis of an alkene, which we can obtain by eliminating the alkyl halide. 
(b) Butanoic acid : oxidation of the corresponding alcohol will give the carboxylic acid, so prepare the alcohol by substitution.
(c) Pentanoic acid : need to gain a C to get C5.  One way to do this is via the reaction of the Grignard reagent with carbon dioxide. Alternatively, substitution with NaCN then hydrolysis would also work.
(d) Hexanoic acid : need to gain 2C to get C6.  Reaction of the Grignard reagent with ethylene oxide gets the right number of C and a primary alcohol ready for oxidation to the acid.