Naming rules and acidity of Cl2CHCH2COOH

Question

How do we name $\ce{Cl2CHCH2COOH}$ ?

Is seeing branched chain our number 1 priority so we name it

1,1-dichloro-3-propanoic acid?

also

Which is more acidic $\ce{Cl2CHCOOH}$ or $\ce{Cl2CHCH2COOH}$ ? I feel it should be the first one as there is more −I effect but not sure.

Answer 1

1- 3,3-dichloropropanoic acid. Please see http://www.masterorganicchemistry.com/2011/02/14/table-of-functional-group-priorities-for-nomenclature/

2- The first one is more acidic, as the inductive effect, due to chlorine atom decrease fast with the distance. (So, your answer is correct!).

Answer 2

With regard to numbering of locants, the principal characteristic group (the characteristic group chosen for citation at the end of a name by means of a suffix or a class name, or implied by a trivial name, e.g. propanoic acid) has seniority over simple substituent groups (e.g. chloro). On this matter, the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book) reads as follows:

P-14.4 NUMBERING

When several structural features appear in cyclic and acyclic compounds, low locants are assigned to them in the following decreasing order of seniority:

(…)

(c) principal characteristic groups and free valences (suffixes);

(…)

(f) detachable alphabetized prefixes, all considered together in a series of increasing numerical order;

(…)

Note that Rule c takes precedence over Rule f.

Therefore, the preferred IUPAC name for the given structure is 3,3-dichloropropanoic acid (not 1,1-dichloro-3-propanoic acid).

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As you have correctly assumed, additional chloro substituents increase the acidity of the carboxylic acid due to their electron-withdrawing inductive effect (−I effect):

  acetic acid: $\mathrm{p}K_\mathrm{a} = 4.756$
  2-chloroacetic acid: $\mathrm{p}K_\mathrm{a} = 2.87$
  2,2-dichloroacetic acid: $\mathrm{p}K_\mathrm{a} = 1.35$
  2,2,2-trichloroacetic acid: $\mathrm{p}K_\mathrm{a} = 0.66$

The acidity of chlorocarboxylic acids is hardly directly affected by the length of the carbon chain:

  2-chloroacetic acid: $\mathrm{p}K_\mathrm{a} = 2.87$
  2-chloropropanoic acid: $\mathrm{p}K_\mathrm{a} = 2.83$
  2-chlorobutanoic acid: $\mathrm{p}K_\mathrm{a} = 2.86$

However, the effect decreases as the distance between the chloro substituent and the carboxyl group increases:

  2-chloroacetic acid: $\mathrm{p}K_\mathrm{a} = 2.87$
  3-chloropropanoic acid: $\mathrm{p}K_\mathrm{a} = 3.98$
  4-chlorobutanoic acid: $\mathrm{p}K_\mathrm{a} = 4.52$

($\mathrm{p}K_\mathrm{a}$ values taken from “Dissociation Constants of Organic Acids and Bases”, in CRC Handbook of Chemistry and Physics, 90th Edition (CD-ROM Version 2010), David R. Lide, ed., CRC Press/Taylor and Francis, Boca Raton, FL.)