Naming rules and acidity of Cl2CHCH2COOH

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.




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!).


 3

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)](http://dx.doi.org/10.1039/9781849733069-fp001)* 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).


![3,3-dichloropropanoic acid](https://i.stack.imgur.com/OdweQ.png)




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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.)


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