# 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.

• Your name is not correct. The carboxylic acid has a higher precedence than chlorine, so your numbering is backwards. – LDC3 Jan 4 '15 at 4:22
• @LDC3 even i thought it should be like that but my book says that more branching is given more priority. – Jai Mahajan Jan 4 '15 at 4:24
• I'm not sure what you mean by more -I effect. How about providing me with more information? – LDC3 Jan 4 '15 at 4:48
• Ok it basically means that it chlorine is electron withdrawing so it is more effective in reducing negative charge on cooh near it so h+ ion is given easily – Jai Mahajan Jan 4 '15 at 4:50
• Since there is an intervening carbon in the second compound, the chlorine atoms have very little effect on the acidity of the acid. – LDC3 Jan 4 '15 at 4:59

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!).

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).

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.)