MCPB, 2,4-MCPB, 4-(4-chloro-o-tolyloxy)butyric acid (IUPAC), or 4-(4-chloro-2-methylphenoxy)butanoic acid (CAS) is a phenoxybutyric herbicide. In the United States it is registered for use on pea crops before flowering, for post-emergence control of broadleaf annual and perennial weeds including Canadian thistle, buttercup, mustard, purslane, ragweed, common lambsquarters, pigweed, smartweed, sowthistle, and morning glory. It has low to moderate acute toxicity, with kidney and liver effects as the main hazard concerns. It is not volatile, persistent, or likely to bioconcentrate.

MCPB

Names
Preferred IUPAC name 4-(4-Chloro-2-methylphenoxy)butanoic acid
Identifiers
CAS Number	94-81-5 Y
3D model (JSmol)	Interactive image
ChemSpider	6938 N
ECHA InfoCard	100.002.151
PubChem CID	7207
UNII	OA1Z4N1842 Y
CompTox Dashboard (EPA)	DTXSID4024193
InChI InChI=1S/C11H13ClO3/c1-8-7-9(12)4-5-10(8)15-6-2-3-11(13)14/h4-5,7H,2-3,6H2,1H3,(H,13,14) N Key: LLWADFLAOKUBDR-UHFFFAOYSA-N N InChI=1/C11H13ClO3/c1-8-7-9(12)4-5-10(8)15-6-2-3-11(13)14/h4-5,7H,2-3,6H2,1H3,(H,13,14) Key: LLWADFLAOKUBDR-UHFFFAOYAB
SMILES CC1=C(C=CC(=C1)Cl)OCCCC(=O)O
Properties
Chemical formula	C11H13ClO3
Molar mass	228.67 g/mol
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). N verify (what is YN ?) Infobox references

A variety of methods have been developed for its analysis.^[1] In the U.S., the maximum residue permitted on peas is 0.1 parts per million.^[2]