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Ch19: Decarboxylation of RCO2H
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML> <HEAD> <META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=ISO-8859-1"> <META NAME="Author" CONTENT="Dr Ian R. Hunt"> <META NAME="GENERATOR" CONTENT="Mozilla/4.06 [en]C-compaq (Win98; I) [Netscape]"> <TITLE>Ch19: Decarboxylation of RCO2H</TITLE> </HEAD> <BODY TEXT="#000000" BGCOLOR="#FFFFFF" LINK="#0000EE" VLINK="#551A8B" ALINK="#FF0000"> <font face="Arial"><type="hidden" value="C=OO>>C"> </font> <CENTER> <TABLE BORDER=0 CELLSPACING=5 > <TR> <TD WIDTH="25"><A HREF="../Carey.html"><IMG SRC="mol.gif" ALT="Return to Contents" BORDER=0 HEIGHT=40 WIDTH=40></A></TD> <TD><B><font face="Arial">Chapter 19: Carboxylic Acids</font></B></TD> <TD><A HREF="ch19-0.html"><IMG SRC="book.gif" ALT="Ch 19 contents" BORDER=0 HEIGHT=40 WIDTH=40></A></TD> </TR> </TABLE> </CENTER> <b><font size="+2" face="Arial">Decarboxylation</font> </b> <CENTER> <font face="Arial"><IMG SRC="decarboxylation.gif" ALT="decarboxylation removes CO2" HEIGHT=56 WIDTH=285></font> </CENTER> <font face="Arial"><B>Reaction type: Elimination</B> </font> <P><font size="+2" face="Arial">Summary</font> <UL> <LI><font face="Arial"> Loss of carbon dioxide is called <B>decarboxylation</B>.</font></LI> <LI><font face="Arial"> Simple carboxylic acids rarely undergo decarboxylation.</font></LI> <LI><font face="Arial"> Carboxylic acids with a carbonyl group at the 3- (or b-) position readily undergo thermal decarboxylation, <I>e.g.</I> derivatives of malonic acid.</font></LI> </UL> <CENTER> <font face="Arial"><IMG SRC="macid.gif" ALT="malonic acid" HEIGHT=69 WIDTH=237></font> </CENTER> <UL> <LI><font face="Arial"> The reaction proceeds via a cyclic transition state giving an enol intermediate that tautomerises to the carbonyl.</font></LI> </UL> <CENTER> <TABLE BORDER=0 WIDTH="600" > <TR> <TD COLSPAN="2" WIDTH="600"> <font face="Arial"><B>DECARBOXYLATION</B> </font> </TD> </TR> <TR> <TD><font face="Arial"><B>Step 1:</B> <BR> Remember curly arrows flow.... Start at the protonation of the carbonyl, break the<B> O-H</B> bond and form the <font face="Symbol">p</font> bond, break the<B> C-C</B> and make the <B>C=C</B>. Note the concerted nature of this reaction and the cyclic transition state.</font></TD> <TD ROWSPAN="2"> <font face="Arial"><IMG SRC="decarbmech.gif" ALT="decarboxylation of malonic acid" HEIGHT=353 WIDTH=122> </font> </TD> </TR> <TR> <TD><font face="Arial"><B>Step 2:</B> <BR> Tautomerisation of the enol of the carboxylic acid leads to the acid product (not shown here).</font></TD> </TR> </TABLE> </CENTER> <HR WIDTH="100%"> <CENTER> <TABLE BORDER=0 CELLSPACING=0 CELLPADDING=0 > <TR> <TD WIDTH="60"> <DIV ALIGN=right><A HREF="ch19-3-3.html"><IMG SRC="butL.gif" ALT="previous page" BORDER=0 HEIGHT=30 WIDTH=30></A></DIV> </TD> <TD WIDTH="60"><A HREF="ch19-4.html"><IMG SRC="butR.gif" ALT="next page" BORDER=0 HEIGHT=30 WIDTH=30></A></TD> </TR> </TABLE> <table align="center"> <tbody> <tr> <td><img src="mol.gif" alt="organic chemistry" width="40" height="40"></td> <td><font face="Arial"; size="-1">© <a href="mailto:irhunt@ucalgary.ca">Dr. Ian Hunt</a>, Department of Chemistry</font></td> <td><a href="http://www.ucalgary.ca/"><img src="../uc_h2.gif" alt="University of Calgary" width="122" height="35" border="0"></a></td> </tr> </tbody> </table> </center> </body> </html>