CHAPTEr EIGHTEEN Enols and enolates and bromination. Moreover, the rates of all three halogenation reactions, although first order in acetone, are independent of the halogen concentration. Thus, the halogen does not participate in the reaction until after the rate-determining step. These kinetic obser vations, coupled with the fact that substitution occurs exclusively at the a-carbon atom, ed Lapworth to propose that the rate-determining step is the conversion of acetone to a more reactive form. its enol isomer: this chapter is an electrostatic CHACHa= Acetone Propen-2-o1(enol Once formed, this enol reacts rapidly with the halogen to form an a-halo ketone OH CH3C= 2 CH3 CCH2X HX Propen-2-0 Halogen a-Halo derivative Hydrogen form of ac of acetone halide PROBLEM 18.3 Write the structures of the enol forms of 2-butanone that react with chlorine to give 1-chloro-2-butanone and 3-chloro-2-butanone far ahead of Both parts of the Lapworth mechanism, enol formation and enol halogenation, are new to us. Lets examine them in reverse order. We can understand enol halogenation ow organic reactions occur. by analogy to halogen addition to alkenes. An enol is a very reactive kind of alkene. Its For an account of Lapworth's carbon-carbon double bond bears an electron-releasing hydroxyl group, which activates nistry, see the it toward attack by electrophiles ovember 1972 issue of the OH on,pp.750-752. CHSC-CH, +Br-Br: ICH3-C-CH,Br:+Br Bromine Stabilized carbocation (enol form The hydroxyl group stabilizes the carbocation by delocalization of one of the ared electron pairs of oxygen: CH, Br CH3-C-CH, Br able resonance More stable 6 electrons on form: all aton tively charged c Participation by the oxygen lone pairs is responsible for the rapid attack on the carbon-carbon double bond of an enol by bromine. We can represent this participation explicitly: Back Forward Main MenuToc Study Guide ToC Student o MHHE Websiteand bromination. Moreover, the rates of all three halogenation reactions, although first￾order in acetone, are independent of the halogen concentration. Thus, the halogen does not participate in the reaction until after the rate-determining step. These kinetic obser￾vations, coupled with the fact that substitution occurs exclusively at the -carbon atom, led Lapworth to propose that the rate-determining step is the conversion of acetone to a more reactive form, its enol isomer: Once formed, this enol reacts rapidly with the halogen to form an -halo ketone: PROBLEM 18.3 Write the structures of the enol forms of 2-butanone that react with chlorine to give 1-chloro-2-butanone and 3-chloro-2-butanone. Both parts of the Lapworth mechanism, enol formation and enol halogenation, are new to us. Let’s examine them in reverse order. We can understand enol halogenation by analogy to halogen addition to alkenes. An enol is a very reactive kind of alkene. Its carbon–carbon double bond bears an electron-releasing hydroxyl group, which activates it toward attack by electrophiles. The hydroxyl group stabilizes the carbocation by delocalization of one of the unshared electron pairs of oxygen: Participation by the oxygen lone pairs is responsible for the rapid attack on the carbon–carbon double bond of an enol by bromine. We can represent this participation explicitly: Less stable resonance form; 6 electrons on positively charged carbon. CH3 CH2Br  C O More stable resonance form; all atoms (except hydrogen) have octets of electrons. CH3 C CH2Br  H O H   Br  Bromide ion CH3 CH2Br  C OH Stabilized carbocation very fast Br Br Bromine CH3C CH2 OH Propen-2-ol (enol form of acetone) -Halo derivative of acetone CH3CCH2X O Halogen X2 Hydrogen halide   HX Propen-2-ol (enol form of acetone) CH3C CH2 OH fast Acetone CH3CCH3 O Propen-2-ol (enol form of acetone) CH3C CH2 OH slow 704 CHAPTER EIGHTEEN Enols and Enolates The graphic that opened this chapter is an electrostatic potential map of the enol of acetone. Lapworth was far ahead of his time in understanding how organic reactions occur. For an account of Lapworth’s contributions to mechanistic organic chemistry, see the November 1972 issue of the Journal of Chemical Educa￾tion, pp. 750–752. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website