= Solution
Label the <carbon> next to the carbonyl $C_\alpha$ and the OH-bearing <carbon> $C_\beta$. The three limiting routes are \b[E1, E1cb and <E2 elimination>]. They differ in whether C$_\beta$–O cleavage, C$_\alpha$–H cleavage, or both occur in the first elementary event. In the following original transition-state structures, dotted lines denote changing bonds, $\delta$ marks developing partial charge, and the bracketed structures carry $\ddagger$. Unmarked skeletal bonds remain present. The dotted line parallel to C$_\alpha$–C$_\beta$ denotes the developing <pi bond>, in addition to its existing <sigma bond>.
\Image[/past-exam-of-the-chemistry-b-course-of-the-university-of-cambridge/2022/paper-2-elimination.png]
{title=The five transition states in E1, E1cb and concerted E2 limiting elimination pathways}
For \b[E1], C$_\beta$–OH heterolysis occurs first. The <electron pair> in that bond goes to O, producing $\mathrm{CH_3COCH_2C^+(H)CH_3+OH^-}$. Its first <transition state> has a partly broken C$_\beta$–O bond, developing positive charge on C$_\beta$ and negative charge on the leaving O. Next, <water> uses a <lone pair> to remove the alpha proton; the C$_\alpha$–H <electron pair> forms the C$_\alpha$\=C$_\beta$ <pi bond>. This second <transition state> has partly broken C$_\alpha$–H, partly formed water-O–H, and a partly formed C$_\alpha$–C$_\beta$ <pi bond>. Positive charge is transferred from the organic <cation> toward the incoming <water>/<hydronium> unit. <hydroxide> from the first step is a spectator in the second.
For \b[E1cb], <water> first removes the alpha proton to form a resonance-stabilized <enolate> plus <hydronium>:
$$
\mathrm{CH_3C(O^-){=}CHCH(OH)CH_3\ \leftrightarrow\ CH_3COC^-(H)CH(OH)CH_3}.
$$
The first <transition state> has partly broken C$_\alpha$–H and partly formed O$_{\rm water}$–H, with developing negative charge delocalized over the alpha-carbon/carbonyl-oxygen system and positive charge on the proton-accepting <water> unit. The <enolate> then expels <hydroxide>: use the carbon-centred contributor to send its alpha-carbon <lone pair> into C$_\alpha$–C$_\beta$, while the C$_\beta$–O pair goes to <oxygen>. This second <transition state> has a partly formed <alkene> <pi bond> and partly broken C$_\beta$–O, with negative charge moving from the <enolate> framework onto the departing <hydroxide>. <hydronium> is a spectator in this second step.
For \b[E2], <water> removes the alpha proton as <hydroxide> leaves in the same elementary step. Three electron-pair movements are simultaneous: <water> <lone pair> to H$_\alpha$, C$_\alpha$–H to the C$_\alpha$–C$_\beta$ <pi bond>, and C$_\beta$–O to O. Its single <transition state> therefore contains \b[all three changing bonds]: C$_\alpha$–H, water-O–H and C$_\beta$–O, together with the developing <pi bond>. The departing O becomes negative, the proton-accepting <water> unit becomes positive, and partial charge at <carbon> depends on the synchrony of proton loss and leaving-group departure. The reacting <conformation> should align C$_\alpha$–H and C$_\beta$–O approximately in an <antiperiplanar conformation> for efficient elimination.
These are formal limiting pathways for the displayed net process, not three claims of equal <reaction rates> in neutral <water>. In particular, unassisted loss of OH$^-$ to a secondary <carbocation> is strongly disfavoured, and <water> is a weak base for making an <enolate>. The <enolate> pathway benefits from resonance stabilization and the final product is a conjugated <enone>, but actual rate and dominant pathway depend on chemical conditions. The displayed <hydroxide> and <hydronium> also rapidly neutralize each other; retaining them separately here tracks which bond cleavage supplied each species. No biological <catalysis> is needed to describe these chemical mechanisms.
Back to article page
