The E1 Reaction

Elimination reactions may follow a different pathway from that given in Section 6.16. Treating tert-butyl chloride with 80% aqueous ethanol at 25°C, for example, gives substitution products in 83% yield and an elimination product (2-methylpropene) in 17% yield  

Whether substitution or elimination takes place depends on the next step (the fast step). 

Aided by the polar solvent, a chlorine departs with the electron pair that bonded it to the carbon. 

This slow step produces the relatively stable 3° carbocatlon and a chloride Ion. 

The Ions are solvated (and stabilized) by surrounding water molecules. 

The abbreviation El staixls for Elimination, unimolecular. The mechanism is called unimolecular because the rate-limiting transition state involves a single molecule rather than a collision between two molecules. The slow step of an EI reaction is the same as in the SnI reaction unimolecular ionization to form a carbocation. In a fast second step, a base abstracts a proton fr n the carbon atom adjacent to the C . The electrons that once fwmed the carbon-hydrogen bond now form a pi bond between two carbon attxns. The general mechanism for the E1 reaction is shown in the following Key Mechanism box. 

The El reaction requires ionization to a carbocation intermediate like the 1, so it fol- 

Step 1 Unimolecular ionization to give a carbocation (rate-limiting). 

Step 2 Deprotonation by a weak base (often the solvent) gives the alkene (fast). 

EXAMPLE El elimination of bromocyclohexane in methanoi. Step 1 Ionization gives a carbocation and bromide ion. 

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Mechanism of the E1 reaction. Two steps are involved, the first of which is rate limiting, and a carbocation intermediate is present. 

First-Order Elimination The E1 Reaction 258 Key Mechanism 6-8 The E1 Reaction 258 Mechanism 6-9 Rearrangement in an E1 Reaction 261 Summary Carbocation Reactions 262 6-18 Positional Orientation of Elimination Zaitsev s Rule 263 6-19 Second-Order Elimination The E2 Reaction 265 Key Mechanism 6-10 The E2 Reaction 266 6-20 Stereochemistry of the E2 Reaction 267... 

In the E1 reaction, C X bond-breaking occurs first, fhe substrate dissociates to yield a carbocalion in the slow rate-limiting step before losing H from an adjacent carbon in a second step. The reaction shoivs first-order kinetics and no deuterium isotope effect and occurs wlien a tertiary substiate reacts in jjolai, nonbasic solution. 

Reaction-energy diagram of the E1 reaction. The first step is a rate-limiting ionization. Compare this energy profile with that of the Sn 1 reaction. Figure 6-8. 

If no strong base is present, with a good solvent a unimolecular ionization is likely, followed by loss of a proton to a weak base such as the solvent. Under these conditions, the E1 reaction usually predominates (always accompanied by the SnI). 

Effect of the Solvent The slow step of the El reaction is the formation of two ions. Like the 1, the E1 reaction critically depends on polar ionizing solvents such as water and the alcohols. 

In the E1 reaction, the rate-limiting step i s formation of a carbocation, and the reactivity order reflects the stability of carbocations. In the E2 reaction, the more substituted halides generally form more substituted, more stable alkenes. 

Kinetics The rate of the E1 reaction is proportional to the concentration of the alkyl halide [RX] but not to the concentration of the base. It follows a first-order rate equation. 

In most unimolecular reactions the Sn1 reaction is favored over the E1 reaction, especially at lower temperatures. In general, however, substitutbn reactbns of tertiary halides do not find wide use as synthetic methods. Such halides undergo eliminations much too easily. 

Sm1 reaction. The R must be tertiary or secondary. Stereochemical result is largely racemization. The E1 reaction competes. HOS is a polar, protic solvent... 

Reaction profile for the E1 reaction between (CH3)3CBr and methanol... 

E1 Reaction C-X bond breaks first to give a carbocation intermediate, followed by base removal of a proton to yield the alkene. 

The following data refer to the reduction of a mercurous complex at a DME. Draw a Heyrovsky-llkovic plot to determine both E1/2 and the number of electrons transferred in the electrode reaction. 

Again, the exclusive formation of six-membered rings indicates that the cyclization takes place by the electrophilic attack of a cationic center, generated from the enol ester moiety to the olefinic double bond. The eventually conceivable oxidation of the terminal double bond seems to be negligible under the reaction conditions since the halve-wave oxidation potentials E1/2 of enol acetates are + 1.44 to - - 2.09 V vs. SCE in acetonitrile while those of 1-alkenes are + 2.70 to -1- 2.90 V vs. Ag/0.01 N AgC104 in acetonitrile and the cyclization reactions are carried out at anodic potentials of mainly 1.8 to 2.0 V vs. SCE. 

The loss of a halide from 2 (step C4) was shown to be a reversible reaction, which was more shifted toward the monohalide cation for X = Br than for X = Cl. The electrochemical steps E2 and E4 were studied using the Mo(V) complex [MoOCl(dtc)2]. The reduction of 2 follows the E1-C1-E2-C2 pathway and the oxidation of [MoO(dtc)2] the E3-C3-E4-C4 pathway [23]. 

The chance, therefore, that two molecules in a collision will be such that the energy of one exceeds E1 and the energy of the other exceeds E2 is e EiRlT x e E lRT. This is equal to e (E +Ed(RT. We may write E1 + E2 = E. We are not now concerned with the separate values Ex and E2 but with their sum E, which is the energy of activation in the bimolecular reaction. 

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