The E1 Mechanism

In this section we explore the stepwise mechanism, or El, pathway  

Kinetic studies show that many eUmination reactions exhibit first-order kinetics, with a rate equation that has the following form  

Much like the S l reaction, the rate is linearly dependent on the concentration of only one compound (the substrate). This observation is consistent with a stepwise mechanism, in which the rate-determining step does not involve the base. The rate-determining step is the first step in the mechanism (loss of the leaving group), just as we saw in the Sj,jl reaction. The base does not participate in this step, and therefore, the concentration of the base does not affect the rate. Because this step involves only one chemical entity, it is said to be unimolecular. Unimolecular elimination reactions are called El reactions  

Reiative stability of primary, secondary, and tertiary carbocations. 

A comparison of energy diagrams for the El reactions of secondary and tertiary substrates. 

The dehydrohalogenation of (CH3)3CI with H2O to form (CH3)2C=CH2 can be used to illustrate the second general mechanism of elimination, the El mechanism. 

Like the SnI mechanism, the kinetics suggest that the reaction mechanism involves more than one step, and that the slow step is unimolecular, involving only the alkyl halide. 

The most straightforward explanation for the observed first-order kinetics is a two-step reaction the bond to the leaving group breaks first before the n bond is formed, as shown in Mechanism 8.2. 

The El and E2 mechanisms both involve the same number of bonds broken and formed. The only difference is the timing. 

In an El reaction, the leaving group comes off before the p proton is removed, and the reaction occurs in two steps. 

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FIGURE 5.6 The E1 mechanism for the acid-catalyzed dehydration of tert-butyl alcohol. 

FIGURE 5.12 The E1 mechanism for the dehydrohalogenation of 2-bromo-2-methylbutane in ethanol. 

The observed first-order kinetics and independence of the nature of the attacking base make the bimolecular E2 or E lcB mechanisms unlikely. The E1 mechanism or fraws-elimination would involve the formation of the amide anion NR2. Even when this cannot be ruled out, this possibility seems less probable in aqueous solutions. A synchronous mechanism, possibly with ring formation, seems to be more probable. 

Elimination reactions often compete with substitution. They involve elimination of the halogen and a hydrogen from adjacent carbons to form an alkene. Like substitution, they occur by two main mechanisms. The E2 mechanism is a one-step process. The nucleophile acts as a base to remove the adjacent proton. The preferred form of the transition state is planar, with the hydrogen and the leaving group in an anti conformation. The E1 mechanism has the same first step as the SN1 mechanism. The resulting carbocation then loses a proton from a carbon atom adjacent to the positive carbon to form the alkene. 

Provide similar information about the E1 mechanism. (Problems 9.19,9.28, and 9.33)... 

Since the E1 mechanism has two steps, there are two energy barriers. 

Much evidence has been obtained in support of the E1 mechanism. For exampie, El reactions show first-order kinetics, consistent with a rate-limiting spontaneous dissociation process ... 

In very rare cases, such as the superacid solutions we described in Chapter 15 (p. 335), the cation is stable because counterions such as BFj and SbFg are not only non-nucleophilic but also so non-basic that they won t even accept a proton. This fact tells us that despite this common way of writing the E1 mechanism, some sort of weak base is necessary even for El. 

In the second step of the E1 mechanism, the carbon atom next to the C must rehybridize to sp as the base attacks the proton and electrons flow into the new pi bond. 

Dehydration usually goes by the E1 mechanism. Rearrangements may occur to form more stable carbocations. 

Increasing the temperature of the reaction favors reaction by the E1 mechanism at the expense of the Sn1 mechanism. 

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

A stoichiometric reaction of tetrakis(triphenylphosphine)platinum(0) with bis(pinacolato)diboron gives cis-diborylplatinum(n) complex in high yield (Scheme 3).38 The diborylplatinum complex then reacts with an alkyne, giving m-diboration product.40,41 These results indicate that the diboration proceeds through the general mechanism shown in Scheme 1 (E1 = E2 = Bpin), which involves the formation of diborylplatinum(n), insertion of an alkyne into the B-Pt bond, and reductive elimination. 

Elimination unimolecular (E1) mechanism (Section 5.17) Mechanism for elimination characterized by the slow formation of a carbocation intermediate followed by rapid loss of a proton from the carbocation to form the alkene. 

If the ft proton is slightly less acidic than required for the (E1)anlon mechanism and k j is comparable to k1 but k2 is still small, the anion forms from the starting material in a rapid equilibrium and the leaving group departs in a subsequent slow step. This is called the (E1cB)b ( R for reversible ) mechanism. Because k2 is much smaller than kx and k 1, we can assume that k2 does not affect the equilibrium concentration of the anion of the substrate, S then... 

For the limit of very fast kinetics, the RPV response is analogous to that of the E mechanism but shifted toward more positive potentials (in the case of a reduction process), the shift magnitude being dependent on the value of the equilibrium constant. This can be observed clearly in Fig. 4.27 by comparing the curve for (k + 2)f2 > 105 and for the E mechanism (empty points). From Eq. (4.245) it can be inferred that the mid-potential value CinidR[.y only depends on the equilibrium constant, and is independent of geometric and kinetic parameters and coincident with. E1 [80]. 

Know the meaning of SN2 mechanism, inversion of configuration, SN1 mechanism, racemization, rate-determining step, E2 and E1 mechanisms. 

There are two types of elimination reactions, E1 and E2 reactions. The mechanism for E1 is a multistep reaction that involves the formation of a carbocation intermediate. The E2 mechanism is a series of steps, bond breaking and bond formation, that occurs simultaneously. Similar to the Sn2 case outlined above, both the haloalkane and the base are involved in the transition state. 

The bromine is bonded to a tertiary carbon and there is not a strong base present, so the reaction will proceed by an SN1 /E1 mechanism. The substitution product should predominate. The El reaction follows Zaitsev s rule, so more 1-methylcyclohexene should be formed than methylenecyclohexane. 

The conversion of geranyl pyrophosphate to myrcene follows an E1 mechanism. Show the steps in this mechanism. 

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... 

CHAPTER 7 Inversion of Configuration in the Sn2 Reaction 244 Racemization in the Sn1 Reaction 252 Hydride Shift in an Sn1 Reaction 253 Methyl Shift in an Sn1 Reaction 254 Rearrangement in an E1 Reaction 261 Dehydrohalogenation by the E2 Mechanism 304 Stereochemistry of the E2 Reaction 306 E2 Debromination of a Vicinal Dibromide 310... 

Problem 8.13 Draw an E1 mechanism for the following reaction. Draw the structure of the transition state for each step. 

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