E2 mechanisms

Dehydrohalogenation is the elimination of a hydrogen and a halogen from an alkyl halide to form an alkene. In Sections 6-17 through 6-21 we saw how dehydrohalogenation can take place by the El and E2 mechanisms. The second-order elimination (E2) is usually better for synthetic purposes because the El has more competing reactions. 

Second-order elimination is a reliable synthetic reaction, especially if the alkyl halide is a poorSf42 substrate. E2 dehydrohalogenation takes place in one step, in which a strong base abstracts a piroton from one carbon atom as the leaving group leaves the adjacent carbon. 

E2 elimination takes place by a concerted one-step reaction. 

A strong base abstracts a proton on a carbon next to the one bearing a halogen. The leaving group (halide) leaves simultaneously. 

EXAMPLE E2 elimination of /-butyl bromide with sodium hydroxide. Na+ QH 

---

On the basis of these observations Ingold proposed a one step bimolecular E2 mechanism for dehydrohalogenation... 

The E2 mechanism is followed whenever an alkyl halide—be it primary second ary or tertiary—undergoes elimination m the presence of a strong base... 

Use curved arrows to track electron movement in the dehydro halogenation of tert butyl chloride by sodium methoxide by the E2 mechanism J... 

The regioselectivity of elimination is accommodated m the E2 mechanism by noting that a partial double bond develops at the transition state Because alkyl groups... 

Further insight into the E2 mechanism comes from stereochemical studies One such experiment compares the rates of elimination of the cis and trans isomers of 4 tert butyl cyclohexyl bromide... 

The E2 mechanism is a concerted process m which the carbon-hydrogen and carbon-halogen bonds both break m the same elementary step What if these bonds break m separate steps s... 

The best examples of El eliminations are those carried out m the absence of added base In the example cited m Eigure 5 12 the base that abstracts the proton from the car bocation intermediate is a very weak one it is a molecule of the solvent ethyl alcohol At even modest concentrations of strong base elimination by the E2 mechanism is much faster than El elimination... 

Like alcohol dehydrations El reactions of alkyl halides can be accompanied by carbocation rearrangements Eliminations by the E2 mechanism on the other hand nor mally proceed without rearrangement Consequently if one wishes to prepare an alkene from an alkyl halide conditions favorable to E2 elimination should be chosen In prac tice this simply means carrying out the reaction m the presence of a strong base... 

Section 5 16 The preceding equation shows the proton H and the halogen X m the anti coplanar relationship that is required for elimination by the E2 mechanism... 

Benzylic halides that are secondary resemble secondary alkyl halides in that they undergo substitution only when the nucleophile is weakly basic If the nucleophile is a strong base such as sodium ethoxide elimination by the E2 mechanism is faster than substitution... 

As depicted, the E2 mechanism involves a bimolecular transition state in which removal of a proton to the leaving group is concerted with departure of the leaving group. In contrast, the rate-determining step in the El mechanism is the unimolecular ionization of... 

Rg. 6.4. Three-dimensional (More O Fenall) diagrams depicting transition-state locations for El, Elcb, and E2 mechanisms. 

It has generally been assumed that phosphorous oxychloride-pyridine dehydrations, the elimination of sulfonates, and other base catalyzed eliminations (see below) proceed by an E2 mechanism (see e.g. ref. 214, 215, 216). Concerted base catalyzed eliminations in acyclic systems follow the Saytzelf orientation rule i.e., proceed toward the most substituted carbon), as do eliminations (see ref 214). However, the best geometrical arrangement of the four centers involved in 2 eliminations is anti-coplanar and in the cyclohexane system only the tran -diaxial situation provides this. 

---