E2 elimination alkyl halides

I Tertiary7 alkyl halides E2 elimination occurs when a base is used, but SN1 substitution and El elimination occur together under neutral conditions, such as in pure ethanol or water. ElcB elimination takes place if the leaving group is two carbons away from a carbonyl group. 

Tertiary alkyl halides E2 elimination occurs when a base such asi OH or RO is used. For example, 2-bromo-2-niiethylpropane gives 97% elimination product when treated with ethoxide ion in ethanol. By contrast, reaction under neutral conditions (heating in pure ethanol) leads to a mixture of products resulting from both S l sub- stitution and El elimination. 

Two special cases need to be considered activation by alkyl halide and by a formal cationic center. The interaction diagrams are shown in Figures 10.6a,b, respectively. These are involved in elimination reactions of alkyl halides, E2 and El, respectively. 

A potential side reaction in malonate alkylation is E2-elimination, which mainly depends on the structure of the alkyl halide. Also, dialkylation may occur if the malonate contains two active hydrogens. Dialkylation may be circumvented by carrying out the deprotonation with one equivalent of NaH. 

Because elimination from a tertiary alkyl halide typically leads to a more substituted alkene than does elimination from a secondary alkyl halide, and elimination from a secondary alkyl halide generally leads to a more substituted alkene than does elimination from a primary alkyl halide, the relative reactivities of alkyl halides in an E2 reaction are as follows ... 

Reagents that are classified as bases can be used with both Lewis and Brpnsted-Lowry acids and one application is the elimination of alkyl halides (E2 reactions sec. 2.9.A) in the presence of a base. Elimination involves conversion of a saturated moiety containing a leaving group to a molecule with a multiple bond. An example of this latter transform is ... 

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... 

FIGURE 5 10 Potential en ergy diagram for concerted E2 elimination of an alkyl halide... 

Typically elimination by the El mechanism is observed only for tertiary and some secondary alkyl halides and then only when the base is weak or m low con centration Unlike eliminations that follow an E2 pathway and exhibit second order kinetic behavior... 

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... 

The first example of a stereo electronic effect in this text concerned anti elimination in E2 reactions of alkyl halides (Section 5 16)... 

Substitution can take place by the S l or the 8 2 mechanism elimination by El or E2 How can we predict whether substitution or elimination will be the principal reac tion observed with a particular combination of reactants The two most important fac tors are the structure of the alkyl halide and the basicity of the anion It is useful to approach the question from the premise that the characteristic reaction of alkyl halides with Lewis bases is elimination and that substitution predominates only under certain special circumstances In a typical reaction a typical secondary alkyl halide such as iso propyl bromide reacts with a typical Lewis base such as sodium ethoxide mainly by elimination... 

FIGURE 8 11 When a Lewis base reacts with an alkyl halide either substitution or elimination can occur Sub stitution (Sn2) occurs when the Lewis base acts as a nu cleophile and attacks carbon to displace bromide Elimi nation (E2) occurs when the Lewis base abstracts a pro ton from the p carbon The alkyl halide shown is iso propyl bromide and elimi nation (E2) predominates over substitution with alkox ide bases... 

As a practical matter elimination can always be made to occur quantitatively Strong bases especially bulky ones such as tert butoxide ion react even with primary alkyl halides by an E2 process at elevated temperatures The more difficult task is to find condifions fhaf promofe subsfifufion In general fhe besf approach is fo choose condi lions lhal favor fhe 8 2 mechanism—an unhindered subslrale a good nucleophile lhal IS nol slrongly basic and fhe lowesl praclical lemperalure consislenl wilh reasonable reaclion rales... 

Section 8 13 When nucleophilic substitution is used for synthesis the competition between substitution and elimination must be favorable However the normal reaction of a secondary alkyl halide with a base as strong or stronger than hydroxide is elimination (E2) Substitution by the Sn2 mechanism predominates only when the base is weaker than hydroxide or the alkyl halide is primary Elimination predominates when tertiary alkyl halides react with any anion... 

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... 

Secondary and tertiary alkyl halides are not suitable because they react with alkox ide bases by E2 elimination rather than by 8 2 substitution Whether the alkoxide base IS primary secondary or tertiary is much less important than the nature of the alkyl halide Thus benzyl isopropyl ether is prepared m high yield from benzyl chloride a pri mary chloride that is incapable of undergoing elimination and sodium isopropoxide... 

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