Elimination (chemistry)

In organic chemistry, a elimination (or β-elimination ) is a reaction which transforms a Alcane substituted (Halogénoalcane S, alcohol S…) in ethylene derivative, even in Alcène, if the starting molecule, in addition to the group therefore, is only one carbonaceous chain of alkane type.

The conditions are, in harder, those of a nucleophilic Substitution, close reaction by many aspects and competitor: elimination occurs in the presence of a bases strong and by heating the reactional mixture.

General equation

One of the principal examples of elimination is the transformation of a halogénoalcane into olefinic hydrocarbon; one speaks then about deshydrohalogenation:

The bases most usually used are soda (Na⁺, HO^-), the potash (K⁺, HO^-), even the ions alcoholates (RO^-) (obtained while plunging Na in an alcohol ROH).

Just like for the nucleophilic Substitution there exists a monomolecular elimination and a bimolecular elimination which one usually notes E1 and E2 respectively.

In the continuation of this article the examples are given with a halogenous derivative as a reagent.

Monomolecular elimination

Kinetics of E1

The monomolecular reaction of elimination is characterized by kinetics of a total nature 1, of partial natures 1 compared to the halogenous derivative and 0 compared to the base B| : v = K. ''. The reaction is done in two stages, of which one is kinetically limiting. The latter, monomolecular, utilizes only the molecule of the halogenous derivative on the side of the reagents, consequently the kinetics of the reaction, in the approximation of the stage kinetically limiting (or determining ), depends only on the concentration of this species.

represents the concentration in halogenous derivative in mol. L^-1

v represents the reaction speed of elimination. It is expressed in mol.s^-1

K is the constant speed: it is in fact that of the stage kinetically limiting (k₁). Its unit, L.s^-1, result from those of the two other terms.

Mechanism of E1

The reaction is done in two stages.

First stage - Departure of the nucléofuge, formation of a carbocation

This stage of formation of the Carbocation is a slow, reversible stage and imposes its speed on the unit of the reaction.

Second phase - Reaction acid-bases

The base tears off the proton. The doublet " rabat" then on the connection carbon-carbon to form a double linking. As the connection simple carbon-carbon can swivel, it can be formed two stéréoisomères. This reaction being much faster than the preceding one, its speed does not influence total speed.

Stéréosélectivité and Régiosélectivité

Stereochemistry

Like the mechanism, the passage by a carbocation shows it, which is a plane structure, removes any element of stereoselectivity contrary (see the continuation) to the bimolecular mechanism of elimination.

But, if two stéréoiseomères geometrical can be formed, they are not formed in the same proportions. The most stable isomer, i.e. that whose energy is lowest, is formed mainly. It is thus the isomer E which is mainly formed. One speaks then about partial stereoselectivity.

Example:

Regiochimy

When several isomers of constitution can be formed, it is formed the most stable olefinic hydrocarbon preferentially thermodynamically. If there are only alkyls groups, it is the olefinic hydrocarbon more substituted. So mesomeries can intervene, it is the olefinic hydrocarbon more combined which is majority. One speaks then about the regioselectivity or the Règle of Zaitsev.

However, sometimes the least stable olefinic hydrocarbon thermodynamically is formed preferentially, one speaks then about the elimination of Hoffmann. It is the case in particular when the base used is encumbered (ex: KtBuO) or when which one has an ion ammonium which is eliminated for example. (Molecule carrying an amine in the presence of MeI of sufficient number of equivalents to form the ammonium salt, then put in the presence of money oxide wet and heated. Formation of olefinic hydrocarbon by elimination of Hoffmann and thus departure of NR (Me) 3.)

Factors influencing the reaction

The parameters influencing E1 elimination are:

the effect R: the stage kinetically determining is the function of the carbocation (and not its reactivity), therefore the more this one is stable, the more the reaction is favoured.
the nucléofuge: the cut of the connection C-group therefore being the key stage of the mechanism, the reaction strongly depends on the nature of this nucléofuge. Better is the group therefore, plus the réction is fast. For example for the dehydration of alcohols, the first stage is the protonation of alcohol to form a oxonium, and thus a better group therefore (H2O>HO-) OSO 2R (R=Me, AR) >I>Br>Cl>OH.

To note that the base does not influence this type of reaction of order 1 and this insofar as it does not intervene at the time of the first stage which determines the total speed of the reaction.

Group alkyl of the halogenous derivative

The mechanism passes by a reactional intermediary of carbocation type. This one is all the more stable as it is substituted. Thus, the reaction speed (thus of formation of the carbocation) progresses of the primary education derivatives, with the secondaries then with the tertiary sectors. In practice, only the latter, and some secondary derivatives, react by this mechanism.

Nucléofuge

The more polarizable connection X-ray is, the more the rupture is easy. Speed thus believes as follows:

\ mathrm {F} << \ mathrm {Cl} < \ mathrm {Br} < \ mathrm {I} ~

(<: reacts more quickly than)

Solvent

Polarity: a polar solvent stabilizes the carbocation; it returns easier the initial reaction thus. Speed thus believes with the polarity of solvent.

Proticité : a solvent protic is rather a handicap: indeed it is likely to return in competition with the carbocation during the reaction acid-bases (strong).

Compete with

This reaction is not only in competition with other elimination, but also with the nucleophilic reactions of substitutions, in particular with the S_N1. The latter occurs indeed under similar conditions (in addition to the heating).

Bimolecular elimination

a bimolecular reaction of elimination is characterized by kinetics of order 2 and a partial nature 1 compared to halogenous derivative X-ray and at the B:^- base: v=k. ''.

E2 is also Stéréospécifique.

the mechanism is composed of only one bimolecular stage:

The reaction is done only when one hydrogen is in anti position compared to the group therefore.

Factors influencing the reaction

the reaction speed of E2 varies only slightly according to the class of halide considered. Nevertheless, more the class is raised and more the reaction is fast.
the reaction speed grows with the force of the base.
the character nucléofuge of the group therefore also has an influence on the reaction.

Remarks

The base used is also a Nucléophile. That explains competition with the reactions of substitutions. At the end of the reaction one can find, mixed, products of eliminations and products of substistutions.

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