Elimination reaction
Elimination reaction (short: elimination ) is a term from organic chemistry . It is a collective term for chemical reactions in which two atoms or groups of atoms are split off (eliminated) from a molecule . For this split off, two bonds must be broken. The elimination is the reverse reaction of the addition .
variants
The most common elimination affects two adjacent atoms in the starting molecule that are bonded to one another, after which a double or triple bond is formed between them. This is known as β-elimination . If both leaving groups leave the same carbon atom, this α-elimination results in a carbene . The γ-elimination at two not directly adjacent carbon atoms leads to cyclopropane derivatives.
Reaction mechanisms in β-elimination
Irrespective of the variants, the elimination can proceed according to various reaction mechanisms , which in this case cannot be determined by the product distribution, but only by parameters such as reaction speed and activation energy . Based on the prevailing reaction conditions such as substrate, attacking particles, temperature and solvent , it is possible to predict which elimination will take place and to what extent competitive reactions will influence the yield.
E1
The E1 mechanism is a first-order reaction; thus the rate of reaction is proportional to the concentration of the substrate. In contrast to other mechanisms, it is irrelevant for the speed in which concentrations the solvent and the attacking nucleophile are present. A reaction that proceeds according to this mechanism can be divided into two sub-steps.
In a first, rate-determining step, the leaving group is split off. This often happens when, for example, a halogen atom emerges as a halide ion , taking up the binding electrons . A positive charge then remains on the carbon atom, creating a carbocation (also known as a carbenium ion). The octet rule is strictly valid for the carbon atom , so that it tries to overcome the electron deficiency situation of the sextet by accepting electrons.
In the second step, this is done by splitting off a further substituent in the β position. This time the binding electrons are “kept”; the leaving group is positively charged. A rehybridization of the carbon from sp 3 to sp 2 takes place at the α and β positions . The binding electrons resulting from the elimination of the cation are distributed between the two p orbitals that are formed and are parallel to one another. A C = C double bond is formed. Often a proton is split off in the second step. It must not be forgotten that this only happens formally, because in fact the proton is abstracted from the attacking nucleophile. This is conclusive when comparing the base strength of the halide ion and the attacking nucleophile, since the stronger base absorbs the proton.
Since a planar carbenium ion occurs as an intermediate , isomerization is possible due to the free rotation around the CC axis . Unless effects such as steric hindrance severely destabilize an isomer, cis and trans isomers are equally likely to occur. It is understandable that the preparative utility of this reaction is very limited, since the isomerization results in a single product only in stereochemically unambiguous cases. This also applies to first-order nucleophilic substitution. The intermediate carbenium ion is so reactive that, in addition to the problem of isomers, there is a high potential for side reactions such as rearrangements .
E1cb
This mechanism also describes a first-order reaction. The fundamental difference to the E1 mechanism is the order of the splitting off. In the E1 cb mechanism, the proton is eliminated first and then the leaving group. An intermediate with a negatively charged carbon, the so-called carbanion , is thus formed . The abbreviation cb stands for conjugated base , in German conjugated base, since the reaction proceeds via the base assigned to the hydrocarbon , namely the carbanion produced by splitting off a proton.
Here, too, stereoisomerism is only determined in the second step, since free rotation around the CC axis is guaranteed.
E2
In the E2 mechanism, the proton and leaving group are split off in concert . There is a transition state in which the attacking nucleophile (“ Lewis base ”) forms a bond with the proton. The unstable transition state is then converted into an alkene by splitting off both groups. The reaction rate is proportional to the product of the substrate concentration and the concentration of the attacking base.
The relative position of the two leaving groups is decisive for the reaction according to E2. They must be on the same level as the core connection axis, i.e. they must be at a 0 ° or 180 ° angle to one another when looking at the CC connection axis (see Newman projection ). Only in this way can the p orbitals overlap in the transition state in order to form the double bond. This condition is met in anti and syn groups. The anti position is usually the preferred one because it ensures faster overlapping of the p orbitals. Syn elimination is mostly observed with alkanes whose leaving group also represents the nucleophile. The stereoisomerism is determined by the substrate or the elimination reagent used, since free rotation is completely excluded.
Competition of reaction mechanisms
There are different factors that make the E1, E1cb or E2 reaction more likely:
Optimal conditions for E1
The probability for the first-order mechanism is high if the intermediate carbocation is stabilized:
- A cation stabilized by hyperconjugation and / or mesomerism appears.
- Polar protic solvent: The cation is stabilized by solvation.
- High temperature: splitting becomes more likely.
- Weak bases : Strong bases lead to E2 kinetics (see below). In addition, a low concentration of the base suppresses the E2 mechanism.
Optimal conditions for E1cb
This mechanism hardly competes with the other two mechanisms because the conditions differ too greatly from one another. It is mainly used in the chemistry of carbonyls:
- The formation of the carbanion must be stabilized by electron-withdrawing substituents. These make the occurrence of a cation extremely unlikely.
- The leaving group can be relatively bad, since the negative charge of the carbanion strongly favors the splitting off.
Optimal conditions for E2
The mechanisms between E1 and E2 are similar; With E2 conditions it is important that the proton is split off before the carbocation can form:
- Low temperatures and polar aprotic solvents make the occurrence of a cation unlikely.
- A strong base in high concentration attacks the proton adjacent to the electrophilic leaving group before it dissociates into the cation and leaving group. If the base is also sterically inhibited, the substitution is suppressed as a competitive reaction (see below).
Stereoselectivity
Using the example of haloalkanes, the elimination reactions compete.
| Preference E1 vs. Favor E2 | ||||
| Haloalkane |  |
 |
 |
|
| - | primarily it is haloalkane | secondary it is haloalkane | tertiary it haloalkanecarboxylic | |
| Preference | E2 | E1 / E2 | E1 (E2) | |
| X = halogen | ||||
The nucleophilic substitution as a possible competitive reaction
E1 competes with the S N 1 reaction , E2 with the S N 2 reaction. This can be controlled, among other things, by the influence of solvents.
Competition E1 / S N 1
E1 is preferred over S N 1 if ...
- Substituents are present that stabilize the C = C bond
- connect many alkyl groups (through the + I effect )
- bulky (sterically demanding) nucleophiles are used, since bond expansion from 109.48 ° (sp 3 ) to 120 ° (sp 2 )
- bad nucleophiles are used (low nucleophilicity)
- the temperature is increased (entropy effect)
Competition E2 / S N 2
E2 is preferred over S N 2 if ...
- Alkyl / phenyl / vinyl substituents in the α or β position
- bulky nucleophiles are used or, in the case of bulky hydrocarbons (tertiary substituted)
- strong nucleophile is used (strong base)
- bad leaving groups present, as these are more strongly bound in the transition state
- Dihedral angle of 180/0 ° (anti-periplanar arrangement)
- more non-polar solvents can be used because the transition state is less solvated
Bimolecular reactions are mostly elimination-directed, since the steric influence of the transition state (S N 2) is very large. Repulsive forces put such a course at a disadvantage.
Regiochemistry
In the case of secondary and tertiary starting materials, the double bond can arise in different directions; there are regioisomers. The products are differentiated according to the number of substituents bound to the double bond as Hofmann or Saytzeff products (see also Saytzeff rule according to Alexander Michailowitsch Saizew , also written Saytzeff or Saytzev).
The Saytzeff product is usually the more stable configuration, since the intermediate carbenium ion, provided it is an E1 elimination, is stabilized by the substituents (+ I effect). The Hofmann product is the preferred configuration when heteroatoms in Saytzeff position destabilize the carbenium ion (−I effect). Furthermore, the Hofmann product can result if - starting from an E2 elimination - the leaving group in Saytzeff position is not in a periplanar arrangement to the proton, or the rest of the base is so large that steric hindrance occurs.
Examples
| reaction | Leaving group | mechanism |
|---|---|---|
| Dehydration | water | E1 |
| base-catalyzed aldol condensation | Hydroxide ion | E1cb |
| Dehydrohalogenation to alkenes | Halide | E2 |
In biochemistry, α or β eliminations of amino acids are carried out via pyridoxal phosphate (coenzyme).
Web links
literature
- Peter Sykes: reaction mechanisms - an introduction , 8th edition, VCH, Weinheim 1982, ISBN 3-527-21090-3 .
Individual evidence
- ↑ Chemgapedia.de: Glossary: Elimination .
- ^ AF Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 101st edition. Walter de Gruyter, Berlin 1995, ISBN 3-11-012641-9 , p. 384.
- ↑ Science online lexica: Elimination in the lexicon of chemistry .
- ^ A b Hans P. Latscha, Uli Kazmaier, Helmut A. Klein: Organic Chemistry . 2002, Springer-Verlag , ISBN 3-540-42941-7 .
- ↑ Chemgapedia.de: Glossary: E1-Elimination .
- ↑ Chemgapedia.de: Glossary: E2 elimination .
- ↑ Competition between substitution and elimination. In: www.uni-muenster.de. Retrieved June 25, 2019 .