Utente:Grasso Luigi/sanbox1/Reazione di eliminazione

Reazione eliminazione di cicloesano a cicloesene con acido solforico e calore

Una reazione di eliminazione è un tipo di reazione organica in cui due sostituenti vengono rimossi da una molecola in un meccanismo ad una o due fasi. Nel meccanismo ad uno step viene detta reazione E2, mentre in quello a due step viene detta reazione E1. I numeri non hanno a che fare con il numero di passaggi nel meccanismo, ma piuttosto con la cinetica della reazione, bimolecolare o unimolecolare. Nei casi in cui la molecola è in grado di stabilizzare un anione ma possiede una scarsa tendenza a lasciare il gruppo, esiste un terzo tipo di reazione detta E1_CB. Infine, la pirolisi degli esteri di xantato e acetato procedono attraverso un meccanismo di eliminazione "interno", detto meccanismo E_i.

Perdita dell'idrogeno (H⁺)

Nella maggior parte delle reazioni di eliminazione organica, almeno un atomo d'idrogeno viene perso per formare il doppio legame: l'insaturazione delle molecole aumenta. È anche possibile che una molecola subisca eliminazione riduttiva, della quale la valenza di un atomo nella molecola diminuisce di due, anche se ciò è più comune nella chimica inorganica.

Una classe importante di reazioni di eliminazione è quella tra gli alogenuri alchilici, con buon gruppo uscente, reagendo con una base di Lewis formano un alchene. L'eliminazione si può considerare come inversa della reazione di addizione. Quando il substrato è asimmetrico, la regioselettività viene determinata dalla regola di Zaitsev o dalla eliminazione di Hofmann se l'atomo di carbonio col maggior idrogeno sostituito non è accessibile.

Meccanismo E2

During the 1920s, Sir Christopher Ingold proposed a model to explain a peculiar type of chemical reaction: the E2 mechanism. E2 stands for bimolecular elimination. The reaction involves a one-step mechanism in which carbon-hydrogen and carbon-halogen bonds break to form a double bond (C=C Pi bond).

The specifics of the reaction are as follows:

• E2 is a single step elimination, with a single transition state.
• It is typically undergone by primary substituted alkyl halides, but is possible with some secondary alkyl halides and other compounds.
• The reaction rate is second order, because it's influenced by both the alkyl halide and the base (bimolecular).
• Because the E2 mechanism results in the formation of a pi bond, the two leaving groups (often a hydrogen and a halogen) need to be antiperiplanar. An antiperiplanar transition state has staggered conformation with lower energy than a synperiplanar transition state which is in eclipsed conformation with higher energy. The reaction mechanism involving staggered conformation is more favorable for E2 reactions (unlike E1 reactions).
• E2 typically uses a strong base. It must be strong enough to remove a weakly acidic hydrogen.
• In order for the pi bond to be created, the hybridization of carbons needs to be lowered from sp³ to sp².
• The C-H bond is weakened in the rate determining step and therefore a primary deuterium isotope effect much larger than 1 (commonly 2-6) is observed.
• E2 competes with the S_N2 reaction mechanism if the base can also act as a nucleophile (true for many common bases).

Scheme 1. E2 reaction mechanism

An example of this type of reaction in scheme 1 is the reaction of isobutylbromide with potassium ethoxide in ethanol. The reaction products are isobutylene, ethanol and potassium bromide.

Meccanismo E1

E1 is a model to explain a particular type of chemical elimination reaction. E1 stands for unimolecular elimination and has the following specificities.

• It is a two-step process of elimination: ionization and deprotonation.
  • Ionization: the carbon-halogen bond breaks to give a carbocation intermediate.
  • Deprotonation of the carbocation.
• E1 typically takes place with tertiary alkyl halides, but is possible with some secondary alkyl halides.
• The reaction rate is influenced only by the concentration of the alkyl halide because carbocation formation is the slowest step, aka the rate-determining step. Therefore, first-order kinetics apply (unimolecular).
• The reaction usually occurs in the complete absence of a base or the presence of only a weak base (acidic conditions and high temperature).
• E1 reactions are in competition with S_N1 reactions because they share a common carbocationic intermediate.
• A secondary deuterium isotope effect of slightly larger than 1 (commonly 1 - 1.5) is observed.
• There is no antiperiplanar requirement. An example is the pyrolysis of a certain sulfonate ester of menthol:

E1 elimination Nash 2008, antiperiplanar relationship in blue

Only reaction product A results from antiperiplanar elimination. The presence of product B is an indication that an E1 mechanism is occurring

• It is accompanied by carbocationic rearrangement reactions

Scheme 2. E1 reaction mechanism

An example in scheme 2 is the reaction of tert-butylbromide with potassium ethoxide in ethanol.

E1 eliminations happen with highly substituted alkyl halides for two main reasons.

• Highly substituted alkyl halides are bulky, limiting the room for the E2 one-step mechanism; therefore, the two-step E1 mechanism is favored.
• Highly substituted carbocations are more stable than methyl or primary substituted cations. Such stability gives time for the two-step E1 mechanism to occur.
• If S_N1 and E1 pathways are competing, the E1 pathway can be favored by increasing the heat.

Specific features : 1 . Rearrangement possible 2 . Independent of concentration and basicity of base

Concorrenza tra meccanismi E1,E2

The reaction rate is influenced by the reactivity of halogens, iodide and bromide being favored. Fluoride is not a good leaving group, so eliminations with fluoride as the leaving group have slower rates than other halogens.

There is a certain level of competition between the elimination reaction and nucleophilic substitution. More precisely, there are competitions between E2 and S_N2 and also between E1 and S_N1. Substitution generally predominates and elimination occurs only during precise circumstances. Generally, elimination is favored over substitution when

• steric hindrance around the α-carbon increases.
• a stronger base is used.
• temperature increases (increase entropy)
• the base is a poor nucleophile. Bases with steric bulk, (such as in Potassium tert-butoxide), are often poor nucleophiles.

In one study the kinetic isotope effect (KIE) was determined for the gas phase reaction of several alkyl halides with the chlorate ion. In accordance with an E2 elimination the reaction with t-butyl chloride results in a KIE of 2.3. The methyl chloride reaction (only S_N2 possible) on the other hand has a KIE of 0.85 consistent with a S_N2 reaction because in this reaction type the C-H bonds tighten in the transition state. The KIE's for the ethyl (0.99) and isopropyl (1.72) analogues suggest competition between the two reaction modes.

Voci correlate

• Reazione di eliminazione-E1cB

Collegamenti esterni

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