The E1 reaction usually occurs under neutral or acidic conditions when an E2 reaction is unfavourable. A good example is dehydration of t-butanol to 2-methylpropene as catalyzed by a strong acid.
Elimination reactions occur by three fundamentally different mechanisms. The E1, E2 and E1-CB differ in the timing of bond breaking and bond forming events, and reaction conditions and structural features in the substrate can help determine which mechanism is most likely to operate. Each type of mechanism is detailed below, with an animation at the end of each sequence to put all of the main mechanistic steps together.
The E1 reaction usually occurs under neutral
or acidic conditions when an E2 reaction is unfavourable. A good
example is dehydration of t-butanol to 2-methylpropene as catalyzed by
a strong acid.
Protonation makes OH a better leaving
group and, under the reaction conditions (polar solvent and often heat),
the molecule can lose water.
The bond between the leaving group and
carbon begins to break, the system going through a late transition state
which looks most like the resultant carbocation intermediate (Hammond postulate).
The carbocation (3o
in this case) is unstable and needs to react further to yield a stable
product. One option open is reaction with a nucleophile to give a
substitution product (the SN1 reaction), or (especially at elevated temperature)
reaction with a base to form the elimination product.
Once the proton is completely abstracted,
the pi bond is completely formed..
The following animation puts these events in sequence:
The E2 reaction is the most common type
of elimination and involves synchronous bond breaking and bond forming
events. The reaction is promoted by raising the temperature, and
can lead to regioisomeric products. The outcome of the reaction depends
on the base employed. The use of small bases usually leads to the
thermodynamically favoured product (Zaitsev elimination), while large bases
usually result in the most accessible proton being removed and the Hofmann
product being formed (often the less thermodynamically favoured product).
In all E2 cases, the base approaches the
substrate and begins to bond with a proton on the carbon (the beta-carbon)
next to the one which is attached to the leaving group (the alpha-carbon).
As the base begins to remove the proton,
the bond between C and H begins to break. At the same time, the pi
bond is starting to form, and the bond between the alpha-carbon
and the leaving group is starting to break. The system passes through
a maximum energy point, the transition state.
Once all bonds are completely formed and
broken, the product alkene results. It is important to remember that
there are no reactive intermediates in the E2 reaction, and that important
factors like stereochemistry (are the proton being removed and the leaving
group antiperiplanar?) and regiochemistry (Zaitsev vs. Hofmann elimination)
are to be considered.
The following animation puts these events in sequence:
The third type of elimination pathway,
the E1-CB is less common than either the E1 or the E2. This type
of mechanism operates when a fast deprotonation is possible to yield a
stabilized conjugate base, for example an enolate, which then undergoes
loss of the leaving group in the rate-determining step. Since the
slow step only involves the conjugate base, the reaction is unimolecular.
The base encounters the acidic proton
and begins to deprotonate.
A resonance stabilized enolate (the conjugate
base) results.
The result is again a more unsaturated
product, in this case an a,b-unsaturated
ketone.
The following animation puts these events in sequence:
