Elimination Reactions56.2.1 E1 vs E2
Reaction rates
E1andE2are two different pathways to creating alkenes from haloalkanes. As withSN 1
andSN2 reactions, one of the key differences is in the reaction rate, as it provides great
insight into the mechanisms.
E1reactions, likeSN1 reactions are 2-step reactions. Also likeSN1 reactions, the rate-
limiting step is the dissociation of the halide from its alkane, making it a first-order reaction,
depending on the concentration of the haloalkane, with a reaction rate of:
Rate=k[RX]On the other hand,E2reactions, likeSN2 reactions are 1-step reactions. And again, as
withSN2 reactions, the rate limiting step is the ability of a nucleophile to attach to the
alkane and displace the halide. Thus it is a second-order reaction that depends on the
concentrations of both the nucleophile and haloalkane, with a reaction rate of:
Rate=k[Nu:][RX](where Nu: is the attacking nucleophile)Zaitsev’s Rule
Zaitsev’s rule^11 (sometimes spelled ”Saytzeff”) states that in an elimination reaction, when
multiple products are possible, the most stable alkene is the major product. That is to say,
the most highly substituted alkene (the alkene with the most non-hydrogen substituents) is
the major product.
BothE1andE2reactions produce a mixture of products, when possible, but generally
follow Zaitsev’s rule. We’ll see below whyE1reactions follow Zaitsev’s rule more reliably
and tend to produce a purer product.
Figure 160 Dehydrohalogenation reaction of (S)-2-bromo-3-methylbutane
The above image represents two possible pathways for the dehydrohalogenation of (S)-
2-bromo-3-methylbutane. The two potential products are 2-methylbut-2-ene and 3-
methylbut-1-ene. The images on the right are simplified drawings of the molecular product
shown in the images in the center.
As you can see on the left, the bromine is on the second carbon and in anE1orE2reaction,
the hydrogen could be removed from either the 1st or the 3rd carbon. Zaitsev’s rule says
that the hydrogen will be removed predominantly from the 3rd carbon. In reality, there will
be a mixture, but most of the product will be 2-methylbut-2-ene by theE1mechanism. By
theE2reaction, as we’ll see later, this might not necessarily be the case.
11 Chapter131.22on page 435