Both E1 and E2 reactions are elimination reactions. This means they produce a double bond in the product. The chart below summarizes key differences between the two reactions and shows how to predict the product correctly.
Mechanism: E2 is a one step reaction. Leaving group leaves and the base takes off hydrogen on the carbon neighboring the leaving group (beta hydrogen). The bond between hydrogen and carbon breaks creating a double bond.
Regioselectivity: In order for E2 to happen, the leaving group and the beta hydrogen need to be antiperiplanar. This means if the leaving group is on a wedge, hydrogen must be on a dash. If the leaving group is on a dash, hydrogen must be on a wedge. If the leaving group is up, hydrogen needs to be down and vice versa. If the hydrogen is not anti to the leaving group, we will have to rotate the single bond until it is. If the single bond is inside the ring and can not be rotated, then that hydrogen can not be used.
Whenever possible, we want to make the most substituted double bond (Zaytsev Rule).
In certain cases, Hoffman elimination will occur where we preferentially make the least substituted double bond. Most professors and books teach that a Hoffman elimination will occur when a strong bulky base is used such as tertbutoxide. However, please refer to your notes to double check what your professor's policy on this is. Many do not follow this rule.
Substrate Preference: E2 can be done on a primary substrate if a strong bulky base is used such as test-butoxide. It will also work with a secondary or tertiary substrate as long as strong base is used.
Nucleophile/Base: E2 prefers strong bases. Examples of strong bases are : OH-, OR-....
Mechanism: E1 is a two step mechanism. First, the leaving group leaves creating a carbocation.
Then the nucleophile take the hydrogen on the carbon neighboring the carbocation (beta hydrogen). The bond between hydrogen and carbon break creating a double bond.
Regioselectivity: The major product for E1 is the most substituted double bond (Zaytzev rule). E1 can also create both trans and cis double bonds (trans are more stable).
Substrate Preference: E1 does not work on primary substrates. It will happen on secondary and tertiary substrates.
Nucleophile: E1 prefers weak nucleophile such as H2O, ROH, Acid.
PS: Some professors say that elimination reactions are favored when heat is present. Therefore, for some students it is easier to identify an elimination reaction. This does not apply to all colleges. Please consult your notes to see if this is applicable to you.
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