In this tutorial, you will learn 6 ways to distinguish between the two elimination reactions, E1 vs E2.
Topics Covered in Other Articles
- Intermediate– a compound that is formed then used during a reaction
- Polar Protic Solvent– solvent containing O-H or N-H bonds
- Stereochemistry– physical orientation of the components in a molecule
- Stereospecificity– stereochemistry of the beginning reactant plays a role in how the molecule can react and determining stereochemistry of the final product
- Substrate– the starting molecule in an organic reaction
Ok, now let’s learn six ways in which we can differentiate E1 from E2 reactions in organic chemistry!
1. Number of Steps
The most obvious way to distinguish E1 vs E2 is by looking at the number of steps in the mechanism. E1 takes place in two steps and has a carbocation intermediate; on the other hand, E2 takes place in one step and has no intermediate.
2. Rate of Reaction
E1: This is a first-order unimolecular reaction, hence the 1 in the name. This means that the rate of reaction depends only on the concentration of the substrate. As the concentration of the substrate increases, so does the reaction rate.
E2: This is a second-order bimolecular reaction, hence the 2 in the name. This means that the rate of reaction depends on both the substrate and the deprotonating base.
E1: There is no specific way the molecule has to be oriented in space because the two steps of the reaction happen independently; therefore, E1 has no stereospecificity.
E2: The leaving group and hydrogen that will be detached must be anti to each other, or 180 degrees apart. This is because the reaction happens in one step, so each part of the mechanism must have room to occur. For this reason, the two should have opposite orientations in space.
4. Bulk of Molecule
E1: When looking at the carbon attaching the leaving group to the molecule, the more bulk it has, the faster it will undergo E1 elimination. Tertiary substrates will be the fastest, followed by secondary substrates; however, primary carbocations are too unstable to be created as an intermediate (click here for more information), so E1 elimination will very rarely occur in this case. This is because the most substituted carbocation is favored; the more bonds it has to molecules other than hydrogen, the more stable it will be. Carbocation stability is important.
E2: E2 follows a similar order; however, since there is no carbocation intermediate, primary substrates can undergo this type of elimination. Tertiary substrates will be favored the most, followed by secondary substrates and last will be primary substrates.
5. Base Strength
E1: The nucleophile, or Lewis base, can be strong or weak. Since this reaction occurs in two steps, the base does not need to displace the leaving group and can solely focus on grabbing onto the hydrogen. Weak bases, such as water, are usually favored for this reaction.
E2: A strong base, such as a hydroxide ion, is required. The single-step reaction needs a strong base in order to assist in the displacement of the polar leaving group.
E1: Polar protic solvents are used in this type of elimination. These types of solvents are good for ionization and can assist in stabilizing the carbocation intermediate.
E2: The type of solvent in this reaction does not matter.