Chapter 9: Nucleophilic Substitution and ß-Elimination
Table of contents of main topics:
Sn1 Versus Sn2 reactions of Haloalkanes
Before we start our discussion of the SN1 and SN2 reactions, let's get introduced to some of the terms.
Both reactions are nucleophilic substitution reactions. What that means is that a nucleophile will replace (substitute) something in the starting material. What does it replace? A leaving group. The molecule with the leaving group is called a substrate (starting material).
The chart above summarizes both reactions and major differences between them. Let's take a more in depth look at each one.
Steps and Mechanism: SN2 is a 1 step reaction. The leaving group leaves and, at the same time, nucleophile attacks the carbon with the leaving group from the back.
Stereochemistry: The reaction results in an inversion. If the leaving group was on a wedge, nucleophile will be on a dash. If the leaving group was on a dash, nucleophile will be on a wedge. If the leaving group was up, nucleophile will be done and vice versa.
Kinetics: The rate of the reaction depends on both the substrate and nucleophile. This reaction is said to be bimolecular.
Substrate: SN2 prefers primary substrate (leaving group is attached to a carbon that is attached to one carbon only). It is ok with secondary substrate but never tertiary. This is because nucleophile attacks from the back and if there are too many groups in the back, nucleophile will simply not be able to get through to the carbon.
Nucleophile: SN2 prefers strong nucleophile. Strong nucleophiles usually have a negative charge and this makes it easer to spot them. Oftentimes, negatively charged anions are together with spectator ions such as Na+, Li+ and K+. For example, NaOH might look neutral. However, Na+ is the spectator ion. OH- is the nucleophile (and it is negatively charged).
Examples of strong nucleophiles are: OH-, RO-, RS-, I-, CN-, I-, Br-
Solvent: SN2 prefers polar aprotic solvent (no hydrogen connected to an electronegative atom). This solvent increases the energy of the nucleophile.
Examples of polar aprotic solvents: DMSO, DMF, CH3CN
Steps and Mechanism: SN1 is a two step reaction (sometimes more than two steps are needed). In step 1, leaving group leaves forming a carbocation. In step 2 nucleophile attaches to the carbocation. If the nucleophile is H2O or ROH, a third step is needed to deprotonate (take off an H) from the group that attached.
Stereochemistry: The reaction gives two products. One with the same stereochemistry (retention). For example if the leaving group was on a wedge, nucleophile would be on a wedge in the product as well. Another product is with an inversion of stereochemistry. For example, if the leaving group was on a wedge, nucleophile in the product would be on a dash. The two products together are called a racemic mixture.
Kinetics: The rate of SN1 reaction depends on the step of the leaving group leaving (rate determining step) and therefore depends on the substrate only (unimolecular).
Substrate: SN1 prefers tertiary substrate, ok with secondary, never primary (unless resonance stabilized, please consult with your professor in regards to this point as it differs from one college to another).
The reason SN1 prefers tertiary substrate is because the more substituted a carbocation is, the more stable it is. The more stable carbocation is, the more likely the reaction is to proceed since creating carbocation is the rate determining step. Therefore, carbocations that are stabilized via resonance are also favored by SN1.
Nucleophile: SN1 prefers weak nucleophile. Generally, weak nucleophile are neutral (not always). Examples of weak nucleophiles are : H2O, ROH, F-
Solvent: SN1 prefers polar protic solvent ( contains a hydrogen attached to an electronegative atom). Polar protic solvents stabilize the carbocation.
Examples of polar protic solvents are: H2O, ROH, NH3, RCOOH.
Both reactions SN1 and SN2 want as good of a leaving group as possible. A good leaving group, when it leaves, is stable on its own, and does not want to attack the molecule back.
Examples of good leaving groups: I->Br->Cl-> OTS-> H2O
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.
Substitution Versus Elimination
Here is a chart you can use to determine what kind of reaction you will predict based on the substrate and nucleophile.