nuc_subs Flashcards
Define nucleophile.
A nucleophile is an electron-rich molecule (or anion) with a lone pair of electrons that is attracted to a positively charged region of a different molecule.
Define electrophile.
An electrophile is an electron-poor molecule (or region of a molecule) which attracts electron-rich molecules (or negatively-charged anions).
Describe what reaction, if any, that will occur with the following reactants.
Although the alkyl halide is primary, this weak base/nucleophile is unable to react until the leaving group leaves (which is not possible for a primary alkyl halide). Therefore no reaction will occur.
Describe what reaction, if any, that will occur with the following reactants.
The electrophile is secondary and the base/nucleophile is weak. Therefore the reaction will occur as an E1 and SN1.
In the case of SN1, the nucleophile (H2O) will not attack until the leaving group (the F-) has left, which allows access to the trigonal planar carbocation. A racemic mixture of butan-2-ol will form.
For the E1, the base (H2O) will not accept a proton until the leaving group leaves to form the carbocation. The major elimination product will be trans-but-2-ene followed by cis-but-2-ene and but-1-ene.
Describe what reaction, if any, that will occur with the following reactants.
The electrophile is tertiary and the base/nucleophile is weak. Therefore the reaction will occur as an E1 and SN1.
In the case of SN1, the nucleophile (H2O) will not attack until the leaving group (the F-) has left, which allows access to the trigonal planar carbocation. 2-methylbutan-2-ol will form.
For the E1, the base (H2O) will not accept a proton until the leaving group leaves to form the carbocation. The major elimination product will be 2-methylbut-2-ene followed by 2-methylbut-1-ene.
Describe what reaction, if any, that will occur with the following reactants.
The electrophile is secondary and base/nucelophie is strong. This reaction will occur E2 and SN2 because the base/nucleophile is strong and will not “wait” for the leaving group to leave before reacting.
In the SN2 reaction, the nucleophile (OH-) will attack opposite side of leaving group and there will be an inversion of configuration. Butan-2-ol will form.
For the E2 (which is usually the mjor mechanism for these conditions), the base (OH-) will react in one concerted step to eliminate the halide and form the alkene. The major elimination product will be trans-but-2-ene followed by cis-but-2-ene and but-1-ene.
Describe what reaction, if any, that will occur with the following reactants.
The electrophile is tertiary and base/nucelophile is strong. This reaction will occur E2 only because the base is strong and will not “wait: for the leaving group to leave before reacting. No substitution reaction will occur, however, because of steric hindrance.
For the E2, the base (OH-) will react in one concerted step to eliminate the halide and form the alkene. The major elimination product will be 2-methylbut-2-ene followed by 2-methylbut-1-ene.
Explain why an E2 mechanism is not possible with a weak base?
An E2 mechanism occurs in one concerted step, which means the proton is removed at the same time as the leaving group leaves. A weak base (as implied by the name) is too weak to remove the proton before the leaving group leaves. When the leaving group leaves, this results in the formation of the carbocation. In these conditions, the beta hydrogens are very acidic, and are then easily removed by a weak base.
Explain why an E1 mechanism is not possible with a strong base?
An E1 mechanism requires that a leaving group leaves first with the formation of a carbocation intermediate. A strong base will react to remove a beta proton in concert with the loss of the leaving group. Therefore there will be no chance for the formation of the carbocation intermediate.
Explain why an SN2 mechanism requires methyl, primary, (or secondary) alkyl halide, whereas an E2 can work with a primary through to tertiary alkyl halide.
The SN2 mechanism involves the attack of the nucleophile form the opposite side of the leaving group in one concerted step. This means there needs to be sufficient room for the nucleophile to access the alpha carbon. This will therefore only happen with a methyl, primary or at most secondary alkyl halide. By contrast an E2 mechanism, although it occurs in one concerted step, only requires access to beta hydrogens, which is unrelated to whether the alkyl halide is primary, secondary, or tertiary at the alpha carbon.
Explain why both an SN1 and E1 mechanism requires a secondary or tertiary alkyl halide.
Both the SN1 and E1 mechanism requires a secondary or tertiary alkyl halide, because both mechanisms require the formation of a carbocation intermediate. This intermediate will only form is there is stabilization through induction and more importantly by hyperconjugation. This sort of stabilization is only effective if the carbocation is secondary or tertiary. An exception to this is when the carbocation is stabilized by resonance in allylic or benzylic carbocations.
Which substitution or elimination reactions can undergo rearrangments?
Substitution and elimination reactions that involve the formation of a carbocation (SN1 and E1) can undergo rearrangements (hydride shift or alkyl shift) if this results in a more stable carbocation.
