I understand the basics of HBr/HI cleavage of ethers.
Ether: R-O-R'
When both substituents are primary or secondary, cleavage occurs by SN2. Br- or I- attack less hindered substituent.
when one of the ether substituents is tertiary, benzyllic, or allylic, the ether cleaves by either E1 or SN1.
My question is: how do I know how the ether will cleave when there is a tertiary or benzyllic or allylic group? How do I know which side will be attacked and will it be E1 or SN1?
Thanks!
When determining how an ether will cleave in the presence of a tertiary, benzyllic, or allylic group, there are a few factors to consider. The decision between E1 (elimination) or SN1 (nucleophilic substitution) mechanisms will depend on the stability of the carbocation intermediate formed and the strength of the nucleophile.
1. Stability of the Carbocation Intermediate:
- Tertiary alkyl groups stabilize positive charges better than secondary or primary alkyl groups due to increased hyperconjugation and inductive effects.
- Benzyllic and allylic carbocations are also stabilized due to resonance delocalization of the positive charge.
- If a more stable carbocation intermediate can be formed, E1 elimination is favored.
2. Strength of the Nucleophile:
- Ethers are typically cleaved by weak nucleophiles, such as Br- or I-, rather than strong nucleophiles like hydroxide (OH-) or alkoxides (RO-).
- If a strong nucleophile is present, it will favor SN1 substitution instead of E1 elimination.
Based on these factors, the following general trends can be observed:
- If the ether substitution is tertiary and the nucleophile is weak, it is likely to undergo E1 elimination rather than SN1 substitution. The less hindered substituent will be attacked by the weak nucleophile, resulting in the formation of an alkene.
- If the ether substitution is benzyllic or allylic and the nucleophile is weak, it is also likely to undergo E1 elimination. The carbocation stability provided by the resonance of the benzyllic or allylic system will promote elimination.
- If a strong nucleophile is present, regardless of the nature of the ether substitution, it is more likely to undergo SN1 substitution. The nucleophile will attack the carbocation intermediate, resulting in the substitution of one substituent with the nucleophile.
It is important to note that these are general trends, and the specific reaction conditions, such as solvent, temperature, and concentration, may also influence the reaction outcome. Additionally, experimental data and reaction kinetics can further help determine the most likely mechanism.