Este caso es una reacción SN1 y no SN2 por varias razones. Primero, el carbono donde está el bromo es bencílico, y eso hace que cuando el Br se va se forme un carbocatión bencílico muy estable gracias a la resonancia con el anillo aromático. Esa estabilidad es lo que más favorece el mecanismo SN1.

Segundo, en un SN2 el nucleófilo tendría que atacar directamente ese carbono, pero como está justo al lado del anillo, hay mucho estorbo estérico y el ataque por la parte de atrás se hace difícil. Por eso el SN2 no es favorable aquí.

Además, la reacción está en condiciones que también ayudan al SN1: un disolvente polar prótico como agua o alcohol estabiliza tanto el carbocatión como el grupo saliente.

En conclusión, esta reacción va por SN1 porque el carbocatión bencílico es muy estable y el ataque SN2 está estorbado por el anillo.

1 hour ago, Alvarop said:

This case is an SN1 reaction and not SN2 for several reasons. First, the carbon where the bromine is is benzyl, and that means that when the Br leaves, a very stable benzyl carbocation is formed thanks to the resonance with the aromatic ring. This stability is what the SN1 mechanism favors the most.

Second, in an SN2 the nucleophile would have to attack that carbon directly, but since it is right next to the ring, there is a lot of steric hindrance and the attack from the back is difficult. That's why SN2 isn't favorable here.

In addition, the reaction is in conditions that also help SN1: a protic polar solvent such as water or alcohol stabilizes both the carbocation and the leaving group.

In conclusion, this reaction goes for SN1 because the benzyl carbocation is very stable and the SN2 attack is hindered by the ring.

What is this? It looks like an answer to a homework question that was not provided.

It is my understanding that benzyl bromide undergoes nucleophilic substitution via a predominately S_N2 mechanism. Although the benzyl carbocation is stabilised by the benzene ring, it is a primary carbocation, and the transition state of the S_N2 mechanism is also stabilised by the benzene ring. Thus, the benzene ring enhances both the S_N1 and the S_N2 mechanisms without necessarily favouring the S_N1 mechanism over the S_N2 mechanism. However, the use of a polar protic solvent does enhance the S_N1 mechanism relative to the S_N2 mechanism, while the use of a polar aprotic solvent enhances the S_N2 mechanism relative to the S_N1 mechanism, so the specific reaction conditions may be significant to the relative contributions of the two mechanisms.

Edited October 4Oct 4 by KJW