Why do nucleophiles always attack from backside in SN2 reactions?

The SN2 reaction occurs in one step through the backside attack. That is, in this reaction, the nucleophile attacks the carbon atom of the substrate from the opposite side of the living group.

The SN2 reaction is completed through the formation of a transition state. In the transition state, the nucleophile, the carbon and the living group are in the same straight line.

Why do nucleophiles always attack from backside in SN2 reactions?

In the transition state, the carbon atom attacked by the nucleophile loses its tetrahedral (sp3) arrangement and remains in the sp2 state.

In this state, the p-orbital of carbon is approximately perpendicular to the plane of the three sp2-hybrid orbitals.

Under this condition, one lobe of this p-orbital participates with the living group and the other lobe with the nucleophile. This is why in the SN2 reaction the nucleophile does not attack from the front.

When the nucleophile attacks from the front, both the nucleophile and the living group in the fictitious transition state have to overlap with a lobe of the p-orbital, and this is not possible in reality.

For the above said reason, nucleophiles always attack from backside in SN2 reactions.

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