Does Sn2 reaction do inversion

Key Concepts

1. One-step Mechanism
   $S_N2$ stands for Substitution, Nucleophilic, Bimolecular. Both nucleophile (Nu$^-$) and substrate (R–L) appear together in the rate-determining step.

2. Back-Side Attack
   The nucleophile approaches the carbon exactly opposite to the leaving group. This geometry minimizes electron–electron repulsion and lines up the orbitals for bond making and bond breaking.

3. Transition State
   A pentacoordinate carbon forms temporarily:

   $\text{Nu} \;\dots\; C \;\dots\; L$

   where Nu–C and C–L bonds are half-formed/half-broken.

4. Inversion of Configuration
   Because the attack is strictly from the back, the stereochemistry flips like an umbrella turning inside-out. If the carbon is chiral, R becomes S or vice-versa (provided the priority order stays the same).

5. Why Not Retention?
   A front-side attack would force Nu$^-$ to pass right through the electron cloud of L, which is highly unfavourable. Hence only back-side attack is observed, leading only inversion.

Imagine a Dance Move

Think of a carbon atom as a dancer holding five balloons – four around his waist (the bonds to other atoms) and one behind his back (the leaving group, like bromine).

A new friend (nucleophile) wants to grab the balloon from the back. Because the front is crowded, the new friend sneaks from the back side and pushes the old balloon away at the same time.

When the new friend finishes, the dancer turns around so that all the balloons are now facing the opposite direction.

That one‐step back-side attack is called an $S_N2$ reaction, and the dancer’s turn is called inversion of configuration (also nick-named Walden inversion).

Step-by-Step Confirmation of Inversion

1. Identify Mechanism
   The reaction is $S_N2$ (single step, nucleophile + substrate together in RDS).
2. Visualise Approach
   Nu$^-$ must come from the side opposite to the leaving group (back-side).
3. Form Transition State
   $[\text{Nu} \;---\; C \;---\; L]^{\ddagger}$ (Nu–C forming, C–L breaking).
4. Leaving Group Departs
   C–L bond breaks completely, Nu–C bond forms completely.
5. Resultant Geometry
   The three remaining substituents around carbon flip; configuration is inverted.

Therefore, an $S_N2$ reaction always gives inversion (Walden inversion) at the reacting stereocentre.