Organic Chemistry Help: Mechanism for SN2 Reactions
Ciao! I’m struggling with understanding SN2 reaction mechanisms. I get the concept that it’s a bimolecular nucleophilic substitution and that it happens in one step, but I keep getting confused about the orbital interactions and inversion of configuration.
The part I really don’t understand is why primary and secondary halides do SN2 readily but tertiary ones don’t. I know steric hindrance plays a role but I can’t visualize it properly.
Also, can someone explain the difference between the nucleophile approaching from the back side vs. what happens if it approaches from the front? My textbook shows the mechanism but I need a better physical understanding.
Do you have any good resources or can someone walk through a specific example? I have a quiz Monday and I want to actually understand this, not just memorize it.
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if u want visualization check out the chemsketch app or wolfram demonstrations. theres one specifically for sn2 showing the 3d inversion. way better than textbook diagrams imo. also khan academy has a decent explanation if u want an alternative take
Pro tip: when you're doing practice problems, draw out ALL the atoms in 3D using wedges and dashes. That forces you to visualize the actual geometry rather than just looking at flat 2D structures. It feels tedious but it really helps with understanding why tertiary carbons don't work.
The backside approach is crucial to SN2. Imagine the C-X bond is an axis. The nucleophile must attack from the opposite side, forming a transition state that looks like a trigonal bipyramidal intermediate. The key is that this geometry forces the carbon to invert - it's like an umbrella flipping inside out.
For tertiary carbons: there are THREE alkyl groups already attached. When the nucleophile tries to approach from the back, those three groups create a wall. It's sterics - pure and simple. The nucleophile literally can't get close enough. Primary carbons only have one alkyl group (the hydrogen counts as less bulky), so access is easy.
For the orbital interactions: the nucleophile's lone pair is donating into the antibonding orbital of the C-X bond, breaking it while simultaneously forming a new C-Nu bond. This all happens concurrently.
Oh wow, the umbrella analogy is perfect! That helps so much. So the nucleophile MUST come from behind, and the steric bulk prevents it from even getting there in tertiary cases. And the transition state is when it's halfway through flipping - that makes it click for me now. Thank you!