Trying to understand why Sn2 is not possible in this reaction

I am trying to understand why the red Br doesn't react? Is it because there can't be a flip of groups? Or steric hindrance?


Thank you!


[![enter image description here](https://i.stack.imgur.com/CjGPo.png)](https://i.stack.imgur.com/CjGPo.png)




Backside attack on the red bromide is not possible. The rest of the norbornyl skeleton completely blocks the trajectory required for $\mathrm{S}\_{N}2$ reaction.


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