Why might 2-bromopentane, when reacting sodium iodide in acetone (SN2 reaction) ...?

In a SN2 reaction, it is necessary for the nucleophile to attack the backside of the carbon atom. The 2-carbon of 2-bromopentane must be somewhat more hindered than the 3-carbon of 3-bromopentane. Use a molecular model kit to clarify.