Question

Between iodopropane and bromopropane which would undergo SN2 reaction faster and why? Do you
think the order will change if we change the solvent?

Answer 1

An incoming nucleophile will often react with whatever electrophile it can reach most easily. In technical terms, a secondary halide is more sterically hindered than a primary halide, so #S_N2# will occur more readily at the primary halide.

An incoming nucleophile will often react with whatever electrophile it can reach most easily. In technical terms, a secondary halide is more sterically hindered than a primary halide, so #S_N2# will occur more readily at the primary halide.We should therefore expect the isopropyl iodide to have the slower reaction rate.

An incoming nucleophile will often react with whatever electrophile it can reach most easily. In technical terms, a secondary halide is more sterically hindered than a primary halide, so #S_N2# will occur more readily at the primary halide.We should therefore expect the isopropyl iodide to have the slower reaction rate.In #(CH_3)_3C CH_2I# (neopentyl iodide), in addition to iodine, the carbon atom is bound to two hydrogen atoms and only one other carbon. This is called a primary halide (primary meaning bound to only one carbon).

An incoming nucleophile will often react with whatever electrophile it can reach most easily. In technical terms, a secondary halide is more sterically hindered than a primary halide, so #S_N2# will occur more readily at the primary halide.We should therefore expect the isopropyl iodide to have the slower reaction rate.In #(CH_3)_3C CH_2I# (neopentyl iodide), in addition to iodine, the carbon atom is bound to two hydrogen atoms and only one other carbon. This is called a primary halide (primary meaning bound to only one carbon).You would therefore expect this compound to have the fastest #"S"_"N"2# reaction rate. But there is a complication.

An incoming nucleophile will often react with whatever electrophile it can reach most easily. In technical terms, a secondary halide is more sterically hindered than a primary halide, so #S_N2# will occur more readily at the primary halide.We should therefore expect the isopropyl iodide to have the slower reaction rate.In #(CH_3)_3C CH_2I# (neopentyl iodide), in addition to iodine, the carbon atom is bound to two hydrogen atoms and only one other carbon. This is called a primary halide (primary meaning bound to only one carbon).You would therefore expect this compound to have the fastest #"S"_"N"2# reaction rate. But there is a complication.The bulky t-butyl group prevents backside attack by the nucleophile.

The steric hindrance is so effective that isopropyl iodide reacts almost 3000 times as fast as neopentyl iodide.