Question

Which alkyl bromide reacts fastest with sodium iodide in acetone:1- bromobutane or neopentyl bromide? Is this consistent with your prediction? If not, what were you expecting? Explain the difference inreactivity even though both of these are primary alkyl bromides.

Which solvent gave the fastest reaction; acetone or the ethanol/acetone mixture? Is this consistent with your prediction? If not, what were you expecting? Explain how the solvent affects the rate in the SN2 reaction.

Explain the reactivity of bromobenzene in the SN2 reaction. Is this consistent with your prediction? If not, what were you expecting?

Answer 1

1-bromobutane and neopentylbormide both are primary alkyl halides.

NaI give Iodide ion which isa good nucleophile.

Acetone is a less polar aprotic solvent .

All these support the SN2 mechanism.

In Sn2 the nucelophile attacks from backside of the substrate, thus steric factors in the substrate play important role.

In neopentyl bromide, the substrae is much crowded, compared to 1-bromobutane.

THUS WE EXPECT 1-BROMOBUTANE TO BE MORE REACTIVE THAN NEOPENTYLBROMIDE.

2)admong acetone and ethanol/acetone mixtures, acetone gives faster reaction than the mixture.

It is due to the fact that Sn2 reaction is favored by polar aprotic solvents than polar protic solvents, due to free availability of nucleophile in aprotic solvents.

In protic solvents like ethanol/water the nucleophiles are solvated and are less available.

Changing the solvent from water/ethanol to DMFa polar protic solvent greatly enhances the rate of SN2 reaction due to the presence of free (naked) nucleophile.

3) bromobenzene in SN2 reaction is very less reactive , due to the high steric hindrance offered by the benzene ring for the backside nucleophilic attack.