Question

1. arrange the halides in order of reactivity under the two sets of conditions

Part 2: A Comparison of the Reactivity of Alkyl Halides 2% ethanolic silver nitrate 15 Nal in acetone 1. n-butyl chloride Clear Milky precipitate 2. sec-butyl chloride After heating, some precipitation 3. t-butyl chloride White precipitate After heating, precipitate Clear No precipitate Milk precipitate 4. n-butyl bromide Clear 5. allyl chloride White precipitate Brown color precipitate 6. chlorobenzene Clear Clear 1. Arrange the halides in order of reactivity under the two sets of conditions. (3 points) 2. Discuss the reason for these observations in your notebook. (3 points)

Answer 1

First, examine the reason behind the observations.

2)

In ethanolic silver nitrate, the substitution of alkyl halides occurs through SN1 mechanism. Ethanol is moderately polar (it is more polar than acetone) , and thus it favors SN1 mechanism since it can stabilise the carbocation which is formed as an intermediate in SN1 reaction. Ag+ ion can also coordinare with the leaving halide group to make it leave more easily. Also, the nuceophile here is NO3- ion, which is a poor nucleophile. This too indicates SN1 mechanism.

In SN1 mechanism, the rate determining step is the formation of carbocation as the halide group leaves.Hence, the more stable the carbocation formed, the faster the reaction will take place. Order of stability of carbocation is:

Allyl > tertiary > secondary > primary .

Hence, order of reactivity in ethanolic silver nitrate:

Allyl chloride ≥ t-butyl chloride > sec-butyl chloride > n-butyl bromide ≥ n-butyl chloride > chlorobenzene

In NaI - acetone, SN2 mechanism will be followed. Acetone is a non polar solvent, thus favoring SN2 mechanism , and I- ion is also a better nucleophile. In SN1 mechanism, the reaction will occur faster as steric crowding around the carbon atom which has the leaving group decreases, since in the transition step, that Carbon is simultaeously coordinated to both the attacking nucleophile and the leaving group. A less crowded Carbon allows for the nucleophile to approach at the required angle.

Hence, order of reactivity in NaI in acetone is:

allyl chloride > n-butyl bromide ≥ n-butyl chloride > sec-butyl chloride > t-butyl chloride > chlorobenzene

Note : Allylic compounds are reactive towards both SN1 and SN2 mechanism. Allylic carbocations are stabilised through resonance and they donot have steric crowding around carbon which is reacting in SN2.

Chlorobenzene is unreactive towards SN1 since phenyl carbocation is not stable at all.

Also, Carbon where substitution takes place in SN2 is sp2 hybridised, which doesnot allow the nucleophile to attack at the required angle for reaction to take place (In SN2 reactions, the nucleophile must attack from the opposite side of the leaving group).