Reaction A (Starting Material 1-butanol: 2.828 min) Reaction B 7285117 a. Determ
ID: 548508 • Letter: R
Question
Reaction A
(Starting Material 1-butanol: 2.828 min)
Reaction B
7285117
a. Determine the mole ratios of the alkyl bromide:alkyl chloride for both reactions.
b. Would the product ratio for the 1-butanol reaction change if an aprotic solvent such as DMSO was used instead of water? If yes, what change do you expect? Explain.
c. Would the product ratio for the 2-methyl-2-propanol reaction change if an aprotic solvent such as DMSO was used instead of water? If yes, what change do you expect? Explain.
d. State which nucleophile is better in each reaction and explain why. Justify the observed product ratios. (Factors to consider: the structure of the alcohols, the type of reaction mechanism, the rate determining steps of the mechanisms, the nature of the nucleophiles, and the nature of the solvent.)
Retention Time Area under the Peak 2.743 min: 1-chlorobutane (Alkyl Chloride) 54757 3.595 min:1-bromobutane (Alkyl Bromide) 816712 1-butanol XI H20 H2S04 NH4BNH4CI 2-methyl-2-propanolExplanation / Answer
In Reaction A
Mol ratio RBr : RCl =816712/159.8 : 54757/70.90 = 5110.8 : 772.31= 6.62:1
In Reaction B
Mol ratio RBr : RCl =7285117/159.8 : 5559736/70.90 = 45588.96 : 78416.58= 1:1.72
The reaction mechanism is SN2 and the rate of the reaction depends on the nucleophilicity of the nucleophile, it’s a single step, 2nd order bimolecular reaction. In H2O, Chloride ions are highly solvated due its smaller size and more electron density making the bromide a better nucleophile giving the product ratio ca 6 to 1.
The product ratio would be reverse if the reaction is carried out in an aprotic solvent in which no salvation effect will be there and chloride will act as stronger nucleophile.
This is an SN1 reaction and the rate of reaction does not depend on the nucleophilicity because the rate determining step is the carbo cation formation but it certainly depends on the solvent because carbo cation formation requires polar protic solvent, so the reaction rate will be slower..
In reaction A, SN2 reaction, bromide is the better nucleophile.
In reaction B, SN1 reaction, the rate of the reaction is independent of nucleophilicity of nucleophile.
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