Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: Benzoic Acid on March 22, 2006, 05:22:34 PM
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Hello, I did some reactions in the lab and the results I got don’t make sense to me for some. Maybe it is because I don’t completely understand the effect of the solvent.
1-Chlorobutane did not react in Sodium Iodide in acetone, which doesn’t make sense to me because this compound is a primary halide… shouldn’t it have reacted in this SN2 solvent?
Iodoethane also did not react in NaI in acetone. However it did react in 1% ethanolic silver nitrate solution, which is an SN1 solvent… what’s the deal?
1-Chloroadamantane reacted in NaI in acetone, again this doesn’t make sense to me because 1-Chloroadamantane is a tertiary compound.
Maybe other students contaminated the vials ;(
Thanks
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1-Chlorobutane did not react in Sodium Iodide in acetone, which doesn’t make sense to me because this compound is a primary halide… shouldn’t it have reacted in this SN2 solvent?
hint: Which is a better leaving group - chloride or iodide?
Iodoethane also did not react in NaI in acetone. However it did react in 1% ethanolic silver nitrate solution, which is an SN1 solvent… what’s the deal?
It is not surprising if you do not observe any reaction, because the product of the reaction is also the starting reagent:
1. R-X => R+ + X- (what happen when X = I)
2. R+ + I- (from NaI) -> R-I
1-Chloroadamantane reacted in NaI in acetone, again this doesn’t make sense to me because 1-Chloroadamantane is a tertiary compound.
Maybe other students contaminated the vials ;(
Thanks
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1-Chloroadamantane would not react Sn1 because of the bond angles--the carbon which the cl is attached to will not be able to become a planar carbocation because the structure of the rest of the molecule would be too strained.
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1-Chlorobutane did not react in Sodium Iodide in acetone, which doesn’t make sense to me because this compound is a primary halide… shouldn’t it have reacted in this SN2 solvent?
hint: Which is a better leaving group - chloride or iodide?
The iodide may be a better leaving group, however, it is a much better nucleophile than the Chloride. A primary alkyl halide (Cl or Br) with NaI and Acetone, should (barring some unusual exception) produce the primary iodide compound. This reaction should drive almost to completion or all the way. This is because NaI is soluble in acetone, however, when the substitution occurs, you produce the insoluble NaCl, which drops out of the solution, and drives the equilibrium toward the right.
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Hi. Apparently I have a similar orgo lab as benzoic acid.
1-chlorobutane with NaI <-- SN2
1-bromobutane with NaI <-- SN2
2-chlorobutane with NaI <--SN1 & SN2
2-bromobutane with NaI <--SN1 & SN2
I know in the post it says that 1-chlorobutane with NaI reacts in SN2 solvent, and it did when we heated the solution up. I hope all of the above is right so far:
The question is known about the kinetics of these reactions? I'm confused with kinetics of these reactions; I understand that under kinetic conditions you look at the carbocation stability but how do you tell if it's first order or second order?
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If the reaction is second order overall (1st order in both alkyl halide and nucleophile), then doubling the concentration of either should double the rate of the reaction. If it is 1st order in the alkyl halide, but zero order in the nucleophile (1st order overall), then doubling the concentration of the nucleophile should have no effect. If you are provided with different concentrations of each material in the lab, you should be able to check to see if the rate changes as you change concentrations of reagents.
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If the reaction is second order overall (1st order in both alkyl halide and nucleophile), then doubling the concentration of either should double the rate of the reaction. If it is 1st order in the alkyl halide, but zero order in the nucleophile (1st order overall), then doubling the concentration of the nucleophile should have no effect. If you are provided with different concentrations of each material in the lab, you should be able to check to see if the rate changes as you change concentrations of reagents.
In lab, we weren't able to change the concentrations, so how would you be able to tell whether 2-chlorobutane and 2-bromobutane is doing a SN1 reaction or a SN2 reaction?
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Adjust the temperature. How do you think this will change SN2 vs SN1 reaction rates?
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Quote from previous post:
"In lab, we weren't able to change the concentrations, so how would you be able to tell whether 2-chlorobutane and 2-bromobutane is doing a SN1 reaction or a SN2 reaction? "
Well, if you had optically active materials, the product of SN2 would still be optically active, whereas SN1 would be racemic But most undergraduate labs do not use optically active materials (at least not 2-halobutanes), so this will probably not be of use to you.
In the absence of experimental data, the general rule is that (for secondary halides) strong nucleophiles in aprotic polar solvents favor SN2, whereas weak nucleophiles in protic solvents favor SN1 reactions. But as previous contributors to this post have said, their data do not seem to support this trend. Contamination (esp by water) can cloud the results.