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### Topic: Determine whether or not it is a Sn1 or Sn2 Reaction? Help, I'm so lost!  (Read 24873 times)

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#### theanonymous

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##### Re: Determine whether or not it is a Sn1 or Sn2 Reaction? Help, I'm so lost!
« Reply #30 on: October 16, 2012, 05:12:42 PM »
The reaction depends ONLY on the concentration of the alcohol, which is why we would call this reaction unimolecular (SN1)

Is that the answer to this question?
Almost.  It is not quite an experiment yet.  Let me ask you a question.  If you double the concentration of bromoalkane, R-Br, does the rate double for the SN1 case?  For the SN2 case?

If you write the two rate equations side-by-side or on two adjacent lines of text, you will see one difference between them that suggests a second experiment.  Two final points.  One I interpreted the initial question to be, "How do you discriminate (decide) between the possibility that the reaction proceeds by the SN1 versus the SN2 mechanism"  A related question is "Does this particular set of conditions favor one mechanism over the other"  I would answer the two questions differently.  Two, I think it is better to say that the nucleophile is bromide ion, Br1-, than to say it is bromine.

Omg this is so overwheliming...i've been working on this problem for 4 hours...
Can you help me formulate the answer?

Would I just say...

If you double the concentration of bromoalkane, R-Br, the rate DOES double for the SN1 case.
However, for the SN2 case, the rate wouldn't double?

I'm getting more confused because I feel like I'm going in circles..
And I have like 4 other problems to finish too.

#### Babcock_Hall

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##### Re: Determine whether or not it is a Sn1 or Sn2 Reaction? Help, I'm so lost!
« Reply #31 on: October 16, 2012, 05:17:00 PM »
For SN2 Reactions, the rate law is: Rate = k[haloalkane][nucleophile]

The conversion of 2-Butyl alcohol to 2-butyl bromide upon treatment with aqueous HBr can't be an SN2 reaction because the haloalkane and nucleophile is both Br (?)

For SN1 Reactions, the rate law is: Rate = k[R original molecule-X the leaving group]
Oops, I made a mistake in my earlier post, which I just now realized.  Sometimes halide ions (usually Br-1 or I1-) become the leaving group in these reactions and sometimes they are the nucleophile.  Sorry about the confusion.

For a general SN2 reaction
rate = k[R-X][nucleophile] (X is the leaving group)
The starting material is the same in either the SN1 or the SN2 case.  The product is also the same, other than stereochemistry.  The product is the bromoalkane, and it can be represented as R-Br.
« Last Edit: October 16, 2012, 05:27:54 PM by Babcock_Hall »

#### theanonymous

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##### Re: Determine whether or not it is a Sn1 or Sn2 Reaction? Help, I'm so lost!
« Reply #32 on: October 16, 2012, 05:20:48 PM »
So the two rate laws side by side would be...

SN2: Rate = k[R-X][nucleophile]
SN2: Rate = k[R-H2O][Br-]

SN1: Rate = k[R-H2O]
The rate of this reaction depends on one thing; how quickly  the leaving group can leave so that the nucleophile Br- can squeeze in.

?

#### theanonymous

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##### Re: Determine whether or not it is a Sn1 or Sn2 Reaction? Help, I'm so lost!
« Reply #33 on: October 16, 2012, 05:23:56 PM »
For SN2 Reactions, the rate law is: Rate = k[haloalkane][nucleophile]

The conversion of 2-Butyl alcohol to 2-butyl bromide upon treatment with aqueous HBr can't be an SN2 reaction because the haloalkane and nucleophile is both Br (?)

For SN1 Reactions, the rate law is: Rate = k[R original molecule-X the leaving group]
Oops, I made a mistake in my earlier post, which I just now realized.  Sometimes halide ions (usually Br-1 or I1-) become the leaving group in these reactions and sometimes they are the nucleophile.  Sorry about the confusion.

For a general SN2 reaction
rate = k[R-X][nucleophile] (X is the leaving group)
The starting material is the same in either the SN1 or the SN2 case.  The product is also the same, other than stereochemistry.  The product is the bromoalkane, and it can be represented as R-Br.

Wait is that the answer? Like for describing two experiements that could allow me to determine whether the reaction proceeeds though an Sn1 or Sn2 reaction??

#### Babcock_Hall

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##### Re: Determine whether or not it is a Sn1 or Sn2 Reaction? Help, I'm so lost!
« Reply #34 on: October 16, 2012, 05:27:18 PM »
Omg this is so overwheliming...i've been working on this problem for 4 hours...
Can you help me formulate the answer?

Would I just say...

If you double the concentration of bromoalkane, R-Br, the rate DOES double for the SN1 case.
However, for the SN2 case, the rate wouldn't double?

I'm getting more confused because I feel like I'm going in circles..
And I have like 4 other problems to finish too.
I made a mistake in two of my previous comments, and I was able to edit only one of them just now (I wrote the wrong rate law).  You starting material is an alcohol, and your product is a bromoalkane, and I had it backwards.  I am multitasking, and my focus was not all that it should have been.

If I double the concentration of the alcohol, R-X, I will double the rate in either the SN2 case or the SN1 case.  Make sure you understand this in terms of the two rate equations before going forward.  Now what happens to the rate if I double the concentration of the nucleophile instead?
« Last Edit: October 16, 2012, 05:54:23 PM by Babcock_Hall »

#### Babcock_Hall

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##### Re: Determine whether or not it is a Sn1 or Sn2 Reaction? Help, I'm so lost!
« Reply #35 on: October 16, 2012, 05:30:30 PM »
So the two rate laws side by side would be...

SN2: Rate = k[R-X][nucleophile]
SN2: Rate = k[R-H2O][Br-]

SN1: Rate = k[R-H2O]
The rate of this reaction depends on one thing; how quickly  the leaving group can leave so that the nucleophile Br- can squeeze in.
From the mathematical point of view, what does the SN2 rate equation have that the SN1 rate equation does not have?

#### theanonymous

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##### Re: Determine whether or not it is a Sn1 or Sn2 Reaction? Help, I'm so lost!
« Reply #36 on: October 16, 2012, 05:39:13 PM »
EDIT:

Would this be a better way to formulate my answer?

Experiment 1)
If you double the concentration of the of the alcohol, R-X, you would double the rate of the reaction in either the SN2 reaction OR the SN1 reaction.

Experiment 2)
Reminder: The rate law for SN2 has the concentration of the nucleophile in it: k[R-X][Br-]
Whereas the rate law for the SN1 just has the alcohol (original molecule with the leaving group) in it: k[R-X].

In contrast to Experiment 1, if you double the concentration of the nucleophile, the rate of the Sn2 reaction would be dependent on BOTH the alcohol AND the nucleophile.
« Last Edit: October 16, 2012, 05:49:38 PM by theanonymous »

#### Babcock_Hall

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##### Re: Determine whether or not it is a Sn1 or Sn2 Reaction? Help, I'm so lost!
« Reply #37 on: October 16, 2012, 05:53:01 PM »
EDIT:

Would this be a better way to formulate my answer?

Experiment 1)
If you double the concentration of the of the alcohol, R-X, you would double the rate of the reaction in either the SN2 reaction OR the SN1 reaction.

Experiment 2)
Reminder: The rate law for SN2 has the concentration of the nucleophile in it: k[R-X][Br-]
Whereas the rate law for the SN1 just has the alcohol (original molecule with the leaving group) in it: k[R-X].

In contrast to Experiment 1, if you double the concentration of the nucleophile, the rate of the Sn2 reaction would be dependent on BOTH the alcohol AND the nucleophile.
I think you have the basic idea down, and you just need to tweak the wording a little bit.  For experiment 2), you correctly say that when the concentration of nucleophile is doubled, the rate of the SN2 reaction would double.  You should say that the rate of the SN1 reaction would stay the same.

#### theanonymous

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##### Re: Determine whether or not it is a Sn1 or Sn2 Reaction? Help, I'm so lost!
« Reply #38 on: October 16, 2012, 05:59:45 PM »
Sweeeeet.
Ok!

Thanks a bunch!!!

I never thought I'd be able to understand this in time. It's starting to make more sense, thanks to you...Now i have to bookmark this page

#### orgopete

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##### Re: Determine whether or not it is a Sn1 or Sn2 Reaction? Help, I'm so lost!
« Reply #39 on: October 16, 2012, 10:17:29 PM »
Hint, if you repeated the reaction with twice the HBr present, how would that affect the rate? If the reaction were SN1, how would that affect the reaction? SN2?

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