Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: theanonymous on October 16, 2012, 09:53:15 AM
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My organic chemistry 1 teacher isn't the best organic chem teacher you could ask for - he's way too smart for our class and he doesn't cover the basic stuff for Organic Chemistry 1. So now, Organic Chemistry is absolute hell for me.
There are about 150 students in our class - and guess what the average was for the 1st test? A freaking 31%. Yes, 31 out of 100... 150-200 students. 31% -.-
Right now, I'm panicking because the end of the semester is almost here and we have one more exam left to take.
My homework grades is the only thing that's supporting my grade for this class, but they're so hard. I'm an excellent note taker as well (an active one) and I have never in my entire life made a 22% on a test.
Anyways, the first question I have is here:
1. 2-Butyl alcohol is converted to 2-butyl bromide upon treatment with aqueous HBr at elevated temperature. Please describe two experiments/measurements you could make to determine whether the reaction proceeds via an Sn1 or an Sn2 reaction. (Assume that it is one or the other, not both.)
What am I supposed to do? Write the reaction???
Does it look like this: http://www.imagebam.com/image/ea98fe215480338
If so, what do I do next?
It says "describe two experiements/measurements you could make to determine whether the reaction proceeds via an Sn1 or an Sn2 reaction" though (?) :(
*delete me*
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My organic chemistry 1 teacher isn't the best organic chem teacher you could ask for - he's way too smart for our class and he doesn't cover the basic stuff for Organic Chemistry 1. So now, Organic Chemistry is absolute hell for me.
There are about 150 students in our class - and guess what the average was for the 1st test? A freaking 31%. Yes, 31 out of 100... 150-200 students. 31% -.-
Right now, I'm panicking because the end of the semester is almost here and we have one more exam left to take.
My homework grades is the only thing that's supporting my grade for this class, but they're so hard. I'm an excellent note taker as well (an active one) and I have never in my entire life made a 22% on a test.
Anyways, the first question I have is here:
1. 2-Butyl alcohol is converted to 2-butyl bromide upon treatment with aqueous HBr at elevated temperature. Please describe two experiments/measurements you could make to determine whether the reaction proceeds via an Sn1 or an Sn2 reaction. (Assume that it is one or the other, not both.)
What am I supposed to do? Write the reaction???
Does it look like this: http://www.imagebam.com/image/ea98fe215480338
If so, what do I do next?
It says "describe two experiements/measurements you could make to determine whether the reaction proceeds via an Sn1 or an Sn2 reaction" though (?) :(
*delete me*
What are the differences between SN1 and SN2 reactions? Can you write complete equations, including intermediates for both? You've made a good start in your depiction of the alcohol - there seems to be something you have added to that that is missing from your bromide.
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What are the differences between SN1 and SN2 reactions? Can you write complete equations, including intermediates for both? You've made a good start in your depiction of the alcohol - there seems to be something you have added to that that is missing from your bromide.
That's the thing...I have absolutely no idea where to go from there!
That's all I have in my notes for 2-butyl-alcohol. :(
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Here's another page of my notes:
http://www.imagebam.com/image/49aabf215486613
Is the answer basically..." Treating 2-Butyl alcohol with SOCl2 and PBr3"?
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That's all I have in my notes for 2-butyl-alcohol. :(
If you are not prepared to read beyond your notes, then I promise you will fail. I'm not trying to be nasty, just realistic. You cannot pass a chemistry course without opening a book.
Get yourself a couple of organic chemistry textbooks from the library. My personal recommendations would be Clayden/Greeves/Warren (http://www.amazon.co.uk/Organic-Chemistry-Jonathan-Clayden/dp/0199270295) and Sykes (http://www.amazon.co.uk/Guidebook-Mechanism-Organic-Chemistry-Peter/dp/0582446953) (for other recommended texts click here (http://www.chemicalforums.com/index.php?topic=2414.0)). Read and make additional notes on the chapters dealing with nucleophilic aliphatic substitution.
Hint: What factors affect the rate of SN1 and SN2 reactions? Think about the differences.
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Do you understand where the names SN1 and SN2 come from? That would be a good place to start. Also, is carbon-2 of the reactant and product a stereogenic center or not? Your drawing of the product does not make this clear.
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:(
This is hopeless..
I have my textbook right in front of me (Organic Chemistry 8th Edition Francis A. Carey Robert M. Giuliano) and I'm reading the chapter where I should be finding an explanation to all this but I feel like it's all fluff.
My teacher literally did /not/ go over the fundamentals of organic chemistry, he just dove right into the hard stuff - or the "stuff that really matters." I'm doing the best I can to keep up with him by looking up explanations online and I'm struggling to find something that's actually substantial!
:(:(:(
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Don't cry. Start reading at the first page and finish with the last page.
I'm sure you can find some web sites with easy explanations to the fundamentals.
By the way; what, for you, are the fundamentals?
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Hint: What factors affect the rate of SN1 and SN2 reactions? Think about the differences.
Alcohol structure has an effect on reaction rate
SN1 - a unimolecular reaction (only one thing undergoes a chemical change); tertiary and secondary carbocations
SN2 - a bimolecular reaction (two things undergoe a chemical change; methyl and primary carbocations
?
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What don't you understand in that quote?
Unimolecular, bimolecular, carbocation. If you don't tell us exactly what you don't get how can we help?
One question at a time, it makes it easier for everyone to understand.
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There are lots of web pages describing SN1 and SN2 reactions - try looking here http://en.wikipedia.org/wiki/SN1_reaction and http://en.wikipedia.org/wiki/SN2_reaction
Look at how the stereoselectivity is different for SN1 and SN2 and look at what favours each. Now try answering your own question again.
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Discoder...
I don't understand how to go about answering this question:
2-Butyl alcohol is converted to 2-butyl bromide upon treatment with aqueous HBr at elevated temperature. Please describe two experiments/measurements you could make to determine whether the reaction proceeds via an Sn1 or an Sn2 reaction. (Assume that it is one or the other, not both.)
I also don't understand how my teacher expects us to know how to think abstractly in an intro course to Organic Chemistry when we have no basis for the stuff we're doing now :(
For example, knowing the structure for 2-Butyl alcohol, 2-butyl bromide, etc etc, arrow pushing mechanisms, etc etc, electron flow, etc etc. All the little stuff that my teacher never elaborated on or explained clearly enough. I don't want to make it seem like I'm complaining about how miserable I am with Organic Chemistry. I'm DESPERATELY trying to piece all this together because I'm such a logical thinker. My teacher makes it all abstract and vague and I'm struggling.
This particular problem is something I don't get because it's no where in my notes and it's no where in my book AND I've also searched online!
I don't know if he's pulling these weird questions out of his head or something but I have absolutely no idea how to approach this question... hence me being here seeking help. I don't want to come across as rude or needy, I'm just trying to make sense of it all.
Ultimately, my teachers inability to provide us with sufficient information BEFORE assigning us extremely complicated/vague homework problems to tackle is what's frustrating me the most. :(
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Hint: What factors affect the rate of SN1 and SN2 reactions? Think about the differences.
Alcohol structure has an effect on reaction rate
SN1 - a unimolecular reaction (only one thing undergoes a chemical change); tertiary and secondary carbocations
SN2 - a bimolecular reaction (two things undergoe a chemical change; methyl and primary carbocations
?
Right according to your definition above the conversion of butane-2-ol to 2-bromobutane must be a unimolecular process. That is SN1.
So what do you think is the mechanism for this reaction, what happens first?
Read this page carefully:
http://en.wikipedia.org/wiki/SN1_reaction (http://en.wikipedia.org/wiki/SN1_reaction)
Google SN1 reaction conditions and see what you get
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There are lots of web pages describing SN1 and SN2 reactions - try looking here http://en.wikipedia.org/wiki/SN1_reaction and http://en.wikipedia.org/wiki/SN2_reaction
Look at how the stereoselectivity is different for SN1 and SN2 and look at what favours each. Now try answering your own question again.
What whattt???
It says "Please describe two experiments/measurements you could make to determine whether the reaction proceeds via an Sn1 or an Sn2 reaction."
??
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Hint: What factors affect the rate of SN1 and SN2 reactions? Think about the differences.
Alcohol structure has an effect on reaction rate
SN1 - a unimolecular reaction (only one thing undergoes a chemical change); tertiary and secondary carbocations
SN2 - a bimolecular reaction (two things undergoe a chemical change; methyl and primary carbocations
?
Right according to your definition above the conversion of butane-2-ol to 2-bromobutane must be a unimolecular process. That is SN1.
So what do you think is the mechanism for this reaction, what happens first?
Read this page carefully:
http://en.wikipedia.org/wiki/SN1_reaction (http://en.wikipedia.org/wiki/SN1_reaction)
Google SN1 reaction conditions and see what you get
OH is displaced by Br (the nucleophile)?
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No, although that is the ultimate reaction.
The OH of the alcohol accepts a proton from the HBr. Then it leaves as H2O to give a secondary carbocation.
What next?
Read this page
http://www.organic-chemistry.org/namedreactions/nucleophilic-substitution-sn1-sn2.shtm
(http://www.organic-chemistry.org/namedreactions/nucleophilic-substitution-sn1-sn2.shtm)
If you wish me to draw the mechanism I will.
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Hint: What factors affect the rate of SN1 and SN2 reactions? Think about the differences.
Alcohol structure has an effect on reaction rate
SN1 - a unimolecular reaction (only one thing undergoes a chemical change); tertiary and secondary carbocations
SN2 - a bimolecular reaction (two things undergoe a chemical change; methyl and primary carbocations
?
Right according to your definition above the conversion of butane-2-ol to 2-bromobutane must be a unimolecular process. That is SN1.
So what do you think is the mechanism for this reaction, what happens first?
Read this page carefully:
http://en.wikipedia.org/wiki/SN1_reaction (http://en.wikipedia.org/wiki/SN1_reaction)
Google SN1 reaction conditions and see what you get
I did and there's a lot of stuff...
hate my life.
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No, although that is the ultimate reaction.
The OH of the alcohol accepts a proton from the HBr. Then it leaves as H2O to give a secondary carbocation.
What next?
Read this page
http://www.organic-chemistry.org/namedreactions/nucleophilic-substitution-sn1-sn2.shtm
(http://www.organic-chemistry.org/namedreactions/nucleophilic-substitution-sn1-sn2.shtm)
If you wish me to draw the mechanism I will.
I would greatly appreciate that!!
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Don't panic, it's really simple.
Here is my mechanism. Others may not like it but it is simple enough.
So what about the experimental conditions to distinguish between SN1 and SN2?
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Don't panic, it's really simple.
Here is my mechanism. Others may not like it but it is simple enough.
So what about the experimental conditions to distinguish between SN1 and SN2?
Sn1
- the rate-determining step is unimolecular
- Sn1 reaction takes place in three steps
That's it?
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No it is not three steps. The intermediate in brackets is only there to show you it's structure. It probably does not exist in the true sense of the word.
But I asked you how you would distinguish between SN1 and SN2 reactions with respect to the reaction conditions. This is also a part of your question. The info is in the links I sent you.
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For SN2 reactions of haloalkanes, the rate law is:
rate = k[haloalkane][nucleophile]
The "2" means bimolecular (first order in the haloalkane and first order in the nucleophile)
I suggest you try this:
1. Identify the haloalkane and the nucleophile in your example.
2. Write the rate law for the SN1 reaction.
rate = ?
Hint: what does the "1" tell you?
EDT
I see that you have made a good start on SN1 reactions in that you know that they are unimolecular. Now you just have to translate the difference between unimolecular and biomolecular reactions into an experiment.
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No it is not three steps. The intermediate in brackets is only there to show you it's structure. It probably does not exist in the true sense of the word.
But I asked you how you would distinguish between SN1 and SN2 reactions with respect to the reaction conditions. This is also a part of your question. The info is in the links I sent you.
Sn2 reactions always proceed by inversion of configuration; two different molecules in a reaction; subsitition of 2 nucleophiles.
Is that it?
Omg at this point I have no idea.
I just keep getting the wrong answer.
Can you please tell me what the answer is?
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Sn2 reactions always proceed by inversion of configuration; two different molecules in a reaction; subsitition of 2 nucleophiles.
So far, so good. Do SN1 reactions proceed with inversion? If not, what do you observe?
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Ohhhhhh!!!!!!
No SN1 reactions don't proceed with inversion!! Racemization occurs right (?)
SN1 Reactions
1) The leaving group attached to the central carbon (C) leaves - a carboncation (C+) is formed
2) The nucleophile comes in and attacks the carbocation (C+). Nucleophile is free to attack from EITHER side; racemization occurs - one enantiomer is converted into two different versions of the molecule (R and S); half the product has retained the original configuration, and about half is inverted.
(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fwww.organic-chemistry.org%2Fnamedreactions%2FSN12.5.gif&hash=6b1992751c16049671b04cbbfa19dde5a6a8471a)
SN2 Reactions
1) Two molecules are involved in the actual transition state
2) 1 leaving group departs while 1 nucleophile comes in and attacks the backside of the molecule
3) Result - inversion of configuration at the stereocenter
(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fwww.organic-chemistry.org%2Fnamedreactions%2FSN12.4.gif&hash=2266daede3f5a977fb9454f4c04a698237679d67)
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Sn2 reactions always proceed by inversion of configuration; two different molecules in a reaction; subsitition of 2 nucleophiles.
So far, so good. Do SN1 reactions proceed with inversion? If not, what do you observe?
So how would I go about "describing two experiments/measurements you could make to determine whether the reaction proceeds via an SN1 or an SN2 reaction"?
Or is the answer just the conditions I posted a few seconds ago?
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One way is to determine the stereochemistry of the product to see whether the product has inverted stereochemistry or is a racemic mixture. I think that doing an experiment with plane-polarized light would be the easiest way to obtain an answer. One down, one to go.
I wrote the rate equation for an SN2 reaction in an earlier message, and the rate equation for SN1 reactions looks similar but not identical. It might be helpful to remember that rate constants (symbolized by the letter k) are just the proportionality constant between the rate of the reaction and the concentration(s) of reactant(s). For SN1 reactions, the rate of the reaction is
rate = ?
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One way is to determine the stereochemistry of the product to see whether the product has inverted stereochemistry or is a racemic mixture. I think that doing an experiment with plane-polarized light would be the easiest way to obtain an answer. One down, one to go.
I wrote the rate equation for an SN2 reaction in an earlier message, and the rate equation for SN1 reactions looks similar but not identical. It might be helpful to remember that rate constants (symbolized by the letter k) are just the proportionality constant between the rate of the reaction and the concentration(s) of reactant(s). For SN1 reactions, the rate of the reaction is
rate = ?
For SN2 reactions of haloalkanes, the rate law is:
rate = k[haloalkane][nucleophile]
The "2" means bimolecular (first order in the haloalkane and first order in the nucleophile)
I suggest you try this:
1. Identify the haloalkane and the nucleophile in your example. Haloalkane and nucleophile is Br (?)
2. Write the rate law for the SN1 reaction.
rate = k[original molecule with the leaving group attached to it]
Hint: what does the "1" tell you?
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?
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So my answer to this whole question would be.
1) We can determine the stereochemistry of the product to see whether the product formed has its configuration inverted (SN2 reaction) or if it is a racemic mixture (SN1 reaction).
2) We can also determine whether or not the reaction is unimolecular or bimolecular by writing out the rate law.
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]
The reaction depends ONLY on the concentration of the alcohol, which is why we would call this reaction unimolecular (SN1). Therefore, the conversion of 2-Butyl alcohol to 2-butyl bromide upon treatment with aqueous HBr is an SN1 reaction.
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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 R-X, 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.
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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. :(:(:(
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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.
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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.
?
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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??
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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?
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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?
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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.
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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.
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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
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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?