SN1 and SN2 Reactions, Part 2
Dec 2006
The SN1 and SN2 examples I have place in substitution reactions should be given further scrutiny. As a practicing synthetic organic chemist, SN1 and SN2 reaction mechanism were not of great concern. Depending on the requirements, reactions would be optimal SN1 or SN2 reactions. Thus, if one needed to form a new bond and if stereochemistry of the product was to be controlled, then an SN2 reaction would be called for. In attempting to accomplish an SN2 reaction, one would use optimal SN2 reaction conditions; good nucleophile, polar-aprotic solvents, and high concentrations.
I note instances in which students are given problems in which they are asked to predict the product(s) of a reaction in which neither of those circumstances are provided. Perhaps these make good questions for some classes, but these only remind me to avoid examples in which this ambiguity might exist. My preference is to provide clear examples of either SN1 or SN2 reactions. I believe students should be able to predict some reactions without any doubt as to what must occur.
As a consequence (even though this is not part of what many students are being asked), students might be able to judge that success and outcome of some reactions might be in doubt. Solvolysis of a secondary chloride in ethanol probably will be a poor reaction (without predicting the product of the reaction). Similarly, addition of sodium bromide to that reaction mixture will not be an effective route for formation of the bromide in an SN2 reaction.
I note instances in which students are given problems in which they are asked to predict the product(s) of a reaction in which neither of those circumstances are provided. Perhaps these make good questions for some classes, but these only remind me to avoid examples in which this ambiguity might exist. My preference is to provide clear examples of either SN1 or SN2 reactions. I believe students should be able to predict some reactions without any doubt as to what must occur.
As a consequence (even though this is not part of what many students are being asked), students might be able to judge that success and outcome of some reactions might be in doubt. Solvolysis of a secondary chloride in ethanol probably will be a poor reaction (without predicting the product of the reaction). Similarly, addition of sodium bromide to that reaction mixture will not be an effective route for formation of the bromide in an SN2 reaction.
SN1 and SN2 Reactions
Dec 2006
Let us consider SN1 and SN2 reactions and concertedness. As my premise that there are no concerted reactions, that is, if time is infinitely divisible, then it is not possible for three objects to collide at exactly the same time. One collision will always precede the other.
For SN1 and SN2 reactions, then the notion of SN1 reactions is easy. Bond breaking precedes bond formation. That is the essence of SN1 reactions. Bond breaking leads to a carbocation intermediate.
However, that would force SN2 reactions to occur with bond making preceding bond cleavage. That fits the transition state model. Further, it implies that the transition state would behave as an intermediate. However, that is quite different than how I have written these problems and what you will find in any textbook. A transition state is the necessary construct one arrives at in a concerted reaction. That is, if bond making and breaking occurred at the same time, then the substituents attached to a carbon atom must invert in forming a new product.
On the other hand, if we consider that bond breaking must occur after bond formation (as the opposite of bond breaking occurring first as in an SN1 reaction), then that can be a convenient construct to test for an SN2 reaction. Are the electrons of the nucleophile sufficiently available to collide with the electrophilic carbon before the carbon-halogen bond can break? Therefore, even though the reactions might be written as concerted (wrongly according to the premise), we might think of SN2 reactions as bond formation then bond cleavage reactions.
For SN1 and SN2 reactions, then the notion of SN1 reactions is easy. Bond breaking precedes bond formation. That is the essence of SN1 reactions. Bond breaking leads to a carbocation intermediate.
However, that would force SN2 reactions to occur with bond making preceding bond cleavage. That fits the transition state model. Further, it implies that the transition state would behave as an intermediate. However, that is quite different than how I have written these problems and what you will find in any textbook. A transition state is the necessary construct one arrives at in a concerted reaction. That is, if bond making and breaking occurred at the same time, then the substituents attached to a carbon atom must invert in forming a new product.
On the other hand, if we consider that bond breaking must occur after bond formation (as the opposite of bond breaking occurring first as in an SN1 reaction), then that can be a convenient construct to test for an SN2 reaction. Are the electrons of the nucleophile sufficiently available to collide with the electrophilic carbon before the carbon-halogen bond can break? Therefore, even though the reactions might be written as concerted (wrongly according to the premise), we might think of SN2 reactions as bond formation then bond cleavage reactions.
What's happening with A Guide to Organic Chemistry Mechan Guide to Organic Chemistry Mechanisms?
Dec 2006
From an email message:
... When you get my book, you will see it is just like the lectures except you don't have to attend. Well, not exactly, but I would be surprised if you couldn't learn organic chemistry on your own using A Guide to Organic Chemistry Mechanisms. The real truth is that everything you have learned in your life, you have learned on your own. There is no other way. So, I think you can learn ochem on your own also.
... I had a high school student, without any organic chemistry, he started learning organic without any problem. He was using one of the prototypes of my book.
... Organic has like three parts, spectroscopy (mostly NMR), things (nomenclature, terms, stereochemistry), and reactions. The reactions are the hard part for most students. The others, I taught less and less. I just put in enough examples so students would become familiar and could do it. Reactions, I just had to work and work on. I think the limit is largely the limit of the willingness of students to actually learn it. You are trying to make neural connections in your brain. You are better than anyone at doing it. Your instructor really can't do it for you.
... When you get my book, you will see it is just like the lectures except you don't have to attend. Well, not exactly, but I would be surprised if you couldn't learn organic chemistry on your own using A Guide to Organic Chemistry Mechanisms. The real truth is that everything you have learned in your life, you have learned on your own. There is no other way. So, I think you can learn ochem on your own also.
... I had a high school student, without any organic chemistry, he started learning organic without any problem. He was using one of the prototypes of my book.
... Organic has like three parts, spectroscopy (mostly NMR), things (nomenclature, terms, stereochemistry), and reactions. The reactions are the hard part for most students. The others, I taught less and less. I just put in enough examples so students would become familiar and could do it. Reactions, I just had to work and work on. I think the limit is largely the limit of the willingness of students to actually learn it. You are trying to make neural connections in your brain. You are better than anyone at doing it. Your instructor really can't do it for you.
How hard is it to create a website?
Dec 2006
I can’t believe how much time I’ve spent in trying to not spend any money. I’ve tried various software packages, from the user friendly iWeb to TextEdit. For a novice like myself, there is no good solution. I wasn’t hoping for much. On the one hand, I just want to be able to connect with anyone interested in A Guide to Organic Chemistry Mechanisms. On the other, I don’t want a really sluggish website. I know how it feels to wonder, does this site exists, will it load, will my browser crash?
The code generated by iWeb has been post-processed as one way to include content inaccessible to iWeb. I have done some of that. The code created by iWeb is somehow bizarre to import into Adobe GoLive. As I have been discovering, all WYSIWYG editors create something other than optimal code.
I’m not sure I am up to writing this website in php which would be useful for some aspects of what I’d like. However, I find organic chemistry easy in comparison to learning these programming languages.
I’m going to try to fix one or two more pages in iWeb and then stand pat until someone again forces me to improve it.
The code generated by iWeb has been post-processed as one way to include content inaccessible to iWeb. I have done some of that. The code created by iWeb is somehow bizarre to import into Adobe GoLive. As I have been discovering, all WYSIWYG editors create something other than optimal code.
I’m not sure I am up to writing this website in php which would be useful for some aspects of what I’d like. However, I find organic chemistry easy in comparison to learning these programming languages.
I’m going to try to fix one or two more pages in iWeb and then stand pat until someone again forces me to improve it.
How to study
Nov 2006
... The key is to know how the reaction takes place. Do the problems in A Guide to Organic Chemistry Mechanisms and be sure you understand the pre-bonds and the curved arrows. If you know that, then you can solve any problem.
I had tried to think how I could tell my class how to study, what they needed to know. I thought that at the lowest level of achievement, I would advise students to only learn the mechanisms. If they didn't understand the mechanisms, then if they encountered a new problem, they wouldn't have any tools to solve it. If they new the mechanism, they would have the tools, but would make errors due to their lack of practice. Then there would be the B students. They would know the mechanisms, and could do some of the problems, but were still prone to errors. The A students understood the mechanisms and probably learned them relatively easily. They could practice the mechanisms on new problems because they had more studying time since it did not take as much time for them to learn the mechanisms initially.
I still think that is the most effective strategy, and that would have been the most effective one in my class because I emphasized solving problems and writing mechanisms. I have been helping other students in other classes who have professors who stress synthesis strategies. I had a student from a big school who had a very good grade in her first semester and I .... She said her professor stressed synthesis problems. I discovered she didn't know anything about the reactions she had to use.
... But I am still nervous about those errors when you see a new problem. I don't know how to get you to avoid those errors. I actually think that people are like basketball players, some people shoot free throws in the 90+% and some people only shoot in the 60+%. Chemistry has a higher percent, but I hear students say afterwards, "Oh, I knew that". It is like the basket, it doesn't move, but it can be hard to hit every time.
I think you should aim at doing as many examples from my book as you can. Don't worry about perfecting one reaction. Just do as many reactions as you can. Cover every type that will be on your test. If there is a reaction not in my book that your professor talked about in class, tell me and I'll write some examples for you.
I had tried to think how I could tell my class how to study, what they needed to know. I thought that at the lowest level of achievement, I would advise students to only learn the mechanisms. If they didn't understand the mechanisms, then if they encountered a new problem, they wouldn't have any tools to solve it. If they new the mechanism, they would have the tools, but would make errors due to their lack of practice. Then there would be the B students. They would know the mechanisms, and could do some of the problems, but were still prone to errors. The A students understood the mechanisms and probably learned them relatively easily. They could practice the mechanisms on new problems because they had more studying time since it did not take as much time for them to learn the mechanisms initially.
I still think that is the most effective strategy, and that would have been the most effective one in my class because I emphasized solving problems and writing mechanisms. I have been helping other students in other classes who have professors who stress synthesis strategies. I had a student from a big school who had a very good grade in her first semester and I .... She said her professor stressed synthesis problems. I discovered she didn't know anything about the reactions she had to use.
... But I am still nervous about those errors when you see a new problem. I don't know how to get you to avoid those errors. I actually think that people are like basketball players, some people shoot free throws in the 90+% and some people only shoot in the 60+%. Chemistry has a higher percent, but I hear students say afterwards, "Oh, I knew that". It is like the basket, it doesn't move, but it can be hard to hit every time.
I think you should aim at doing as many examples from my book as you can. Don't worry about perfecting one reaction. Just do as many reactions as you can. Cover every type that will be on your test. If there is a reaction not in my book that your professor talked about in class, tell me and I'll write some examples for you.
eMail reply about curved arrows
Nov 2006
Just a little info about the curved arrows. I know they are different in my book than your instructor uses, but I'd like you to follow the format I used in the book. I am also attaching a document explaining how they came about. On my use of the curved arrow, I am a little ahead of my time. However, the one thing my students told me to not change was the curved arrows.
In order to succeed, you have to find the reactions like you are doing in class. Then start with Part A, add the curved arrows. You should be able to figure out how the electrons are moving from the 'pre-bond' and the resulting charges. Do a few examples. If you think you are going to have a hard time with this, photocopy the pages you are going to do so you can repeat it. You should be able to learn the logic of the reactions from Part A. Then proceed to Part B. If you know the logic, then you can add the pre-bonds, curved arrows and draw the resulting intermediates. In Part B, photocopying is a better idea. I had students tell me they made several copies of all the pages they worked on. The objective is to learn examples of reactions. When you see a new reaction, you will apply the same logic to that reaction as you did to the ones in my book. Obviously, you must be able to do one reaction of a given mechanism in order to be able to solve a new problem.
If you have any questions or problems with my book or any homework, ask me and I'll try to help you.
In order to succeed, you have to find the reactions like you are doing in class. Then start with Part A, add the curved arrows. You should be able to figure out how the electrons are moving from the 'pre-bond' and the resulting charges. Do a few examples. If you think you are going to have a hard time with this, photocopy the pages you are going to do so you can repeat it. You should be able to learn the logic of the reactions from Part A. Then proceed to Part B. If you know the logic, then you can add the pre-bonds, curved arrows and draw the resulting intermediates. In Part B, photocopying is a better idea. I had students tell me they made several copies of all the pages they worked on. The objective is to learn examples of reactions. When you see a new reaction, you will apply the same logic to that reaction as you did to the ones in my book. Obviously, you must be able to do one reaction of a given mechanism in order to be able to solve a new problem.
If you have any questions or problems with my book or any homework, ask me and I'll try to help you.
Reply to email about A Guide to Organic Chemistry Mechan Guide to Organic Chemistry Mechanisms
Nov 2006
I haven't heard from you in a while. I would like to know that you are doing the problems in my book. I am especially interested to learn that you are doing them as I laid them out, Parts A to B to C. For students that do not have any trouble with organic, they can skip part of that process, but in order to learn reactions, it is vital (VITAL!) that you understand which bonds are being made, and which are being broken. The curved arrows MUST be consistent with that process. The more problems you do, the easier it will be for you to recognize a pattern throughout organic chemistry.
If you would like me to suggest problems based upon different topics, I can do that. I don't have a copy of your textbook, so I can't go through it and write a concordance. (Another future objective.)
I am also going to attach a note about why I use pre-bonds. I know they aren't used and that is a problem I have with being in the forefront. However, I hope you can see they make reactions easy to understand. In Chapter 2, I did insert a section on writing English sentences to match the curved arrows (see page 8). If you are a little confused about the curved arrow, go back and do that exercise.
If you would like me to suggest problems based upon different topics, I can do that. I don't have a copy of your textbook, so I can't go through it and write a concordance. (Another future objective.)
I am also going to attach a note about why I use pre-bonds. I know they aren't used and that is a problem I have with being in the forefront. However, I hope you can see they make reactions easy to understand. In Chapter 2, I did insert a section on writing English sentences to match the curved arrows (see page 8). If you are a little confused about the curved arrow, go back and do that exercise.
An electrophilic substitution question
Nov 2006
This what I love, easy questions. An amino group is an o/p director, but ammonium (NH3+) is a meta director. When you do a nitration in sulfuric/nitric acid, the amino group becomes protonated. It changes from an o/p to a meta director.