Posts Tagged ‘organic chemistry’


Steps of a Free Radical Reaction

Friday, February 7th, 2014

This is one of the best depictions of a free radical reaction I have seen.   It shows what can go on in this reactions and how we get from starting material to desired final product.

Steps of a free radical reaction

 

Initiation = 1 neutral provides two radicals.  This is what starts the entire reaction.  This is also the only initiation step that can occur as CH4 is not going to participate in that type of reaction.

Propagation = 1 neutral + 1 radical provides a different neutral and a different radical.  In this reaction, the most likely propagation is chlorine abstracting a proton from methane to give HCl and the methyl radical.  The next step is where the methyl radical breaks up two Cl atoms.  What I really like about this depiction is that it shows that the Cl* from reaction 3 can be recycled back into step 2.  This means that the reaction is self-propagating.  This also means that IN THEORY you could have one initiation reaction, followed by a bunch of propagation, ending with one termination reaction.  Of course, in real life, for many reasons, this does not happen as there are lots of initiation reactions.

Termination = 2 radicals providing one neutral.  The part to remember here is that any two radicals can get together to terminate the reaction and form a neutral species.  Since we have 2 types of radicals in the reaction (Cl* and CH3*) , there are three combinations of potential termination steps.  Reaction 4 gives us back starting material, so it is fine.  Reaction 6 gives us product, so it is also fine.  Reaction 5 give us a byproduct, which strangely enough can replace methane in the propagation step and give us another by-product.

Think about this picture and figure out all of the side reactions that might occur to fowl up the reaction.  Then, (for you advanced students) think about what ways exist that you can minimize those side reactions.

Hope this was helpful to you all, and as always, happy reacting.

 

 

 

 

 

 

 

THE world’s most interesting chemist

Thursday, February 6th, 2014

The world's most interesting chemist

 

I didn’t get good at drawing them until after I almost failed exam 2.

Detect Hydrogen Sulfide Using Nucleophilic Aromatic Substitution

Wednesday, June 26th, 2013

This is some very cool chemistry.  And this post is dedicated to all of those who think that the organic chemistry you learned in your class was useless.

A professor and his graduate student at the University of Oregon have developed a hydrogen sulfide detector that can detect H2S in the parts per billion range (ppb).   This detector would be very important in the study of biologically contaminated water samples.  H2S is a colorless gas that smells like rotten eggs and is world-renown for its ability to make people sick.

This is very cool chemistry and it is performed using nucleophilic aromatic substitution.  Most of you are familiar with electrophilic aromatic substitution from second semester class, so some of you may recognize nucleophilic aromatic substitution.  Here is a quick example to refresh your memory.

In this reaction, methoxide anion is displacing fluorine in order to create the new aromatic ring.  The defining characteristic of nucleophilic aromatic substitution is that you need electron withdrawing groups on your aromatic ring to make the reaction occur.  In the above example, NO2 is our electron withdrawing group.  I am not sure what they are using in their probe (spoiler alert: i have not red the article yet), but i am sure it is something similar.

The crux of the reaction, said the study’s  graduate student Leticia A. Montoya, is the reaction process in which the probe reacts with H2S to produce a distinctly identifiable purple compound. “This method allows you look selectively at hydrogen sulfide versus any other nucleophiles or biological thiols in a system,” Montoya said. “It allows you to more easily visualize where H2S is present.”

The cite for JOC is: Leticia A. Montoya, Taylor F. Pearce, Ryan J. Hansen, Lev N. Zakharov, Michael D. Pluth. Development of Selective Colorimetric Probes for Hydrogen Sulfide Based on Nucleophilic Aromatic SubstitutionThe Journal of Organic Chemistry, 2013; :

“Ace Organic Chemistry Mechanisms with EASE”: a new e-book for the iPad, only $1.99

Wednesday, May 1st, 2013

Hey everybody.  We have just published our latest e-book called “Ace Organic Chemistry Mechanisms with EASE”, which teaches students a step-wise and methodical approach to solving organic chemistry mechanisms and synthesis problems called the EASE method.  The e-book has over 2 hours of video, 100 + practice problems, and is only $1.99 for a limited time. It is for students of every level and is a must for finals.

To learn more, click here:  organic chemistry mechanisms book

If you download the preview, you can see a preview movie for the book.  We are very proud of the new book, as it takes a very difficult subject like organic chemistry mechanisms and makes it understandable to all.  Hope you like it too.

Organic Chemistry Help: Resonance between equivalent atoms means equal bond lengths.

Wednesday, October 17th, 2012

Let’s talk resonance in organic chemistry.  

Once most students hear this tip, it makes perfect sense to them, but it isn’t one that you might think about on your own.  Take a look at the structure below, and ask yourself: are the two N-O bonds the same length?

resonance in organic chemistry

Since freshman chemistry, we have been told that double bonds between two atoms are shorter than a single bond between the same two atoms.  Hence, the N-O double bond should be shorter than the N-O single bond.  But let’s look at some resonance forms:

resonance and bond length

Here, we can more clearly see that the nitro group is moving between the three resonance structures.  Structure 3, where the charge is spread evenly between the two oxygens is a valid structure and shows that the bond two oxygen atoms in the molecule are equivalent and have the same bond length (124 pm).

We care about this principle when it can be applied to more complex organic molecules where it is not obvious that the atoms are equivalent.  Take the cyclopentadiene anion:

At first glance, this appears to have three different carbon atoms.  However, once you start looking at resonance structures, you can see that the anion can be moved to any of the carbons in the ring.  This makes them all equivalent, via resonance.  This is confirmed through analytical studies which show that all C-C bonds are approximately 137pm long.  Additionally, as this fits Huckel’s rule of 4N+2, the molecule is also aromatic.

 

Take Home Message: If you see symmetry or aromaticity, think equivalent atoms

 For more help with resonance, please see our homepage at organic chemistry

Organic Chemistry Podcast

Wednesday, October 17th, 2012

Hi Everybody -

AceOrganicChem.com is very proud to announce the release of their new video podcasts.  Each week the podcast will go over the topics that you are seeing in your class at that time.   The podcasts are relatively short (10 mins or so) and feature a quick lesson, a virtual whiteboard demo, practice problems and a helpful website.  We hope to make this as helpful for as many students as possible and it is free.  The podcast can be found in the iTunes store by clicking this link Organic Chemistry Help Podcast by AceOrganicChem.com  or searching “AceOrganicChem podcast”

As always, thanks a lot and happy reacting.

Graduate School for Organic Chemistry?? Not a bad idea if I do say so myself.

Saturday, May 21st, 2011

Graduate School for Organic Chemistry??

A presumptuous congratulations on passing this class.  For some of you it was not easy.  For others, it might have been enjoyable.  And for a select few of you, it was so awesome that you would love to take the class again just for fun.  Right now, I am talking to that last group of students.

If you enjoyed your organic chemistry class, you might want to consider a way to continue the party. That way is to get a graduate degree in chemistry.  Here are some of the benefits to it:

1) Recruiting trips: Rent the 80′s hit movie “Johnny Be Good”.  Your recruiting trips to prospective graduate schools will not be quite that crazy, but each school you are accepted to will fly you out for the weekend to wine and dine you.  This includes meeting the faculty & current graduate students, seeing the campus, hearing about research that you might be interested in and seeing what life as a grad student would be like.  It is a great way to spend part of your senior year and is the first step to picking the perfect graduate school for you.  It is also an amazing opportunity to talk to the graduate students that are already there and find out how life at that school really is.

2) You get paid to go to school:  Almost every university that offers a graduate degree in chemistry will pay you go to school there.  No joke.  In exchange for teaching undergraduate classes and/or doing research in order to obtain your degree, these schools will pay you a stipend.  Generally, it is not much money, but it will be enough for most of you to live on.   Depending on the school, this stipend can range from $15K to $35K/year and tuition is usually covered in that (or is very cheap).  Considering that you are being paid to be a
student, this isn’t such a bad deal.

3) You get to put off starting real life:  If you get a masters degree, it will take you 18 months to three years to complete.  If you get a PhD, it will take you between 4-6 years.  This is all time in which you are still a college student and can continue to party like it is 1999.

4) You will increase your earning potential for your entire career:  With an advanced degree on your resume, you can demand higher salaries for your entire working career.

5) You don’t necessarily even need to become a chemist with your degree:  A sizable percentage of those who get advanced degrees in chemistry never actually become bench chemists, or even stay in the field of chemistry.  I know people that have become engineers, pharmaceutical sales reps, medical examiners, and even FBI agents.  The great part about it is that you have flexibility and aren’t pigeon-held into a chemistry job.

Overall, more education never hurts anyone, especially when someone else is paying for you to do it.  If you are even remotely interested in hearing more about this, I would strongly suggest learning more about a graduate degree in the sciences.  For most schools, you can visit their websites and get more information.  If you decide to start the process toward going to graduate school, you want to
take the GRE exam sometime in your junior year and start applying in the fall of your senior year

Over 15 hours of organic chemistry help for only $12.99

Wednesday, May 5th, 2010

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Organic Chemistry Help: Symmetric diols came from the Pinacol reaction

Tuesday, April 27th, 2010

Some of your professors will try to sneak this one in on you during an exam:  Your professor will ask you to synthesize the symmetric diol shown below and you will either not know how or you will come up with a convoluted, difficult or wrong answer. 

While not obvious, there is a very simple way to create a symmetric diol using the Pinacol reaction.  Using a number of reducing agents, ketones and aldehydes can be coupled with themselves to form a symmetric diol, as shown below:

 

The reaction is also called the Pinacol Coupling Reaction and can be used on most aldehydes and ketones, but not on acid halides or carboxylic acids.  Now to address the original synthetic problem above:

 

The first step is reaction of the reaction of ethyl lithium with ethanal to form 2-butanol.  This is then oxidized to 2-butanone using Jones’ oxidation.  Now, under Pinacol Coupling conditions, 2-butanone is then reacted with itself using magnesium to form the final product. 

Take Home Message:  Symmetric 1,2 diols came from the Pinacol reaction.           

 

For more organic chemistry help, go to organic chemistry

Happy New Semester!! Some helpful organic II tips.

Friday, January 1st, 2010

Hey Everybody–  Once again, very sorry for not posting for so long.  My New Years Resolution is going to be to post at least one helpful organic chemistry tip per week for the rest fo the semester.  Anywho, many of you are home relaxing on break, but will return to the scary prospect of an organic II course.  Here are a couple of helpful hints for getting through it:

1) Organic terms are going to get thrown around this semester like nothing.  If you don’t understand the language your prof is speaking in, you are not going to understand a thing.  My advice is to go back and review some stuff from your first semester course to make sure you understand terms like nucleophile, electrophile, ketone ect…so that when the prof uses that word, you are not scanning your brain for a translation.

2) Organic II is reaction and synthesis heavy.  Remember that all an organic chemistry reaction is (at its core) is the movement of electrons.  Identify the nucleophile (who has the electrons), your electrophile (who wants the electrons) and your electron flow, and you are 90% done with the problem. 

3) Stay calm.  The only thing we have to fear is fear itself.

For more help with organic chemistry, I highly recommend the organic chemistry aides at AceOrganicChem.com or click here organic chemistry

Good luck, and as always, happy reacting!