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.

 

 

 

 

 

 

 

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

Watch out for the Flunking Professor

Saturday, August 11th, 2012

Because the sciences have become so popular in recent years, many of the larger universities have several professors teaching organic chemistry, which means in financial terms, this is a buyer’s market.  You have choices on whose class you take and whose class you avoid.  Of course the best way to learn who is good, who is bad, and who is ugly is to ask the boys and girls who have already taken the class.  In lieu of that, here are a couple of tell-tale signs that you are about to enter the realm of “Professor Flunks-alot”:

1) On the first day, the professor brags about how many students fail/drop the class.

2) The professor is anti-medical school.  (Organic chemistry is a big med school prep subject)

3) The professor does not have a degree in organic chemistry; some schools, due to staffing needs, will run someone out there with a degree in another field of chemistry, or worse—a biology degree.

4) The professor is not receptive to student questions in office hours.

5) The professor does not incorporate examples of more recent organic chemistry into the lectures.  This is a tricky one.  It might show that the instructor is a little out of touch, or doesn’t care enough to find more interesting examples to present.

6) The professor focuses a lot on physical organic chemistry (orbitals and such).  First, this is not the main part of organic chemistry, which rotates around the synthesis of new molecules.  Second, it is very boring.

If you determine that you have a bad professor, the first thing to decide is if there is a better one out there, preferably teaching the course this semester.  If it is easy, and you feel comfortable, switch to the other class.  If you consider yourself a masochist, tough guy, or just can’t switch, then sit back and make the best of it.

If you really need to get out and can spend a little extra money, many universities will accept a junior college transfer credit.  Call the registrar’s office at your school, ask if they will accept organic chemistry from the local community college and take it there.  More often than not, the community college will offer a simpler version of the course, which you can take back with you to avoid the whole mess of a jerk professor.

Organic chemistry help: free iPhone app

Tuesday, April 5th, 2011

Hey everybody. So there is a free organic chemistry app for the iPod/iPhone in the iTunes app store. It is called “Organic Chemistry Essentials” and it rocks. There are over 15 different sections to help with organic 1 and 2 classes and even has a section of nerd humor.

As of today, it was downloaded over 40,000 times and has a 4.5 star rating. We highly recommend this. Give a try, it is free so you have nothing to lose.

Organic Chemistry Help: “Eject! Eject! Eject!” Carbonyls with an ejectable group are added to twice.

Sunday, February 6th, 2011

These functional groups remind me of 1985 when Maverick flew through the jet wash and Goose and he had to eject from their F-14 Tomcat.  What does this title mean?  What we are trying to say is that carbonyls can be classified two different ways: ejectable or non-ejectable.  What this means is that sometime when a carbonyl is attacked by a nucleophile the carbonyl will eject one of its substituents before it reduces the carbonyl to an alcohol.  After the group has been ejected, then a second equivalent of nucleophile will reduce the carbonyl to alcohol.

In essence, this means if a carbonyl has an ejectable group on it, a nucleophile will add twice to that carbonyl.  Some examples of ejectable and non-ejectable systems are below:

In terms of synthesis, we will then observe the following:

As shown above with the ketone and the aldehyde (which have non-ejectable substituents), the Grignard reagent can only add once to the carbonyl, giving an alcohol as the product in both instances.  However, in the case of the acid halide and ester (which have ejectable groups attached) the first equivalent of Grignard kicks off the ejectable group to give a ketone.  This ketone can then be reacted with another equivalent of Grignard reagent to give the final product, which is a tertiary alcohol.   In the example above, we have added two equivalents of Grignard to the starting material in two different steps.  However, if the alcohol is your desired end-product, you can do this all in one step by adding two or more equivalents of Grignard reagent.

Take Home Message: Nucleophiles will substitute twice at the carbonyl if the starting material is an acid halide or an ester.

Free Organic Chemistry App for iPhone/iPod Touch

Saturday, October 30th, 2010

Hi everybody, so AceOrganicChem.com has teamed up with an iPod developer and has released an app to give a little on-the-go aide in organic chemistry.  The app is called “Organic Chemistry Essentials”, the rating so far is 4.5/5 stars from users, and is free. 

It has 10-15 sections and gives tricks and tips to nail organic chemistry.  It is a great app, and best of all it is free.  I strongly recommend it.    Link is organic chemistry app

Here is one review on it: “Simply laid out, but it is very helpful, and the humor (both within the lesson and on its own) provide compic relief along with helping the reader to remember.  Very fun”

As always, for help with all of organic chemistry, please go to organic chemistry

Organic Chemistry Help: Fischer Projections are a Black Tie Affair

Tuesday, September 21st, 2010

Emil Fischer is considered by many to be the greatest organic chemist to ever live.  His problem was that he created a way of looking at organic molecules that is very confusing to undergraduates.  These structures are necessary to learn and are very helpful when looking at certain molecules (such as carbohydrates), but they are also very easy to jumble.  This is because Fischer structures are drawn as crosses, which could lead one to erroneously think that the central carbon is flat, when it is actually still tetrahedral.

The easiest way to look at these is to think of them as bowties that have been strung together:

3-dimensionally speaking, the substituents that are on the sides of the structure are depicted at the end of the bowtie and are represented as “coming out of the paper”.  The backbone is composed of dashed lines, which are meant to represent that those portions “are going into the paper”.  This is now a much easier way to view these structures, as it is more apparent what area each substituent occupies.

The useful part of the bowtie projection is that it is now easier to assess the stereochemistry at each chiral center.   It should be much easier to visualize that the bottom chiral center is “R”.  This was not as obvious when viewing the Fischer projection as a cross

For more help like this, please go to organic chemistry

Are you taking organic chemistry this semester??

Monday, August 16th, 2010

Hey everybody. Are you taking organic chemistry this semester? It is never to early to get a head start. My best advice is don’t fall behind early in the course or you will be in big trouble later on.

Suggestion 1: Get a good review book at start peeking at it now. Be sure to follow it throughout the year and you will be able to keep up.

Suggestion 2: do a TON of practice problems. You can’t do to many of these. It is a great way to study.

Find help and get a head start at organic chemistry

The Good, The Bad and The Ugly of Summer Organic chemistry classes

Monday, June 7th, 2010

Congrats on getting done with your finals!  Not an easy task sometimes.  But on the topic of today’s post, I was recently asked what I thought about students taking o-chem classes over the summer.  This is not an easy “yes or no” question, and definitely depends on the student.  Here are some of the considerations:

The Good:

1. It is only usually 5 weeks long. 

2. If you are not working this summer, it is much better than just sitting around doing nothing.

3. If you are not majoring in chemistry and don’t want to go to med school, it is a great way to get it out of the way quickly.

 The Bad:

1. If you ARE majoring in chem, it is very easy to forget everything that you learned in the class because you crammed it all into 5 weeks.

2. Classes are usually at least 3 hours per day, plus homework every night and an exam once a week.  This can be overwhelming.

The Ugly:

1. If you get a bad prof, keep the hemlock close.

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Wednesday, May 5th, 2010

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