Browsing: o-chem help

Organic Chemistry Help: More on Electrophilic Aromatic Substitution

Hey Everybody, here is a good trick to keep in your back pocket if you run across an EAS question where you have something in the ortho position, but not the para.

Here is a good trick to do it: First, bromonate your benzene ring under standard condition, then sulfonate using SO3/H2SO4.  This will make the para bromo sulfonate.  Now the next substituent, our chlorine, will be directed ortho to the bromine.  We now have a trisubstituted arene ring and can remove the sulfonate unsing acidic water.  This gives us a nice route to the ortho di-halide without having to justify why we got a mixture of the ortho and para products. 

 

For more information on this, please go to organic chemistry

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Organic Chemistry Help: Resonance and SN1/SN2

Hi everybody, I wanted to talk briefly today about resonance and sterics and how it can affect and SN1 or SN2.  For background, I hope everyone knows when it comes to SN1 reactions, tertiary substrates are the fastest and primary substrates are the slowest (because of carbocation stability).  Conversely, when it comes to SN2, it is all about steric hindrance, so primary is the fastest and tertiary is the slowest.  But what happens when there are other factors involved?

As shown here, the benzyl cation was a primary cation, but can undergo resonance stabilization that moves the cation all throughout the ring.  This serves to further stabilize it and makes the benzyl cation have the reactivity of a secondary carbocation when it comes to SN1.

Lesser known is the neopentyl bromide, which is a primary substrate so it should react quickly via SN2, but it does not.  This is because, even though it is primary, it has a very large t-butyl group close, which blocks the reaction site.  This makes neopentyl bromide less reactive than one would expect.  In fact, it has reactivity somewhere between a secondary and tertiary substrate, for SN2 reactions.

For more information on this, please visit Organic Chemistry.

 

As always, good luck and happy reacting.

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Organic Chemistry Help: Retrosynthesis on an enolate

Hi everybody, one of the questions I hear alot is about enolate product.  How do you do the retrosynthetic analysis on one?  How do you even know it came from an enolate reaction?  Here is an example to look at:

As you can see here, a possible enolate product is an α,β-unsaturated ketone.  When you see one of these types of ketones, disconnect first at the double bond and “insert” an oxygen atom, as shown above.  Now, it becomes more evident which two carbonyls were participants in the original reaction.
Remember that in most cases, an α,β-unsaturated ketone can be derived from an enolate reaction.
Hopefully this was somewhat helpful.  For more information on this and other topics, please visit organic chemistry.
Good luck and happy reacting.

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Organic Chemistry Help: Electrophilic Aromatic Substitution

Hey Everybody, this is one of the tricks that organic chemistry profs always catch o-chem students on when doing EAS.  Put a group in the meta position, when there is an ortho,para director already on the ring.  For example:

 

 How do you put a methyl group in the meta position in this problem?  Here’s the trick:  Start with a meta director on your ring, place the methyl group in the meta position, then change that meta director into an amine. 

 A nitro group is the logical choice for the “meta-switcheroo”….keep it in mind next time you run into a problem  like this. 

As always, we think organic chemistry help is a great resource for all of you organic chemistry needs.

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An often-made SN1 mistake

Hi everybody, just wanted to give out a quick tip when looking at SN1 reactions.  Many students believe that SN1 reactions give totally racemic products, but this is not actually the case. 

We all know that the SN1 reaction proceeds through a carbocation intermediate, which cannot hold the stereochemistry that the carbon atom previously possessed.  (In other words, if you start with a chiral carbon and go through a cation, you will have a trigonal, achiral cation) 

 However, this is not the whole story.  Many times, the leaving group can come back to the cation on the same side.  This partially blocks one face of the cation.  The net result is that your carbocation CANNOT be equally attacked from both sides.  What you get out is both R and S at that carbon, but not in equal ratios.  This means that the SN1 reaction will lose some chirality, but will not usually become totally racemic.

 Good luck on those exams and happy reacting.

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Organic Chemistry Tip of the Week

Hey Everybody–

 Here is a minor distinction between two reactions that I have lately seen some students get crossed up on. 

Cold KMnO4, when reacted with a double bond, give the diol.

Hot KMnO4, when reacted with a double bond, cleaves the double bond to give two carboxylic acids. 

Temperature is the key here, don’t lose points on this.

 For more information on this, please go to organic chemistry

Good luck and happy reacting.

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Starting Organic Chemistry II

Hey Everybody, welcome to semester II.  Hopefully everybody survived the first semester and are ready to clobber “Organic Chemistry Part Deux”.

 One of the questions I get asked alot are about the differences between Semester I and II.  In semester I, you are building the foundation of your house so you need to learn things like nomenclature, stereochemistry, and functional groups.  Most people do some reactions (SN1, SN2, E1, E2) but it is very limited. 

 Semester II will focus on learning reactions.  This is a very good thing for the memorizers out there.  In addition to the simple reactions, you will also have multi-step syntheses and retrosyntheses (working backwards) to complete.  This becomes a big stumbling point for many people, because putting it all together can be difficult.  In future post, I will go over the best way to tackle a multistep problem, but that won’t be until middle semester. 

So until the next post, good luck and happy reacting.

For more information on this, please visit organic chemistry help

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Organic Chemistry Help: Deciphering an NMR

YOUR CLASS NOTES AND EXAMS ARE WORTH MONEY, FIND OUT HOW WITH THIS FREE GUIDE

Happy New Year!  It has been a while since we have put a new organic chemistry post up, so I thought I would put a little guide up now that finals are over.  The question is:  How did you decipher an H1 NMR spectrum?  Well, here is a good, uniform way to tackle the problem. 

 

Step 1: Calculate the degree of unsaturation in the molecule.  This is sometimes called the Sum of Double Bonds and Rings or SODAR.  You will most times be given a molecule formula, and can calculate your total number of double bonds and rings in the molecule using the formula (2#C + 2 – #H – #X + #N)/2 where

 

            #C = the number of Carbons

            #H = the number of Hydrogens

            #X = the number of Halogens

            #N = the number of Nitrogens

 

In this, you do not count the oxygen or sulfur atoms.  For example, the molecular formula C6H6NOCl would be (2*6 + 2 – 6 –1 +1)/2 = 4, meaning that there are 4 double bonds and/or rings.  It is helpful to remember that benzene rings equal to 4 on the SODAR scale, so if you have a SODAR that is 4 or larger, think benzene ring.

 

Step 2: Look for arene protons.  The number of protons between 6ppm-7.5ppm, known as the AR region, can give many clues to your molecule.  A mono-substituted benzene ring will have 5 protons in the AR region.  A di-substituted benzene will have 4 protons in the AR region.  However there are even clues to what type of di-substituted benzene it is.  If the peaks in the AR region are 2 perfect doublets, it is most likely para substituted.  If you have a singlet in the AR region, you most likely have a meta-substituted benzene.  If you just have a mess, it is most likely ortho substituted. 

 

Step 3: Look for the 2 A’s, aldehydes and alcohols.  This is actually simpler than it sounds, and can give you some nice clues.  Aldehydes are sharp singlet peaks that show up past 9ppm.  Alcohols are broad singlets that can show up anywhere in the spectrum, but will “exchange” with D2O, meaning that they will disappear if D2O is added.  Most organic chemistry profs will signify this by writing “exchange” over your spectrum.

 

Step 4:  Add up the integrations in your spectrum and make sure it equals the number of protons that you have.  For example, if you have 10 H’s in your formula, but can only have an integration equal to 5 on your spectrum, you need to realize that each integration is equal to 2 protons.

 

Step 5: Start to make fragments and then add up the fragments.  Using the integration and splitting of each peak, you can start to write down fragments of the molecule.  For example, if you have a singlet with an integration of 3, you know that you have a methyl group (3 H’s) next to something with no protons.  If you have a doublet with an integration of 2, you have a CH2 that is next to a CH.  Once you have all of your fragments, start to piece them together and you will be figure out what your molecule is. 

 For some good practice tests, please see organic chemistry.

 

 

 

 

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Always, sometime, never.

Disclaimer:  This posting applies to Undergraduate organic chemistry.  This does NOT apply to crazy physicists who create all sorts of insanity in a laboratory that cannot exist outside a xenon forcefield.   

I am going to give some organic chemistry advice that I like to refer to as:

 “ALWAYS, Sometimes, NEVER.”

ALWAYS/NEVER:

  – Hydrogen ALWAYS has only one bond to it.

  – Carbon NEVER has more thank four bonds.

  – Alkaline metals (Li, Na, ect) and alkali earth metals (Be, Mg, Ca, ect) can NEVER be negatively charged.

  – Noble gases are NEVER a part of any organic molcule.

  – Electrons ALWAYS flow from negative to positive.

  – Arrows ALWAYS point from negative to positive.

Sometimes:

  – Carbon can have 4 bonds (neutral), 3 bonds (positive, negative), or even 2 bonds (carbene)

  – Halogens USUALLY have one bond, but can occassionally have two.

  – Nitrogen usually has 3 bonds (neutral), 4 bonds (positive) or 2 bonds (negative)

  – Oxygen usually has 2 bonds, but can have only 1 bond (negative) or 3 bonds (positive)

  – Phosphorous is USUALLY an oxophile, meaning if it can react with oxygen, it will.

More organic chemistry help can always be found on the web too.

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Practice tests and practice questions: Good or is the time better spent playing Halo 3?

For the record, all I know is that Halo 3 is a video game that everyone loves.  Anywho…

The short answer to that question is YES, practice tests are great. But the real question that should have been asked is “what practice questions do I want”?  First, you want to get old exams from YOUR professor.  Different organic chemistry professors focus in on different things.  Your professor might focus in on physical organic chemistry (orbitals and such) where as another prof might focus on synthesis.

Next, if you have the time, will, and extra food, surf the net to find other profs at other schools and their tests.  If you can answer both sets (yours and the other guys) of questions, you are well on your way to a nice fat A. 

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