Tag: organic chemistry help

Organic Chemistry Help: Carbonyls are added to twice when they have an “ejectable” group on it

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:

Some useful examples of this can be found in your textbook. But for more information on this, please see organic chemistry

Thanks alot, and as always, happy reacting.

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Organic Chemistry: There are only FOUR important IR peaks….that’s IT

Amazingly enough, IR is not used much by professional organic chemists.  This is because all IR can show is different functional groups.  Thus, IR cannot tell the difference between any of the molecules shown below:

All of the molecules above will show an OH peak and various C-H stretches, but each spectra will look striking similar.  Hence, we should recognize the limits of the instrument and not try to use it for more than it is intended.  Further, there are really only four peaks in an IR that we look for to tell us something about our unknown molecule.  Here, we show them in order of importance.

1)           Carbonyl peak (1750-1650cm-1):  This will be a very sharp, prominent peak and shows that a carbonyl is present in somewhere in your unknown molecule.  What is less obvious is which type of carbonyl it is.  It is not as easy to distinguish between ketones, esters, aldehydes, ect.

2)           OH peak (3500-3200cm-1): This is a large, broad stretch which cannot be mistaken for any other functionality.  One problem to be aware of is that the OH of water will also show up here, in the event that your unknown is not totally dry.  Remember, that this can be from an alcohol OH or a carboxylic acid OH.

3)           C-O peak (1300-1040cm-1): Usually a large, sharp peak, this can be from an alcohol, carboxylic acid, ether, or an ester.

4)           Nitriles (2250-2230cm-1) and alkynes (2100-2280cm-1) peaks: Usually rather small peaks, but easy to spot as they are the only peaks in that area.

For more help with this and other organic chemistry topics, please go to organic chemistry

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Memorize your nomenclature and essential vocabulary

Learning organic chemistry is like trying to work in a foreign country; if you don’t know the language, it is going to be very difficult to learn how to do your job.  Imagine that you have just been transported to the mythical country of “ochemia”, a small island nation in the south Pacific, where your job is to write chemistry reactions.

Frequently, in a chemistry lecture, professors start tossing out strange organic chemistry terms far too quickly.  Because students aren’t fluent in “ochemia” yet, they need to translate each word in their head to understand what the instructor has just said.  By the time this mentally translation is done, the student has just missed the next sentence and has lost half of the lecture.  Our goal is to get as fluent as we can in the language of chemistry as quickly as we can.  Here are some terms it will be helpful to memorize so that you don’t have to do a mental translation when you hear them:

Meth = 1

Eth= 2

Prop = 3

But = 4

Pent = 5

Hex = 6

Hept = 7

Oct = 8

Non = 9

Dec = 10

Nucleophile = has electrons, has a negative or partial negative charge

Halogen = F, Cl, Br, I

Aprotic solvents = do not contain OH or NH bonds

Protic solvents = contain OH or NH bonds

Lewis Acid = electron acceptor

Lewis Base = electron donor

Carbonyl group =  (C=O)

Cis = same side of a double bond or ring

Trans = opposite sides of a double bond or ring

Electrophile = wants electrons, has a positive or partial positive charge

As always, for more help please go to organic chemistry

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Good for Nothing Alkanes

 

 

Let’s face facts: alkanes don’t have many uses.  In real life, they are non-polar solvents, larger-chain ones can be used to make wax, and they are good for burning/they make good fuels.  As far as organic chemistry goes, alkanes are very boring.  They have only three uses in your class:

1)      Solvents: Alkanes are very good non-polar solvents.

 

2)      Halogenation: Alkanes can be reacted with bromine or chlorine under free-radical conditions to obtain an alkyl halide.

 

3)      Combustion: This is the fuel part.  Complete combustion is an alkane reaction with oxygen to obtain CO2 and water, as shown in the following example:  C3H8 +5O2 –> 3CO2 + 4H2O + Heat

 

There is only a small possibility that you will see a combustion reaction on one of your exams.  Therefore, the only real uses for alkanes in your undergraduate organic chemistry class will be as a solvent or in a halogenation reaction.  Hence, if you have an alkane in one of your exam reactions, it should be very simple to determine its role.  If there is a halogen (usually either Br2 or Cl2) , then beware of a halogenation reaction.  Otherwise, it is most likely safe to say that if you have an alkane in your reaction, it is a solvent and does not participate as a reactant.

 

 

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Beware of the bad acid trip: Know your strong acids

A problem we see students constantly running into is that they do not readily recognize strong acids.  This is a terrible mistake and should never happen.  You will need to quickly recognize strong acids and which atom they will be protonating.  As far as strong acids go, you should immediately recognize the Magnificent Seven:

 

STRONG ACIDS: HI, HCl, HBr, HNO3, H2SO4, HClO3, H3PO4

 

Just think that that it’s the three hydrogen halides and the four acids that have N, S, Cl, and P as central atoms.  Many students remember the other four acids with the mnemonic, something you would never say to a girlfriend: “Never Say ‘Please Clean’ ”. 

Once you have recognized that you have a strong acid present in your reaction, it is necessary to determine what it is protonating.  Remember that H+ is electron-deficient (Lewis acidic) and will look for an electron-rich (Lewis basic) atom to protonate.  This could be a nitrogen atom, such as an amine, or an oxygen atom in a carbonyl or an alcohol.  Most often, the atom being protonated will have a lone pair somewhere.

 

Take Home Message: Know the strong acids, the mnemonic Never Say ‘Please Clean’ might help

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