Organic chemistry general stuff

[bigballer27]

So I reviewed all the ochem but it doesn't really seem to help me.


Can someone just give the reactions we need to know...for example


Acid+ester= carb acid
carb acid plus halogen= acid chloride?


that's the only ones I know..and Ive noticed that carb acids are a big deal on the mcat...can someone just give all/as many as possible of the rxns we should know as such?

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[Geekchick921]

Check out the last few pages in this.
http://www.aamc.org/students/mcat/preparing/bstopics.pdf

 

[loveoforganic]

"carb acid plus halogen= acid chloride?"

example??

 

[docelh]

I wouldn't take him too literally. (the other example was acid + ester = acid)

 

[bigballer27]

"carb acid plus halogen= acid chloride?"

example??

sorry, you're right, its not + halogen, but should be:
RCOOH +SOCl2 (or PCl3) --> Gives acid halide, RC0Cl

Above poster, i went to the link and there were a few wierd things ive never heard of:

under alcohols:
1. pinacol rearrangement in polyhydroxyalcohols, synthetic uses?!?!
2.preparation of mesylates and tosylates?!?!
can anyone clarify?


also added to the list can be carb acid plus carb acid --> acid anhydride

 

[docelh]

Acid + Acid --> Anhydride + H2O. Endothermic conditions.

 

[bigballer27]

Acid + Acid --> Anhydride + H2O. Endothermic conditions.

yeah endothermic reactions apply, loss of water +heat

when you oxidize an alcohol. with dichromate for ex, how do you know if you will get an aldehyde, or all the way to carb acid?

 

[docelh]

Depents on the reagent. Chromic acid will complete to acid. PCC will stop at aldehyde.

 

[bigballer27]

Depents on the reagent. Chromic acid will complete to acid. PCC will stop at aldehyde.

is there a rule or some way to tell?

sorry, as you can probably tell, my ochem is my weakest area

 

[docelh]

No.

You either memorize the reactions, or you fully understand how reaction mechanisms work so you don't have to memorize the reactions.

My explanation above pertains only to primary alcohols. Secondary alcohols oxidize to ketones.

 

[bigballer27]

on this site, http://www.aamc.org/students/mcat/pr...g/bstopics.pdf

these topics are listed, and i don't know them, can you explain?

1. pinacol rearrangement in polyhydroxyalcohols, synthetic uses?!?!
2.preparation of mesylates and tosylates?!?!

 

[docelh]

My mesylate/tosylate knowledge is a bit hazy at the moment.

Pinacol is an elimination reaction that leads to a carbonyl.

 

[TFS]

the pinanol reaction is going from a diol to a carbonyl via rearrangement.

going from a diol to a carbonyl is inherently more stable. under acidic conditions, one of the OH groups gets protonated, forming a great leaving group: water. once water leaves, you are left with a carbocation, which is unstable.

so you have one carbon that only has three bonds, and an adjacent carbon with a hydroxy group that has the potential to become a carbonyl group, which would form 2 bonds (desired) - but in order to do that, that carbon would need to lose one of its bonds, which it does by having one of its alkyl groups shift over to the carbocation carbon.

hmm...hope that made sense.

the picture should make a lot more sense:
http://upload.wikimedia.org/wikiped...ment.png/500px-800px-Pinacol_rearragement.png


preparation of mesylates and tosylates is analogous to using SOCl2 to make an alcohol more reactive by converting the OH, which is a crappy leaving group into a good leaving group, Cl-. im not familiar with mesylates, but im guessing its a good leaving group like Tos.

 

[BerkReviewTeach]

on this site, http://www.aamc.org/students/mcat/pr...g/bstopics.pdf

these topics are listed, and i don't know them, can you explain?

1. pinacol rearrangement in polyhydroxyalcohols, synthetic uses?!?!
2.preparation of mesylates and tosylates?!?!

Pinacol Rearrangement
Sort of pointless that they took the time to give this a name, because it's a typical rearrangement where you get an alkyl shift, because there are no hydrogens on the key carbons in the rearrangement. It's shown on page 40 of organic chemistry book 2 (in the carbonyl and alcohol chapter) and also in the lecture handout (fourth sample passage) read that page and do that passage, and that will expose you to pretty much everything you need to know.

Preparation of Mesylates and Tosylates
These are reactions used to make the OH group of a simple alcohol a better leaving group. The O attacks a Mes-Cl or Tos-Cl, and displaces the chlorine, leaving a bulky and electron poor group in place of the OH group. The key feature here is that the stereochemistry of the alcohol-carbon remains the same, because the reaction took place on O and not C. It's a way to make a better leaving group while retaining the same stereochemistry (unlike using PBr3 or SOCl2, which invert stereochemistry when reacting with chiral alcohols). The general topic is addressed on page 43 and the reaction is listed on page 49 of organic chemistry book 2 (in the carbonyl and alcohol chapter) and Passage II is based on tosylating and mesylating an alcohol. There's also a page in the stereochemistry lecture handout (last sample before the practice questions). Read those pages and do that passage, and that should explain it at a deep enough level that you'll be ready for anything the MCAT throws.

 

[JoshuaR]

To the OP, overall, you don't have to know a ton of crazy stuff.

The basics: substitution, elimination, addition, stability, acidity, picturing molecules (example: enantiomers), carbonyl chemistry including alpha carbon hydrogen stuff, aromatic stuff including eas and the like, biological stuff like peptides, sugars...

Be solid on all of that, then go into deeper study.

 

[bigballer27]

thanks for all the help guys. i spent the last 2 days going through a bunch of stuff, and its not too complicated, but another question regarding R/S configuration:

when the lowest substituent (H usually), is NOT in the back, so it is either in the plane of the page, or out of the page, i cant figure out how to rotate the molecule in my head to place it in the back. My teacher taught me to just take the configuration if it isnt in the back and ALWAYS just switch it.
So if the molecule was originally R with the H in the plane of the page, or coming out of the page, then the actual config is S...does that ALWAYS work, or can someone explain how to rotate the molecules?

 

[loveoforganic]

If the lowest priority is coming toward you out of the plane of the page, then you just do your priority rotation normally, then assign the opposite configuration. As far as visualizing rotation, do you have a model kit? That's the best way to be able to picture what the paper represents.

 

[BerkReviewTeach]

thanks for all the help guys. i spent the last 2 days going through a bunch of stuff, and its not too complicated, but another question regarding R/S configuration:

when the lowest substituent (H usually), is NOT in the back, so it is either in the plane of the page, or out of the page, i cant figure out how to rotate the molecule in my head to place it in the back. My teacher taught me to just take the configuration if it isnt in the back and ALWAYS just switch it.
So if the molecule was originally R with the H in the plane of the page, or coming out of the page, then the actual config is S...does that ALWAYS work, or can someone explain how to rotate the molecules?

NO!!! This does not always work! Perhaps they explained it differently than you worded it here. That technique works all of the time if the H is coming out of the plane. But it will only work half of the time when H is in the plane. There are much better techniques. Two are really hard to explain via internet, because one involves using your hand and the other involves doing four sample determinations to come up with a grid of all possibilities. Figure 3-7 on page 191 in Organic Chemistry Book I shows the summary of all of the possibilities, so if that method makes sense, USE IT!

For now, the following approach should work well for you.

(1) If priority #4 is in back, then draw the circle and apply R for clockwise and S for counterclockwise.

(2) If priority #4 is in front, then draw the circle and apply S for clockwise and R for counterclockwise.

(3) If priority #4 is in in the plane, switch it with the group in back to generate a new (and different) chiral center. The new chiral center is easy to solve, because priority #4 is in back. The original compound has the opposite stereochemistry of the new one we made, because whenever you interchange two substitutents on a chiral carbon, the stereochemistry inverts. This example is shown in the attachment.

View attachment 14509 (removed the attachment because I needed space and this thread seems to have ran its course)

 

[bigballer27]

NO!!! This does not always work! Perhaps they explained it differently than you worded it here. That technique works all of the time if the H is coming out of the plane. But it will only work half of the time when H is in the plane. There are much better techniques. Two are really hard to explain via internet, because one involves using your hand and the other involves doing four sample determinations to come up with a grid of all possibilities. Figure 3-7 on page 191 in Organic Chemistry Book I shows the summary of all of the possibilities, so if that method makes sense, USE IT!

For now, the following approach should work well for you.

(1) If priority #4 is in back, then draw the circle and apply R for clockwise and S for counterclockwise.

(2) If priority #4 is in front, then draw the circle and apply S for clockwise and R for counterclockwise.

(3) If priority #4 is in in the plane, switch it with the group in back to generate a new (and different) chiral center. The new chiral center is easy to solve, because priority #4 is in back. The original compound has the opposite stereochemistry of the new one we made, because whenever you interchange two substitutents on a chiral carbon, the stereochemistry inverts. This example is shown in the attachment.

View attachment 14509

yeah perfect, that method works great. THANKS!

 

[bigballer27]

hey quick question

when you have a ketone, how do you know when a base will abstract a hydrogen (creating an enol i think), versus when it will attack the partial positive carbonyl carbon?

 

[loveoforganic]

That usually has to do with steric hinderance - more hinderance, less nucleophilicity.

 

[debbie26]

hey all the best with organic tho..talk to u later

 

[bigballer27]

so am i correct in saying that diastereomers have different boiling point and solubilities, but enantiomers don't, so you need "resolution" to separate enantiomers?

also, what is the difference between coupling and splitting?
- liek a question about a "doublet of triplets or a "doublet of doublets?"

 

[loveoforganic]

1st question - correct, although solubilities and boiling points are still likely to be rather close - especially boiling point.

2nd question - coupling (an interaction between two atoms) causes splitting (a property of an nmr spectrum)