Why does organic chemistry seem so random?

[MangoPlant]

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Sometimes the carbonyl group (C=O) gets protonated, other times the hydroxy group (C-OH) gets protonated and leaves.. both of these happen in water/acid. There are a bunch of other examples where I've always wondered "why doesn't this happen instead..".

To those who did well in organic chemistry: did you just memorize the reactions or is there some way to actually predict whats going to happen despite everything seeming so random?

 

[gmcguitar4]

You have to understand it man. Memorizing the reactions will just confuse you. You just have to understand what happens in base and acid catalyzed rxns. If you don't understand basic acid/base stuff then go back to your basics and practice!

 

[d12]

It's crazy at first, but there is a method to the madness! Try "ochem as a 2nd language" and be comfortable with the rules of acidity.

 

[konotsu]

Both carbonyls and alcohol groups can be protonated - carbonyls are more basic (more likely to be protonated) because carbonyls have a resonance form where there's a negative charge on the oxygen.

Also, alcohol groups, when protonated, MAY be a good leaving group (water!), depending on the solvent or the rest of the molecule structure.

Like everyone else has said so far - there really is a method to the madness. Do not try to memorize things by rote - it will make you miserable and hate the subject even more.

 

[Geebeejay]

What everyone else said. Keep practicing those mechanisms, and remember general rules and trends.

 

[chemsmith]

Both carbonyls and alcohol groups can be protonated - carbonyls are more basic (more likely to be protonated) because carbonyls have a resonance form where there's a negative charge on the oxygen.

Also, alcohol groups, when protonated, MAY be a good leaving group (water!), depending on the solvent or the rest of the molecule structure.

Like everyone else has said so far - there really is a method to the madness. Do not try to memorize things by rote - it will make you miserable and hate the subject even more.

a carbonyl group does not mean carboxylic acid (o resonance) and even then we are talking about protonation, not deprotonation


http://evans.harvard.edu/pdf/evans_pka_table.pdf

look at pka of conjugate acids of methanol and the ketone listed, similar species will follow the same trends listed here

 

[MangoPlant]

I guess I should actually try to understand instead of just memorize. Is a college o-chem textbook generally enough or should I buy some other supplement (like ochem as a 2nd language) to understand? My textbook seems to encourage straight up memorization. Also this is for o-chem 2 if that matters..

 

[Planes2Doc]

Orgo was extremely random for me until the day it all clicked. This was also the day I had a new professor and he showed us electron pushing in full detail. After that moment, the class was awesome.

If you need some motivation in the class, watch Breaking Bad.

 

[La Presse]

What do you guys think is the best supplemental material/textbook for ochem?

 

[113]

Sometimes the carbonyl group (C=O) gets protonated, other times the hydroxy group (C-OH) gets protonated and leaves.. both of these happen in water/acid. There are a bunch of other examples where I've always wondered "why doesn't this happen instead..".

If you understand equilibrium, it's not really random. The carbonyl can be continually protonated/deprotonated in solution and there is some mix of both forms, but the deprotonated form predominates. Same for the alcohol.

 

[konotsu]

a carbonyl group does not mean carboxylic acid (o resonance) and even then we are talking about protonation, not deprotonation


http://evans.harvard.edu/pdf/evans_pka_table.pdf

look at pka of conjugate acids of methanol and the ketone listed, similar species will follow the same trends listed here

A stand-alone carbonyl would have a resonance form (e.g. acetone), even if it's not a carboxylic acid

 

[AdrianVeidt]

Orgo was extremely random for me until the day it all clicked. This was also the day I had a new professor and he showed us electron pushing in full detail. After that moment, the class was awesome.

If you need some motivation in the class, watch Breaking Bad.

this and nothing else

 

[yehhhboiii]

I think it seems that way because reactions were discovered pretty haphazardly at the beginning. Early chemists didn't start from first principles and make predictions about reactions but they knew that this does this so let's see if we can do this.

 

[goofball]

I think it seems that way because reactions were discovered pretty haphazardly at the beginning. Early chemists didn't start from first principles and make predictions about reactions but they knew that this does this so let's see if we can do this.

If someone had told me this 2 years ago it might have changed my outlook on Organic.

 

[Docility]

What do you guys think is the best supplemental material/textbook for ochem?

I highly recommend:

1. Organic Chemistry as a Second Language by Klein

2. Chad's videos

Hope that helps! 🙂

 

[twizzlers713]

Depends on conditions man. It's all covered in Breaking Bad, as mentioned before hahaha

 

[circulus vitios]

A lot of it seems like bull**** hand waving borne of empirical observation, at least at the introductory level.

 

[Omppu27]

Go ahead and remember the mechanisms but you'll need to know what's actually going on... you might have to show a mechanism for a reaction you've never seen.

 

[chemsmith]

A stand-alone carbonyl would have a resonance form (e.g. acetone), even if it's not a carboxylic acid

Are you talking about a resonance on a protonated carbonyl or an uncharged carbonyl? A protonated carbonyl doesnt have one w/ a negative charge and as for a neutral carbonyl; alcohol has a more prevalent resonance with its alkoxide ion and hydronium. Hence why alcohols are more basic than carbonyl oxygens (not the other way around as you claimed above). A single resonance form provides no useful information on its own.

again, see the pka values for protonated species:

http://evans.harvard.edu/pdf/evans_pka_table.pdf

 

[SnoopyMD]

You definitely need to memorize the mechanisms, but it pays off to understand what's happening in each one until you can explain in backwards and forwards. Work plenty of practice problems. It helps to have done well in Organic I; if that happens, then Organic II is pretty much just the same template reaction over and over with different moieties and quirks.

So in conclusion, it has to be a combination of memorization (the mechanisms, what reagents do, etc.) and understanding (how NMR works, acid/base chemistry). If the foundation isn't there from the first semester, the second semester will be rough. If the first semester went well, then second semester will be a breeze (at least in my experience).

 

[Arbor Vitae]

Sometimes the carbonyl group (C=O) gets protonated, other times the hydroxy group (C-OH) gets protonated and leaves.. both of these happen in water/acid. There are a bunch of other examples where I've always wondered "why doesn't this happen instead..".

To those who did well in organic chemistry: did you just memorize the reactions or is there some way to actually predict whats going to happen despite everything seeming so random?

It depends on whether it's base or acid catalysis. Acid protonates the hydroxyl whereas base deprotonates the alpha carbon.

 

[deleted357625]

Not good advice, but I memorized.

 

[wonderdog]

Try "ochem as a 2nd language" and be comfortable with the rules of acidity.

This book got me through OChem. It comes highly recommended from me!

 

[MangoPlant]

This book got me through OChem. It comes highly recommended from me!

I'm going to try this book.

Can someone confirm that it is by David Klein? This is the one I was going to buy on amazon:

http://www.amazon.com/gp/product/1118144341/ref=pd_lpo_k2_dp_sr_1?pf_rd_p=486539851&pf_rd_s=lpo-top-stripe-1&pf_rd_t=201&pf_rd_i=111801040X&pf_rd_m=ATVPDKIKX0DER&pf_rd_r=0E3HN6RS2RGZ5BYB9518

 

[Aerus]

I'm going to try this book.

Can someone confirm that it is by David Klein? This is the one I was going to buy on amazon:

http://www.amazon.com/gp/product/1118144341/ref=pd_lpo_k2_dp_sr_1?pf_rd_p=486539851&pf_rd_s=lpo-top-stripe-1&pf_rd_t=201&pf_rd_i=111801040X&pf_rd_m=ATVPDKIKX0DER&pf_rd_r=0E3HN6RS2RGZ5BYB9518

Yeah, that's the one I have.

 

[ahutni3]

Sometimes the carbonyl group (C=O) gets protonated, other times the hydroxy group (C-OH) gets protonated and leaves.. both of these happen in water/acid. There are a bunch of other examples where I've always wondered "why doesn't this happen instead..".

To those who did well in organic chemistry: did you just memorize the reactions or is there some way to actually predict whats going to happen despite everything seeming so random?

The thing that always made everything make sense to me was learning about the different orbitals and anti orbitals that house the electrons. Luckily the Ochem program at my undergrad was computer based so it was easier to visualize, but I'm sure you can find some representations online. Once you can visualize those and understand them, you can predict just about any reaction mechanism. That and make sure you have a VERY solid foundation in acid base chemistry

 

[deleted393595]

To those who did well in organic chemistry: did you just memorize the reactions or is there some way to actually predict whats going to happen despite everything seeming so random?

It's not.

 

[Arbor Vitae]

I'm going to try this book.

Can someone confirm that it is by David Klein? This is the one I was going to buy on amazon:

http://www.amazon.com/gp/product/1118144341/ref=pd_lpo_k2_dp_sr_1?pf_rd_p=486539851&pf_rd_s=lpo-top-stripe-1&pf_rd_t=201&pf_rd_i=111801040X&pf_rd_m=ATVPDKIKX0DER&pf_rd_r=0E3HN6RS2RGZ5BYB9518

The trick is to know the pKa's of the the different parts of your starting material. Throw in some resonance and everything falls into place. Don't try memorizing reactions. Just try to understand the fundamentals of how compounds react to eachother. It's a very visual subject. It helps to be able to visualize the patterns in structures. Then there become no reactions to memorize because you just understand the language.

 

[SoulinNeed]

On a basic level, it's all about stability. Once you grasp that, then things start making sense.

 

[ashylarry23]

hahah

read for understanding man, not for brute memorization

 

[PingPongPro]

When I first learned it, it made no sense to me.

After I learned it, it seemed pretty intuitive.

After not studying it for 2+ years, it makes no sense to me now.

 

[lobo.solo]

You need to memorize a few things here and there, but fundamentally try to understand the electron flow and the conditions necessary for the reactions. I don't think is random at all.

 

[deleted393595]

One thing why certain reactions happen and others don't is often since the reaction conditions are different. For example, you're likely to have -OH elimination at high temperatures, which favor E1, and -OH SN1 substitution at low temperatures. Also, a lot of these reactions happen in an equilibrium and the reaction conditions favor one side of the equilibrium over the other, which is why you end up favoring one reaction pathway over the other (E1 over SN1 at high temps)

 

[HSUReddie]

Just wanted to echo what other people were saying - Klein's second language books were a huge help to me. Also like someone else said... I think for some people there is just a moment where it all clicks, and it becomes a little easier.

Isn't there a full textbook by Klein? That was like "still in the works" a couple of years ago when I was in orgo so I'm not quite sure if it's any good.

 

[orgohacks]

Sometimes the carbonyl group (C=O) gets protonated, other times the hydroxy group (C-OH) gets protonated and leaves.. both of these happen in water/acid. There are a bunch of other examples where I've always wondered "why doesn't this happen instead..".

--> the events you are referring to DO happen, but they're in equilibrium and represent cul-de-sacs on the main reaction pathway. In carbonyl chemistry, acid does two things: 1) increases the rate of carbonyl addition, and 2) increases the rate of elimination by making species into good leaving groups.
There are lots of common patterns in carbonyl chem, as shown here:
http://www.masterorganicchemistry.com/2011/04/20/summary-sheet-7-21-carbonyl-mechanisms-on-1-page/

To those who did well in organic chemistry: did you just memorize the reactions or is there some way to actually predict whats going to happen despite everything seeming so random?

Re: prediction, keep in mind that what you get taught in ochem is the tip of the iceberg. Underneath the details you learn in a 36 hour course are hundreds of thousands of hours of human effort discovering reactions and reactivity pathways, often through chance and trial and error. It *does* make sense, looking in the rear view mirror, why things happen the way they happen, but it might not be obvious to the beginner trying to predict.

There are a few key mental models that will carry a lot of intellectual freight for you: understanding factors that stabilize positive and negative charge, acidity/basicity, drawing resonance forms (and evaluating their stability), understanding nucleophilicity and electrophilicity, understanding leaving group ability, steric hindrance. and molecular orbital theory.