Ranking Acidities- Best/Easiest Method?

[virtualmaster999]

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Hey everyone!

I thought I would ask this since it is probably a troubling concept for many students. What is the best overall way to determine the most or least acidic in a group (for Organic Chem)? I know Chad emphasized using CARDIO, but it seems there are some times this does not work. I know there are also different "situations" where you rank acidities: resonance, alpha hydrogens, donating/ withdrawing groups, ring structures (I guess categories would work here). Is there an overall best method to incorporate everything? Is the cARDIO method the best option?

Any advice helps. Thanks in advance!

 

[tooth_hurty]

To my understanding, CARDIO is the best approach and works for pretty much every problem. I suppose the only time it gets tricky is trying to differentiate the influences of resonance and induction when there are EDGs and EWGs present.

Do you have a specific "situation" in mind that doesn't follow CARDIO?

 

[virtualmaster999]

To my understanding, CARDIO is the best approach and works for pretty much every problem. I suppose the only time it gets tricky is trying to differentiate the influences of resonance and induction when there are EDGs and EWGs present.

Do you have a specific "situation" in mind that doesn't follow CARDIO?

Hmm not anything specific off the top of my head (besides the NH3+ and carboxylic acid example in Chad's). So you're saying that you should be able to use CARDIO for mostly every situation (like the ones I listed above)? And how do you go about the influences of resonance and induction when EDG/ EWG are present?

 

[tooth_hurty]

Hmm not anything specific off the top of my head (besides the NH3+ and carboxylic acid example in Chad's). So you're saying that you should be able to use CARDIO for mostly every situation (like the ones I listed above)? And how do you go about the influences of resonance and induction when EDG/ EWG are present?

I'm assuming you meant carboxylic acid vs. NH4+? Yeah this is one exception, as well as the case of phenol being less acidic than a carboxylic acid. I'm not aware of any other exceptions to CARDIO besides these two cases. CARDIO is a very effective guide for acidity and I think it should cover anything we encounter on the DAT. I do think it's good idea to commit those two exceptions to memory though.

As far as influences of EDG and EWG, I would still just use CARDIO and/or reason it out depending on the question. For example, butanol vs tert-butanol...since the OH of tert-butanol has 3 neighboring methyl groups that are donating electron density, these strengthen the O-H bond and make it a slightly weaker acid than butanol. A more obvious case would be methanol>butanol acidity. You may have already known all this lol but I hope it helped

 

[DAT Destroyer]

Hey everyone!

I thought I would ask this since it is probably a troubling concept for many students. What is the best overall way to determine the most or least acidic in a group (for Organic Chem)? I know Chad emphasized using CARDIO, but it seems there are some times this does not work. I know there are also different "situations" where you rank acidities: resonance, alpha hydrogens, donating/ withdrawing groups, ring structures (I guess categories would work here). Is there an overall best method to incorporate everything? Is the cARDIO method the best option?

Any advice helps. Thanks in advance!

I wish I could give you a single answer, but I cannot. Electronegativity and Resonance may work together or oppose each other. As a general guideline acidity will increase if groups such as NO2, CN, SO3H, Halogens, carbonyls are present. The closer these groups are to a COOH group for example,,,,,the more acidic a compound. If removal of a hydrogen can result in an anion that has resonance,,,,,,,be alert...this can make it acidic. Removal of an H from the OH of a carboxy acid yields an anion that has resonance.... Another example,,,,,,the Hydrogens adjacent to a carbonyl group are acidic and called alpha hydrogens. Removal of them result in an enolate anion. If there are two carbonyl groups in a 1,3 position......the carbon having the Hydrogen between them will be more acidic since it is a double alpha. Consider 1, 3-cyclopentadiene..removal of a Hydrogen from the sp3 carbon results in an Aromatic anion....pKa of about 16 !!!!! Knowing a few pKa values can also help...Alcohols are 16......Carboxy acids are 5......Aldehydes and Ketones around 17.....Phenols around 10......Alkynes around 25.......Alkenes around 44.......Alkanes around 52.

I hope this helps.....get out a few texts,,,,,and try a few problems. The books by Klein, Carey, Bruice, Wade, and McMurray all have problems with simple explanations. Use resources such as these written by PhD chemists.

Dr. Romano

 

[virtualmaster999]

Thank you all! Also just to make sure, EDG decrease acidity and ewg increase acidity right?


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[LittleKay]

EWG (Strong: NO2, SO3H, COOH, ETC + Weak: F Cl Br I) + Resonance increase acidity (compared to other organics)

EDG (NH2, NR2, OH, NHCOR, OR, OCOR, R) decrease acidity