Is propanoic acid stronger than ethanoic?

[josephcollins]

Hi ppl,
Is propanoic acid stronger than ethanoic?Because propanoic acid has a longer R-group, it will have a greater inductive electron releasing effect than ethanoic acid, so the electron density is greater in propanoic acid than for ethanoic acid. With a region of greater e density, will then propanoic acid be stronger since it is easier for it to donate a H? Could someone please check this reasoning for me?

Does anyone here know the side reaction in the oxidation of 2-hydroxypropanoic acid that gives the green colour of this reaction? I know that it must involve the Cr3+ ions of potassium dichromate but I'm not sure about what they react with?

Also, is H20 an organic product of the reaction between KOH and 2-fluoropropanoic acid?

Thanks, Joe

 

[chem_tr]

I think that the more inductive effect on side groups, the less the acidity; so propanoic acid should be less acidic than acetic acid. However, solubility in organic solvents increase with increasing methylene groups. We can conclude that propanoic acid is weaker than acetic acid in aqueous media, but stronger than that in non-aqueous media.

Greater inductive effect on C of COOH group will push oxygen's electrons away, so hydrogen will be more bound to oxygens; this means that it will be less acidic than the former case (less inductive effects).

2-Hydroxypropanoic acid, also known as lactic acid (milchsäure;milk acid) can be oxidized to give pyruvic acid (2-oxopropanoic acid); here, a secondary alcohol group is oxidized to ketone. As a result, chromium(VI) ions are reduced to chromium(III), giving the characteristic green color.

2-Fluoropropanoic acid is an acid, which can be summarized to HA, and this can easily be reacted with KOH to give potassium 2-fluoropropanoate and water. Your assumption is correct.

 

[josephcollins]

Thanks a lot for the help, my friend, I very much appreciate it. I have just to clarify about the chromate though: I need to give the equation for the reaction that produces the colour change, an unbalanced one. All I know is that it involves the Cr3+, could you please suggest the other reactants? Thanks, Joe

 

[chem_tr]

Dichromate (Cr⁶⁺) and lactic acid (C⁰) will undergo a redox reaction to give Cr³⁺ and pyruvic acid (C²⁺). Do the balance and show your work. Note that a balanced electron exchange will ensure that a balanced redox reaction to be written. Also worth noting here is that dichromate has two chromium(VI) centers, so you will need to double the electron exchange.

As I said above, do something and show us.

//////////////////////this one came to my mind after seeing Movies' message, so I am putting it separately////////////////////////////

The redox reaction proceeds best in sulfuric acid media; so you'll have hydrogen ion on the left side and water on the right. They will help you about balancing.

 

[movies]

Chem_tr has answered this question with exceptional detail, I just wanted to add one thing: propanoic acid and acetic acid will be very, very close in acidity (pKa's of 4.88 and 4.76, respectively).

For the oxidation, you know the chromium species that you start with, and that should be chromium (VI+), if I'm counting correctly. The product chromium species will be chromium (IV+). Typically the product Cr species is chromic acid, CrO₃H₂.

 

[chem_tr]

Unfortunately, I disagree with you about the redox product here, movies. I suppose a deep green color of Cr(III), starting from orange-red (which means that Cr(VI) is present) will be seen as an important color change.

Logarithmic scales are usually confusing, since a difference of 0.12 here means that the one with lower value will be 1.58 times more acidic than the greater one. If it were 0.3, then we could say that there were 2 times of greater acidity.

 

[GCT]

I think that the more inductive effect on side groups, the less the acidity; so propanoic acid should be less acidic than acetic acid. However, solubility in organic solvents increase with increasing methylene groups. We can conclude that propanoic acid is weaker than acetic acid in aqueous media, but stronger than that in non-aqueous media.

I would imagine the trend would be the same in aq/nonaq media. With more electron withdrawing groups, the corresponding anion is deemed more stable, one can draw the analogy vice versa, with a longer non-polar chain (conversely to electron donating).

 

[movies]

Chem_tr, I agree that the green color suggests Cr (III+), but in the oxidation mechanism you only give 2 electrons to the Cr (VI+) species. I suppose there might be a way to disproportionate to Cr (III+), but I'm not sure how that would work.

Indeed the logarithmic scale is a little misleading, but a difference of less than 1 pKa unit isn't a very big difference for practical concerns.

GCT, the trend should be the same up to a point. I would suspect that something like dodecanoic acid would be more acidic that something like formic acid if pentane were the solvent, simply based on solubility. A formate anion is a relatively dense negative charge whereas a long alkyl chain could mask the carboxylate anion more efficiently. I don't know if this has been proven experimentally though, it's just my intuition.

 

[Protervum]

But then I have a question: Ok Cr(VI) H2Cr04 oxidizes an alcohol molecule to its corresponding ketone or aldehyde (and even it can oxidizes it again to a carboxylic acid), reducing our H2CrO4 to Cr(IV) H2CrO3, ok, ok, but then Cr(IV) H2CrO3 can take two paths: dismutate to a chromium III and a chromium V species (where chromium V can even oxidize more alcohol and reduce to chromium III H3CrO3 ); Or chromium (IV) can oxidize an alcohol an it reduces to chromium II H2CrO which can react with a chromium (VI) H2CrO4 to give both chromium III and chromium V species (This chromium V can even oxidize more alcohol)...
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Ok, Ok, this is clare for me but My question is:
How Chromium (IV) H2CrO4 can dismutate or disproportionate to Chromium (III) H3CrO3 and Chromium (V) H3CrO4? Is there a mechanism involved? or why this game of loose and gain of oxygen (and hence, up and down of oxidation states) for chromium sapecies can take place? Note that we´re working in acidic medium so I think H20 and H30+ are involved in a kind of mechanism...¬¬...°__°! But what you think? I expect your ideas !...


Ohh! sorry If I fall in grammar mistakes in this message, I´m from Mexico City and I don´t speak English so good! °__°!... ^__^!