brightsky's Chem Thread

[Mao]

Oh man, that was pretty hard. This is the solution:

Code: [Select]

  H H CH3
  | | |
H-C-C-C-O-H
  | | |
  H H CH3

2-methylbutan-2-ol

Note that the OH peak can be found anywhere between ppm. Also, tertiary alcohols can delocalise the charge withdraw from oxygen quite well, hence why the peaks are reasonably close to each other. In a primary or secondary alcohol, the separation between peaks tend to be a lot more distinct.

[brightsky]

Oh man, that was pretty hard. This is the solution:

Code: [Select]

  H H CH3
  | | |
H-C-C-C-O-H
  | | |
  H H CH3

2-methylbutan-2-ol

Note that the OH peak can be found anywhere between ppm. Also, tertiary alcohols can delocalise the charge withdraw from oxygen quite well, hence why the peaks are reasonably close to each other. In a primary or secondary alcohol, the separation between peaks tend to be a lot more distinct.

thanks mao! would you be able to elaborate on the bit in bold. don't quite follow...

also, consider, say, propan-1-ol (CH3CH2CH2OH). would the peak due to the two emboldened hydrogen atoms be split by the single hydrogen attached to the oxygen? i read somewhere that if the propan-1-ol sample were pure, then the peak would be split. then i ran into the concept of decoupling, which don't quite fully understand. why is it that only non-equivalent hydrogen atoms attached to CARBON atoms  have the ability to split peaks?

thanks!

[lzxnl]

I've read in the chemistry textbook that OH hydrogens do not split other peaks, nor are their own peaks split.

[teletubbies_95]

Is there a reason why?

[brightsky]

So, I was always under the impression that aspirin is insoluble, or has very low solubility (I think the pKa value of aspirin is quoted to be 3.5 at room temperature or something).

But we all know that aspirin, when ingested, reacts with water in the alkaline conditions of the small intestine to produce salicylic acid and ethanoic acid. We also know that the condensation reaction between salicylic acid and ethanoic acid has an appreciable back reaction (which is one of the reasons why we use ethanoic anhydride instead).

But surely this is a contradiction? Or am I missing something?

After you synthesise aspirin, you must isolate it via filtration. Would you wash the precipitate with cold distilled water or something else like acetone in order to maximise yield?

[lzxnl]

Firstly, pKa values are not the same as solubility. Look at the alkanoic acids. Methanoic acid has the lowest pKa by far. Then we have ethanoic acid at 4.76. The following acids have very similar pKas, but their solubility in water drops off quickly.

As an extreme example, ethanol has a pKa of around 16. So what? It's miscible with water.
A substance doesn't have to ionise to dissolve in water.

But we all know that aspirin, when ingested, reacts with water in the alkaline conditions of the small intestine to produce salicylic acid and ethanoic acid. We also know that the condensation reaction between salicylic acid and ethanoic acid has an appreciable back reaction (which is one of the reasons why we use ethanoic anhydride instead).

It's more that the usage of ethanoic anhydride produces ethanoic acid, something which can also react with salicylic acid, so this reaction hampers the aspirin hydrolysis reaction.
We use aspirin because salicylic acid is a major irritant. It burns. In the alkaline conditions of the intestines where it is produced, it will be neutralized by the base.

Washing aspirin with acetone would not be a good idea. Acetone is an organic compound which will dissolve other organic compounds like aspirin. Water, however, won't, especially cold water.

[brightsky]

Firstly, pKa values are not the same as solubility. Look at the alkanoic acids. Methanoic acid has the lowest pKa by far. Then we have ethanoic acid at 4.76. The following acids have very similar pKas, but their solubility in water drops off quickly.

As an extreme example, ethanol has a pKa of around 16. So what? It's miscible with water.
A substance doesn't have to ionise to dissolve in water.

It's more that the usage of ethanoic anhydride produces ethanoic acid, something which can also react with salicylic acid, so this reaction hampers the aspirin hydrolysis reaction.
We use aspirin because salicylic acid is a major irritant. It burns. In the alkaline conditions of the intestines where it is produced, it will be neutralized by the base.

Washing aspirin with acetone would not be a good idea. Acetone is an organic compound which will dissolve other organic compounds like aspirin. Water, however, won't, especially cold water.

Can you explain the emboldened part a little further? I've always been under the impression pKa determines solubility.

I'm unsettled precisely by the fact that aspirin undergoes hydrolysis. Aspirin + Water --> Salicylic Acid + Ethanoic acid. Now, when we produce aspirin using salicylic acid and ethanoic anhydride, the limiting reagent is invariably the salicylic acid, and so the reaction mixture we are left with may theoretically contain ethanoic anhydride, aspirin and ethanoic acid. Now, both ethanoic anhydride and aspirin are liquids, while aspirin is a solid, which allows for easy separation. However, to maximise yield, we need to wash the aspirin precipitate thoroughly. But the question is, with what? Surely, if we wash it thoroughly with distilled water, even cold, at least some of the aspirin will react with the water to produce salicylic acid and ethanoic acid, thus reducing yield.

[lzxnl]

I'm unsettled precisely by the fact that aspirin undergoes hydrolysis. Aspirin + Water --> Salicylic Acid + Ethanoic acid. Now, when we produce aspirin using salicylic acid and ethanoic anhydride, the limiting reagent is invariably the salicylic acid, and so the reaction mixture we are left with may theoretically contain ethanoic anhydride, aspirin and ethanoic acid. Now, both ethanoic anhydride and aspirin are liquids, while aspirin is a solid, which allows for easy separation. However, to maximise yield, we need to wash the aspirin precipitate thoroughly. But the question is, with what? Surely, if we wash it thoroughly with distilled water, even cold, at least some of the aspirin will react with the water to produce salicylic acid and ethanoic acid, thus reducing yield.

The bold part confuses me.
You've forgotten something. The reaction of salicylic acid and ethanoic acid to produce aspirin requires a catalyst. Both acid and base work. Without the catalyst, the reaction is painfully slow. Similar for the back reaction. With just cold water and no catalyst, the back reaction essentially won't happen.

PKa means how weakly bonded a proton is to the compound, or how stable the conjugate base is. Now, when it's really small, as is the case for most weak acids, the VAST majority of molecules will still be in their neutral, molecular form. Therefore the tiny amount that does dissociate really won't make much of an impact. We're talking less than 1% for acids like ethanoic acid.
Octanoic acid has a long hydrocarbon tail which will mostly be insoluble in water. In fact, it forms a dimer as the polar sides line up and form hydrogen bonds, while the hydrocarbon tails form dispersion forces with each other. This structure isn't easily broken by water. Its pKa, according to http://www.zirchrom.com/organic.htm, is 4.89, VERY similar to ethanoic acid's 4.76.

[brightsky]

The bold part confuses me.
You've forgotten something. The reaction of salicylic acid and ethanoic acid to produce aspirin requires a catalyst. Both acid and base work. Without the catalyst, the reaction is painfully slow. Similar for the back reaction. With just cold water and no catalyst, the back reaction essentially won't happen.

Ahhhh! Yes of course, how could I forget the catalyst!! Thanks nliu!

PKa means how weakly bonded a proton is to the compound, or how stable the conjugate base is. Now, when it's really small, as is the case for most weak acids, the VAST majority of molecules will still be in their neutral, molecular form. Therefore the tiny amount that does dissociate really won't make much of an impact. We're talking less than 1% for acids like ethanoic acid.
Octanoic acid has a long hydrocarbon tail which will mostly be insoluble in water. In fact, it forms a dimer as the polar sides line up and form hydrogen bonds, while the hydrocarbon tails form dispersion forces with each other. This structure isn't easily broken by water. Its pKa, according to http://www.zirchrom.com/organic.htm, is 4.89, VERY similar to ethanoic acid's 4.76.

Hmm..I thought pKa was a measure of the extent of hydrolysis. Large pKa => the molecule/ionic compound reacts almost completely with water. Small pKa => the molecule/ionic compound doesn't react much with water. I understand why octanoic acid has very low water solubility, but surely this means that the majority of octanoic acid molecules would remain unreacted when placed in water, entailing a low pKa value. Is not hydrolysis the same as dissolution in this case?

Also, another question: Is there a possibility of a small amount of salicylic acid remaining unreacted? If there is, how can we separate the salicylic acid from the aspirin? Surely, filtration alone is not sufficient, since both salicylic acid and aspirin exist at room temperature as solids.

[Mao]

> Also, tertiary alcohols can delocalise the charge withdraw from oxygen quite well, hence why the peaks are reasonably close to each other.

Tertiary carbon are much better at supporting charge than secondary or primary carbons. Carbon is relatively "electro-neutral", it doesn't withdraw much electrons, it doesn't donate much. When you put an electronegative atom (oxygen) near it, carbon will donate some electron density, and the next carbon will donate some, and so forth. The more 'connected' the carbon is, the better it is at 'smearing out' the electronegativity of oxygen. When electronegativity is more 'smeared out', the chemical shifts become less pronounced.

also, consider, say, propan-1-ol (CH3CH2CH2OH). would the peak due to the two emboldened hydrogen atoms be split by the single hydrogen attached to the oxygen? i read somewhere that if the propan-1-ol sample were pure, then the peak would be split. then i ran into the concept of decoupling, which don't quite fully understand. why is it that only non-equivalent hydrogen atoms attached to CARBON atoms  have the ability to split peaks?

I've read in the chemistry textbook that OH hydrogens do not split other peaks, nor are their own peaks split.

nliu is correct.

The common reason (when I was taught) was some explanation involving "deshielding". That is wrong.

The real reason is that the hydroxyl proton is not permanently attached to the oxygen. It is constantly swapping in/out with other protons in the solvent via "proton exchange". The hydroxyl proton does participate in coupling, the peaks in reality do split, but its rate of swapping in/out is so fast that our NMR machines cannot capture it, and instead it just gets a blurred singlet. See: http://www.chemistry.ccsu.edu/glagovich/teaching/316/nmr/couplingoxygen.html

[Mao]

I'm unsettled precisely by the fact that aspirin undergoes hydrolysis. Aspirin + Water --> Salicylic Acid + Ethanoic acid. Now, when we produce aspirin using salicylic acid and ethanoic anhydride, the limiting reagent is invariably the salicylic acid, and so the reaction mixture we are left with may theoretically contain ethanoic anhydride, aspirin and ethanoic acid. Now, both ethanoic anhydride and aspirin are liquids, while aspirin is a solid, which allows for easy separation. However, to maximise yield, we need to wash the aspirin precipitate thoroughly. But the question is, with what? Surely, if we wash it thoroughly with distilled water, even cold, at least some of the aspirin will react with the water to produce salicylic acid and ethanoic acid, thus reducing yield.

This is why we use cold water, the reaction rate is small enough that it doesn't matter. 100% yield is a fantasy.

Hmm..I thought pKa was a measure of the extent of hydrolysis. Large pKa => the molecule/ionic compound reacts almost completely with water. Small pKa => the molecule/ionic compound doesn't react much with water. I understand why octanoic acid has very low water solubility, but surely this means that the majority of octanoic acid molecules would remain unreacted when placed in water, entailing a low pKa value. Is not hydrolysis the same as dissolution in this case?

You got it the wrong way around. Small pKa --> readily ionises, large pKa --> does not readily ionise. You will learn the theory behind pKa properly in U4.

Solubility does depend on ionisation, but not solely on ionisation. In this case, ionisation helps, but other factors such as hydrophobicity also contributes. It is impossible to tell how much each of these contribute, and we do not have a way yet to reliably predict the solubility of a compound in modern chemistry. What you are seeking is a way to predict the solubility of every compound using a single metric, which is impossible (for now).

Also, another question: Is there a possibility of a small amount of salicylic acid remaining unreacted? If there is, how can we separate the salicylic acid from the aspirin? Surely, filtration alone is not sufficient, since both salicylic acid and aspirin exist at room temperature as solids.

Yes, there generally is. We don't mind because it's really small.

[brightsky]

A few questions regarding IR spec:
1. When the sample absorbs IR radiation, and gets promoted to a higher vibrational energy level, does it then re-emit the photon it absorbed as in UV-vis and atomic emission spec, or is the energy simply used up?
2. What influences the shape of the absorption bands (i.e. the sharpness/broadness thereof)? For instance, why is the O-H (alcohol) band broad, and the O-H (acid) band even broader?
3. How exactly is the IR spectrum generated? The schematic diagram in the textbook shows a monochromator interposed between the sample and the infrared detector, suggesting the selection and detection of SINGLE wavelengths. But we all know that the IR spectrum is continuous, and shows transmission of light from around 500/cm to 4000/cm...

Thanks!

[Mao]

> When the sample absorbs IR radiation, and gets promoted to a higher vibrational energy level, does it then re-emit the photon it absorbed as in UV-vis and atomic emission spec, or is the energy simply used up?

No, IR excites vibrational energy levels (think: guitar string periodic vibrations). Energy is dissipated via non-radiative paths

> What influences the shape of the absorption bands (i.e. the sharpness/broadness thereof)? For instance, why is the O-H (alcohol) band broad, and the O-H (acid) band even broader?

This is to do with the number of Boltzmann states available in a particular solvent. Bonds to "loosely-held" hydrogens (particularly on electronegative atoms) tend to be more flexible, and thus have vibrational states across a wider range of energies, giving a broad peak.

> How exactly is the IR spectrum generated? The schematic diagram in the textbook shows a monochromator interposed between the sample and the infrared detector, suggesting the selection and detection of SINGLE wavelengths. But we all know that the IR spectrum is continuous, and shows transmission of light from around 500/cm to 4000/cm...

We make thousands of single measurements at different wavelengths, thus compiling a graph. This process takes about 10~30 seconds on a modern machine. There are more fancy methods such as Fourier Transform IR, which only require a single measurement to capture the entire spectrum, but the theory for that is way way past VCE.

[brightsky]

Name an isomer of pent-2-ene which has only one peak in its proton NMR spectrum.

[SocialRhubarb]

Cyclopentane?