[sharbeldam]

In this lab we are supposed to extract caraway essential oil, I understood the first part (Steam distillation) up until we pour our distillate into separatory funnel and drying our organic layer and determining the yield of my essential oil.

But the next part is extracting carvone from the oil, so we dissolve our oil in ethanol and add DNP that apparently reacts only with carvone to produce imine (carvone 2,4 DNP).

My questions are:
1)why Will the carvone 2,4 DNP precipitate, and limonene stays in liquid (oil) ?
2)why do we wash the precipitate (2,4 dnp) with NaHCO3 and water as we filter it (using buchner filter)?

My thoughts (Tries)?
1)Carvone 2,4 dnp can make hydrogen bond with ethanol but limonene cant so it's really hard for me to know why would carvone 2,4 DNP and limonene not... or why is limonene soluble in ethanol and dnp of carvone is not.

[hollytara]

Alkenes like limonene do not usually react with 2,4-dinitrophenylhydrazine.  Carbonyls (aldehydes and ketones like Carvone) react fairly quickly.  The resulting 2,4-dinitrophenylhydrazones are known to form crystalline precipitates in nearly all cases.  The2,4-DNP test is used in a variety of ways: to test for presence of aldehyde/ketone, developing TLC plates, and identification via formation of derivatives in the old ways of Organic Chemistry (Shriner-Fuson-Curtin-Morrill being the Bible if I remember my names correctly): each aldehyde or ketone had a MP/BP and the highly crystalline 2,4-DNP derivatives had their unique MP: it was rare that two aldehydes/ketones of similar BP/MP had the same MP for the derivative and so the pair of MP was often a good ID for a known compound. 

[sharbeldam]

Thank you so much for your great answers as always

" The resulting 2,4-dinitrophenylhydrazones are known to form crystalline precipitates in nearly all cases "
Is there a solubility logic behind it? when we do the recrystalization of it via heating ethanol, it means that it dissolves in hot ethanol but not in cold ethanol, but my question is why doesnt it dissolves in ethanol and limonene does? limonene is a non-polar alkene and carvone 2,4 DNP has amine and some N=O so it's polar and can form hydrogen bonds with ethanol and also has lipophilic non polar parts... you would think by that logic it should be more soluble in ethanol no?

I would appreciate it if there is a logic behind it, if there isn't and i should just accept it the way it is, i dont mind as well, it just bugs me haha

[sharbeldam]

Anyone has any idea why limonene is soluble in ethanol and 2,4 dnp of carvone isn't? I even asked my professor and she said " it's just how it is ". but if anyone has idea why limonene (non polar) which can form no hydrogen bonds with ethanol is more soluble than a molecule that can form these bonds and also has non polar parts.

molecule is attached to image below if it helps

[Guitarmaniac86]

Anyone has any idea why limonene is soluble in ethanol and 2,4 dnp of carvone isn't? I even asked my professor and she said " it's just how it is ". but if anyone has idea why limonene (non polar) which can form no hydrogen bonds with ethanol is more soluble than a molecule that can form these bonds and also has non polar parts.

molecule is attached to image below if it helps

Other than polarity, what other factors affect whether something dissolves? Think about energy of dissolution and the energy required to form a lattice? In the case of 2,4-DNP-carvone, does the lattice energy overcome the energy needed to dissolve it? If it does, would you expect it to dissolve?

[sharbeldam]

Please tell me if I understood you right, you are saying that is not soluble in ethanol because it has a high lattice energy probably due to being bigger (more VDW forces) and having more hydrogen bonds/dipole dipole between its molecules themselves, and that leads to it crystalizing in ethanol?

[Guitarmaniac86]

Please tell me if I understood you right, you are saying that is not soluble in ethanol because it has a high lattice energy probably due to being bigger (more VDW forces) and having more hydrogen bonds/dipole dipole between its molecules themselves, and that leads to it crystalizing in ethanol?

Yes, exactly. I think the lattice energy is greater than the energy of dissolution, so energetically crystallising out of solution would be favoured. In the case of limolene, it wont crystallise out because at room temperature it is a liquid with a melting point below -70 celsius, so it will not ever crystalize out in ethanol.

[AWK]

Limonene is miscible with all alcohols except methanol, so talking about limonene solubility is incorrect.

Carvone 2,4-Dinitrophenylhydrazone dissolves well in boiling: ethanol or ethyl acetate.

The solubility of compounds depends very much on the energy of interactions in the crystal lattice.
I will give examples of amino acid solubility in water (solubility in g/L at 25, 50 and 100°C, respectively)
L-Phe - 30, 42, 99
DL-Phe - 14, 21, 99 (at 100°C both show the same solubility).

And now let's compare the solubility of three isomeric L-tyrosines at 25°C.
p-, o-, m-: 0.45, 2.0, 5.1 (identical summary formulas, identical functional groups). There is one more polar group in tyrosines than in phenylalanine, and the solubility of all isomeric L-tyrosines in water is much lower than L-Phe.

[sharbeldam]

Thank you both for great explanation.

[hollytara]

To follow up on AWK, there is a good correlation in general between high melting point and low solubility, both derived from the lattice energy. 

In olden times, 2,4-DNP chemistry was developed in part because of the high crystallinity of the aldehyde and ketone derivatives when measuring MP was one of the few techniques available in a typical organic lab!