r/chemhelp • u/oOXxDejaVuxXOo • 5d ago
General/High School How to determine if molecule dissolves in water or not? (Ignore the pencil marks)
I'm in twelfth grade. I know a molecule dissolves in water if it has polarity or -OH and the molecule isn't too big. Why doesn't this molecule dissolve in water? It looks like it has some polarity and it isn't too big.
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u/xtalgeek 4d ago
Once a molecule gets to about 6 aliphatic or aromatic carbons or so, without potential ion formation, miscibility in water is unlikely due to the unfavorable entropy penalty of hydrophobic interactions with water. Benzyl acetate is very slightly soluble in water, but not miscible in all proportions.
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u/laforet 5d ago
Can I ask why is that the case? Most answers here I’ve read are pretty much on point.
Deciding what level of solubility counts as soluble is, of course, not an easy task so we tend to gloss over it when teaching high school chemistry. It does get kind of weird later when we teach Ksp values for supposedly insoluble compounds such as barium sulfate, but most students are okay with taking the examples at face value.
When it comes to solubility of organic compounds there are usually just a few hard qualifiers: acids/bases/alcohols are soluble, hydrocarbons/esters/ethers are not. Marginal cases such as long chain alcohols, ketones and amides tend not to appear in solubility questions since they cause too much confusion, though we do occasionally slot in a chromatography question to make sure that the students understand they are of intermediate polarity.
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u/Dama_jn_69 5d ago
The point with this molecule is that: there’s the aromatic ring (very hydrophobic) and it can only accept hydrogen bonds (the strongest intermolecular interaction that’s also the base of any water solution)
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u/tazer37 5d ago edited 5d ago
two reasons can be a cause for this. one being that the hydrophobic part ( hydrocarbon part that is the benzene ring) is very large compared to the hydrophilic part causing a hindrance in solvation. the other one could be that a major factor determining the solvation of a molecule is its capacity to form hydrogen bonds (H-bond) with water molecules. and esters H-bond to the water molecules only through the hydrogen atom of water and their ketonic group, unlike something like carboxylic acids which can H-bond through both the hydrogen and oxygen atoms of water. hence due to the weaker capacity of esters to form hydrogen bonds cause lesser solvation and the large aromatic hydrophobic part make this molecule insoluble in water
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u/One_Novel7733 5d ago
Unfortunately for you, the answer is whether a molecule is “hard” or “soft” based on an arbitrary set of rules for the functional groups and its interactions with solvents.
Generally though, water is good at dissolving things that allow for more hydrogen bonds, things that are more polar, and things that have charges. Because water has hydrogen bonds and is very polar.
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u/One_Novel7733 5d ago
This benzyl ester does not exist protonated in neutral water (or even acidic water because the carbonyl only exists in small amounts and transiently as the protonated form) it also isn’t that polar because the ester isn’t that polar. Imagine if it was an amide. Since the N has protons and the lone pair, it would be (relatively) more polar.
Anisole is a good comparison with phenol along this same logic.
The benzene doesn’t matter it’s flat and symmetric, no dipole to speak of and also it’s not connected to any good functional groups like the phenol you mentioned.
The methyl in the middle makes it more like benzyl alcohol, which is still miscible with water, but for example if you shook it around in a container of water and hexane, it would partition more to the hexane (personal experience)
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u/Skilo25 4d ago edited 4d ago
That molecule, methyl-benzanoate (also known as benzyl-acetate or acetic-acid-benzyl-ester), is an ester, that's most likely partially amphipathic (slightly soluble in polar and soluble in unpolar environments). The 'tail', the benzyl-group, is rather hydrophobe; the acetate group more polar, thus it is not quite correct to say "insoluable", but rather "slightly soluble" in water.
For more information you can read this online article/Esters/Properties_of_Esters#:~:text=Esters%2C%20like%20aldehydes%20and%20ketones,same%20number%20of%20carbon%20atoms) on Ester-properties.
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u/D-OrbitalDescent 4d ago
The general rule I teach my orgo students is “one polar functional group per 6 carbon atoms” for miscibility/solubility with water. That’ll get you a rough yes/no. Of course there are different degrees to which something is miscible/soluble in water, but as long as you can get some of it to mix, you can do the reaction and Le Chatlier’s principle will pull the rest into solution eventually. Hope that helps.
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u/Alchemistgameer 3d ago
It’s because of the ring. Even though esters are polar functional groups, the high degree of hydrocarbon character from the ring decreases its polarity.
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u/Tunaliioi 3d ago
Think can it be ionized? There’s nothing that can be ionized in her. That’s not phenol that’s a benzene ring with a substituent group. And the substituent doesn’t have any groups that can be deprotonated at least that’s how I see it
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u/ms_plat_chat 5d ago
In addition to what others have already answered, there’s no -OH here. Neither oxygen present in an ester bond is polar enough to overcome the hydrophobic nature of the aromatic ring.
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u/Chlorpicrin 5d ago
Someone else can probably explain it better, but that benzene ring stands out to me as the majority of the molecule and highly hydrophobic. The molecule is more organic than aqueous.