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Suggest an explanation for the observations that ethanol($\ce{C_2H_5OH}$), is completely miscible with water and that ethanethiol ($\ce{C2H5SH}$) is soluble only to the extent of 1.5 g per 100 mL of water.

My answer:

The difference in the solubility of ethanol and ethanethiol in water can be explained by considering the nature of their intermolecular forces:

Ethanol

  • Hydrogen Bonding: Ethanol has an -OH (hydroxyl) group, which can form strong hydrogen bonds with water molecules. Hydrogen bonding occurs when a hydrogen atom is attracted to a highly electronegative atom like oxygen. This strong interaction makes ethanol completely miscible with water.
  • Polarity: Both ethanol and water are polar molecules, meaning they have a partial positive charge on one end and a partial negative charge on the other. This polarity allows them to mix well, following the principle "like dissolves like".

Ethanethiol

  • Weaker Hydrogen Bonding: Ethanethiol has an -SH (thiol) group instead of an -OH group. The sulfur atom in the thiol group is less electronegative than oxygen, resulting in weaker hydrogen bonding with water molecules.
  • Polarity: While ethanethiol is still somewhat polar, the weaker hydrogen bonding and the larger size of the sulfur atom compared to oxygen reduce its solubility in water. This results in ethanethiol being only slightly soluble in water, with a solubility of about 1.5 g per 100 mL.

In summary, the strong hydrogen bonding and polarity of ethanol make it completely miscible with water, whereas the weaker hydrogen bonding and different polarity of ethanethiol limit its solubility in water.

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    $\begingroup$ Why do you ask this question ? You provide the answer ! $\endgroup$
    – Maurice
    Commented Sep 8 at 15:30
  • $\begingroup$ This is what we call an "AMIRITE" question, and is somewhat frowned upon. Ideally you can explain in more detail what you find difficult or confusing. $\endgroup$
    – Buck Thorn ♦
    Commented Sep 18 at 9:02

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Your explanations are basically correct, but there is more to this problem than meets the eye. The solubility of ethanethiol in water is actually not all that low.

The data in this question actually corresponds to more than $0.2$ molar, and even the more conservative result given in Wikipedia ($0.7$%) exceeds $0.1$ molar. Sulfur has about the same electronegativity as carbon and we would expect the solubility of ethanethiol to be similar to that of propane based on your explanations. But on a molar basis ethanethiol is about $100$ times that of propane using the data in Wikipedia.

Although sulfur has about the same electronegativity as carbon and thus compounds of sulfur, carbon and hydrogen are expected to be largely nonpolar, the sulfur atom being larger makes it more polarizable: it and bonds to it will generate induced dipoles more readily than carbon in a polar environment such as water solvent. So replacing a carbon in propane with a similarly electronegative but more polarizable sulfur makes the thiol more soluble than propane. A similar effect appears in this table when hydrogen sulfide ($\ce{H2S}$) is compared with methane ($\ce{CH4}$).

Of course this polarizability effect is not enough to match the effects of a permanent dipole and strong hydrogen bonding from ethanol, and so we have the solubility order

$\ce{CH3CH2CH3} << \ce{CH3CH2SH}<< \ce{CH3CH2OH}.$

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  • $\begingroup$ Higher acidity of thiols also increases the solubility a bit. $\endgroup$
    – Mithoron
    Commented Sep 8 at 18:22
  • $\begingroup$ Most of the dissolved thiol (or H2S) will be undissociated. Polarizability of the sulfur may allow more charge transfer from hydrogen bonded thereto and thus allow for the observed slight dissociation. $\endgroup$
    – Oscar Lanzi
    Commented Sep 8 at 18:46

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