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We know that like dissolves like. And dichloromethane is a polar solvent and water is also a polar solvent. Also there ought to be a strong hydrogen bonding between the chlorine and hydrogen atoms. So why is it immiscible?

Vishnu
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Sreetama ghosh hazra
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    The quick way to answer is that DCM isn't polar enough, as well as the strength of the H-bond it might form in water. A detailed answer involves the enthalpy end entropy terms analysis of the dissolution but I am afraid it will remain rather qualitative. And see also https://chemistry.stackexchange.com/questions/38137/dichloromethane-solubility-in-water where interestingly an azeotrop is mentioned. It is quite possible that ideally mixing results in a ordered phase, so that the heat of solubilisation doesn't compensate for the reduced entropy. – Alchimista Mar 30 '19 at 09:07
  • @Alchimista What do you mean by ordered phase? – Sreetama ghosh hazra Mar 30 '19 at 09:12
  • @Alchimista. Dipole moment of DCM is 1.47 and that of water is 1.85. – Sreetama ghosh hazra Mar 30 '19 at 09:14
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    It can be that entropy is lower as for the water might have to arrange more strictly to accommodate the dcm molecules. This would give a positive delta G as for the intermolecular forces are weaker for sure. – Alchimista Mar 30 '19 at 09:16
  • And if you insist on polarity, dcm is heavier than water. Still if you let a drop on tge bench,it simply disappears in seconds. Thus, H-bonding is much weaker as compared to water. – Alchimista Mar 30 '19 at 09:24
  • Forget. I wanted to say that it has a low boiling point and evaporates very fast. It was a drop on the bench, not a layer in a funnel. I wild try to better explain later, but if you didn't get a grasp of the situation yet, probably my answer won't help much. Wait for another user. – Alchimista Mar 30 '19 at 09:29
  • Do you know about frontier MO theory? if yes, check out this to understand why DCM resists solvation by nucleophilic solvents like water – Yusuf Hasan Mar 30 '19 at 11:27
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    For an immiscible compound, DCM is quite soluble in water (~20g will dissolve in a litre of water). As will chloroform (~10g/L) which is enough to make a recreational drink (or was in Victorian times before we worked out how toxic it was). – matt_black Mar 30 '19 at 20:37
  • @matt_black it depends on temperature right? 25.6g/L at room temperature.. – Sreetama ghosh hazra Mar 30 '19 at 20:47
  • @Sreetamaghoshhazra yes temperature matters. I was approximating. – matt_black Mar 30 '19 at 20:48
  • @YUSUFHASAN DCM dissolves in benzene right? – Sreetama ghosh hazra Mar 30 '19 at 20:50
  • @matt_black data shows with temperature solubility increases.. Is it only since the entropy is also increasing.. And hence the solvation enthalphy... – Sreetama ghosh hazra Mar 30 '19 at 20:55
  • I add this comment for further clarification. Somehoe cunterintuitive, is the occurrence of hydrogen bonding (whatever extent it happens) that, together with the endothermic character of the process, unfavours mixing. This h-bond occurrence reduce the positive entropy term associated with mixing. A hypothetical compound with comparable size and dipole as DCM but not suitable for H-bond would be much more soluble. – Alchimista Mar 31 '19 at 09:26
  • @Alchimista hypothesis will be true if aprotic solvent is used. H bonding favors solubility here. isn't? – Sreetama ghosh hazra Mar 31 '19 at 09:39
  • Also. But most of the h-bonding DCM to solvent involves its hydrogens rather than chlorine, I guess. But don't try to make a predictive rule out of my example. It served to let you consider deltaG in finer details. – Alchimista Mar 31 '19 at 09:44

2 Answers2

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In the following figure the free energy computed from solubilities available in the wikipedia are plotted against T. The positive values of $\Delta G$ imply that formation of a 1 molal solution from the pure components is an endergonic process (requires energy input), consistent with the limited solubility of DCM in water.

enter image description here

From the slope and intercept we see that the enthalpy of solubilization of DCM in water is negative (exothermic) while the entropy change is also negative. The negative entropy change, as explained in the comments, opposes mixing. The exothermic character means that increasing T actually discourages solubilization (the negative entropic effect is emphasized at higher T).

This does not address the mechanism of solubilization, of course. However, it supports the argument in the comments that formation of water cages or conformationally restrictive hydrogen bonding to the solute imposes an entropic penalty upon solubilization.

Buck Thorn
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    Nice formulation for what I tried to convey. I am just surprised by the fact that the mixing has an exothermic character, as for I would have expected water to water interaction to be stronger as well, so that the entropy term could impose at the reltively small concentrations also mentioned in comments. – Alchimista Mar 31 '19 at 18:26
  • Thanks @Alchimista This is admittedly just an extension of the argument you raised. I did not come across a more systematic description of the solvation process in this particular system, the details are missing. – Buck Thorn Mar 31 '19 at 19:50
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Dichloromethane is non-polar.

The physical causes of solubility of substances in each other can semiquantitatively be expressed by the Hansen solubility parameters.

$\begin{array}{} \text{Water}:&&\delta_d=15.6&\delta_p=16.0&\delta_h=42.3&R_0=47.8\\ \text{Dichloromethane}:&&\delta_d=18.2&\delta_p=6.3 & \delta_h=6.1&R_0=20.2 \end{array}$

$$R_0=20.2,\ \ \ R_a=37.8$$

$$RED=\frac{R_a}{R_0}=1.87$$

RED < 1 would show solubility of both substances in each other. But RED > 1, so both substances are little soluble in each other.

The solubility parameters above show, the energies from polar and from hydrogen bridging bond intermolecular attractions are higher in water.

Generally, chlorinated hydrocarbons have lower energies from polar and hydrogen bridging bond intermolecular attractions.

The strongest hydrogen bridging bonds are formed between the atoms of the strongly electronegative elements $\ce{F}$, $\ce{O}$ and $\ce{N}$. But the atoms of other electronegative elements, e.g. $\ce{Cl}$-, $\ce{S}$- and $\ce{C}$-atoms, are capable of weak hydrogen bridging bonds at suitable molecular structure.

The atomic radii of the third period elements ($\ce{P}$, $\ce{S}$, $\ce{Cl}$) are much larger than those of the second period elements ($\ce{N}$, $\ce{O}$, $\ce{F}$). $\ce{Cl}$-atoms have lower charge density therefore. They are weaker proton acceptors therefore.

Hydrogen bridging bonds can be proved e.g. by molecular spectroscopy.

Li Bian: Proton Donor Is More Important Than Proton Acceptor in Hydrogen Bond Formation: A Universal Equation for Calculation of Hydrogen Bond Strength. J. Phys. Chem. A 107 (2003) (51) 11517–11524

Weinhold, F.; Klein, R. A.: What is a hydrogen bond? Mutually consistent theoretical and experimental criteria for characterizing H-bonding interactions. Mol. Phys. 110 (2012) (9-10) 565-579

Gilli, G.; Gilli, P.: The Nature of the Hydrogen Bond: Outline of a Comprehensive Hydrogen Bond Theory. Oxford University Press, Oxford, 2013

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