Question

Why does CS2 have higher boiling point than OCS (C as central atom)?

Answer 1

`"CS"_2` molecules experience less intermolecular forces than `"OCS"` per molecule pair, but in the bulk solution, there are likely more interactions possible for `"CS"_2` because of its greater symmetry, which overcomes the apparently minor influence from the slight polarity of `"OCS"`.

Explanation:

Carbon disulfide has a symmetric molecular geometry. Sulfur is in the same group as oxygen (new IUPAC group `16`) and these two elements share many chemical properties. The geometry of the triatomic compound sulfur forms with carbon- `"CS"_2`- has a molecular geometry identical to that of carbon dioxide `"CO"_2` which is symmetric about the central carbon atom.

Molecules of carbon disulfide are non-polar and experience London Dispersion (a.k.a. Van Der Waal forces) as the only kind of intermolecular interaction possible. Carbon-sulfur bonds within the molecule is (slightly) polar (`chi("S") - chi("C") = 0.03`)

`stackrel(" " delta^+)("C") "->-" stackrel(" "delta^-)("S")`

However, the symmetric molecular geometry lines the two dipoles of equal strength up in a straight line, such that they cancel each other and the molecule exhibits no net dipole:

`stackrel( delta^- )("S") "->-" stackrel( delta^+)(" C") " -<-" stackrel( delta^+)(" S")`

On the other hand, despite having an identical geometry, the dipole due to the presence of a carbon-sulfur single bond (`chi("S") - chi("C") = 0.03`) is weaker than that of a carbon-oxygen single bond (`chi("O") - chi("C") = 0.89`) (Wikipedia). The two dipoles do not cancel despite having lined up along the bonding axis.

The molecule is thus (slightly) polar (with dipole moment `vecmu = "0.7146 D"`):

`stackrel(delta^+ )("S-C") "-" stackrel(delta - )("O")`

Among molecules of similar sizes, those of strong intermolecular interactions tend to have high melting and boiling points. It takes high temperatures to supply sufficient amounts of energy that would break intermolecular bonds allowing for the decrease in the rigidity of the structure.

However, as it turns out, recalling that molecules are generally amongst others,

• the increased molar mass of `"CS"_2` compared to `"OCS"` increases its boiling point.
• the slightly higher polarity of `"OCS"` compared to `"CS"_2` leads to only a slightly raised boiling point.
• the greater symmetry of `"CS"_2` leads to more interactions in the bulk solution, increasing the boiling point of `"CS"_2` further.

Apparently, the trends of `CS_2`'s increased boiling point from greater symmetry and larger molar mass balance out to overcome the trends of `OCS`'s increased boiling point from being polar and decreased boiling point from being less symmetrical. Likely, the difference in the bulk interactions is the primary cause here.