Michael S. answered • 13d
B.S. in Chemistry, Indiana University; Organic Chem Teaching Intern
They dissolve into the hexane layer.
Why — "like dissolves like"
Br2, Cl2, and I2 are homonuclear diatomics. Both atoms in each molecule are identical, so their electronegativities are identical, so there is no bond dipole at all — these molecules are perfectly nonpolar. The only intermolecular force available to them is London dispersion.
Hexane (C6H14) is also nonpolar and also relies on dispersion forces. A halogen molecule slipping between hexane molecules costs almost nothing energetically — the interactions it forms are the same kind it gives up.
Water is the opposite case. Its molecules are held together by hydrogen bonds, which are strong. Inserting a nonpolar X2 molecule means breaking those hydrogen bonds and getting only weak dispersion interactions in return. Energetically that is a bad trade, so the halogens are largely excluded from the aqueous layer.
Which layer is which
Hexane sits on top. Its density is about 0.66 g/mL against water's 1.00 g/mL, so the organic layer floats. Whenever your solvent is a hydrocarbon or an ether, expect it on top; with something like dichloromethane (1.33 g/mL) the organic layer sinks instead. Always check the density — guessing which layer is the product is a classic way to throw away your compound in a separatory funnel.
What you will actually see
This is the point of the experiment — the halogens are coloured in hexane, and each is distinctive:
• Cl2 → very pale yellow, nearly colourless
• Br2 → orange to reddish-brown
• I2 → bright violet/purple
That violet iodine colour in hexane is striking, and it is worth noting that iodine in water or alcohol looks brown instead, because those solvents interact with I2 enough to shift its absorption. The colour is telling you about the solvent environment, not just the halogen.
So in the test tube: shake, let it settle, and the colour migrating into the upper hexane layer confirms both the identity of the halogen and its nonpolar character in one observation. This is exactly the trick used to detect which halogen a displacement reaction produced.
One honest qualification: "insoluble in water" is a simplification. Chlorine and bromine dissolve in water to a small extent, and chlorine also reacts with it (Cl2 + H2O ⇌ HCl + HOCl), which is what makes chlorinated water a disinfectant. Iodine is the least water-soluble of the three — which is why lab iodine solutions are usually made as I3− with added KI rather than as I2 alone. The partitioning still strongly favours hexane in every case.
