Why does the alkyl halide layer switch from the top layer to the bottom layer at the point where water is used to extract the organic layer?
When performing a liquid-liquid extraction, the organic layer (in this case, the alkyl halide) and the aqueous layer (water) are immiscible, which means they do not mix together. As a result, they form two separate layers.
Initially, the alkyl halide layer is positioned on top due to its lower density compared to water. Density is a physical property that determines the relative heaviness of a substance. The organic layer, being less dense, floats on top of the denser aqueous layer.
However, the situation changes when water is used to extract the organic layer. When water is added, the two liquid layers mix together, and the compounds distribute themselves between the solvents based on their solubilities and interactions.
In this case, when water is added, it can dissolve some of the alkyl halide due to its polar nature. The alkyl halide can form temporary interactions with the polar water molecules. As a result, some of the alkyl halide molecules become solubilized in water, causing the overall density of the organic layer to decrease.
This decrease in density makes the aqueous layer, containing dissolved alkyl halide, become heavier than the organic layer. Consequently, the layers switch positions, and the aqueous layer, now containing some of the alkyl halide, becomes the bottom layer, while the purified organic layer floats on top once again.
The switch in layer order occurs as a consequence of altering the density of the organic layer by dissolving some of the alkyl halide in water.