In a Williamson ether synthesis, the alkoxide attacks the alkyl halide in an SN2 process, so backside attack on a carbon with a good leaving group is key. SN2 works best when the carbon is not heavily hindered, allowing the nucleophile to approach directly. Primary alkyl halides are ideal because they offer minimal steric hindrance, enabling fast SN2 displacement of the halide by the alkoxide to form the ether. Secondary substrates can react but more slowly due to increased hindrance, while tertiary substrates are usually too hindered for SN2 and can lead to elimination instead. Aryl halides are not suitable for SN2 because the carbon bearing the halogen is sp2-hybridized, making backside attack very difficult. So the most suitable substrate for SN2 to form the ether is a primary alkyl halide.

When a tertiary alkyl bromide is heated in water, the bond to the bromide can break to form a stable tertiary carbocation. This unimolecular ionization step is the rate-determining part of the process. Water then acts as the nucleophile, attacking the carbocation to form the oxonium intermediate, which is deprotonated by water to yield tert-butanol. The reasons this pathway wins are that a tertiary carbocation is highly stabilized, so SN1 is favored over SN2 (which would be hindered by the bulky tert-butyl group), and water is a weak base, so elimination via E2 is not competitive under these conditions. The major product is tert-butanol.

Replacing the hydroxyl group on an alcohol with chloride is how you make a haloalkane. The reagents thionyl chloride (SOCl2), phosphorus trichloride (PCl3), or phosphorus pentachloride (PCl5) do this effectively by turning the poor leaving group OH into a much better leaving group and supplying chloride to complete the substitution. Mechanistically, SOCl2 reacts with ROH to form a chlorosulfite intermediate, which then undergoes chloride attack to give RCl while SO2 and HCl are expelled. With PCl3 or PCl5, the alcohol is converted to ROCl (or undergoes a sequence that ultimately yields RCl), with byproducts such as HCl and phosphate species. In all cases, chloride becomes the nucleophile that displaces the improved leaving group, giving the haloalkane. Performing the reaction under reflux provides the necessary energy to drive the substitution to completion and helps manage the evolved gases or byproducts, ensuring the process proceeds efficiently.

The key idea is how strong the oxidant is and how much water is present. PCC (pyridinium chlorochromate) in a dry, non-aqueous solvent is a mild oxidant, so it converts a primary alcohol to an aldehyde and stops there under controlled conditions. The low water content prevents formation of the hydrated form that would lead to further oxidation to the carboxylic acid, so you get the aldehyde selectively. Jones oxidation uses chromium trioxide in aqueous acid, a much stronger, water-rich system. The aldehyde formed from a primary alcohol is readily oxidized further under these conditions to the carboxylic acid, so it does not stop at the aldehyde. Since Jones oxidation pushes beyond the aldehyde and PCC under proper conditions stops at the aldehyde, the method that selectively yields the aldehyde is PCC in a suitable solvent.