Ethers Epoxides Thioethers

Characteristic IR absorption band for an ether

A strong $C-O$ stretch at $1000\text{--}1200\,\text{cm}^{-1}$ in the absence of both $O-H$ and $C=O$ absorptions.

Reason ethers possess significantly lower boiling points than isomeric alcohols

Ethers lack a hydrogen atom directly bonded to oxygen ($O-H$), meaning they cannot form intermolecular hydrogen bonds with each other.

Crown ethers tailored to selectively bind $Li^+$, $Na^+$, and $K^+$ cations

$12\text{-crown-}4$ selectively binds $Li^+$, $15\text{-crown-}5$ binds $Na^+$, and $18\text{-crown-}6$ binds $K^+$, matching cavity diameter to cation size.

Optimal combination of reactants for a successful Williamson ether synthesis

A metal alkoxide ($RO^-$) and an unhindered primary ($1^\circ$) alkyl halide or tosylate to minimize competing $E2$ elimination.

Reason aryl halides cannot act as the electrophile in a Williamson ether synthesis

Aryl halides cannot undergo $S_N2$ nucleophilic substitution because the leaving group is bonded to an $sp^2$-hybridized carbon.

Regiochemical outcome of the alkoxymercuration–demercuration of an alkene

Markovnikov addition, resulting in the alkoxy group ($-OR$) bonding to the more substituted carbon atom.

Cleavage products when an aryl alkyl ether is heated with excess strong hydrogen halide ($HX$)

Phenol and an alkyl halide ($RX$), because phenol cannot react further via $S_N2$ displacement at an $sp^2$ carbon.

Hazardous products formed when ethers undergo slow autoxidation upon exposure to air

Hydroperoxides and dialkyl peroxides ($ROOR$), which are shock-sensitive and dangerously explosive upon heating or concentration.

Comparison of nucleophilicity and basicity between thiolates ($RS^-$) and alkoxides ($RO^-$)

Thiolates are stronger nucleophiles but weaker bases than alkoxides, allowing them to give clean $S_N2$ products even with secondary alkyl halides.

Stereochemical outcome of alkene epoxidation using a peroxyacid (such as MCPBA)

Syn addition across the double bond that strictly preserves the original alkene stereochemistry (cis alkene gives cis epoxide; trans gives trans).

Regiochemical difference between acid-catalyzed and base-catalyzed epoxide ring openings

Under acidic conditions, the nucleophile attacks the more substituted carbon (which carries more partial positive charge); under basic conditions, the nucleophile attacks the less hindered carbon due to steric factors.

Stereochemical relationship between the attacking nucleophile and the leaving oxygen in an epoxide opening

Anti (trans), because the nucleophile attacks via backside $$