Simulation · Chemistry · Class 12
Which C–O bond does HI break?
From the lesson Ethers: properties and reactions in Alcohols, Phenols and Ethers. Change the values and watch what happens.
Which C–O bond does HI break?Chemistry · Class 12
The idea behind it
NCERT §7.6.2–7.6.3
- The C–O bonds are polar, so ethers have a net dipole moment, but this weak polarity barely raises the boiling point: it stays close to that of an alkane of similar mass and far below that of an alcohol.
- Boiling points: n-pentane 309.1 K, ethoxyethane 307.6 K, butan-1-ol 390 K. Alcohols are linked by hydrogen bonds; ether molecules are not.
- Ethers of low mass dissolve in water about as well as alcohols of similar mass, because ether oxygen can accept hydrogen bonds from water. Ethoxyethane and butan-1-ol dissolve to 7.5 g and 9 g per 100 mL of water; pentane hardly dissolves.
- Ethers are among the least reactive functional groups. The C–O bond is cleaved only under drastic conditions with an excess of a hydrogen halide; the reactivity order is HI > HBr > HCl.
- A dialkyl ether with HX gives two alkyl halides. The ether oxygen is first protonated; the halide ion then attacks by SN2 at the less substituted carbon, so the smaller alkyl group ends up as the halide.
- When one group is tertiary the cleavage goes by SN1 through the stable tertiary carbocation, and the tertiary group becomes the halide.
- Anisole with HI gives phenol and methyl iodide: the O–C₆H₅ bond has partial double-bond character and does not break, and phenyl halides never form this way.
- The alkoxy group activates the ring and directs to ortho and para. Anisole brominates in ethanoic acid even without an iron(III) bromide catalyst, the para isomer forming in 90% yield.
- Anisole undergoes Friedel–Crafts alkylation and acylation with AlCl₃, and a mixture of concentrated sulphuric and nitric acids gives ortho- and para-nitroanisole.