Account for the following `:`
(i) Ethers have significant dipole moments.
(ii) Ethers have lower boiling points than the corresponding isomeric alcohols but approximately equal to a paraffin with similar molecular mass structure.
(iii) HI is better reagent than HBr for cleavage of ethers.
(iv) Why a non-symmetrical ether is not prepared by heating a mixture of ROH and R'OH in acid ?
(v) Why is it possible to prepare tertiary butyl ethyl ether in good yield by heating tertiar butanol and ethanol ?
(vi) Ether are soluble in conc. `H_(2)SO_94)` but ethers separate out from the solution on addition of water.
(vii) Why id diethyl ether used as a solvent for (i) `BF_(3)` and (ii) RMgBr ?
(viii) Sometimes explosion occrs during distillation of an ether sample.
(ix) Why `[(CH_(3))_(3)C]_(2)O` cannot be prepared either by Williamson's reaction or by dehydration of tertiary butyalcohol ?
(x) Why ArO-R ethers are clearved with HI to give RI and ArOH rather than ArI and R-OH ?
(xi) Which of the following is the correct method for synthesising methyl tert-butyl ether and why ?
(a) `(CH_(3))_(3)C-Br+NaOMe rarr`
`(b) CH_(3)+NaO -T-Bu rarr`
(xii) 2,2-Dimethyl oxirane can be cleaved by acid `(H^(+))` . Write mechanism.
(xiii) Write the equations of the reaction of hydrogen iodide with `:`
(i) 1-Propoxypropane (ii) Methyoxybenzene
9iii) Benzyl ethyl ether

Explanation

(i) Ethers are weakly polar. The C-O-C bond angle is about 383K and the C-O moments do not cancel each other .
(ii) ROH molecules have strong intermolecular attractive forces because of hydrogen bonding that is absent in ethers having two polarity. Hence, the boiling points of ethers are low.
(iii) HI is a stronger acid than HBr and thus, oxonium salt is readily formed with greater yields. `I^(-)` is also a better nucleophile than `Br^(-)` in `S_(N^(2))` reaction.
(iv) A mixture of three ethers, `R-O-R,R'-O-R'` and `R-O-R'` is obtained.
(v) When one alcohol is `3^(@)`, the oxonoium ion easily loses water to form carbocation, whichis solvated by `2^(@)` or `1^(@)` alcohol to form mixed ether.
`Mg_(3)COoverset(+)(H_(2))underset((-H_(2)O))(rarr)Me_(3)overset(+)(C) underset(-H^(+))overset(HOCH_(2)CH_(3))(rarr)Me_(3)C-O-CH_(2)CH_(3)`.
(vi) Water is a stronger base than ether and removes the proton from `R_(2)OH^(+)`
`R_(2)OH^(+)+H_(2)O rarr R_(2)O +H_(3)^(+)O`
(viii) Ethers form peroxide with oxygen. The boiling point of peroxide is higher than that of ether . It is, thus, left as residue in the distillation of ether which is very unstable and decomposes violently on heating .
(ix) The product in both the cases is isoubutylene as the tert-butyl carbocation elemination an `H^(+)`. The reaction between `(CH_(3))_(3)overset(+)(C)` on `(CH_(3))_(3)COH` to give an ether is sterically hindered.
The instability of di-, tert-butyl ether in sulhuric acid may also be due to steric crowding of `CH_(3)` groups ?

(xi) The ether formation involves nucleophilic substitution of alkoxide ion for halide ion.

`3^(@)` alkyl halide can also involve elimination of HX to give alkene in the presence of a base. So, it is better to start with `3^(@)` alkoxide and `1^(@)` alkyl halide, i.e., equation (b).
(xii) The oxirane ring is cleaved via `S_(N^(2))` mechanism.