Preparation and Properties of Ethers

(I) Preparation of Ethers:

From alkenes, by addition of alcohol, in presence of H₂SO₄ :
Here, the rearrangement of carbocation may give unexpected ether.

From alkylhalide:
(i) Using Ag₂O
2 RCl + Ag₂O → 2AgCl + R—O—R
(ii) Williamson synthesis (Main method):

(b) H₂C = CH—CH₂—X react much easily because stability of allyl carbocation.
(c) 2° Alkyl halides produce both ether and alkene
(d) 3° alkyl halides produce alkene.

If t-BuOH is used in place of C₂H₅OH, alkene is produced as Hofmann product.

From Alcohols:
(i) Using Al₂O₃ at about 513 K.
\tt 2ROH\xrightarrow{{Al_2O_3/513K}}R-O-R+H_2O
(ii) Using conc. H₂SO₄ at 403 to 413K. [Laboratory method]
(a) 1° Alcohols follow SN² mechanism
\tt 2C_2H_5OH\xrightarrow[130^0-140^0C]{{conc\ H_2SO_4}}C_2H_5-O-C_2H_5+H_2O
b. 2° and 3° alcohols follow SN¹ mechanism forming carbocation.
c. When two different alcohols react and one is 3° alcohol, its "O" will go in the form of water because it is more basic.
e. Cyclic ethers are generally prepared by Intra molecular Williamson synthesis.

Some cyclic ethers are

Properties of ethers:
a. These have low polarity, boiling points almost same as that of comparable molecular masses but much lower than isomers alcohols.
b. Chemical properties: Because of no special group, these are generally considered inert.

\tt \left(CH_3\right)_3C-O-C\left(CH_3\right)_3\xrightarrow[25^0C]{{HCl\ conc}}2\left(CH_3\right)_3C-Cl+H_2O

This reaction with conc. HI/373 K converting CH₃O — (or) C₂H₅ — O — groups to CH₃I (or) C₂H₅I is used to estimate methoxy and ethoxy groups by zeisel method, where in the iodo products are further reacted with ethanolic AgNO₃. The precipitates of AgI are dried and weighed

No. of methoxy groups = \tt \frac{wt. of\ AgI}{wt\ of\ ether}\times\frac{31(mass\ of\ CH_3O)}{235(molar\ mass\ of\ AgI)}

Any old sample of ether may contain peroxide group which causes explosion on heating such a sample, before using is tested FeSO₄ followed by KCNS that gives blood red colour.
Peroxide as ether + FeSO₄ + H₂SO₄ → Fe₂(SO₄)₃ + H₂O + normal ether
Fe³⁺ + 3CNS⁻ → Fe(CNS)₃ (Blood red)
Excess of FeSO₄ / H₂SO₄ removes the total peroxide and the normal ether can be used in experiment.