Q.What is metamerism? Explain metamerism with suitable examples of ethers.

Br$_2$/AcOH on anisole gives p-bromoanisole (electrophilic substitution, o,p-directed). Soda-lime decarboxylates sodium acetate to methane.

(i) Liquid bromine in acetic acid on anisole: anisole undergoes electrophilic aromatic bromination. The $-OCH_3$ group is a strong $o,p$-director and ring activator, so bromination occurs mainly at the para position (with some ortho):

$C_6H_5OCH_3 + Br_2 \xrightarrow{CH_3COOH} p\text{-}BrC_6H_4OCH_3 + HBr$

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Diethyl ether is manufactured industrially by continuously feeding ethanol into hot conc. sulphuric acid and distilling off the ether as it forms.

In the continuous etherification (industrial) process, ethanol vapour is continuously passed into excess hot concentrated sulphuric acid (acting as a dehydrating catalyst) maintained at about 413–443 K:

$C_2H_5OH + H_2SO_4 \rightarrow C_2H_5OSO_3H + H_2O$

$C_2H_5OSO_3H + C_2H_5OH \xrightarrow{413-443\,K} C_2H_5-O-C_2H_5 + H_2SO_4$ …

Ethers ($R-O-R'$) are classified by whether the two attached groups are identical (simple) or different (mixed), and by whether the groups are alkyl and/or aryl.

Ethers are organic compounds in which two alkyl or aryl groups are linked through an oxygen atom, general formula $R-O-R'$ (the functional group is $-O-$ linking two carbon chains, unlike alcohols where oxygen bears a hydrogen). They are classified as:

1. Simple (symmetrical) ethers — both groups attached to oxygen are identical, e.g. diethyl ether $C_2H_5-O-C_2H_5$.
2. Mixed (unsymmetrical) ethers — the two attached groups are different, e.g. methyl ethyl ether $CH_3-O-C_2H_5$. …

Continuous etherification keeps feeding fresh ethanol into hot conc. $H_2SO_4$ so ether distils off as fast as it forms, effectively regenerating the acid catalyst.

Diethyl ether is manufactured industrially by the continuous etherification process: ethanol is first mixed with excess concentrated sulphuric acid, forming ethyl hydrogen sulphate:

$C_2H_5OH + H_2SO_4 \rightarrow C_2H_5OSO_3H + H_2O$

This mixture is maintained at about $413\ K$ ($140^\circ C$), and fresh ethanol vapour is continuously fed in from the top of the reaction vessel. Each incoming molecule of ethanol displaces a molecule of ether from the ethyl hydrogen sulphate (an $S_N2$ substitution), and $H_2SO_4$ is regenerated:

$C_2H_5OSO_3H + C_2H_5OH \xrightarrow{413K} C_2H_5-O-C_2H_5 + H_2SO_4$

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Metamers have the same functional group but different alkyl groups distributed on either side of it, with the same molecular formula.

Metamerism is a type of structural isomerism shown by compounds belonging to the same homologous series (having the same functional group and molecular formula) but differing in the nature of the alkyl/aryl groups attached on either side of the functional group/atom.

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