Dear Student,
Answer : Isomer of ethanol is methoxymethane also known as dimethyl ether.
Explanation : Isomers are the compounds which have same molecular formula but differ in their structural formula. In present question, ethanol has molecular formula C2H6O with structural formula CH3-CH2-OH. It has -OH (alcohol) as functional group.
Now, its isomer must have same molecular formula (i.e. C2H6O) but a different structure which for ethanol can be CH3-O-CH3 (methoxy methane). This compound has ether as functional group.
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