Question

Dimethyl ether Ch3-o-CH3 and ethanol ch3ch2oh have the same molecular formula. Dimethyl ether evaporates faster than ethanol, which is at same temperature. Explain these observations??

Answer 1

Answer:-

Since we know that dimethyl ether (CH3-O-CH3) and ethanol (CH3-CH2-OH) both are organic compounds but ethanol (CH3-CH2-OH) molecule has the ability to form intermolecular hydrogen bonding (H- bonding) between the ethanol (CH3-CH2-OH) molecules because ethanol (CH3-CH2-OH) has a free polar hydroxyl group (--O---H+) which forms large extent of inter- molecular hydrogen bonding (H- bonding) between the ethanol (CH3-CH2-OH) molecules as compared to the weak polarity of C-O bond in dimethyl ether (CH3-O-CH3). The intermolecular forces between the dimethyl ether (CH3-O-CH3) molecules are weak van-der waals forces i.e dipole -dipole interactions because dimethyl ether has no free hydrogen atom directly attached to the oxygen atom therefore it unable to form intermolecular H- bonding. As we know that evaporation or boiling point of any compound is directly related to the intermolecular forces between the compound molecules. Boiling point of any compound increase with increase in intermolecular forces between the compound molecules.

Therefore attractive forces between the ethanol (CH3-CH2-OH) molecules i.e H- bonding are stronger than the attractive forces between the dimethyl ether (CH3-O-CH3) molecules which are weak van-der waals forces i.e dipole -dipole interactions. So boiling point of ethanol (CH3-CH2-OH) is greater than the boiling point of dimethyl ether (CH3-O-CH3). Due to low boiling point of dimethyl ether (CH3-O-CH3), it would be evaporated faster than the ethanol (CH3-CH2-OH). (i.e the answer)