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Expalin : (a) Alcohols have higher boiling points than alkanes, ehters and alkyl halides of comparable molecular mass. (b) ROH with three or fewer carbon atoms are water soluble but those with five or more carbon atoms are insoluble. (c ) When volumes of ethanol and water are mixed, the total volume is less than the sum of the two individual volumes. (d) Ethanol cannotbe used as a solvent with Grignard reagent or LiAlH_(4). (e) The relative acidity of alcohols is in the order of 3^(@) lt 2^(@) lt 1^(@) lt CH_(3)OH. (f) Sodium metal cannot be used to remove last traces of water from ethanol. |
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Answer» SOLUTION :(a) O-H bond of alocohols is highly POLARIZED. This gives rise to intrahydrogen bonding, i.e., molecules are brought nearer to each other and held together by attractive forces. No such hydrogen bonding EXISTS in alkanes, ethers and alkyl halides. This is the reason why the boiling points of alcohols are HIGHER than alkanes, ehters and alkyl halides of comparable mass. (b) The water solubility of alcohols is atributed to hydrogen bonding with water. However, with increase of carbon atoms, the hydrophobic part increases which predominates the hydrophilic part, ie., OH part of alcohols and thus, the solubility decreases. (c ) Hydrogen bonding between ethanol and water permits the TWO unlike molecules to move closer in the closer in the solution than can ethanol to ethanol and water to water molecules and thus, the total volume decreases. (d ) Ethanol being acidic readily with strongly basic R of Grignard reagent. `CH_(3)CH_(2)OH +RMgX rarr underset(" Alkane")(R-H)+CH_(3)CH_(2)OMgX` `LiAlH_(4)` reacts with alcohol to form hydrogen. `4CH_(3)CH_(2)OH +LiAlH_(4) rarrLiAl(OCH_(2)CH_(3))_(4)+4H_(2)uarr` (e) The order of decrease of acidity from `CH_(3)OH` to `3^(@)` alcohol is attributed due to `+I` effect of alkyl group which intensify the charge on the base `RO^(-)` and the removal of proton becomes difficult. (f) Ethanol beinga acidic reacts with sodium metal, although not as vigorously as water and form sodium ethoxide with evolution of hydrogen. `2CH_(3)CH_(2)OH+2Na rarr 2CH_(3)CH_(2)ONa+H_(2)` |
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