Because hydrocarbons cannot form hydrogen bonds. Water forms hydrogen bonds, which are a very strong type of intermolecular bonding and involve the transfer of hydrogen ions between molecules. To dissolve a compound in water, it must also form hydrogen bonds. Hydrocarbons are non-polar, which means they can only form London forces – a very weak type of intermolecular force. To dissolve a hydrocarbon in water, the hydrogen bonds between the water molecules must be broken, and the London forces must form between the hydrocarbon and water. The breaking of a strong bond with the formation of a weak bond is strongly endothermic. Highly endothermic reactions are not enthalpically favorable and are therefore unlikely to be spontaneous.
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