All the wines and spirits contain alcohol in different proportions. Alcohols are organic compounds with an hydroxyl (OH) group. There are thousands of compounds in this category. But the alcohol which wines and distillates of fermented matter obtained from the carbohydrates is Ethanol or ethyl alcohol with chemical formula written as C2H5-O-H. Compare it to formula of water H-O-H. If you replace one hydrogen atom in water with ethyl (C2H5) group, you get ethyl alcohol. Introduction of non polar ethyl group, reduces the polarity of the alcohol in comparison to water. This causes the alcohol to be the best solvent for many less polar organic compounds. In fact alcohol is used as a solvent in organic work.
So if the wine, whisky, rum, vodka etc were only ethyl alcohol, they would all taste the same. So where does the different taste come from? The answer lies in the variety of grain used for fermentation, the wood used for drying the fermented grain and the wooden barrels made of wood from selective trees, duration of ageing in these barrels as well as the temperature and humidity.
Whiskey is produced using barley. The grain is soaked in water to release the sugars. And yeast is applied to ferment the sugars and breakdown it into lower sugars and finally to ethyl alcohol. The spirit is then distilled to concentrate the alcohol. The distillates are then kept for years in the oak barrels for ageing before bottling.
During firing to dry the soaked mass, the aroma of wood fuel enters into the ground mass. Then thousands of organic compounds are leaches by alcohol from wood during ageing process. All these contribute a unique aroma and taste to whiskies.
Organic compounds belonging to classes namely ketones, esters, phenolic categories are mainly responsible for the distinct taste. Some of these are given below:
Whiskey lactones : These are cis and trans 3-methyl-4-Octanolide. These are ketones and are imported from Oak wood. Trans isomer imparts woody, coconut flavor while cis isomer imparts strong spicy coconut aroma.
Aldehydes: Syringaldehyde and vanillin. Syringaldehyde lends a spicy Woody aroma while vanillin gives vanilla tone. Furfural almonds like grainy flavor and hexanal offers grassy notes.
Esters: Ethyl hexanoate gives sweet apple flavor. Isoamyl acetate gives banana like flavor. Since many esters impart cloudiness to whiskey they are removed chill filtering.
Phenolic compounds: Guaiacol and eugenol impart smoky flavor and bitter taste. Cresols enter the grains when they are peat fired imparting medicinal smell.
Many other compounds like Beta demascenone and phenethyl alcohol give floral notes. Diacetyl adds a buttery taste. Many sulphur compounds also contrite the flavor.
For more graphical description see the illustration