C6H12O6 → 2C2H5OH + 2CO2 is the formula for fermentation or in simple terms, one molecule of glucose or fructose converts into two molecules each of ethanol (alcohol) and carbon dioxide, as a result of the addition of yeast.
In additon, there is a whole heap of other chemistry going on at the same time…
Esters are aromatic compounds that are part of the glucose molecules that are released as the yeast consumes the glucose, These esters are the cause of the citrus, fruity and floral characteristics that are found in young wines.
Tannins are the antioxidant polyphenols that naturally occur in the skins and pips of grapes. They are extracted by the alcohol as it starts being produced and cause dryness and astingency. Other by products of fermentation cause changes in the structure of the tannins which enhances the bitterness and dryness
Acetaldehyde is formed as one of the later products of the yeast’s action which enhances the fruity aromas of the wine. If the levels are too high, then flavours of bruised apples can result.
Anthocyanins are a family of compounds that are found in the skins of red grapes which produce the colour and also have beneficial antioxidant properties.
Sulphites are also produced during fementation of grapes as a natural defence mechanism against unwanted micro-organisms. They serve to protect the wine against microbial infestation and unwanted oxidation.
Amino Acids contain nitrogen and occur naturally in unfermented grape juice (known as must). Most of them are used by the yeast during fermentation to build proteins.
(PS – I only just passed GCE ‘O’ Level Chemistry so please forgive any errors in my explanation)
(PPS – forget all this nonsense and get on with drinking wine and enjoying it)