If it smells - it's chemistry

Like Proust, for whom the scent of a madeleine dipped into tea recalled childhood Sunday mornings with his aunt, most of us are familiar with the smell of cut grass, or a piece of blue cheese, or an amazing white wine, or Chanel No 5 perfume. Smell is the most chemical of all the senses, intimately rooted in our memories. But do we know how we smell these 'fragrances'?

Such smells can be attributed to specific molecules, though the smell of many substances comes from a combination of many molecules. Moreover small changes in molecules can make an amazing difference to their smell. 

Simple alkylpyrazines like 2,3-dimethylpyrazine (1) and 2,6-dimethylpyrazine (2) are responsible for the nutty smells of roasted peanuts and baked bread; just a small tweak, and 2-ethyl-3,5-dimethylpyrazine (3) is one of the 'chocolate' molecules that rounds out the roasted smell of coffee. Substituting slightly different groups to give 2-isobutyl-3-methoxypyrazine (4) gives green bell peppers their distinctive smell, which can be detected as low as 0.002 parts per billion; and 2-acetylpyrazine (5) smells of roasted popcorn.