Whirlpool hops are primarily used not for bitterness but instead for extracting volatile hops oils. While some hops bitterness is isomerized when the wort is still near boiling temperature, hops utilization falls off quickly once the temperature drops below about 176°F (80°C). A long-boil hops addition is a much better way to add bitterness to your hops if that is your goal.
This brings us back to the hops oils. The four primary hops oils are myrcene, humulene, caryophyllene, and farnesene. The largest of these is myrcene, which makes up 40–60 percent of the hops oils in most varieties. Myrcene has an herbal note that can be described as green, balsamic, or hoppy and can also have piney/citrusy hints as is the case with many American varieties. Cascade, for instance, has a high myrcene content of around 50 percent. Noble hops, in contrast, have low myrcene content.
Humulene is the classic noble hops oil and is widely used in the perfume industry. It has a strong herbal component and has a boiling point just below that of water and tends to produce a spicy flavor if boiled long. Caryophyllene is the counterpoint to humulene, providing often spicy, earthy, woody tones. It is strong in many English varieties such as East Kent Goldings and Northdown. Finally farnesene, which is typically a factor only in noble hops, can provide flowery, green apple, or woody notes.