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In this task you will be given a text passage from a trivia quiz. You should choose the category this question belongs to from these categories: History, Science, Social Science, Fine Arts, Literature. You can only choose one category, so if in doubt, please choose the closest match.

Halogen addition to them creates a mixture of a 1-2 and 1-4 addition product. One preparation of them involves butane undergoing thermal dehydration. Another synthesis of the conjugated variety of this chemical group involves elimination of unsaturated alcohols and alkyl halides. They include a number of useful reagents, including ones named for Chan, Dane, Brassard, Rawal, and Danishefsky. In addition to the conjugated variety, they also occur in isolated and cumulated varieties. Conjugation energy guarantees that the conjugated form is the most stable. For ten points, what is this organic functional group whose 1,3 variety are a principal reactant in the Diels-Alder reaction, and which is characterized by a pair of double bonds.
Output:
Science