The Bombardier Beetles Noxious Spray

The bombardier beetle defends itself by spraying a jet stream of hot (100°C), noxious p-benzoquinones at an attacker. The beetle mixes p-hydroquinones and hydrogen peroxide from one abdominal reservoir with enzymes from another reservoir. The enzymes convert hydrogen peroxide to oxygen, which in turn oxidizes the p-hydroquinones to p-benzoquinones and explosively propels the irritating spray at the attacker. Photos by T. Eisner and D. Aneshansley (Cornell University) have shown that the amazing bombardier beetle can direct its spray in virtually any direction, even parallel over its back, to ward off a predator. 

STRATEGY AND ANSWER The presence of a cyclohexane ring with a donble bond in it suggests that the compound could be made by a Diels-Alder reaction. Suitable reactants here would be p-benzoquinone as the dienophile and 1,3-butadiene as the diene. 

Chlorobenzene, for example, can be boiled with sodium hydroxide for days without producing a detectable amount of phenol (or sodium phenoxide). Similarly, when vinyl chloride is heated with sodium hydroxide, no substimiion occurs  

We can understand this lack of reactivity on the basis of several factors. The benzene ring of an aryl halide prevents back-side attack in an Sn2 reaction  

Phenyl cations are very unstable thns SnI reactions do not occnr. The carbon-halogen bonds of aryl (and vinylic) halides are shorter and stronger than those of alkyl, allylic, and benzylic halides. Stronger carbon-halogen bonds mean that bond breaking by either an S l or Sn2 mechanism will require more energy. 

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