Exam 24: Organic Compounds

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Most arenes that contain a single six-membered ring are volatile liquids, such as benzene and the xylenes, although some arenes with substituents on the ring are solids at room temperature. Although arenes are usually drawn with three C=C bonds, benzene is about 150 kJ/mol more stable than would be expected if it contained three double bonds. This increased stability is due to the delocalization of the π electron density over all the atoms of the ring. Compared with alkenes, arenes are poor nucleophiles. They do not undergo addition reactions like alkenes instead, they undergo a variety of electrophilic aromatic substitution reactions that involve the replacement of -H on the arene by a group -E, such as -NO₂, -SO₃H, a halogen, or an alkyl group, in a two-step process. The first step involves addition of the electrophile to the π system of benzene, forming a carbocation. In the second step, a proton is lost from the adjacent carbon on the ring.

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Many important organic reactions involve radicals, such as the combustion of fuels. Probably the best known is reacting a saturated hydrocarbon, such as ethane, with a halogen, such as Br₂. The overall reaction is as follows:
CH₃CH₃ + Br₂ CH₃CH₂Br + HBr
Radical chain reactions occur in three stages: initiation, propagation, and termination. At high temperature or in the presence of light, the relatively weak Br-Br bond is broken in an initiation step that produces an appreciable number of Br atoms (Br∙). During propagation, a bromine atom attacks ethane, producing a radical, which then reacts with another bromine molecule to produce ethyl bromide. The sum of the two propagation steps corresponds to the balanced chemical equation for the overall reaction. There are three possible termination steps: the combination of
(1) two bromine atoms,
(2) two ethyl radicals, or
(3) an ethyl and a bromine radical.