Chapter 7: Alkenes: Structure and Reactivity
The C=C double bond: one sigma bond and one pi bond
Both carbons of a carbon–carbon double bond are sp² hybridized and therefore trigonal planar, with bond angles of approximately 120°. Each carbon uses three sp² hybrid orbitals to form σ bonds — one to the other alkene carbon and two to substituents — so the atoms directly bonded to the double bond lie in a common plane. One unhybridized 2p orbital remains on each carbon, oriented perpendicular to that plane. Side-on overlap of these two parallel p orbitals forms the π bond, whose electron density lies above and below the σ framework rather than along the internuclear axis. This density is…
Restricted rotation and E/Z isomers
Restricted rotation about the carbon–carbon double bond fixes the relative positions of the groups at its two ends. When each alkene carbon bears two different substituents, this rigidity produces stereoisomers: compounds with identical connectivity but different, non-interconvertible geometry. The E/Z system assigns these configurations unambiguously. On each alkene carbon, the two substituents are ranked by the same Cahn–Ingold–Prelog priority used for R and S in Chapter 5, comparing atomic number at the first point of difference. Only the question changes: not the direction the priorities…
Electrophilic addition and the carbocation intermediate
Electrophilic addition is the general mechanism shared by most reactions in this chapter, and it is Chapter 2's association move run on a π bond: the alkene is the electron donor, the electrophile the acceptor. In the first step, the nucleophilic π electrons attack an electrophile, most commonly a proton supplied by an acid H–X. Formation of the new bond consumes the π bond and leaves one carbon with three bonds and a positive charge, giving a carbocation. The carbocation is sp² hybridized and planar, and the site of the former π bond is now a vacant p orbital. Because this orbital is empty…
Markovnikov addition of HBr
The addition of HBr to an unsymmetrical alkene shows how carbocation stability controls regiochemistry. Protonation of propene can occur in two ways: addition of the proton to the terminal carbon places the positive charge on the internal (secondary) carbon, whereas addition to the internal carbon places it on a terminal (primary) carbon. The first pathway predominates because a secondary carbocation is more stable than a primary one, and this preference is set in the rate-determining first step. In the second step, bromide adds to the vacant p orbital of the secondary carbocation, so bromine…
Acid-catalyzed hydration
Acid-catalyzed hydration proceeds through the same carbocation pathway as the addition of HBr, with water serving as the nucleophile. In aqueous acid, the alkene is first protonated to give the more stable carbocation; for propene this is again the secondary cation. A molecule of water then donates a lone pair to the vacant p orbital, producing a protonated alcohol (an oxonium ion). Loss of a proton to solvent gives the neutral alcohol and regenerates the acid catalyst. Because the intermediate is the same, the regiochemistry is the same: the hydroxyl group becomes bonded to the more…
Halogenation and anti addition
Halogenation requires a refinement of the simple carbocation model. As the π electrons approach a molecule of Br2, one bromine is displaced as bromide, but the remaining bromine does not leave an open carbocation. Instead it bridges both former double-bond carbons as a three-membered cyclic bromonium ion, which blocks one face of the two-carbon unit. With one face shielded, the displaced bromide attacks a carbon from the opposite face and opens the ring. The two new carbon–bromine bonds are therefore formed on opposite faces of the original double bond; that is, the bromine atoms add anti.…
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