CHEM 203. Topics Discussed on Oct. 16

Transcription

1 EM 203 Topics Discussed on Oct. 16 ydrogenation (= saturation) of olefins in the presence of finely divided transition metal catalysts (Ni, Pd, Pt, Rh, Ru...): generic alkene R 1 finely divided Pd (or Pt or Ni, or...) R 1 a generic alkane The hydrogenation of alkenes as a method to form alkanes Ability of elemental metals such as Ni, Pd, Pt, etc., to react with 2 through oxidative addition: a metal atom on the surface of a microscopic metal particle or of an appropriate support (Mt = Ni, Pd, Pt...). The oxidn. state of the metal is 0 Mt oxidative addition (0) of Mt to 2 a metal hydride 2 has reacted with Mt, which is now at the ox. st. of +2 Greater electronegativity of relative to most metals Mechanistic outline of the hydrogenation reaction: Mt (0) oxidative addition R 1 surface of a metal particle (Mt = Ni, Pd, Pt...). The ox. st. of the metal is 0 R 2 R 4 A reaction similar to a hydroboration takes place of Mt to 2 reductive elimination of Mt 2 has reacted with Mt, which is now at the ox. st. of +2 R 2 R 4 Mt (0) Mt(II) has a vacant orbital that can draw e density from the π system of an alkene the alkyl-mt- product is unstable: it decomposes to Mt(0) plus an alkane. Overall syn addition of 2 Strictly syn course of the hydrogenation reaction Technological importance of the hydrogenation of alkenes

2 Lecture of Oct 16 p. 2 Principle: the hydrogenation reaction is an exothermic process (Δ <0): finely divided Pd (or Pt, or...) R 1 bonds broken (Δ, ) bonds formed (Δ, ) = π bond ca ca. 95 ea. ca. 100 overall Δ react 30 Important physical properties of alkenes that transpire from a thermochemical study of the hydrogenation reaction: a. the stability of an alkene increases with increasing substitution around the = system; i.e., a more highly substituted alkene is more stable (=less energetic) than a less highly substituted isomer Example 1: the hydrogenation of positional isomers of butene to butane: Δ 30.3 Δ 27.6 trans-2-butene contains almost 3 kcal / mol less energy than 1-butene: it is the more stable isomer Example 2: the hydrogenation of positional isomers of methylbutenes to 2-methylbutane: Δ 30 Δ 27 the olefinic linkage in 2-methyl-2-butene is more highly substituted: a stabilization of ca. 3 kcal / mol results Putative origin of the greater stability of more highly substituted alkenes relative to less highly substituted isomers: hyperconjugative interactions between σ and π* = orbitals:

3 Lecture of Oct 16 p. 3 "allylic hydrogens" those bound to an sp 3 atom, which in turn is attached to an olefinic sp 2 -carbon atom R R R lobes of the π* = orbital (phases omitted for clarity) b. trans alkenes are more stable (=less energetic) than their cis isomers. Example: the hydrogenation of cis- and trans-2-butenes to butane: Δ 28.6 Δ 27.6 trans-2-butene contains 1 kcal / mol less energy than cis-2-butene: it is the more stable isomer Putative origin of the greater stability of trans-alkenes relative to their cis isomers : absence of steric interactions between alkyl groups: cis-isomer: electronic clouds around the 3 groups are compressed against each other: more energetic arrangement trans-isomer: electronic clouds around the 3 groups are well away from each other: less energetic arrangement c. benzene is unusually stable. Δ 28.6 Δ 57 Δ 49.8 instead of 85.8!!!! benzene contains 36 kcal / mol less energy than expected "Aromaticity" of benzene and of related compounds possessing 6 π electrons delocalized within a ring

4 Lecture of Oct 16 p. 4 Inertness of the "olefinic" bonds of benzene (and related compounds) to the various reagents encountered in EM 203 Examples: O 2 O + O= 2 1. O 2 2. Zn, + 2 SO 4 3 O O 1. B 3 1. OsO 4 O 2. 2 O 2 aq. NaO styrene 2. aq. NaSO 3 O 2 2 l 2 O l O the double bonds of benzene do not react with any of the above reagents!! * * * Addition of to olefins in the presence of alkyl peroxides (but not hydrogen peroxide): anti- Markownikov addition R-O-O-R Weakness of the peroxidic linkage in alkyl peroxides (Δ diss RO OR' 40 ) and facile homolytic dissociation thereof at or near room temperature Use of one-barbed arrows to indicate the movement of individual electrons when writing mechanisms, e.g.: R O O R' R O + O R' σ O-O bond a pair of oxygen radicals

5 Lecture of Oct 16 p. 5 Radicals: species in which an atom possesses a complement of only 7 valence electrons and a formal charge of zero: R-O is an oxygen radical, in the sense that the O atom has only 7 valence electrons Oxygen, carbon, halogen centered radicals Radicals as extremely reactive, electron-deficient species that tend to react in such a way as to re-establish an octet of electrons around the electron-deficient atom