"Electrophilic" addition to an alkene involves forming two new sigma bonds, and thus ultimately involves two electron pairs. Are these new bonds formed in one step or two?

The simplest case of electrophilic addition is initiated by the proton, as an electrophile, forming a bond to one of the carbons by using the pi electrons of the double bond. This generates the same kind of localized carbocation intermediate that is involved in the E1 elimination reaction and raises the same possibilities for rearrangement and loss of stereochemical memory. In this case the subsequent addition of a nucleophile (e.g. halide or alcohol), which supplies the electrons for the second new sigma bond, is a distinct reaction step. In the following diagram we pretend that we can distinguish the sets of red and blue electrons as a way of emphasizing that two independent steps are involved in the addition initiated by the H⁺ electrophile.

Unlike H+, all other electrophiles possess electrons, which often occupy a sufficiently high HOMO properly oriented to give an additional nucleophilic character to the initial step of the reaction. Typically this generates a bridged intermediate (or product) involving bonding to both of the carbons, which avoids the localized carbocation and its annoying tendency to rearrange and lose stereochemical memory.

Specifying "proper orientation" is key. For example, placing a single bond and a double bond side by side does not allow effective HOMO/LUMO mixing. Instead HOMO (s) mixes with HOMO (p) and LUMO (s*) with LUMO (p*).

In favorable cases donation of the (red) pi electrons of the alkene to the LUMO of the electrophile is accompanied by "back donation) from the HOMO of the electrophile to the pi* LUMO of the alkene. Both of these mixings decrease the bonding between the carbons (change C=C to C-C) and create bonding between the electrophile (which is also a nucleophile) and the alkene.

Expressing the same change in electron distribution using valence bonds and curved arrows results in a three-membered ring. Full utilization of the "nucleophilic" electrons in the initial step can cause both new sigma bonds to be formed simultaneously, as in the following three examples, where the HOMO and LUMO of the electrophile are shown in blue and red, respectively before the curved-arrow formula for the reaction:

---

Hydroboration and Carbenoids

Even if overlap with the nucleophilic orbital of the electrophile is small enough that there is only modest back-donation in forming an initial "pi-complex" intermediate, the second "nucleophilic" step which changes sigma bonding can be easy enough to give the products expected from a concerted formation of both bonds (asymmetry in the complex can explain "Markovnikov" orientation).

---

Catalytic Hydrogenation and Polymerization

Transition metal atoms with d-orbital HOMOs and p-(or s- or dz²)-orbital LUMOs can do the same sort of trick to form a metalocyclopropane:

Metals that are effective as catalysts for hydrogenation of alkenes can do the same thing with the H-H single bond:

For bookkeeping purposes, the hydrogen is considered to be more electronegative and to possess the bonding electrons in the M&endash;H bond. Inorganic chemists focus on the metal ion (not on the hydride or alkide group bound to it) so they call this process "oxidative addition" since the metal atom "loses" and unshared pair of electrons and is oxidized. The reverse of the process, in which two elements lose their bonds to metal and become bonded to one another is called "reductive elimination".

If a metal atom has done added both H₂ and an alkene, it is possible for reductive elimination to form a bond between two atoms that were not originally partners. The first such reaction in the example below converts a metalocyclopropane to and alkyl metal. The second releases the alkane formed by hydrogenation of the original alkene and also releases the metal atom ready catalyze hydrogenation of another molecule of alkene.

The metal atom that catalyzes hydrogenation can be part of a solid metal particle (heterogeneous catalysis) or part of a molecule in solution (homogeneous catalysis).

Analogous transformations occur in polymerization of alkenes under metal catalysis.

As you may imagine, there is an enormous amount of lore having to do which which metals or organometallic compounds work with which alkenes. As in so much of chemistry, it is important to know some facts about what specific systems work. Then one can extrapolate from these facts to predict whether more-or-less closely related systems might work better or worse.

For example, analogy to the reactions of H-BH₂ and ICH₂-ZnI above makes it seem that a Grignard reagent RMgX should addto an alkene to put R on one carbon and MgX on the other. But this reaction does not occur, although Grignard reagents do add readily to the C=O double bond putting R on C and MgX on O.

What can we learn from this failure?

It appears that B and Zn are better at forming covalent bonds with C than Mg is, while Mg (further to left in the periodic table) prefers to form more ionic bonds with O. (Remember the exchange reactions of Section 8.8B in the text where the Mg of a Grignard reagent surrenders R to a more covalently disposed element like Hg or Cd and takes in return an ionic halide.)

Our goal in Chem 125 is just to learn in general terms how such reactions work and to learn enough specific examples to solve simple problems in synthetic organic chemistry. Individuals aiming at a career in synthetic chemistry or organometallic chemistry will master much more lore in subsequent courses.