Fixing Chlorine - Addition to Alkene

Frame 20

As Dumas had identified that the gas was a HCl gas produced by bleaching the candles with chlorine, this slide deals with a possible way for the Cl2 molecule to be "fixed" (or as we would say "bonded") to a hydrocarbon chain (whence during burning it might form HCl gas). This is often called an "addition reaction", when an alkene has its double bond broken and the carbons on each side of the former double bond each form a single bond with a new atom.

This slide shows the addition reaction of a chlorine (Cl2) molecule and the "olefiant gas" (ethene) molecule. The slide shows the reaction mechanism first with Lewis structures and curved arrows, then with Spartan HOMO/LUMO drawings. The first "two steps" of the reaction shown in the Lewis drawings actually happen at the same time; the HOMO/LUMO drawings attempt more directly to show them happening at once.

Last year's (2007) wiki described the first part of the reaction in terms of the Lewis structures and curved arrows:

"First, the slide identifies the LUMO of Cl2, the sigma star bond, and the HOMO of C2H4, the pi bond (as a model for a double bond in a hydrocarbon chain). The two orbitals overlap, breaking the pi bond and creating a Cl-C bond; a HOMO from the bonded Cl (lone pair) and LUMO from the other CH2 (unfilled p orbital) then overlap creating the second Cl-C bond. (A Cl- ion is also formed when the Cl-Cl bond breaks.)"

This is how it actually happens according to the Spartan diagrams:

In the "first step", the Cl2 molecule's LUMO, the σ* antibonding for the Cl-Cl bond (unusually low because of high nuclear charge on the chlorines), is attacked by the electrons in the π orbital of the C=C double bond in the ethene molecule (a high HOMO due to poor overlap). In the "second step", but in reality at the same time, a HOMO of the Cl2 molecule, a lone pair on a chlorine atom, attacks the LUMO for ethene: the π* antibonding orbital (unusually low due to poor overlap in the bonding orbital). One Cl-C bond does not form before the other, as described last year and implied by the Lewis drawings; rather, it all happens together.

(Last year's wiki said the following: "This first interaction creates a y bond (similar to two BH3s bonding) between the Cl and the 2 Cs." Is this actually true? There are four electrons involved in this process (two HOMOs and two LUMOs), not two, so we do not have a 3-center, 2-electron bond; instead we get two 2-center, two-electron bonds. If the "y-bond" occurs only in the "first step" before it is "replaced", then this is not actually formed because both processes actually happen at once.)

The rest of the reaction is simple enough that Lewis structures and curved arrows can suffice to describe it; Spartan diagrams are not necessary and one is only shown to demonstrate that the Cl-C σ* LUMO will be attacked at the carbon, not the chlorine. Last year's explanation can suffice:

"There are now two LUMOs, each Cl-C bond has a sigma star orbital, and a HOMO from the free Cl- can overlap with one of these LUMOs, breaking a Cl-C bond and creating a bond between a C and the Cl- ion. This is addition of Cl2 to an alkene."

There are many reasons why the Cl-C σ* LUMO is unusually low: poor overlap in the σ bond becuase of the triangular shape, high nuclear charge on Cl+, and the fact that the Cl+ is missing electrons (remember that chlorine is very electronegative). This means that the energy of the Cl-C bond is incredibly low, and this transitory phase probably lasts an extremely short time before the Cl- ion from the other chlorine in the Cl2 molecule comes and has one of its lone pairs attack the LUMO.

The molecule eventually formed, Cl-CH2-CH2-Cl, was called the "oil of Dutch chemists". The IUPAC name is 1,2-dichloroethane. In any case, we can think of the addition reaction as the chlorines being "added" to the ethene and breaking the double bond; note that there are no side products. (There are many other "addition reactions", involving many different molecules, such as Br2, H2O, or HCl, breaking double bonds in alkenes.)

With an alkane such as methane (CH4) there is no high HOMO to overlap, so the Cl-Cl bond must be broken by exciting one of electrons into the sigma star orbital with light, breaking the bond. (note: when only one electron moves, you use a singly-barbed arrow)

A SOMO from one of these free Cl atoms can overlap with a sigma star HOMO on one of the C-H bonds, breaking the C-H bond and creating an H-Cl bond and a CH3. The CH3 produced now has a SOMO so it can overlap with the LUMO of a Cl-Cl molecule, creating a C-Cl bonding and leaving a free Cl atom, which can now interact with another CH4 molecule, creating this "free radical chain."

This process creates the CH3Cl molecule, which substitutes a Cl for an H in a methane molecule.

(the crossed-out material is actually on slide 21 this year)

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