Simple Pi Bonds

The pi bond is the electron source. For allylic sources, the electron pair donor stabilizes the resultant cation. The simple alkene reactivity trend reflects the stabilization of the resultant carbocation by alkyl substitution and delocalization with other double bonds. The ranking of simple double bonds as electron sources basically is the more substituted, the more reactive. 

Dienes are conjugated systems of two pi bonds. Above, the simplest diene, 13-butadiene, adds the electrophile to the end to produce a resonance stabilized allylic carbocation. As with alkenes, the more substituted the diene is, the more reactive. 

To make the choice on where to add an electrophile to a diene pi system, draw all possible carbocations and then pick the more stable one according to the trends you learned in Section 4.2.4. This is one of many times that we will use the trends to make a decision about the route a reaction will take. 

Predict where an electrophile would attack the following compound. 

Answer Draw out all four possible carbocations that would be formed upon electrophilic attack and rank their stability. 

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The least common of the simple pi bond sources are allenes, R2C=C=CR2, whose two perpendicular double bonds must again be considered separately. Allene reactions have a bit of a strange twist to them, literally. There are two possible sites for the electrophile to attack on the allene above. It would seem to be no contest between a resonance-stabilized ally lie carbocation and the less stable vinyl carbocation. But a look at the orbital arrangement below for the reaction reveals that all is not right with the allylic system in fact, it is twisted 90° out of alignment, so no resonance stabilization can occur at the transition state for addition. 

C=C Simple pi bonds R2C=CR2 RHC=CHR H2C=CH2 The more stable the resultant carbocation, the more reactive the pi system. Electrophilic Additions. Markovnikov s rule used for regiochemistry Other pi systems c=c-c=c c=c c=c=c... 

Lone pair Z Base and Adjacent CH Complex Metal Hydride MH4- Organo- metallic R-M Allylic Z-C=C Simple Pi Bonds C=C CsC Aromatics 0... 

Simple pi bonds are usually not good enough nucleophiles to react with alkyl halides. If a Brpnsted acid or a Lewis acid is added to improve the electron sink, then addition can happen, but it is often complicated with polymerization. 

Simple pi bonds are not good enough nucleophiles to react with nitriles. It is basically a case of little push and little pull, so no electron flow occurs. If the pi bond is made more nucleophilic by addition of a pi donor (making it an allylic source) then the... 

Simple Pi Bond Sources Reacting with L-C=Y Sinks... 

Sources SinkJJ Lone pair Nu Base bi and adjacent CH Metal hydride MH4 Orgsmometal. R-M Allylic a C=C-Z Simple pi bonds C=C Aromatic ArH... 

Two p Orbitals a Simple Pi Bond Three p Orbitals the Allyl Unit Four p Orbitals the Diene Unit HOMO and LUMO General Trends of MO Nodes and Phase... 

When two p orbitals overlap in a side-by-side configuration, they form a pi bond, shown in Figure 7.7. This bond is named after the Greek letter 7t. The electron clouds in pi bonds overlap less than those in sigma bonds, and they are correspondingly weaker. Pi bonds are often found in molecules with double or triple bonds. One example is ethene, commonly known as ethylene, a simple double-bonded molecule (Figure 7.8). The two vertical p orbitals form a pi bond. The two horizontal orbitals form a sigma bond. 

A simple example of the effect of an attached saturated group X on a pi bond,... 

However, the important new feature of metal alkylidenes (4.51) is metal-carbon pi-bonding. As discussed in Section 2.8, pi bonds between transition metals and main-group elements are of d -p type, much stronger than corresponding p —pn bonds between heavier main-group elements. Compared with simple metal hydrides and alkyls, metal-carbon pi-bonding in metal alkylidenes affects the selection of metal d orbitals available for hybridization and skeletal bond formation, somewhat altering molecular shapes. 

The geometries in Figs. 4.86 and 4.87 suggest an important distinction in the multicenter hapticity character of ligand attachment to the metal atom. Hapticity refers to the number of atoms in a ligand that are coordinated to the metal. In the Ir+ diammine complex (Fig. 4.86(a)), the metal attaches to each of two nN donor lone pairs in simple monohapto (one-center, q1) fashion. However, in the Ir+ complexes with HCCH or CML the metal attaches to the face of the pi bond or three-center allylic pi system in dihapto (two-center, r 2) or trihapto (three-center, q3) fashion, respectively. The hapticity label q" therefore conveniently denotes the delocalized n -center character of the donated electron pair(s) and the geometry of the resulting coordination complex. 

Double bonds involve one sigma bond and one pi bond. A simple molecule that contains a double bond is ethene, H2C=CH2. Ethene reacts with water to form ethanol ... 

Pi bonding between metal and ligands provides a simple raison d etre lor strong field ligands, an issue that crystal field theory could not resolve. If we examine the strong field end of the spectrochemical series (page 405), we find ligands such as nitrite ion, cyanide ion, carbon monoxide, phosphites, and phosphines. The latter two owe their positions in the series to their ability to serve as rt acceptors, as described above, which increases the value of A, relative to what it would be in u [Pg.756]

A simple compound with a triple bond is ethyne (acetylene), HC=CH. The Lewis structure for ethyne is shown in Figure 3.14a. It is a linear molecule. One of the CC bonds is a sigma bond. The other two are pi bonds. 

For the double bonds to be completely conjugated, the annulene must be planar so the p orbitals of the pi bonds can overlap. As long as an annulene is assumed to be planar, we can draw two Kekule-like structures that seem to show a benzene-like resonance. Figure 16-3 shows proposed benzene-like resonance forms for cyclobutadiene and cyclooctatetraene. Although these resonance structures suggest that the [4] and [8]annulenes should be unusually stable (like benzene), experiments have shown that cyclobutadiene and cyclooctatetraene are not unusually stable. These results imply that the simple resonance picture is incorrect. 

Pi-bond nucleophiles use the pair of electrons in a it bond, usually a C=C bond, to form a cr bond between one of the atoms in the it bond and the electrophilic atom. The formal charge and total electron count of the nucleophilic atom of the 77 bond do not change, but the other atom of the 7r bond is made electron-deficient, and its formal charge increases by 1. The 7r bonds of simple alkenes and arenes are weakly nucleophilic 7r bonds that are directly attached to heteroatoms, such as in enolates (C=C-0 ), enols (C=C-OH), enol ethers (C=C-OR), and enamines (C=C-NR2), are much better nucleophiles. 

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