Electron-Rich Pi Bonds

Resonance structures show us that either the double bond or the heteroatom can serve as an electron source. Because allylic sources can bite at two different sites they are called ambident nucleophiles. Usually the double bond is the attacking nucleophile the decision about which end of the allylic source attacks the electrophile will be discussed in Section 9.4. 

We will see later that some electron sinks are in equilibrium with a species that can serve as an allylic electron source, Z==C-C-H H-Z-C=C this equilibrium is called tautomerization and is discussed in Chapter 7.3.8 under path combinations. 

The rankings of allylic electron sources reflect the electron-releasing group trends 

The last one in the series, the vinyl halide, is actually a poorer electron source than a simple alkene because halogens are poorer donors than alkyl groups. 

Which of the following, CH2=CH-0 , CH2=CH-NR2, CH2=CH-C1, is the most reactive allylic source Which is the least  

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The pi bond as a nucleophile. A strong electrophile attracts the electrons out of the pi bond to form a new sigma bond, generating a carbocation. The (red) curved arrow shows the movement of electrons, from the electron-rich pi bond to the electron-poor electrophile. 

Methylene ( CH2) is the simplest of the carbenes uncharged, reactive intermediates that have a carbon atom with two bonds and two nonbonding electrons. Like borane (BH3), methylene is a potent electrophile because it has an unfilled octet. It adds to the electron-rich pi bond of an alkene to form a cyclopropane. 

Next look for lone pair donors and the electron-rich pi bonds. 

With the exception of hydrogenation, the addition reactions of alkenes presented in this text occur by an electrophilic addition mechanism. The electrophile (H+ or X+) attacks the electron-rich pi-bond of the double bond. The pi electrons are used to form a single bond between the carbon and attacking species the other carbon becomes a carbocation. The carbocation is then neutralized by halide ion or water the addition is complete. In bromination reactions, the bromine adds in a trans fashion. 

The positively charged nitronium ion is attracted to the electron-rich pi bonds of the benzene ring. A bond forms between one of the carbon atoms and the nitronium ion, breaking one of benzene s pi bonds ... 

As the Br2 molecule gets closer to the alkene, this temporary effect becomes more pronounced. Now we can understand why Br2 functions as an electrophile in this reaction there is a temporary 5+ on the bromine atom that is closer to the pi bond of the alkene. When the electron-rich alkene attacks the electron-poor bromine, we get the following hrst step of our mechanism ... 

Many of the reactions of alkynes are similar to the corresponding reactions of alkenes because both involve pi bonds between two carbon atoms. Like the pi bond of an alkene, the pi bonds of an alkyne are electron-rich, and they readily undergo addition reactions. Table 9-4 shows how the energy differences between the kinds of carbon-carbon bonds can be used to estimate how much energy it takes to break a particular bond. The bond energy of the alkyne triple bond is only about 226 kJ (54 kcal) more than the bond energy of an alkene double bond. This is the energy needed to break one of the pi bonds of an alkyne. 

In this last example, the flow starts with the electron-rich sulfur anion and forms a carbon-sulfur bond with the CH2 group. The pi bond breaks and forms a new pi bond. The flow finishes by breaking the carbon-oxygen pi bond and forming a new lone pair on the electronegative oxygen. Every time an arrow forms a bond to an atom that already has a complete octet, another bond to that atom must break, so the octet is not exceeded. 

Pi-complexes, also called donor-acceptor complexes, are often a weak association of an electron-rich molecule with an electron-poor species. The donor is commonly the electron cloud of a pi bond or aromatic ring the acceptor can be a metal ion, a halogen, or another organic compound. In the absence of solvent, as can occur in an enzyme cavity, the cation-pi interaction can be stronger than hydrogen bonding. The cation snuggles into the face of the aromatic pi cloud (see aromaticity. Sections 1.9.3 and 12.3). 

Electron donor group attached More electron rich Nucleophilic pi bond HOMO more accessible... 

The best electron sources are usually nonbonding electron pairs. They are electron rich, and no bonds need be broken to use them as electron sources. Other excellent electron sources are highly ionic sigma bonds and also pi bonds highly polarized by excellent electron-releasing groups. 

Double and triple bonds are active reaction sites because they are rich in electrons and the electrons are accessible due to the nature of pi-bonds. 

The addition reaction between HCI and ethylene. The initial interaction is between the positive end of HCI (blue) and the electron-rich region of ethylene (red), which is associated with the pi electrons of the C=C bond. 

Bonding in the carbonyl group (a) carbonyl carbon is sp -hybridized, (b) C=0 group consists of sigma and pi bonds, (c) electrostatic potential map illustrates the electron-poor nature (blue) of the carbonyl carbon and the electron-rich nature (red) ot the oxygen. 

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