Rr-Carbon

Also, conjugation of a delocalizing ferrocene group with the carbonyl should cause the intense n— -rr carbon-oxygen transition to move to longer wavelengths and bury the feeble CO , . band hence, no such band is identifiable in acetylferrocene and the 267 m maximum most likely correlates with acetophenone s 239 mjx band. 

The PMAA gel with a low degree of crosslinking and high q is harder than that with a high degree of crosslinking and low q. The increase in q should lead to an increase in the molecular motion of the network polymer in the PMAA gel. Yasunaga and Ando [31] measured the Ti value for the network polymer in the PMAA gel as a function of q. The Ti values for the C=0 (rr) and CH3 (rr) carbons increase with an increase in q, while the... 

BPP theory of NMR relaxation [12]. Therefore, an increase in q means an increase in the inverse of temperature. According to the BPP theory, Ti decreases with increasing temperature and increases after passing through a minimum as the temperature is increased further. As shown in Fig. 20.11, the decrease in Ti for the C—O (rr) and CH3 (rr) carbons of PM A A in the gel with the increase in <7 means that the motion of the side chain carbons is in the extreme narrowing region at the lefthand side of the BPP... 

This suggests sp hybridization at carbon, and a o + rr carbon-oxygen double bond analogous to that of aldehydes and ketones. 

Deuterium Exchange at an rr-Carbon (Section 16.12B) Acid- or base-catalyzed deuterium exchange at an a-carbon involves formation of an enol or enolate anion intermediate. 

Brankovic SR, Marinkovic NS, Wang JX, Adzic RR. Carbon monoxide oxidation on bare and Pt-modified Ru(1010) and Ru(0001) single crystal electrodes. J Electroanal Chem 2002 532 57-60. 

Formation of a Tr-allylpalladium complex 29 takes place by the oxidative addition of allylic compounds, typically allylic esters, to Pd(0). The rr-allylpal-ladium complex is a resonance form of ir-allylpalladium and a coordinated tt-bond. TT-Allylpalladium complex formation involves inversion of stereochemistry, and the attack of the soft carbon nucleophile on the 7r-allylpalladium complex is also inversion, resulting in overall retention of the stereochemistry. On the other hand, the attack of hard carbon nucleophiles is retention, and hence Overall inversion takes place by the reaction of the hard carbon nucleophiles. 

Pd(II) compounds coordinate to alkenes to form rr-complexes. Roughly, a decrease in the electron density of alkenes by coordination to electrophilic Pd(II) permits attack by various nucleophiles on the coordinated alkenes. In contrast, electrophilic attack is commonly observed with uncomplexed alkenes. The attack of nucleophiles with concomitant formation of a carbon-palladium r-bond 1 is called the palladation of alkenes. This reaction is similar to the mercuration reaction. However, unlike the mercuration products, which are stable and isolable, the product 1 of the palladation is usually unstable and undergoes rapid decomposition. The palladation reaction is followed by two reactions. The elimination of H—Pd—Cl from 1 to form vinyl compounds 2 is one reaction path, resulting in nucleophilic substitution of the olefinic proton. When the displacement of the Pd in 1 with another nucleophile takes place, the nucleophilic addition of alkenes occurs to give 3. Depending on the reactants and conditions, either nucleophilic substitution of alkenes or nucleophilic addition to alkenes takes place. 

Facile reaction of a carbon nucleophile with an olefinic bond of COD is the first example of carbon-carbon bond formation by means of Pd. COD forms a stable complex with PdCl2. When this complex 192 is treated with malonate or acetoacetate in ether under heterogeneous conditions at room temperature in the presence of Na2C03, a facile carbopalladation takes place to give the new complex 193, formed by the introduction of malonate to COD. The complex has TT-olefin and cr-Pd bonds. By the treatment of the new complex 193 with a base, the malonate carbanion attacks the cr-Pd—C bond, affording the bicy-clo[6.1,0]-nonane 194. The complex also reacts with another molecule of malonate which attacks the rr-olefin bond to give the bicyclo[3.3.0]octane 195 by a transannulation reaction[l2.191]. The formation of 194 involves the novel cyclopropanation reaction of alkenes by nucleophilic attack of two carbanions. 

Another reaction occurs by the attack of a soft nucleophile at the central carbon to form the 7r-allylpalladium complex 7, which undergoes further reaction with the nucleophile typical of rr-allylpalladium complexes to form the alkene 8,... 

No reaction of soft carbon nucleophiles takes place with propargylic acet-ates[37], but soft carbon nucleophiles, such as / -keto esters and malonates, react with propargylic carbonates under neutral conditions using dppe as a ligand. The carbon nucleophile attacks the central carbon of the cr-allenylpal-ladium complex 81 to form the rr-allylpalladium complex 82, which reacts further with the carbon nucleophile to give the alkene 83. Thus two molecules of the a-monosubstituted /3-keto ester 84, which has one active proton, are... 

The interaction between a substituent and the ring carbon to which it is bonded could be related to some electronic characteristics of the unsubstituted ring and especially to the net charge of its various sites. In that respect the rr-net charges diagram discussed in Section 1.5 indicates that the electron-withdrawing power of the ring-carbon atoms will decrease in the order, 2>4>5. 

As portrayed m Figure 2 20 the two carbons of acetylene are connected to each other by a 2sp-2sp cr bond and each is attached to a hydrogen substituent by a 2sp-ls CT bond The unhybndized 2p orbitals on one carbon overlap with their counterparts on the other to form two rr bonds The carbon-carbon triple bond m acetylene is viewed as a multiple bond of the ct + rr + rr type... 

Section 2 20 Carbon is sp hybridized in ethylene and the double bond has a ct com ponent and a rr component The sp hybridization state is derived by mix mg the 2s and two of the three 2p orbitals Three equivalent sp orbitals result and their axes are coplanar Overlap of an sp orbital of one car bon with an sp orbital of another produces a ct bond between them Each carbon still has one unhybridized p orbital available for bonding and side by side overlap of the p orbitals of adjacent carbons gives a rr bond between them... 

Bonding m alkenes is described according to an sp orbital hybridization model The double bond unites two sp hybridized carbon atoms and is made of a ct component and a rr component The ct bond arises by over lap of an sp hybrid orbital on each carbon The rr bond is weaker than the CT bond and results from a side by side overlap of p orbitals... 

Isomeric alkenes may be either constitutional isomers or stereoisomers There is a sizable barrier to rotation about a carbon-carbon double bond which corresponds to the energy required to break the rr component of the double bond Stereoisomeric alkenes are configurationally stable under normal conditions The configurations of stereoisomeric alkenes are described according to two notational systems One system adds the prefix CIS to the name of the alkene when similar substituents are on the same side of the double bond and the prefix trans when they are on opposite sides The other ranks substituents according to a system of rules based on atomic number The prefix Z is used for alkenes that have higher ranked substituents on the same side of the double bond the prefix E is used when higher ranked substituents are on opposite sides... 

We can consider the hydroboration step as though it involved borane (BH3) It sim phfies our mechanistic analysis and is at variance with reality only m matters of detail Borane is electrophilic it has a vacant 2p orbital and can accept a pair of electrons into that orbital The source of this electron pair is the rr bond of an alkene It is believed as shown m Figure 6 10 for the example of the hydroboration of 1 methylcyclopentene that the first step produces an unstable intermediate called a tt complex In this rr com plex boron and the two carbon atoms of the double bond are joined by a three center two electron bond by which we mean that three atoms share two electrons Three center two electron bonds are frequently encountered m boron chemistry The tt complex is formed by a transfer of electron density from the tt orbital of the alkene to the 2p orbital... 

Electron delocalization m allylic carbocations can be indicated using a dashed line to show the sharing of a pair of rr electrons by the three carbons The structural formula IS completed by placing a positive charge above the dashed line or by adding partial pos itive charges to the carbons at the end of the allylic system... 

An orbital overlap description of electron delocalization mil dimethylallyl cation H2C=CH—C(CH3)2 is given m Figure 10 2 Figure 10 2a shows the rr bond and the vacant p orbital as independent units Figure 10 2b shows how the units can overlap to give an extended rr orbital that encompasses all three carbons This permits the two rr electrons to be delocalized over three carbons and disperses the positive charge... 

Additional evidence for electron delocalization m 1 3 butadiene can be obtained by considering its conformations Overlap of the two rr electron systems is optimal when the four carbon atoms are coplanar Two conformations allow this coplananty they are called the s cis and s trans conformations... 

In addition to its three sp hybrid orbitals each carbon has a half filled 2p orbital that can participate m tt bonding Figure >b shows the continuous rr system that encompasses all of the carbons that result from overlap of these 2p orbitals The six tt electrons of benzene are delocalized over all six carbons... 

The picture of benzene as a planar framework of ct bonds with six electrons m a delo cahzed rr orbital is a useful but superficial one Six elecfrons cannof simulfaneously occupy any one orbifal be if an afomic orbifal or a molecular orbifal We can fix fhis wifh the more accurate molecular orbital picture shown m Figure 114 We learned m Section 2 4 that when atomic orbitals (AOs) combine to give molecular orbitals (MOs) the final number of MOs musf equal fhe original number of AOs Thus fhe six 2p AOs of SIX sp hybridized carbons combine fo give six tt MOs of benzene... 

In orbital terms as represented m Figure 11 10 benzyl radical is stabilized by delo cahzation of electrons throughout the extended tt system formed by overlap of the p orbital of the benzylic carbon with the rr system of the ring... 

Even though resonance tells us that the negative charge m cycloheptatnenyl anion can be shared by all seven of its carbons this delocalization offers little m the way of sta bilization Indeed with eight rr electrons cycloheptatnenyl anion is antiaromatic and rel atively unstable... 

Acetylenic hydrogens are unusual in that they are more shielded than we would expect for protons bonded to sp hybridized carbon This is because the rr electrons circulate around the triple bond not along it (Figure 13 9a) Therefore the induced magnetic field is parallel to the long axis of the triple bond and shields the acetylenic proton (Figure 13 9b) Acetylenic protons typically have chemical shifts near 8 2 5... 

Aldol condensation offers an effective route to a p unsaturated aldehydes and ketones These compounds have some interesting properties that result from conjugation of the carbon-carbon double bond with the carbonyl group As shown m Figure 18 6 the rr systems of the carbon-carbon and carbon-oxygen double bonds overlap to form an extended rr system that permits increased electron delocalization... 

The carbonyl group withdraws rr electron density from the double bond and both the carbonyl carbon and the p carbon are positively polarized Their greater degree of charge separation makes the dipole moments of a p unsaturated carbonyl compounds signifi cantly larger than those of comparable aldehydes and ketones... 

The diminished rr electron density m the double bond makes a p unsaturated aide hydes and ketones less reactive than alkenes toward electrophilic addition Electrophilic reagents—bromine and peroxy acids for example—react more slowly with the carbon-carbon double bond of a p unsaturated carbonyl compounds than with simple alkenes... 

The same tertiary carbocation serves as the precursor to numerous bicyclic monoterpenes A carbocation having a bicychc skeleton is formed by intramolecular attack of the rr electrons of the double bond on the positively charged carbon... 

Extraction by carboxyUc acids (qv) is carried out in a neutral or weaMy acidic medium. The most widely used carboxyUc acid is RR (CH2)CCOOH, where Rplus represents seven carbon atoms. Trade names are Versatic 10 (Shell Chemicals) and Neodecanoic acid (Exxon Chemicals). CarboxyUc acids can be used either in chloride or in nitrate media and have a better selectivity for light lanthanides than for heavy lanthanide separation. 

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