Hybridized C—H Bonds

TABLE 3.4. Gas-Phase Affacid Values of sp and sp Hybridized Carbon Acids  

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Carbanions Derived from sp Hybridized C-H Bonds. There are... 

Effect of hybridization on overlap and bond Molecule Hybridization C—H bond energy (Id not"1) C—H bond length (pm)... 

Metallacyclobutene complexes of both early and late transition metals can, in some cases, be prepared by intramolecular 7-hydrogen elimination, although the intimate mechanism of the reaction varies across the transition series. For low-valent late metals, the reaction is generally assumed to proceed via the oxidative addition of an accessible 7-C-H bond (Scheme 28, path A), but for early metals and, presumably, any metal in a relatively high oxidation state, a concerted cr-bond metathesis is considered most probable (path B). In this process, the 7-C-H bond interacts directly with an M-X fragment (typically a second hydrocarbyl residue) to produce the metallacycle with the extrusion of H-X (i.e., a hydrocarbon). Either sp3- or spz-hybridized C-H bonds can participate in the 7-hydrogen elimination. 

Applications of controlled radical reactions - including oxidation - deal almost exclusively with C=C double bonds. Indeed, a multitude of examples have been reported for the selective transformation of this functional group. Contrasting with this situation, only a very limited number of selective ( stereocontrolled ) radical reactions involving sp3-hybridized C-H bonds are known. Particularly useful functionalizations along these lines include the hydroxylation and the acyloxylation of alkyl chains. The reason for their limited success is of course due to the high stability of the C-H bond compared with that of the olefinic C=C unit most electrophilic reagents which readily add to unsaturated substrates are not able to oxidize a C-H bond. 

The acidity of an acetylenic hydrogen stems from the nature of the sp Formation Of hybrid = C—H bond. Table 9-3 shows how the acidity of a C—H bond varies with. ... ... 

Why is a stronger base needed in this dehydrohalogenation The transition state for the second elimination reaction includes partial cleavage of a C - H bond. In this case, however, the carbon atom is sp hybridized, and sp hybridized C-H bonds are stronger than sp hybridized C-H bonds. As a result, a stronger base is needed to cleave this bond. 

All reactions of alkynes occur because they contain easily broken ft bonds or, in the case of terminal alkynes, an acidic, sp hybridized C—H bond. 

Because sp hybridized C - H bonds are more acidic than sp and sp hybridized C - H bonds,... 

Recall from Section 2.5D that the acidity of a C-H bond increases as the percent s-character of C increases. Thus, sp hybridized C-H bonds (having a C atom with 50% s-character) are more acidic than sp and sp hybridized C-H bonds (having C atoms with 33% and 25% s-character, respectively). 

Because acetylene has two sp hybridized C-H bonds, two sequential reactions can occur to form two new carbon-carbon bonds, as shown in Sample Problem 11.6. 

Alkynes also undergo oxidative cleavage of the a bond and both n bonds of the triple bond. Internal alkynes are oxidized to carboxylic acids (RCOOH), whereas terminal alkynes afford carboxylic acids and CO2 from the sp hybridized C - H bond. 

A and B show peaks in the same regions for their C=0 group and sp hybridized C-H bonds. 

The IR spectra of hexane, 1-hexene, and 1-hexyne illustrate the important differences that characterize the IR spectra of hydrocarbons above 1500 cm. Although all three compounds contain C-C bonds and sp hybridized C-H bonds, the absorption peaks due to C=C and C=C readily distinguish the alkene and alkyne. 

Note, too, that the C-H absorptions in alkanes, alkenes, and alkynes have a characteristic appearance and position. The sp hybridized C-H bonds are often seen as a broad, strong absorption at < 3000 cm , whereas sp and sp hybridized C-H bonds absorb at somewhat higher frequency. 

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