Sp hybridized C-H bonds

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

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. 

Table 14.1 illustrates that absorptions for a given type of C-H bond occur in a narrow range of chemical shift values, usually 1-2 ppm. For example, all sp hybridized C-H bonds in alkanes and cycloalkanes absorb between 0.9 and 2.0 ppm. By contrast, absorptions due to N-H and O-H protons can occur over a broader range. For example, the OH proton of an alcohol is found anywhere in the 1-5 ppm range. The position of these absorptions is affected by the extent of hydrogen bonding, making it more variable. 

Because allylic C—H bonds are weaker than other sp hybridized C-H bonds, the allylic carbon can be selectively halogenated by using iV-bromosuccinimide (NBS, Section 10.15) in the presence of light or peroxides. Under these conditions only the allylic C-H bond in cyclohexene reacts to form an allylic halide. 

Draw the conjugate bases of pyrrole and cyclopentadiene. Explain why the sp hybridized C - H bond of cyclopentadiene is more acidic than the N - H bond of pyrrole. 

Benzylic C - H bonds are weaker than most other sp hybridized C — H bonds, because homolysis forms a resonance-stabilized benzylic radical. 

With internal alkynes two carboxylic acids are formed as products. With terminal alkynes, the sp hybridized C—H bond is converted to CO2. 

An acid-base reaction can also be used to prepare sp hybridized organolithium compounds. Treatment of a terminal alkyne with CH Li affords a lithium acetylide. Equilibrium favors the products because the sp hybridized C-H bond of the terminal alkyne is more acidic than the sp hybridized conjugate acid, CH4, that is formed. 

C-H bond on the a carbon is more acidic than many other sp hybridized C-H bonds, because the resulting enolate is resonance stabilized. Moreover, one of the resonance structures is especially stable because it places a negative charge on an electronegative oxygen atom. 

Reactivity from the naphthyl hydride ruthenium complex is believed to occur via initial reductive elimination of naphthalene followed by oxidative addition of a C-H bond. These activation processes (requiring reductive elimination) occur at temperatures of 150°C in alkane or arene solvents, depending on the desired product ". This complex shows general C-H activation behavior with sp, sp, and sp hybridized C-H bonds . ... 

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 ( stereocontroUed ) radical reactions involving sp -hybridized C-H bonds are known. Particularly useful functionahzations along these Unes include the hydroxylation and the acyloxylation of alkyl chains. The reason for their Umited success is of coiuse due to the high stabihty of the C-H bond compared with that of the olefinic C=C unit most electrophiUc 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 hybrid =C—H bond. Table 9-2 shows how the acidity of a C—H bond varies with its hybridization, increasing with the increasing s character of the... 

The application of hypervalent iodine(III) mediated and catalyzed amination of hydrocarbon substrates has developed into a useful tool for organic synthesis. Reactions comprise direct amination of sp-, sp -, and sp -hybridized C-H bonds and numerous oxidative transformations of alkenes, butadienes, and allenes. Some of these methods have been developed directly in the form of catalytic transformations, which adds to underline the synthetic potential of the field. Where applicable, the possibility for enantioselective transformations has been demonstrated for some cases. One can be optimistic that hypervalent iodine chemistry is able to complement existing methodology for oxidative amination reactions in a practical manner over the next few years. 

In 2006, Ackermann and coworkers pointed out that bulky substituents on the nitrogen atoms of secondary chlorophosphanes facilitate the efficient palladium-catalyzed a-arylation of ketones bearing sp -hybridized C-H bonds in the a-position with electron-rich aryl chlorides (Scheme 8.23). The active catalyst for these reactions was generated from Pd(dba)2 and the bulky sterically hindered ligand shown in Scheme 8.23 [55]. 

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