Carbon triple bond

Acetylenes are another series of unsaturated hydrocarbons which include compounds containing a carbon-carbon triple bond, for example acetylene Itself ... 

Carbon can also form multiple bonds with other carbon atoms. This results in unsaturated hydrocarbons such as olefins (alkenes), containing a carbon-carbon double bond, or acetylenes (alkynes), containing a carbon-carbon triple bond. Dienes and polyenes contain two or more unsaturated bonds. 

The addition of acetylides to oxiranes yields 3-alkyn-l-ols (F. Sondheimer, 1950 M.A. Adams, 1979 R.M. Carlson, 1974, 1975 K. Mori, 1976). The acetylene dianion and two a -synthons can also be used. 1,4-Diols with a carbon triple bond in between are formed from two carbonyl compounds (V. Jager, 1977, see p. 52). The triple bond can be either converted to a CIS- or frans-configurated double bond (M.A. Adams, 1979) or be hydrated to give a ketone (see pp. 52, 57, 131). 

Multiple bonds are very common m organic chemistry Ethylene (C2H4) contains a carbon-carbon double bond m its most stable Lewis structure and each carbon has a completed octet The most stable Lewis structure for acetylene (C2H2) contains a carbon-carbon triple bond Here again the octet rule is satisfied... 

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... 

FIGURE 2 20 Bonding in acetylene based on sp hybridization of carbon The carbon-carbon triple bond is viewed as consisting of one cr bond and two tt bonds... 

Hydrocarbons that contain a carbon-carbon triple bond are called alkynes Non cyclic alkynes have the molecular formula C H2 -2 Acetylene (HC=CH) is the simplest alkyne We call compounds that have their triple bond at the end of a carbon chain (RC=CH) monosubstituted or terminal alkynes Disubstituted alkynes (RC=CR ) have internal triple bonds You will see m this chapter that a carbon-carbon triple bond is a functional group reacting with many of the same reagents that react with the double bonds of alkenes... 

Natural products that contain carbon-carbon triple bonds are numerous Two examples are tariric acid from the seed fat of a Guatemalan plant and cicutoxin a poi sonous substance isolated from water hemlock... 

An sp hybridization model for the carbon-carbon triple bond was developed in Section 2 21 and is reviewed for acetylene in Figure 9 2 Figure 9 3 compares the electrostatic potential maps of ethylene and acetylene and shows how the second tr bond m acetylene causes a band of high electron density to encircle the molecule... 

In the presence of excess hydrogen halide gemmal dihalides are formed by sequen tial addition of two molecules of hydrogen halide to the carbon-carbon triple bond... 

Alkynes are hydrocarbons that contain a carbon-carbon triple bond Sim pie alkynes having no other functional groups or rings have the general formula C H2 -2 Acetylene is the simplest alkyne... 

Acetylene is linear and alkynes have a linear geometry of their X—C=C—Y units The carbon-carbon triple bond m alkynes is com posed of a CT and two tt components The triply bonded carbons are sp hybridized The ct component of the triple bond contains two electrons m an orbital generated by the overlap of sp hybndized orbitals on adja cent carbons Each of these carbons also has two 2p orbitals which over lap m parrs so as to give two tt orbitals each of which contains two electrons... 

Carbon-carbon triple bonds can be cleaved by ozonolysis The cleavage products are carboxylic acids... 

In presence of carbon-carbon double and triple bonds —C=C—C=C— In presence of two carbon-carbon triple bonds —C=C—C=C— Double bond... 

Azomethines add with great ease and stereospecificity to carbon-carbon double bonds affording pyrazolidines. They also add to carbon-carbon triple bonds with the formation of pyrazolines. Benzimidazolium iV-imines, e.g. (622), are a special case of azomethines. Its reaction with DMAD yields l-(2-methylaminophenyl)pyrazole (624), which is formed by cleavage of the initial adduct (623) (75JHC225). 

G. H. Schmid, The Chemistry of the Carbon-Carbon Triple Bond, Part 1, S. Patai, ed., John Wiley Sons, New Wirk, 1978, Chapter 3. 

Alkenes and alkynes are similar in structure to the alkanes except the alkenes contain a carbon-to-carbon double bond (C=C) and the alkynes contain a carbon-to-carbon triple bond (C=C). The name prefixes are exactly the same as for the alkanes with the same number of carbons, but the endings are -one for compounds with double bonds and their derivatives and -yne for compounds with triple bonds... 

Acetylene and terminal alkynes are CH-acidic compounds the proton at the carbon-carbon triple bond can be abstracted by a suitable base. Such a deprotonation is the initial step of the Glaser reaction as well as the Eglinton... 

Shortly after the tetravalent nature of carbon was proposed, extensions to the Kekule-Couper theory were made w7hen the possibility of multiple bonding between atoms was suggested. Emil Erlenmeyer proposed a carbon-carbon triple bond for acetylene, and Alexander Crum Brown proposed a carbon-carbon double bond for ethylene. In 1865, Kekule provided another major advance when he suggested that carbon chains can double back on themselves to form rings of atoms. 

When two sp-hybridized carbon atoms approach each other, sp hybrid orbitals on each carbon overlap head-on to form a strong sp-sp a bond. In addition, the pz orbitals from each carbon form a pz-pz it bond by sideways overlap and the py orbitals overlap similarly to form a py-py tt bond. The net effect is the sharing of six electrons and formation of a carbon-carbon triple bond. The two remaining sp hybrid orbitals each form a bond with hydrogen to complete the acetylene molecule (Figure 1.16). 

An alkyne is a hydrocarbon that contains a carbon-carbon triple bond. Acetylene.. H—C= C—H, the simplest alkyne, was once widely used in industry as the starting material for the preparation of acetaldehyde, acetic acid, vinyl chloride, and other high-volume chemicals, but more efficient routes to these substances using ethylene as starting material are now available. Acetylene is still used in the preparation of acrylic polymers but is probably best known as the gas burned in high-temperature oxy-acetylene welding torches. 

You might recall from Section 1.9 that a carbon-carbon triple bond results from the interaction of two sp-hybridized carbon atoms. The two sp hybrid orbitals of carbon lie at an angle of 180° to each other along an axis perpendicular to the axes of the two unhybridized 2py and 2pz orbitals. When two sp-hybridized carbons approach each other, one sp-sp a bond and two p-p -rr bonds are... 

The length of the carbon-carbon triple bond in acetylene is 120 pm, and the strength is approximately 835 kj/mol (200 kcal/mol), making itthe shortest and strongest known carbon-carbon bond. Measurements show that approximately 318 kj/mol (76 kcal/mol) is needed to break a rbond in acetylene, a value some 50 kj/mol larger than the 268 kj/mol needed to break an alkene n bond. 

An alkyne is a hydrocarbon that contains a carbon-carbon triple bond. Alkyne carbon atoms are sp-hybridized, and the triple bond consists of one sp-sp a bond and two p-p tt bonds. There are relatively few general methods of alkyne synthesis. Two good ones are the alkylation of an acetylide anion with a primary-alkyl halide and the twofold elimination of HX from a vicinal dihalide. 

Alkyne (Chapter 8 introduction) A hydrocarbon that contains a carbon-carbon triple bond, RC CR. 

The metathesis of linear alkynes has also been reported, e.g. the metathesis of propyne, 1-pentyne, 2-pentyne, and 2-hexyne (31-33). This reaction can be visualized as the cleavage and formation of carbon-carbon triple bonds ... 

Ozone also adds to the carbon-carbon triple bond of acetylenic compds, the usual products being diketones and carboxylic acids. In polynuclear aromatic compds, the various carbon bonds and atoms have different reactivities. The reaction with ozone is more complex and the compn of products is difficult to predict... 

The Chemistry of the Carbon-Carbon Triple Bond (two parts)... 

Several stable Group 6 metal-ketene complexes are known [14], and photo-driven insertion of CO into a tungsten-carbyne-carbon triple bond has been demonstrated [15]. In addition, thermal decomposition of the nonheteroatom-stabilized carbene complexes (CO)5M=CPh2 (M=Cr, W) produces diphenylke-tene [16]. Thus, the intermediacy of transient metal-ketene complexes in the photodriven reactions of Group 6 Fischer carbenes seems at least possible. 

---