Acetylene orbital overlapping

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). 

Many of the Lewis structures in Chapter 9 and elsewhere in this book represent molecules that contain double bonds and triple bonds. From simple molecules such as ethylene and acetylene to complex biochemical compounds such as chlorophyll and plastoquinone, multiple bonds are abundant in chemistry. Double bonds and triple bonds can be described by extending the orbital overlap model of bonding. We begin with ethylene, a simple hydrocarbon with the formula C2 H4. 

Composite orbital overlap view of the a and bonds of acetylene. The bonds are shown in an exploded view that makes the C—C a bond visible. 

Novel photochemical (and thermal) reactions of macrocyclic oxa-sila-acetylenic ring systems (expected to show unusual optical properties because of electronic effects arising from orbital overlap of the acetylenic n system with the silicon a bonds and the oxygen lone-pair electrons) were described. While thermolysis in the presence of a transition metal carbonyl compound gave cyclization to both benzenoid and fulvene species, photolysis in the presence of the transition metal carbonyl compound (which catalyzes 1,2-silyl shifts across a carbon-carbon triple bond) gave fulvene and vinylidene products, the latter being readily photolyzed to the fulvene 159 (equation 101). 

In the second place, the electronic nature of the R group can modify the polarity of the carbon-metal bond. The importance of x-bonding in the structure of metal alkyls is discussed in Section III.B. As far as chemical reactivity is concerned, however, a greater electron-attracting power of R should enhance the polarity of the carbon-metal bond and thus increase its chemical reactivity. In addition, if the R group is unsaturated, the reactivity of the organometallic compound changes markedly. Situation of the unsaturation on the carbon atom of C—M bond as in phenyl, vinyl, or acetylenic R—M types usually results in some stabilization of the bond if x-metal orbital overlap can occur ... 

Because the triple bond counts as one effective repulsive unit, each carbon has two effective pairs, which requires a linear arrangement. Thus each carbon atom requires5/7 hybridization, leaving two unchanged p orbitals (see Fig. 9.16). One of the oppositely oriented (see Fig. 9.14) sp orbitals is used to form a bond to the hydrogen atom the other sp orbital overlaps with the similar sp orbital on the other carbon to form the sigma bond. The two pi bonds are formed from the overlap of the twop orbitals on each carbon. This accounts for the triple bond (one sigma and two pi bonds) in acetylene. 

Figure 2.20 Bonding in ethyne (acetylene), C2H2. Overlap of the Is orbitals of H with the sp hybrid orbitals on C (part a) results in a

Benzyne is an extremely reactive species due to the nature of its triple bond. In normal acetylenic species, such as ethyne, the unhybridized p orbitals are parallel to each other above and below the molecular axis, and this facilitates maximum orbital overlap. In benzyne, however, the p orbitals are distorted to accommodate the triple bond within the ring system, thus reducing their effective overlap. Benzyne can also be drawn as a diradical, where the triple bond is drawn as a double bond with a single electron on each carbon. Benzyne can exist as either an ortho-, a meta- or a poro-benzyne, where the diradical can be a 1,2-, a 1,3- or a 1,4-diradical species, respectively. The 1,4-diradical species has been identified in the Bergman cychzation [3]. In this chapter, we will focus solely on the 1,2-dehydro-benzene (o-benzyne species). The term aryne here will be used to refer specifically to 1,2-dehydrobenzenes and derivatives. 

Figure 1.22 shows a Lewis structure and an orbital overlap diagram for acetylene, C2H2. A carbon-carbon triple bond consists of one sigma bond and two pi bonds. The sigma bond is formed by the overlap of sp hybrid orbitals. One pi bond is formed by the overlap of a pair of parallel 2p atomic orbitals. The second pi bond is formed by the overlap of a second pair of parallel 2p atomic orbitals. 

Acetylenes appear to be more reactive acceptors than olefins [12], perhaps because orbital overlap is easier to achieve with the cylindrically symmetrical acetylene. Cyclization conditions are mild enough that highly functionalized systems can be tolerated [13] (15). As with olefins, electron-withdrawing groups further activate acetylenic acceptors [14] (14). 

Chemical bonds are characterized by their length and energy. In this section we are concerned with the structural features that change the normal values of these properties. Any structural feature that shortens the distance between 2 atoms increases the energy needed to dissociate the atoms. This happens because the closer the atoms are, the more their orbitals overlap. The generalization is substantiated by the observed distances and energies of the bonds between the 2 carbon atoms in ethane, ethylene, and acetylene ... 

After each carbon atom has formed two O bonds, each still has two half-filled 2p orbitals. The half-filled 2p orbitals overlap side by side to give two tt bonds. One set of 2p orbitals overlaps in ftont and back of the molecule to form one tt bond. The second set of 2p orbitals overlaps above and below the molecule to form the second Tt bond. Thus, the carbon atoms in acetylene are linked by one G bond and two Jt bonds to give a triple bond. 

Two different types of orbital overlap occur when multiple bonds are described by the valence bond method. In our discussion we will use as specific examples the carbon-to-carbon double bond in ethylene, C2H4, and the carbon-to-carbon triple bond in acetylene, C2H2. 

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 21 Carbon is sp hybridized m acetylene and the triple bond is of the ct + Tt + Tt type The 2s orbital and one of the 2p orbitals combine to give two equivalent sp orbitals that have their axes m a straight line A ct bond between the two carbons is supplemented by two tr bonds formed by overlap of the remaining half filled p orbitals... 

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... 

In the third type of hybridisation of the valence electrons of carbon, two linear 2sp orbitals are formed leaving two unhybridised 2p orbitals. Linear a bonds are formed by overlap of the sp hybrid orbitals with orbitals of neighbouring atoms, as in the molecule ethyne (acetylene) C2H2, Fig. 1, A3. The unhybridised p orbitals of the carbon atoms overlap to form two n bonds the bonds formed between two C atoms in this way are represented as Csp Csp, or simply as C C. 

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