Bonding in Allenes

The three carbons of allene lie in a straight line, with relatively short carbon-carbon bond distances of 131 pm. The central carbon, since it bears only two substituents, is ip-hybridized. The terminal carbons of allene are ip -hybridized. 

FIGURE 10.6 Bonding and geometry in 1,2-propa-diene (allene). 

The nonplanarity of allenes has an interesting stereochemical consequence. 1,3-Disubstituted allenes are chiral they are not superposable on their mirror images. Even an allene as simple as 2,3-pentadiene (CH3CH=C=CHCH3) has been obtained as separate enantiomers. 

Examine models of both enantiomers of 2,3-pentadiene to verify that they are nonsuper-posable. 

The Cahn-Ingold-Prelog R,S notation has been extended to chiral allenes and other molecules that have a stereo-genic axis. Such compounds are so infrequently encountered, however, we will not cover the rules for specifying their stereochemistry in this text. 

The enantiomers shown are related as a right-hand and left-hand screw, respectively. 

Bonding and geometry in 1,2-propadiene (allene). The green and yellow colors are meant to differentiate the orbitals and do not indicate their phases. 

The j -trans conformation is 12 kJ/mol (2.8 kcal/mol) more stable than i -cis, with van der Waals strain between the C(l) and C(4) interior hydrogens contributing to the decreased stability of the i -cis conformation. The two conformations are interconvertible by rotation about the C(2)-C(3) single bond with an activation energy for the 5-trans i -cis conversion of 25 kJ/mol (6 kcal/mol). This energy cost reflects the loss of ir-electron delocalization in going from a coplanar C=C—C=C arrangement to a nonplanar one at the transition state. 

Use Hammond s postulate (Section 4.8) to estimate the angle between the two C=C units at the transition state for rotation about the C(2)-C(3) bond in 1,3-butadiene. Is it 90°, less than 90°, or more than 90°  

The Cahn-Ingold-Prelog R-S notation has been extended to include molecules with a chirality axis. See the article by Mak in the November 2004 issue of the Journal of Chemical Education for a brief discussion of assigning / or S to chiral molecules that do not contain a chirality center. 

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Fluormation likewise significandy destabilizes the multiple bonds in allenes and acetylenes [105] Fluoro- and difluoroacetylene are dangerously explosive, and hexafluoro-2-butyne is very susceptible to both concerted and biradical addition reacbons [106, 107] (see pp 757 and 767)... 

Although at first glance addition to the central carbon and formation of what seems like an allylic carbonium ion would clearly be preferred over terminal addition and a vinyl cation, a closer examination shows this not to be the case. Since the two double bonds in allenes are perpendicular to each other, addition of an electrophile to the central carbon results in an empty p orbital, which is perpendicular to the remaining rr system and hence not resonance stabilized (and probably inductively destabilized) until a 90° rotation occurs around the newly formed single bond. Hence, allylic stabilization may not be significant in the transition state. In fact, electrophilic additions to allene itself occur without exception at the terminal carbon (54). 

As shown in Fig. 10, the lA2 state is predicted to be the lowest singlet state of 4a.55,57 The CASSCF vibrational analysis reveals one imaginary frequency for this state, corresponding to an out-of-plane a2 vibration, which leads to the cyclic allene 3a.57 Thus,1 A2-4a is a transition state for the enantiomerization of 3a. The analogous process of internal rotation about the C-C bonds in allene is also predicted to occur via an open-shell1A2 transition state.72... 

Allene, CH2 = C=CH2, has the structure shown below. Explain how the bonding in allene requires the two =CH2 groups at its ends to be at right angles to each other. 

Active methylene compounds can be added to polar double bonds such as those in acrylate esters and methyl vinyl ketone as has been described in the previous section. Active methylene compounds can also be added to carbon-carbon multiple bonds in allenes and alkynes with the aid of the transition metal complexes as the catalyst. The addition of methylmalononitrile to 3-phenyl-l,2-butadiene takes place in the presence of Pd2(dba)3-CHCl3 to give the corresponding addition product with E-stereochemistry (Eq. 67) [137 a]. The C-C bond formation occurs exclusively at the terminal position of the allenes. Trost et al. independently reported the similar results with respect to palladium-catalyzed addition of C-H bonds in active methylene compounds to allenes [137 b. ... 

The allene molecule H2C=C=CH2 is linear (the three C atoms lie on a straight line). What are the hybridization states of the carbon atoms Draw diagrams to show the formation of sigma bonds and pi bonds in allene. 

Allenes (see p. 6 for a discussion of bonding in allenes) can be chiral. An allene having nonidentical substituents at both sp carbons gives nonsuperimposable mirror images. 

Solution (a) The general range of heats of hydrogenation of alkenes is from -27 to -31 kcal/mole. Here we are dealing with the hydrogenation of double bonds between two sp -hybridized carbons and an sp-hybrid zed carbon. We will assume a AH (heat of hydrogenation) value of -28 to -30 kcal/mole for each double bond in allene. 

Make a sketch of the structure of this molecule that is analogous to Figure 9.25. In addition, answer the following three questions (a) Is the molecule planar (b) Does it have a nonzero dipole moment (c) Would the bonding in allene be described as delocalized Explain. 

A detailed discussion of the structural chemistry of allenes and cumulenes (1 b) has revealed that upon arbitrary substitutions the cumulenic double bonds in allenes, butatrienes, and ketenes retain their linear arrangements and their bond lengths within experimental errors. Furthermore, the antiplanar arrangements of the ligands in allenes (and surely also in pentatetraenes) are not affected by substitutions. 

In 2010, our group examined the silylzincation of allenes (Table 13) [92], Since there were two double bonds in allene molecules involving an sp-carbon, the... 

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