Allenes hybridization

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

Propadiene (H2C=C=CH2) also called allene, is the simplest cumulated diene The two tt bonds m an allene share an sp hybridized carbon and are at right angles to each other Certain allenes such as 2 3 pentadiene (CH3CH=C=CHCH3) possess a chirality axis and are chiral... 

Allerie, P12C — C = CH2> is somewhat unusual in that it has two adjacent double bonds. Draw a picture showing the orbitals involved in the u and -n- bonds of allene. Is the central carbon atom sp2- or sp-hybridized What about the hybridization of the terminal carbons What shape do you predict for allene ... 

What kind of hybridization must the central carbon have Sketch the bonding it orbitals in allene. What shape do you predict for allene ... 

Allen, M. J., Tozer, D. J., 2000, Kohn-Sham Calculations Using Hybrid Exchange-Correlation Functionals with Asymptotically Corrected Potentials , J. Chem. Phys., 113, 5185. 

The Lewis dot formalism shows any halogen in a molecule surrounded by three electron lone pairs. An unfortunate consequence of this perspective is that it is natural to assume that these electrons are equivalent and symmetrically distributed (i.e., that the iodine is sp3 hybridized). Even simple quantum mechanical calculations, however, show that this is not the case [148]. Consider the diiodine molecule in the gas phase (Fig. 3). There is a region directly opposite the I-I sigma bond where the nucleus is poorly shielded by the atoms electron cloud. Allen described this as polar flattening , where the effective atomic radius is shorter at this point than it is perpendicular to the I-I bond [149]. Politzer and coworkers simply call it a sigma hole [150,151]. This area of positive electrostatic potential also coincides with the LUMO of the molecule (Fig. 4). 

The attack of the nucleophile on the acceptor-substituted allene usually happens at the central sp-hybridized carbon atom. This holds true also if no nucleophilic addition but a nucleophilic substitution in terms of an SN2 reaction such as 181 — 182 occurs (Scheme 7.30) [245]. The addition of ethanol to the allene 183 is an exception [157]. In this case, the allene not only bears an acceptor but shows also the substructure of a vinyl ether. A change in the regioselectivity of the addition of nucleophilic compounds NuH to allenic esters can be effected by temporary introduction of a triphenylphosphonium group [246]. For instance, the ester 185 yields the phos-phonium salt 186, which may be converted further to the ether 187. Evidently, the triphenylphosphonium group induces an electrophilic character at the terminal carbon atom of 186 and this is used to produce 187, which is formally an abnormal product of the addition of methanol to the allene 185. This method of umpolung is also applicable to nucleophilic addition reactions to allenyl ketones in a modified procedure [246, 247]. 

Most, perhaps all, of the reactions that simple alkenes undergo are also available to allenes. By virtue of their strain and of the small steric requirement of the sp-hybrid-ized carbon atom, the reactions of allenes usually take place more easily than the corresponding reactions of olefins. Because the allenes can also be chiral, they offer opportunities for control of the reaction products that are not available to simple alkenes. Finally, some reaction pathways are unique to allenes. For example, deprotonation of allenes with alkyllithium reagents to form allenyl anions is a facile process that has no counterpart in simple alkenes. These concepts will be illustrated by the discussion of cyclization reactions of allenes that follows. 

In principle, three basically different types of reaction modes are applied for cross-coupling reactions of allenes. First, cross-couplings of allenes with suitable halogen or metal substituents at one of the sp2-hybridized carbons furnish products still bearing the intact cumulene it-system. On this basis, numerous reactions for conversions of precursor 1 or 3 into substituted allenes 2 have been developed (Schemes 14.1 and 14.2). 

Tius and co-workers elegantly applied a variant of the Nazarov reaction to the preparation of cyclopentenone prostaglandins (Scheme 19.39) [46]. Moreover, it was demonstrated that the chirality of non-racemic allenes is transferred to an sp3-hybridized carbon atom. Preparation of allenic morpholinoamide 214 and resolution of the enantiomers by chiral HPLC provided (-)- and (+)-214. Compound (-)-214 was exposed to the vinyllithium species 215 to afford a presumed intermediate which was not observed but spontaneously cyclized to give (+)- and (—)-216 as a 5 1 mixture. Compound (+)-216 was obtained with an 84% transfer of chiral information and (-)-216 was obtained in 64% ee. The lower enantiomeric excess of (—)-216 indicates that some Z to E isomerization took place. This was validated by the conversion of 216 to 217, where the absolute configuration was established. The stereochemical outcome of this reaction has been explained by conrotatory cyclization of 218 in which the distal group on the allene rotates away from the alkene to give 216. 

Trask, B.J., Allen, S., Massa, H., Fertitta, A., Sachs, R., van den Engh, G., and Wu, M. (1993) Studies of metaphase and interphase chromosomes using fluorescence in situ hybridization. Cold Spring Harb. Symp. Quant. Biol. 58, 767-775. 

Tung VC, Chen L, Allen MJ et al (2009) Low-temperature solution processing of graphene-carbon nanotube hybrid materials for high-performance transparent conductors. Nano Lett 9 1949-1955... 

However, the most powerful tool to study such protein-protein interactions is the yeast two-hybrid system (Lields and Song, 1989 Allen et ah, 1995). In its current form, the system utilizes the efficacy of yeast ge-... 

Allen, J.B., Walberg, M.W., Edwards, M.C., and Elledge, S.J. (1995) Finding prospective partners in the library the two-hybrid system and phage display find a match. Trends Biochem Sci 20 511-516. 

Reed and Allen, using their bond polarity index, have assigned values of 0.000, 0.027, and 0.050, respectively (compared to H —0.032 and F 0.189) [108]. Without attempting to be too quantitative, convenient values of the core energies of hybrid atomic orbitals, in units, recognizing that changes in coordination number also occur, are approximately... 

Element Pauling 4 Sanderson X Allred- Rochow Xttv1 Allen X p Orbital hybrid Pauling scale Volts b Volfs/ electron... 

Element Pauling Sandersan x% Allred-Rachow v d Allen p Orbital hybrid Pauling Volts b Volts/ electron... 

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