Allene: 1,2-Propadiene

Treatment of allene (1,2-propadiene) with 2equiv. of butyllithium leads to an intermediate dilithio species which adds to ketones and aldehydes to afford homopropargylic alcohols in high yield (Table 9.3) [7]. This intermediate also reacts with geranyl chloride to afford the alkynyl coupling product uncontaminated by the allene regioisomer. 

Unlike ordinary olefins, allene (1,2-propadiene) (1) undergoes thermal dimerization to afford 1,2-dimethylenecyclobutane (2), first reported by Lebedev in 1913 [1], The yield of the dimer 2 was improved up to 95% by Dolbier, who used a benzene solution of 1 for the dimerization reaction [2]. 

The isomers of the simplest allene, 1,2-propadiene 1, are propyne 2 and cydopro-pene 3 (Scheme 1.2). Their isomerization engergies have been measured and calculated [2-4]. Compound 2 is clearly the most stable isomer, 1 lies 2.1 kj higher and 3 about 22.3 kj. Hence in principle, if reversible, thermodynamics of an equilibrium should favor the alkyne. However, several factors can influence this in two ways, i.e. a change of the relative thermodynamic stability, for example by substituents, or a... 

The behavior of allene (1,2-propadiene) and its derivatives as ligands in transition metal complexes has been investigated for more than 20 years (197,198). Allene may be coordinated by only one C=C double bond to a metal (197,199), or it may link to two metal centers as a 2 2 or 3 1 electron donor. Dinuclear /i-f/2l2-allene complexes are known for Mo (200,201), W (201), Mn (202-203), and Rh (199,204). In [Fe2(CO)7(/ -f/3 1-C3H4)] a bonding mode (205,206). In the course of the reaction, often two or more allene molecules become connected with transition metal complexes. Two allenes linked by C-2 yield tetramethyl-eneethane (207,208), which is stabilized by coordination on Fe (205,208), Ni (270), or Pd (210,211). 

The polymerisation of allene (1,2-propadiene), the simplest cumulene, is of interest not least because polyallene can be considered to be a versatile intermediate polymer blank , if formed with the chemical structure resulting from polymerisation by the scheme... 

Allene (1,2-propadiene), H2C=C=CH2, has two adjacent double bonds. What kind of hybridization must the central carbon have Sketch the bonding 7T orbitals in allene. What shape do you predict for allene ... 

If the alkene is a cumulative diene, that is, allene (1,2-propadiene [CH2=C=CH2]) or a substituted allene, addition can occur once and/or twice (Equation 9.24),... 

The decay behavior of allenes (1,2-propadienes) is quite different from that of the conjugated 1,3-dienes. Figure 8 shows the decay of ArS in cyclohexane for the reaction with methyl-substituted allenes [46]. By adding allene, the decay of ArS is accelerated even in the degassed solution, suggesting that the reaction proceeds irreversibly. Such irreversibility occurs when the incipient C atom-centered radical becomes a resonance stable allyl-type radical by rotation of the C-C bond, as shown in Scheme 9. In the aerated solution, the decay of ArS is further accelerated, indicating that the irreversibility due to the rotation is not completely established the addition of O2 further shifts the equilibrium to the peroxy radical side by trapping the incipient short-lived C atom-centered radicals. 

From a consideration of molecular orbital theory, deduce the three-dimensional structure of allene (1,2-propadiene), H2C=C=CH2. 

The photoelectron and optical spectra of allene (1,2-propadiene) have been of considerable Aeoietical and experimental interest because a significant portion of Aese spectra is not only quite conq)lex but also raAer peculiar. The molecular point group of allene in Ae ground state is D2d, wiA Ae self-consistent field (SCF) approximation described by Ae orbital occupation a hi2a Qb2Aa bile, RenK)val of an electron from Ae highest occupied nK)lecular orbital 2n (or 2e) of... 

We start with a discussion of allene (propadiene), the simplest diene of all. Its gas phase enthalpy of formation is 190.5 1.2 kJmol-1. We wish to compare this quantity with that of related monoenes. The first comparison addresses the relative stability of one and two double bonds in a 3-carbon chain. Conceptually, this may be expressed as the enthalpy of the formal reaction 9... 

Adjacent polyenes are hydrocarbons with double bonds between each pair of atoms in the polyene system they are called cumulenes2. Two types of cumulenes exist those with an even number of adjacent double bonds and those with an odd number. The former can exhibit chirality, the latter geometric isomerism. Allenes (propadienes) are the simplest members of the even-numbered type of cumulenes. 

D2d A molecule is D2d if it has a C2 axis and two perpendicular C2 axes (as for D2 above), plus two dihedral mirror planes these are mirror planes that bisect two C2 axes (in general, that bisect the C2 axes perpendicular to the principal axis). Example allene (propadiene). Staggered ethane is D3d (it has D3 symmetry elements plus three dihedral mirror planes. Dnd symmetry can be hard to spot. 

Dienes. Uosaturated organic compds of general formula CnH2n-2 containg two double bonds and belonging to the group of polyenes. Typical examples are diolefins, such as allene(propadiene), C3H4 divinyl( 1,3-butadiene), piper ylene( 1,4-pen tad iene),... 

Of the three isomeric C3H4 hydrocarbons allene (propadiene, 1), cyclopropene (2), and propyne (3), cyclopropene (2) possesses the highest heat of formation. ... 

Acrylic acid Adipic acid Allene (propadiene)... 

Allene (propadiene) [463-49-0] M 40.1, m -146°, b -32°. Freeze allene in liquid nitrogen, evacuate, then thaw out. This cycle is repeated several times, then the allene is frozen in a methylcyclohexane/liquid nitrogen bath and pumped for some time. It has also been purified by HPLC. [Cripps Kiefer Org Synth 42 12 1962,... 

Allene (propadiene) is condensed under high vacuum into a 2-litre round-bottomed flask and, after several freeze-and-pump cycles to remove residual oxygen and volatile impurities, the reaction vessel is sealed (internal pressure at room temperature approximately 1000 mb). After 20 hours at 460° C (a high-temperature drying oven) the inner surface of the flask is covered completely by a deep-black, shiny film which can be removed in patches and whose electrical conductivity is 10 S cm. When this film is doped by treating if for 30 minutes with a saturated solution of iodine in carbon tetrachloride its appearance changes to a shiny gold. After solvent removal, the conductivity is found to have increased by a factor of 10. Several other alkynes were polymerized under above conditions and the results of these experiments are summarized in Table 3.7. 

Allene (propadiene) is condensed under high vacuum into a 2-litre round bottomed flask and, after several freeze-and-pump cycles to remove residual oxygen and volatile impurities, the reaction vessel is sealed (internal pressure at room temperature approximately 1000 mbar). 

L,2-propadiene, allene, CH2=C = CH2, CjH4. Colourless gas prepared by the electrolysis of potassium itaeonate, or by the action of zinc and alcohol on 1,3-dibromopropane. It is easily isomerized to propyne (methylacetyl-ene), and is produced as a mixture with this substance from some reactions. 

The recent discovery that propadiene can be converted into a lithio compound, which is stable enough at low temperature to allow functionalization, has made it an important starting compound for the synthesis of derivatives. A fortunate circumstance is that allene can be obtained in > 30% yield in an extremely simple... 

Addition of arylhydroxylamines to electrophilic allenes such as methyl propadienoate or l-methancsulfonyl-l,2-propadiene is another route to 0-vinyl derivatives[2]. The addition step is carried out by forming the salt of the hydroxylamine using NaH and the addition is catalysed with LiO CCFj. The intermediate adducts are cyclized by warming in formic acid. Yields are typically 80% or better. 

Cumulated dienes are those m which one carbon atom is common to two carbon-carbon double bonds The simplest cumulated diene is 1 2 propadiene also called allene and compounds of this class are generally referred to as allenes... 

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... 

Chemical Designations - Synonyms Allene-methylacetylene mixture MAPP gas Methylacetylene-allene mixture Propadiene-methylacetylene mixture Chemical Formula. CH3C=CH+CH2=C=CHj Ohservahle Characteristics - Physical State (as shipped) Liquefied compressed gas Color. Colorless Odor Offensive, like acetylene. 

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