1.2- Alkadienes also

These compounds, are known as diolefins, triolefins, etc., also. The alkadienes also are commonly called dienes. The dienes have the general formula CnH2n-2 which indicates a higher degree of unsaturation than the alkenes. 

Alkadienes also have structural isomers as well as ordinary alkenes (Table 4). 

Alkadienes, alkatrienes, and alkatetraenes (poly-enes). These are unsaturated aliphatic hydrocarbons containing two, three, or four C = C double bonds, respectively. Alkadienes are also called diolefins or dienes, and alkatrienes are also known as triolefins or tricncs. Alkenes containing multiple double bonds fall under the general class of poly-enes. Double bonds that alternate with single bonds in a straight chain are said to be conjugated. Examples are... 

Alkynes, moving down the alltane and alkene protocol, and with the formula, CnH2n-2> Have a triple bond between two carbons, such as ethyne, CH CH or bucyne, CH3-CSC-CH3. Confusing the protocol are the dienes, which also have formula, CnH2n-2j but have two double bonds in their molecules such as butadiene (CH2=CH—CH=CH2) or pentadiene, but hardly anyone uses the archaic word alkadiene. 

It is generally agreed that the kinetics and the distributions of deuter-ated products from the reactions of alkynes or alkadienes with deuterium are satisfactorily interpreted in terms of the consecutive addition of two hydrogen atoms, of unspecified origin, to the adsorbed hydrocarbon to yield the monoolefin. The identity of the distributions of deuteroethyl-enes from the reaction of acetylene with equilibrated and non-equil-ibrated hydrogen—deuterium mixtures also provides strong evidence for such a mechanism [91]. 

Conjugated dienes also undergo photochemical cycloaddition reactions. Related thermal cycloadditions of alkadienes have been discussed in Sections 13-3A, 21-10A, and 21-10D, but the thermal and photochemical reactions frequently give different cyclic products. Butadiene provides an excellent example of the differences ... 

The same transition metal systems which activate alkenes, alkadienes and alkynes to undergo nucleophilic attack by heteroatom nucleophiles also promote the reaction of carbon nucleophiles with these unsaturated compounds, and most of the chemistry in Scheme 1 in Section 3.1.2 of this volume is also applicable in these systems. However two additional problems which seriously limit the synthetic utility of these reactions are encountered with carbon nucleophiles. Most carbanions arc strong reducing agents, while many electrophilic metals such as palladium(II) are readily reduced. Thus, oxidative coupling of the carbanion, with concomitant reduction of the metal, is often encountered when carbon nucleophiles arc studied. In addition, catalytic cycles invariably require reoxidation of the metal used to activate the alkene [usually palladium(II)]. Since carbanions are more readily oxidized than are the metals used, catalysis of alkene, diene and alkyne alkylation has rarely been achieved. Thus, virtually all of the reactions discussed below require stoichiometric quantities of the transition metal, and are practical only when the ease of the transformation or the value of the product overcomes the inherent cost of using large amounts of often expensive transition metals. 

The photocycloaddition of cyclic and acyclic 1,3-alkadienes or furans to aromatic rings has been shown in some cases. Not only symmetrically allowed (4 + 4) photocycloaddition but also (4 + 2) photocycloaddition to benzene, naphthalene, and anthracene rings have been reported. These results are also shown in the following subsections. 

Catalyzed hydrogenation of alkynes, alkenynes, and alkadienes.1 This catalyst effects highly c/s-selective hydrogenation of triple bonds of alkynes and alkenynes, with easy recovery of the complex by filtration. It also effects only 1,2-addition in hydrogenation of even hindered 1,4-substituted 1,3-butadienes. 

Derivatives of benzene can also be named as phenyl derivatives of alkanes, alkenes, alkadienes, alkynes, etc. Examples of these two types of names are shown below. 

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