Alkene Series

Note that there is no one-carbon alkene corresponding to methane, since hydrogen can never form more than one covalent bond, and there is no other carbon atom in the structural formula. Therefore, the first compound in the alkene series is ethene, while the corresponding two-carbon compound in the alkane series, ethane, is the second compound in the series, with methane the first. 

You may recall that we discussed the bonding in ethene in Chapter 7. The double bond in ethene and other alkenes consists of a sigma bond and a pi bond. The ethene molecule is planar. There is no rotation about the double bond, since that would require breaking the pi bond. The bond angle in ethene is 120°, corresponding to sp2 hybridization about each carbon atom. The geometries of ethene and the next member of the alkene series, QHg, are shown in Figure 22.6. 

Compounds consisting of only carbon and hydrogen have the simplest compositions of all organic compounds. These compounds are called hydrocarbons. It is possible to classify the hydrocarbons into four series, based on the characteristic structures of the molecules in each series. These series are known as (1) the alkane series, (2) the alkene series, (3) the alkyne series, and (4) the aromatic series. There are many subdivisions of each series, and it is also possible to have molecules that could be classified as belonging to more than one series. 

The alkene series of hydrocarbons is characterized by having one double bond in the carbon chain. The characteristic formula for members of the series is C H2 . Since there must be at least two carbon atoms present to have a carbon-to-carbon double bond, the first member of this series is ethene, C2H4, also known as ethylene. Propene (propylene), C,Hh, and butene (butylene), C4HK, are the next two members of the series. Note that the systematic names of these compounds denote the number of carbon atoms in the chain with the name derived from that of the alkane having the same number of carbon atoms (Table 21-2). Note also that the characteristic ending -ene is part of the name of each of these compounds. 

The unsaturated open-chain hydrocarbons include the alkene or olefin series, the diene series, and the alkyne series. The alkene series is made up of chain hydrocarbons in which a double bond exists between two carbon atoms. The general formula for the series is CnH2n, where n is the number of carbon atoms. As in the paraffin series, the lower members are gases, intermediate compounds are liquids, and the higher members of the series are solids. The alkene series compounds are more active chemically than the saturated compounds. They react easily with substances such as halogens by adding atoms at the double bonds. 

The members of the alkyne series contain a triple bond between two carbon atoms in the molecule. They are very active chemically and are not found free in nature. They form a series analogous to the alkene series. The first and most important member of the series is acetylene, C2H2. 

Structure effects on the rate of selective or total oxidation of saturated and unsaturated hydrocarbons and their correlations have been used successfully in the exploration of the reaction mechanisms. Adams 150) has shown that the oxidation of alkenes to aldehydes or alkadienes on a BijOj-MoOj catalyst exhibits the same influence of alkene structure on rate as the attack by methyl radicals an excellent Type B correlation has been gained between the rate of these two processes for various alkenes (series 135, five reactants, positive slope). It was concluded on this basis that the rate-determining step of the oxidation is the abstraction of the allylic hydrogen. Similarly, Uchi-jima, Ishida, Uemitsu, and Yoneda 151) correlated the rate of the total oxidation of alkenes on NiO with the quantum-chemical index of delo-calizability of allylic hydrogens (series 136, five reactants). 

The cycloalkanes also are known as naphthenes, cycloparaffins, or alicyclic hydrocarbons. In the petroleum industry, this class of hydrocarbons is known as naphthenes. Naphthenes have saturated rings. The general formula for the ring without substituents is CnH2n. This is the same as the general formula for the alkene series however, the structural configurations differ completely and, thus, the physical and chemical properties are not at all similar. 

Alkene Series Constants from equation 4 Constants from equation 7 Experimental AHhyd (g)f... 

An admitted limitation to analysis by equation 11 is the sparsity of data. Whereas all the corresponding alkane data were available to analyse with the alkenes in equation 7, the same is not true when we attempt to pair alkene isomers to analyse by equation 11. The regression constants are naturally affected by the different-sized data sets. An alternative equation for such circumstances is equation 12, which uses the best available data in each alkene series and avoids the problem of having to eliminate non-paired alkene data. 

Scheme 2. Major MS fragmentation pattern for Al(6)-alkene series (R = H or OH).

Other alkaloids in the Al(6)-alkene series have been reported. Two isomeric alkaloids (C20H27NO3) were obtained from Cephalotaxus harringtonia K. Koch var. harringtonia. Their spectroscopic properties closely resembled those of schelhammericine except that their NMR spectra revealed the presence of two aromatic methoxyl groups in place of the methylenedioxy group of schelhammericine. The alkaloids were therefore 3-epihomo-2,7-... 

Draw the structural diagram for the third member of each homologous series. Hint The first member of the alkene series is ethene, C2H4. 

Methyl groups at the position of electrophilic attack exert exactly the same enthalpic and entropic effects as in the alkene series (Table 4), and one can summarize that the attack of carbocations at alkenes and at allylsi-lanes, allylgermanes, and allylstannanes follows the same mechanism. The differences between these classes of nucleophiles are encountered after the rate-determining step While ordinary carbocations (produced from alkenes) usually accept a chloride ion to give addition products, the /3-metal-substituted carbocations are generally demetalated to yield the Se2 products. It has been reported, however, that j8-silyl-substituted carbe-nium ions with bulky substituents at silicon may also act as chloride acceptors with the consequence that in these cases allylsilanes yield addition products in the same way as ordinary alkenes do [159],... 

The importance of steric interactions with cis hgands can be more clearly seen in alkene substituent effects. The barrier increases significantly in the PtCl(acac)(ene) series where ene = substituted ethylene compared to ene = ethylene. The m-Cl2(DMSO)Pt(alkene) series (6)-(9), studied by Boucher aud Bosuich, is particularly instructive. The rotatiou iu the ethylene begins to produce broadening about -47 °C, whereas the propene conformers broaden at —20 °C and... 

The structures, typical reactions, properties and uses of the alkane and alkene series, including substitution and addition reactions. 

A full discussion of isomerism is given in Unit 6.4 Try drawing and naming some higher members of the alkene series, say n = 6 ... 

The first molecule of the alkene series is when n - 2 and is the compound ethene C2H , because when n = 1 there cannot be a compound. There are three possible structures when C = 4 ... 

Which is the simplest compound of the alkene series in which... 

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