Pentene isomers, structures

Problem 6.7 Supply structural formulas and systematic names for all pentene isomers including stereoisomers. 

The 300 MHz H NMR and 20 MHz 13C NMR spectra of poly(4-methyl-l-pentene) have been found to be more complex than the corresponding spectra of poly(3-methyl-l-butene) due to the presence of an additional isomer structure in the polymer. Investigation of the 20 MHz 13C NMR spectrum of the polymer has indicated that placement of units in different triad sequences is die cause of multiple methyl proton resonances which have been observed in the H NMR spectra of poly(3-methyl-l-butene) and poly(4-methyl-l-pentene). The use of a computer program for simulating and plotting spectra has enabled measurements of polymer composition to be made of poly(4-methyl-l-pentene) s prepared under a variety of synthesis conditions. 

In discussing processes in olefins, it is convenient to divide the reactions into two classes simple particle transfer and carbon-addition reactions. The relative importance of these two types of reaction is also dependent on the structure of the reactants. The presence of the isobutene type structure (CH2=C(CH3)CH2—) in either the reactant ion or neutral molecule favours the simple particle transfer reaction, mainly because of the large cross-section for the formation of parent-plus-one ions. The 2-butene neutral molecule is more like isobutene than 1-butene with regard to the relative importance of simple particle transfer and carbon addition reactions [284]. However, the magnitude of the cross-section for simple particle transfer reactions in 2-butene (2-P + 2-M reaction) is much closer to that in 1-butene (1-P + 1-M reaction) than to that in isobutene (iso-P + iso-M reaction) [284, 299]. The same is true for pentene isomers i.e. the cross-sections for simple particle transfer reactions in 1-pentene and 2-pentenes are almost the same and are much lower than that in 2-methyl-l-butene. The simple particle transfer cross-sections for other branched pentenes, i.e. 2-methyl-2-butene and 3-methyl-l-butene, are even smaller than those for 1- and 2-pentenes, while the proportion of transfer reactions is higher than the corresponding proportions for 1- and 2-pentenes. The proportion is, of course, highest for 2-methyl-l-butene which has the isobutene type structure. 

With pentene isomers, the carbon addition products are mainly Cg, C7 and Cg ions [132, 290]. However, Cg ions are absent in branched pentenes except for 2-methyl-l-butene where Cg ions constitute more than 10% of the total product ions at 1.7 eV ion exit energy [132]. No Cg ions are observed in any of the isomers [132, 290]. The photoionization study by Koyano et al. [132] reports an observation of a very small concentration of dimeric ion(s) (Cj 0 ion(s)) with all of the pentene isomers. With hexenes, the main carbon addition products are C7, Cg and C9 ions [290], the relative importance of these ions being dependent on the structure. The relative probabilities of these carbon addition reactions... 

Identify each of the following pairs as structural isomers, geometrical isomers, or not isomers (a) butane and cyclobutane (b) cyclopentane and pentene ... 

Two structural isomers can result when hydrogen bromide reacts with 2-pentene. (a) Write their structural formulas, (b) What name is given to this type of reaction ... 

With decreasing pressure two principal effects are to be noted. Retention of stereospecificity is diminished in the products 1,1-dimethylcyclopropane and pentene-2, and the proportions of 2-methylbutene-2, 2-methylbutene-l, and, to a slight extent, pentene-2 increase, largely at the expense of the cis addition isomer of 1,2-dimethylcyclopropane. These are the results to be expected from geometric and structural isomerization of the initial products. The thermal isomerization of cis-... 

Not readily reconcilable with these models is the finding by Calderon and coworkers, who observed that cw-4-methyl-2-pentene, a hindered alkene, underwent a faster isomerization than metathesis21 92 (formation of 40% trans isomer at 4% conversion). The most striking feature of the transformation is the highly stereoselective formation of trans-2,5-dimethyl-3-hexene throughout the course of reaction regardless of the structure of the starting alkene ... 

The rate of H addition to 1-pentene is roughly equal to the addition to 1-butene, of the H addition to cis and trans isomers of 2-pentene as to cis-and frans-butenes. Cycloalkenes add a H atom in a similar way to simple alkenes of comparable structure. H attacks either the terminal or the internal C atom of 1,3-butadiene the first way predominates, probably due to allylic or hyperconjugative stabilization of the generated radical. 

A mixture of 0.55 g (1.8 mmol) of phenylselenyl triflate and 0.18 g (1.8 mmol) of 3-methyl-4-penten-l-ol (5) in 32 mL of CH2C12 is stirred at 0°C for 15 min. After dilution with 50 mL of Et20, the organic layer is washed with sat. brine. Drying over MgS04 and removal of the solvent leaves a residue which is purified by column chromatography (silica gel, petroleum cther/EtOAc 9 1) yield 0.369 g (79%) d.r. (trans lets) 80 20. The structures of the trans- and cw-isomers are confirmed by differential NOE 1H NMR at 500 MHz. 

Tae-en, the amine analogue of edta, also called penten, originally synthesised and studied by Schwarzenbach et al. (130, 131) is able to form only a single geometric isomer when acting as a sexadentate ligand coordinated to the Co(III) ion. It exists as a pair of enantiomers. Its absolute configuration was determined by Muto et al. (132). The X-ray structure established that the (+>589-[Co(tae-en) complex is... 

Cis-trans isomers (also called geometric isomers) result from restricted rotation (Section 3.4). Restricted rotation can be caused either by a double bond or by a cyclic structure. As a result of the restricted rotation about a carbon-carbon double bond, an alkene such as 2-pentene can exist as cis and trans isomers. The cis isomer has the hydrogens on the same side of the double bond, whereas the trans isomer has the hydrogens on opposite sides of the double bond. 

Of particular commercial interest, in connection with the effect of the olefin structure on the nature of the products of the Oxo reaction, is the behavior of diisobutylene. Diisobutylene is a mixture of 2,4,4-trimethyl-pentene-2 (I) and 2,4,4-trimethylpentene-l (II). Only one product results from treatment of the mixture of isomers ... 

In this problem, 1-methylcyclopentene, an alkene, is the starting product. This must be converted into the alcohol so that it may be then transformed into the sodium alkoxide for use in the Williamson synthesis. (One does not want to convert 1-methylcyclo-pentene into an alkyl halide because a secondary or tertiary structure would result, and this is not very efficient, as mentioned previously.) Now, there exist a number of ways to synthesize an alcohol from an alkene. The method of choice, however, is the one that provides us with the required stereochemistry. Recall that the final product must be a trans isomer. If hydroboration-oxidation of the alkene is used, addition occurs in a cis fashion and the trans isomer will be the net result. That is, the -H and -OH add trans to the methyl group. 

Evaluate A certain reaction yields 80% frans-2-pentene and 20% c/s-2-pentene. Draw the structures of these two geometric isomers, and develop a hypothesis to explain why the isomers form in the proportions cited. 

Problem 12.46. Which of the following exist as a pair of cis and Irons isomers Show structures of the cis and trans isomers, (a) 4-Methyl-2-pentene, ib) 2,4-dimethyl-2-pentene, (c) 4-methyl-2-pentyne, (dichloro-cyclopropane, (e) 1,2-dichlorocyclopropane. 

A number of unexplained factors warrant mention. Orientation of elimination differs for secondary and tertiary structures. The peculiar predominance of cis- rather than /ra/ii-olefin may arise from the relative stabilities of the proton-olefin complexes. but a more certain conclusion would be possible if the stereochemistry of the dehydration in the acyclic series had been determined. Assumption of the anti stereospecificity known to be favoured by the cyclohexyl systems may be unsound especially in the light of the recent stereochemical findings in base-catalysed elimination reactions (Section 2..1.1(e)). The solution of the problem of the cis/trans ratios may lie in the duality of mechanism, namely the syn-clinallanti complexity. Certainly recent results on the dehydration of threo- and eo t/iro-2-methyl-4-deutero-3-pentanols on thoria show syn-clinal rather than anti stereospecificity as indicated by deuterium analysis of the cis- and /rn/iJ-4-methyl-2-pentenes, but in these cases the trans isomer was formed in a three-fold excess over the m-olefin . Of course, the dehydration reactions on the less acidic thoria may not be good models for alumina but a knowledge of stereochemistry in the acyclic series might prove an invaluable aid in the elucidation of the mechanism. There is obviously plenty of scope for future kinetic investigations which at the moment sadly lag behind preparative studies. 

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