Alkenes constitutional isomers

The different elimination paths often produce different alkene constitutional isomers as products regiochemistry). For the E2 reaction the C-H bond and the C-L bond must lie in the same plane, preferably anti to each other for steric reasons. The protons, shown in bold, on either carbon adjacent to the leaving group can be in proper alignment for reaction thus different products are produced (the cis product is also formed). 

Isomeric alkenes may be either constitutional isomers or stereoisomers There is a sizable barrier to rotation about a carbon-carbon double bond which corresponds to the energy required to break the rr component of the double bond Stereoisomeric alkenes are configurationally stable under normal conditions The configurations of stereoisomeric alkenes are described according to two notational systems One system adds the prefix CIS to the name of the alkene when similar substituents are on the same side of the double bond and the prefix trans when they are on opposite sides The other ranks substituents according to a system of rules based on atomic number The prefix Z is used for alkenes that have higher ranked substituents on the same side of the double bond the prefix E is used when higher ranked substituents are on opposite sides... 

Except for the biochemical exanple just cited, the stnactures of all of the alcohols in Section 5.9 (including those in Problem 5.13) were such that each one could give only a single alkene by p elimination. What about elimination in alcohols such as 2-methyl-2-butanol, in which dehydration can occur in two different directions to give alkenes that are constitutional isomers Here, a double bond can be generated between C-1 and C-2 or between C-2 and C-3. Both processes occur but not nearly to the sane extent. Under the usual reaction conditions 2-methyl-2-butene is the major product, and 2-methyl-1-butene the minor one. 

I Rearrangement reactions occur when a single reactant undergoes isomeric product. An example is the conversion of the alkene 1-butene into its constitutional isomer 2-butene by... 

Zirconocene-catalyzed kinetic resolution of dihydrofurans is also possible, as illustrated in Scheme 6.8 [18]. Unlike their six-membered ring counterparts, both of the heterocycle enantiomers react readily, albeit through distinctly different reaction pathways, to afford — with high diastereomeric and enantiomeric purities — constitutional isomers that are readily separable (the first example of parallel kinetic resolution involving an organome-tallic agent). A plausible reason for the difference in the reactivity pattern of pyrans and furans is that, in the latter class of compounds, both olefmic carbons are adjacent to a C—O bond C—Zr bond formation can take place at either end of the C—C 7T-system. The furan substrate and the (ebthi)Zr-alkene complex (R)-3 interact such that unfavorable... 

When the same substituents are at each end of the double or triple bond, it is called symmetrical. Unsymmetrical means different substituents are at each end of the double or triple bond. Electrophilic addition of unsymmetrical reagents to unsymmetrical double or triple bonds follows Markovnikov s rule. According to Markovnikov s rule, addition of unsymmetrical reagents, e.g. HX, H2O or ROH, to an unsymmetrical alkene proceeds in a way that the hydrogen atom adds to the carbon that already has the most hydrogen atoms. The reaction is not stereoselective since it proceeds via a planar carbocation intermediate. However, when reaction proceeds via a cyclic carbocation intermediate, it produces regiospecific and stereospecific product (see below). A regioselective reaction is a reaction that can potentially yield two or more constitutional isomers, but actually produces only one isomer. A reaction in which one stereoisomer is formed predominantly is called a stereoselective reaction. 

For three parts, there are two constitutional isomers that will give the desired alcohol. For two parts, only one alkene will give the desired alcohol. 

The hydroboration of alkenes, in which the Ca=C is not symmetrically substituted can lead to constitutionally isomeric trialkylhoranes. This is because the new C—B bond can form either at the Ca or at the (A of the Ca=C double bond. In the oxidation/hydrolysis sequence that follows, constitutionally isomeric alcohols are produced. In one of them, the OH group binds to Ca and in the other it binds to C. If only one constitutional isomer of the trialkylbo-rane and consequently only one constitutional isomer of the alcohol is to he produced, the hydroboration step must take place regioselectively. Whether regioselectivity occurs is determined by steric and electronic effects. 

Note the difference between tautomers and resonance forms. Tautomers are constitutional isomers—different compounds with different structures—while resonance forms are different representations of a single structure. Tautomers have their atoms arranged differently, while resonance foims differ only in the position of their electrons. Note also that tautomers are rapUUy interconvertible. Thus, keto and enol isomers are tautomers, but alkene isomers such as 1-butene and 2-butene are not, because they don t interconvert rapidly under normal circumstances. 

For example, 2-bromo-2-methylpropane has three p carbons (three CH3 groups), but because all three are identical, only one alkene is formed upon elimination of HBr. In contrast, 2-bromobu-tane has two different p carbons (labeled pi and P2), so elimination affords two constitutional isomers by loss of HBr across either the a and pi carbons, or the a and p2 carbons We learn about which product predominates and why in Section 8.5. 

Problem 8.4 Label each pair of alkenes as constitutional isomers, stereoisomers, or identical. 

Recall from Section 8.1 that a mixture of alkenes can form from the dehydrohalogenation of alkyl halides having two or more different p carbon atoms. When this occurs, one of the products usually predominates. The m or product is the more stable product—the one with the more substituted double bond. For example, elimination of the elements of H and I from 1-iodo-1-methylcyclohexane yields two constitutional isomers the trisubstituted alkene A (the major product) and the disubstituted alkene B (the minor product). 

A reaction is regioselective when it yields predominantly or exclusively one constitutional isomer when more than one is possible. The E2 reaction is regioselective because the more substituted alkene predominates. 

When an alcohol has two or three different P carbons, dehydration is regioselective and follows the Zaitsev rule. The more substituted alkene is the major product when a mixture of constitutional isomers is possible. For example, elimination of H and OH from 2-methyl-2-butanol yields two constitutional isomers the trisubstituted alkene A as major product and the disubstituted alkene B as minor product. 

With an unsymmetrical alkene, HX can add to the double bond to give two constitutional isomers. CH,... 

Because addition converts sp hybridized carbons to s[ hybridized carbons, however, sometimes new stereogenic centers are formed from hydrohalogenation. For example, Markovnikov addition of HCl to 2-ethyl-1-pentene, an achiral alkene, forms one constitutional isomer, 3-chloro-3-methylhexane. Because this product now has a stereogenic center at one of the newly formed sp hybridized carbons, an equal amount of two enantiomers—a racemic mixture—must form. 

With unsymmetrical alkenes, two constitutional isomers are possible from addition of X and OH, but only one is formed. The preferred product has the electrophile bonded to the less sub-... 

Problem 15.22 Draw all constitutional isomers formed when each alkene is treated with NBS + hv.. . CH,CH=CHCH. b. 

An advantage in using the Wittig reaction over other elimination methods to synthesize alkenes is that you always know the location of the double bond. Whereas other methods of alkene synthesis often give a mixture of constitutional isomers, the Wittig reaction always gives a single constitutional isomer. 

When constitutional isomers are possible, the major alkene has the less substituted double bond in a Hofmann elimination. 

For example, Hofmann elimination of the elements of H and N(CH3>3 from 2-methylcyclopen-tanamine yields two constitutional isomers the disubstituted alkene A (the major product) and the trisubstituted alkene B (the minor product). 

Any H adjacent to the carbocation can be lost, often producing a different alkene. This loss is often reversible, so the El often produces an equilibrium mixture of alkenes, the most stable alkene predominating (Zaitsev s rule). When a reaction produces a mixture of different constitutional isomers, we use the term regiochemistry to refer to product ratios in that mixture. When the El produces a 1,2-disubstituted alkene, the trans stereoisomer predominates because it is less hindered, so more stable. 

Unfortunately, 4,5-dihydro-32/-pyrazoles can not only produce cyclopropanes but also tautomeric 4,5-dihydro-l//-pyrazoles (d -pyrazolines) and acyclic constitutional isomers of the cyclopropanes. This is illustrated for the 4,5-dihydro-3//-pyrazoles 11 which result from [3-1-2] cycloaddition of a-diazo esters to electron-deficient alkenes. If = H, they undergo rapid tautomerism to form 4,5-dihydro-l//-pyrazoles 12 or 13, both of which are not subsequently... 

Sections Isomeric alkenes may be either constitutional isomers or stereoisomers. 

Ans. Alkenes and cycloalkanes have the same general formula and are constitutional isomers of each other. Knowing that the molecular formula of some unknown such as C5H10 fits the general formula C H2 tells us that it is not an alkane and that it is either an alkene or a cycloalkane. The molecular formula alone does not allow us to distinguish an alkene from a cycloalkane. However, alkenes and cycloalkanes have very different chemical properties. Cycloalkanes have similar chemical behavior as alkanes since both have the same kinds of bonds (C — C and C — H single bonds). The chemistry of alkenes is very different due to the presence of the carbon-carbon double bond (Sec. 12.6). 

Ans. This question is broader than Problem 12.5 since it is not limited to alkenes. Alkenes and cycloalkanes are constitutional isomers since both follow the same general molecular formula C H2 . The answer to this question consists of the alkenes in the answer to Problem 12.5 plus the cycloalkanes ... 

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