Carbon double bond

Rule 2. When atoms attached directly to a double-bonded carbon have the same priority, the second atoms are considered and so on, if necessary, working outward once again from the double bond or chiral center. For example, in l-chloro-2-methylbutene, in CH3 the second atoms are H, H, H and in CH2CH3 they are C, H, H. Since carbon has a higher atomic number than hydrogen, the ethyl group has the next highest priority after the chlorine atom. 

The chemistry of propylene is characterized both by the double bond and by the aHyUc hydrogen atoms. Propylene is the smallest stable unsaturated hydrocarbon molecule that exhibits low order symmetry, ie, only reflection along the main plane. This loss of symmetry, which implies the possibiUty of different types of chemical reactions, is also responsible for the existence of the propylene dipole moment of 0.35 D. Carbon atoms 1 and 2 have trigonal planar geometry identical to that of ethylene. Generally, these carbons are not free to rotate, because of the double bond. Carbon atom 3 is tetrahedral, like methane, and is free to rotate. The hydrogen atoms attached to this carbon are aUyflc. 

The dipole moments are very similar in magnitude but differ slightly in direction, depending on the location of the in-plane hydrogen atom (attached to the non-double bonded carbon). ... 

Here are the complete results for the vinyl series (the double-bonded carbons are center numbers 1 and 2 in all cases) ... 

Carbon atom at the head of the double bond. Carbon atom on the other end of the double bond. Third carbon atom. 

Next, we ll specify the hydrogens on Cl and C2. The bond angles formed with the double-bonded carbons and each of these hydrogens is 120°. We ll pick simple dihedral angles for each of them ... 

For the Birch reduction of mono-substituted aromatic substrates the substituents generally influence the course of the reduction process. Electron-donating substituents (e.g. alkyl or alkoxyl groups) lead to products with the substituent located at a double bond carbon center. The reduction of methoxybenzene (anisole) 7 yields 1-methoxycyclohexa-1,4-diene 8 ... 

Step 2 Number the carbon atoms in the chain. Begin at the end nearer the double bond or, if the double bond is equidistant from the two ends, begin at the end nearer the first branch point. This rule ensures that the double-bond carbons receive the lowest possible numbers. 

The lack of rotation around carbon-carbon double bonds is of more than just theoretical interest it also has chemical consequences. Imagine the situation for a disubstifitted alkene such as 2-butene. Disubstitilted means that two substituents other than hydrogen are bonded to the double-bond carbons.) The two methyl groups in 2-bulene can be either on the same side of the double bond or on opposite sides, a situation similar to that in disubstitutecl cycloalkanes (Section 4.2). 

Cis-trans isomerism is not limited to d/substituted alkenes. It can occur whenever both double-bond carbons are attached to two different groups. If one of the double-bond carbons is attached to two ick-ntical groups, however, then cis-trans isomerism is not possible (Figure 6.4). 

Rule 1 Considering the double-bond carbons separately, look at the two atoms directly attached to each and rank them according to atomic number. An atom with higher atomic number receives higher priority than an atom with lower atomic number. Thus, the atoms commonly found attached to a double bond are assigned the following order. Note that when different isotopes of the same element are compared, such as deuterium (2H) and pro tin m ( H), the heavier isotope receives priority over the lighter isotope. 

Rule 2 If a decision can t be reached by ranking the first atoms in the substituent, look at the second, third, or fourth atoms away from the double-bond carbons until the first difference is found. A — CH2CH3 substituent and a -CH3 substituent are equivalent by rule 1 because both have carbon as the first atom. By rule 2, however, ethyl receives higher priority than methyl because ethyl has a carbon as its highest second atom, while methyl has only hydrogen as its second atom. Look at the following examples to see how the Tule works ... 

Look at the two substituents connected to each double-bond carbon, and determine their priorities using the Cahn-lngold-Prelog rules. Then see whether the two high-priority groups are on the same or opposite sides of the double bond. 

When both double-bond carbon atoms have the same degree of substitution, a mixture of addition products results. 

The twofold dehydrohalogenation takes place through a vinylic halide intermediate, which suggests that vinylic halides themselves should give alkynes when treated with strong base. (Recall A vinylic substituent is one that is attached to a double-bond carbon.) This is indeed the case. For example ... 

According to Zaitsev s rule, formulated in 1875 by the Russian chemist Alexander Zaitsev, base-induced elimination reactions generally (although not always) give the more stable alkene product—that is, the alkene with more alkyl substituents on the double-bond carbons. In the following two cases, for example, the more highly substituted alkene product predominates. 

In all the preceding cases, the double or triple bond of the dienophile is next to the positively polarized carbon of an electron-withdrawing substituent. Electrostatic potential maps show that the double-bond carbons are less negative in these substances than in ethylene (Figure 14.8). 

Figure 14.8 Electrostatic potential maps of ethylene, propenal, and propenenitrile show that electron-withdrawing groups make the double-bond carbons less negative.

Cahn-Ingold-Prelog sequence rules (Sections 6.5, 9.5) A series of rules for assigning relative priorities to substituent groups on a double-bond carbon atom or on a chirality center. 

Vinylic (Section 8.3) A term that refers to a substituent at a double-bond carbon atom. For example, chloroethylene is a vinylic chloride, and enols are vinylic alcohols. 

The CH3 carbon is sp3 the double-bond carbons are sp2 the C=C—C and C=C-H bond angles are approximately 120° other bond angles are near 109°,... 

Geometric, or cis-trans, isomerism is common among alkenes. It occurs when both of the double-bonded carbon atoms are joined to two different atoms or groups. The other two structural isomers of C4H8 shown under (1) on page 597 do not show cis-trans isomerism. In both cases the carbon atom at the left is joined to two identical hydrogen atoms. 

Write structures for all the structural isomers of compounds with the molecular formula C4H6ClBr in which Cl and Br are bonded to a double-bonded carbon. 

The ability of C—C to serve as a neighboring group can depend on its electron density. When the strongly electron-withdrawing CF3 group was attached to a double bond carbon of 37, the solvolysis rate was lowered by a... 

The acid-catalyzed hydrolysis of enol esters (RCOOCR =CR) can take place either by the normal Aac2 mechanism or by a mechanism involving initial protonation on the double-bond carbon, similar to the mechanism for the hydrolysis of enol ethers given in 10-6, ° depending on reaction conditions. In either case, the products are the carboxylic acid RCOOH and the aldehyde or ketone R2" CHCOR. ... 

However, a number of examples have been found where addition of bromine is not stereospecifically anti. For example, the addition of Bf2 to cis- and trans-l-phenylpropenes in CCI4 was nonstereospecific." Furthermore, the stereospecificity of bromine addition to stilbene depends on the dielectric constant of the solvent. In solvents of low dielectric constant, the addition was 90-100% anti, but with an increase in dielectric constant, the reaction became less stereospecific, until, at a dielectric constant of 35, the addition was completely nonstereospecific.Likewise in the case of triple bonds, stereoselective anti addition was found in bromination of 3-hexyne, but both cis and trans products were obtained in bromination of phenylacetylene. These results indicate that a bromonium ion is not formed where the open cation can be stabilized in other ways (e.g., addition of Br+ to 1 -phenylpropene gives the ion PhC HCHBrCH3, which is a relatively stable benzylic cation) and that there is probably a spectrum of mechanisms between complete bromonium ion (2, no rotation) formation and completely open-cation (1, free rotation) formation, with partially bridged bromonium ions (3, restricted rotation) in between. We have previously seen cases (e.g., p. 415) where cations require more stabilization from outside sources as they become intrinsically less stable themselves. Further evidence for the open cation mechanism where aryl stabilization is present was reported in an isotope effect study of addition of Br2 to ArCH=CHCHAr (Ar = p-nitrophenyl, Ar = p-tolyl). The C isotope effect for one of the double bond carbons (the one closer to the NO2 group) was considerably larger than for the other one. ... 

For carbenes or carbenoids of the type R—C—R there is another aspect of stereochemistry. When these species are added to all but symmetrical alkenes, two isomers are possible, even if the four groups originally on the double-bond carbons maintain their configurations ... 

The Lewis structure shows that methyl methaciylate has the formula C5 Hg O2, with 40 valence electrons. You should be able to verily that the two CH3 groups have. s -hybridized carbons, the inner oxygen atom is s hybridized, the outer oxygen atom uses 2 p atomic orbitals, and the three double-bonded carbons are s p hybridized. These assignments lead to the following inventory of a bonds and inner-atom lone pairs ... 

These observations are explainable by a pathway in which one end of a bromine molecule becomes positively polarised through electron repulsion by the n electrons of the alkene, thereby forming a n complex with it (8 cf. Br2 + benzene, p. 131). This then breaks down to form a cyclic bromonium ion (9)—an alternative canonical form of the carbocation (10). Addition is completed through nucleophilic attack by the residual Br (or added Ye) on either of the original double bond carbon atoms, from the side opposite to the large bromonium ion Br , to yield the meso dibromide (6) ... 

Y = NMe3) stated that alkene will predominate which has least alkyl substituents on the double bond carbons , i.e. (37) above (b) Saytzev (1875 working on RBr compounds, i.e. Y = Br) stated that alkene will predominate which has most alkyl substituents on the double bond carbons , i.e. (38) above. Both generalisations are valid as the figures quoted above indicate. It is thus clear that the composition of the alkene mixture obtained on elimination is influenced by Y, the nature of the leaving group, and an explanation is required about how this influence may be exerted. 

Double bonds, carbon-carbon, electrophilic bromination of structure, solvent and mechanism, 28, 171... 

Substitution on the double-bond carbon a to the oxygen increased the difficulty of the reaction, and formyl attachment occurred exclusively on the /3-carbon [Eq. (42)]. Thus, these substituted olefins also followed the rule of Keulemans (49). 

Rosenthal and co-workers (91, 92) studied the cobalt hydroformylation of various unsaturated carbohydrates. As with other a,j8-unsaturated ethers, addition of the formyl group occurred almost exclusively at the double-bond carbon a to the oxygen. High yields of product were obtained, but hydrogenation to alcohol was facile, even under mild conditions, as noted in Eq. (43) ... 

A recent approach used by Heimbach regards each C—C coupling process as a heteroring closure to which Woodward-Hoffmann rules can be applied. Regioselectivity in cyclooligomerization can be predicted on the basis of the least electron density in the LUMO of the double-bond carbon atoms of an inserting olefin (6). 

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