Triple bond location

A unique odd-even effect of the monomers in the diyne polycyclotrimer-ization was observed. In the aliphatic diynes with an odd number of methylene spacers, their triple bonds locate in the same side, which facilitates the back-biting reaction (Scheme 21). In contrast, in the diynes with an even number of methylene units, their triple bonds locate in the opposite sides these unfavorable positions frustrate the back-biting reaction. Consequently, hb-P32(4) possessed triple bond residues in its final structure, whereas its... 

Alkynes are hydrocarbons that contain a carbon-carbon triple bond. A triple bond can be thought of as a cylinder of electrons wrapped around the a bond. The functional group suffix of an alkyne is yne. A terminal alkyne has the triple bond at the end of the chain an internal alkyne has the triple bond located elsewhere along the chain. Internal alkynes, with two alkyl substituents bonded to the sp carbons, are more stable than terminal alkynes. We now have seen that alkyl groups stabilize alkenes, alkynes, carbo-cations, and alkyl radicals. 

A preliminary idea about the acceleration effect of the surface Pd atoms on the hydration of acrylonitrile with Cu catalysts is thought to be as follows. The coordination of the C—C double bond of the acrylonitrile to the Pd atom in the bimetallic nanoclusters makes the C—N triple bond locate close to the Cu species, facilitating the hydration catalyzed by the Cu species. This is a good example of the ensemble effect of bimetallic nanoclusters. The electronic effect of the neighboring atoms in the bimetallic nanoclusters upon the catalytic activity may also be important in this process. 

A terminal alkyne has the triple bond at the end of the chain an internal alkyne has the triple bond located elsewhere along the chain. 

Molecular models such as the one shown often do not explicitly show double and triple bonds Write a Lewis structure for this hydrocarbon showing the location of any multiple bonds Specify the hybndization state of each carbon (You can view this model in more detail on Learn mg By Modeling)... 

The first step in the addition of an electrophile such as HBr to an alkyne involves protonation and subsequent formation of an intermediate vinyl cation. Where does propyne protonate Compare energies of 1-methylvinyl and 2-methylvinyl cations. Which is more stable Why Measure CC bond distance in the more stable cation. Does the cation incorporate a full triple bond (as in propyne) or a double bond (as in propene). Examine atomic charges and electrostatic potential maps to locate the positive charge in the two cations. Is the more stable ion the one in which the charge is better delocalized Use the charges together with information about the ions geometry to draw Lewis structures (or a series of Lewis structures) for 1-methylvinyl and 2-methylvinyl cations. 

Intramolecular addition of the amide group to the triple bond in pyrazoles is more difficult, and results in closure of the 5-lactam rather than the y-lactam ring. The reaction time of the 4-phenylethynylpyrazole-3-carboxylic acid amide under the same conditions is extended to 42 h (Scheme 129) (Table XXVII). The cyclization of l-methyl-4-phenylethynyl-l//-pyrazole-3-carboxylic acid amide, in which the acetylene substituent is located in the 7r-electron-rich position of the heterocycle, is the only one complete after 107 h (Scheme 130) (90IZV2089). 

The VSEPR model is readly extended to species in which double or triple bonds are present A simple principle applies Insofar as molecular geometry is concerned, a multiple bond behaves like a single bond. This makes sense. The four electrons in a double bond, or the six electrons in a triple bond, must be located between the two atoms, as are the two electrons in a single bond. This means that the electron pairs in a multiple bond must occupy the same region of space as those in a single bond. Hence the extra electron pairs in a multiple bond have no effect on geometry. 

In Section 7.2, we saw that insofar as geometry is concerned, a multiple bond acts as if it were a single bond. In other words, the extra electron pairs in a double or triple bond have no effect on the geometry of the molecule. This behavior is related to hybridization. The extra electron pairs in a multiple bond (one pair in a double bond, two pairs in a triple bond) are not located in hybrid orbitals. 

In cases where a double or triple bond is located a to the carbanionic carbon, the ion is stabilized by resonance in which the unshared pair overlaps with the 71 electrons of the double bond. This factor is responsible for the stability of... 

As in the case of the base-catalyzed reaction, the thermodynamically most stable alkene is the one predominantly formed. However, the acid-catalyzed reaction is much less synthetically useful because carbocations give rise to many side products. If the substrate has several possible locations for a double bond, mixtures of all possible isomers are usually obtained. Isomerization of 1-decene, for example, gives a mixture that contains not only 1-decene and cis- and franj-2-decene but also the cis and trans isomers of 3-, 4-, and 5-decene as well as branched alkenes resulting from rearrangement of carbocations. It is true that the most stable alkenes predominate, but many of them have stabilities that are close together. Acid-catalyzed migration of triple bonds (with allene intermediates) can be accomplished if very strong acids (e.g., HF—PF5) are used. If the mechanism is the same as that for double bonds, vinyl cations are intermediates. 

First we need to locate the part of the molecule where resonance is an issue. Remember that we can push electrons only from lone pairs or bonds. We don t need to worry about all bonds, because we can t push an arrow from a single bond (that would violate the first commandment). So we only care about double or triple bonds. Double and triple bonds are called pi bonds. So we need to look for lone pairs and pi bonds. Usually, only a small region of the molecule will possess either of these features. 

The first and most prominent source is known as thermal NO or Zeldovich-NO. The label thermal refers to the high temperatures required to break the N2 triple bond in its reaction with O atom and its location of appearance in a flame. 

The lower number of the two carbon atoms of the triple bond is used to designate the location of the triple bond. 

It has been shown that although the positive charge is located on cation, this structure contributes more than that containing a triple bond. 

Step 4 Find the prefix Name each branch as an alkyl group, and give it a position number. If more than one branch is present, write the names of the branches in alphabetical order. Put the position number of any double or triple bonds after the position numbers and names of the branches, just before the root. This is the prefix. Note Use the carbon atom with the lowest position number to give the location of a double or triple bond. 

Pepe, C. Dif, K. The Use of Ethanethiol to Locate the Triple Bond in Alkynes and the Double Bond in Substituted Alkenes by Gas Chromatography/Mass Spectrometry. Rapid Commun. Mass Spectrom. 2001, 75,97-103. 

From work performed in 1983 by Burnier and Jorgensen [15], the following ab initio calculations for the HOMO and LUMO energies of the synthons were developed. The function n(x, parent) returns the number of atoms of type x in the parent. This function is abbreviated below as simply n(x) where the parent is understood. The symbols UU, O, N, S represent triple bonds, oxygen, nitrogen, and sulfer, respectively. The subscripts c and t denote central and terminal locations respectively in the parent for the elements which they modify. For brevity, the terms diene-synthon and dienophile-synthon will be replaced with diene and dienophile respectively. 

Because of their relatively large size, the heterocycles covered in this section display a natural tendency to possess heavily buckled, frequently flexible frames except in cases where bridging and/or strategically located unsaturation, such as the presence of triple bonds and/or trans double bonds, impart a certain degree of skeletal rigidity which quite often results in substantial molecular flattening. 

Simple acyl cations RCO+ have been prepared45 in solution and the solid state.40 The acetyl cation CH3CO is about as stable as the f-butyl cation (see, for example, Table 5.1). The 2,4,6-trimethylbenzoyl and 2,3,4,5,6-pentamethylbenzoyl cations are especially stable (for steric reasons) and are easily formed in 96% H2S04.47 These ions are stabilized by a canonical form containing a triple bond (G), though the positive charge is principally located on the carbon,48 so that F contributes more than G. 

Species such as 5 and 6 are called benzynes (sometimes dehydrobenzenes), or more generally, arynes, and the mechanism is known as the benzyne mechanism. Benzynes are very reactive. Neither benzyne nor any other aryne has yet been isolated under ordinary conditions,34 but benzyne has been isolated in an argon matrix at 8 K,35 where its ir spectrum could be observed. In addition, benzynes can be trapped e.g., they undergo the Diels-Alder reaction (see 5-47). It should be noted that the extra pair of electrons does not affect the aromaticity. The original sextet still functions as a closed ring, and the two additional electrons are merely located in a tt orbital that covers only two carbons. Benzynes do not have a formal triple bond, since two canonical forms (A and B) contribute to the hybrid. 

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