Head to tail

At a fixed distance r, the angular factor in equation (A 1.5.12) leads to the greatest attraction when the dipoles are lined up in a linear head-to-tail arrangement, 9 whereas the Imear tail-to-tail geometry, 0 =... 

Ti,0g = 0, is the most repulsive. A head-to-tail, parallel arrangement, 0 = 0g = is attractive but less... 

Example Anoth er example of Iron tier orbital theory uses the reaction ol phenyl-butadiene with ph en ylethylene. This reaction is a [4-1-21 pericyclic addition to form a six-membered ring. It could proceed with the two phenyl rings close to each other (head to head) or further away front each other (head to tail). 

The dimerization of isoprene is possible, but the reaction of isoprene is slower than that of butadiene. Dimerization or telomerization of isoprene, if carried out regioselectively to give a tail-to-liead dimer 18 or a head-to-tail... 

The reaction of isoprene with MeOH catalyzed by Pd(acac)2 and Ph3P is not regioselective, giving a mixture of isomers[37]. However, l-methoxy-2,6-dimethyl-2,7-octadiene (35), the head-to-tail dimer, was obtained in 80% yield, accompanied by the tail-to-tail dimer (15%) using 7r-allylpalladium chloride and BU3P. On heating, 35 was converted into 2.6-dimethyl-1,3,7-octatriene (36) by an elimination reaction[38]. 

Formic acid behaves differently. The expected octadienyl formate is not formed. The reaction of butadiene carried out in formic acid and triethylamine affords 1,7-octadiene (41) as the major product and 1,6-octadiene as a minor product[41-43], Formic acid is a hydride source. It is known that the Pd hydride formed from palladium formate attacks the substituted side of tt-allylpalladium to form the terminal alkene[44] (see Section 2.8). The reductive dimerization of isoprene in formic acid in the presence of Et3N using tri(i)-tolyl)phosphine at room temperature afforded a mixture of dimers in 87% yield, which contained 71% of the head-to-tail dimers 42a and 42b. The mixture was treated with concentrated HCl to give an easily separable chloro derivative 43. By this means, a- and d-citronellol (44 and 45) were pre-pared[45]. 

The reaction of isoprene with CO2 in the presence of oraanotin ethoxide and DBU by the use of dicyclohexyl( 3-pyridylethyl)phosphine (106) affords the isomeric esters 107 and 108 by head-to-tail and tail-to-tail dimerizations. Tin ethoxide forms tin carbonate, which seems to be an effective carrier of CO2[100]. 

The structural feature that distinguishes terpenes from other natural products is the iso prene unit The carbon skeleton of myrcene (exclusive of its double bonds) corresponds to the head to tail union of two isoprene units... 

The German chemist Otto Wallach (Nobel Prize m chemistry 1910) determined the structures of many terpenes and is credited with setting forth the isoprene rule ter penes are repeating assemblies of isoprene units normally joined head to tail... 

Although the term terpene once referred only to hydrocarbons current usage includes functionally substituted derivatives as well grouped together under the general term isoprenoids Figure 26 6 (page 1086) presents the structural formulas for a number of representative examples The isoprene units m some of these are relatively easy to identify The three isoprene units m the sesquiterpene farnesol, for example are mdi cated as follows m color They are joined m a head to tail fashion... 

Many terpenes contain one or more rings but these also can be viewed as collec tions of isoprene units An example is a selmene Like farnesol it is made up of three isoprene units linked head to tail... 

Tail to tail linkages of isoprene units sometimes occur especially m the higher terpenes The C(12)—C(13) bond of squalene unites two C15 units m a tail to tail man ner Notice however that isoprene units are joined head to tail within each C15 unit of squalene... 

Generally polymers involve bonding of the most substituted carbon of one monomeric unit to the least substituted carbon atom of the adjacent unit in a head-to-tail arrangement. Substituents appear on alternate carbon atoms. Tacticity refers to the configuration of substituents relative to the backbone axis. In an isotactic arrangement, substituents are on the same plane of the backbone axis that is, the configuration at each chiral center is identical. 

Structures [VI] and [VII], respectively, are said to arise from head-to-tail or head-to-head orientations. In this terminology, the substituted carbon is defined to be the head of the molecule, and the methylene is the tail. Tail-to-tail linking is also possible. The term orienticity is also used to describe positional isomerism. 

For most vinyl polymers, head-to-tail addition is the dominant mode of addition. Variations from this generalization become more common for polymerizations which are carried out at higher temperatures. Head-to-head addition is also somewhat more abundant in the case of halogenated monomers such as vinyl chloride. The preponderance of head-to-tail additions is understood to arise from a combination of resonance and steric effects. In many cases the ionic or free-radical reaction center occurs at the substituted carbon due to the possibility of resonance stabilization or electron delocalization through the substituent group. Head-to-tail attachment is also sterically favored, since the substituent groups on successive repeat units are separated by a methylene... 

The free-radical polymerization of acrylic monomers follows a classical chain mechanism in which the chain-propagation step entails the head-to-tail growth of the polymeric free radical by attack on the double bond of the monomer. 

Some details of the chain-initiation step have been elucidated. With an oxygen radical-initiator such as the /-butoxyl radical, both double bond addition and hydrogen abstraction are observed. Hydrogen abstraction is observed at the ester alkyl group of methyl acrylate. Double bond addition occurs in both a head-to-head and a head-to-tail manner (80). 

Replacement of Labile Chlorines. When PVC is manufactured, competing reactions to the normal head-to-tail free-radical polymerization can sometimes take place. These side reactions are few ia number yet their presence ia the finished resin can be devastating. These abnormal stmctures have weakened carbon—chlorine bonds and are more susceptible to certain displacement reactions than are the normal PVC carbon—chlorine bonds. Carboxylate and mercaptide salts of certain metals, particularly organotin, zinc, cadmium, and antimony, attack these labile chlorine sites and replace them with a more thermally stable C—O or C—S bound ligand. These electrophilic metal centers can readily coordinate with the electronegative polarized chlorine atoms found at sites similar to stmctures (3—6). 

Any of the four monomer residues can be arranged in a polymer chain in either head-to-head, head-to-tail, or tail-to-tail configurations. Each of the two head-to-tail vinyl forms can exist as syndiotactic or isotactic stmctures because of the presence of an asymmetric carbon atom (marked with an asterisk) in the monomer unit. Of course, the random mix of syndiotactic and isotactic, ie, atactic stmctures also exists. Of these possible stmctures, only... 

The physical properties of any polyisoprene depend not only on the microstmctural features but also on macro features such as molecular weight, crystallinity, linearity or branching of the polymer chains, and degree of cross-linking. For a polymer to be capable of crystallization, it must have long sequences where the stmcture is completely stereoregular. These stereoregular sequences must be linear stmctures composed exclusively of 1,4-, 1,2-, or 3,4-isoprene units. If the units are 1,4- then they must be either all cis or all trans. If 1,2- or 3,4- units are involved, they must be either syndiotactic or isotactic. In all cases, the monomer units must be linked in the head-to-tail manner (85). 

Propagation occurs by head-to-tail addition of monomer ... 

Terpenes are characterized as being made up of units of isoprene in a head-to-tail orientation. This isoprene concept, invented to aid in the stmcture deterrnination of terpenes found in natural products, was especially useful for elucidation of stmctures of more complex sesquiterpenes, diterpenes, and polyterpenes. The hydrocarbon, myrcene, and the terpene alcohol, a-terpineol, can be considered as being made up of two isoprene units in such a head-to-tail orientation (1). 

Chain Structure. The chemical composition of poly (vinyhdene chloride) has been confirmed by various techniques, including elemental analysis, x-ray diffraction analysis, degradation studies, and in, Raman, and nmr spectroscopy. The polymer chain is made up of vinyhdene chloride units added head-to-tail ... 

Vlayl acetate polymerises chiefly ia the usual head-to-tail fashion, but some of the monomers orient head-to-head and tail-to-tail as the chain grows. The fraction of heat-to-head addition iacreases with temperature. For example, a 1.15 mol % head-to-head stmcture and a 1.86 mol % stmcture were determined at 15°C and 110°C, respectively (86). 

Molecular Structure and Monomer Addition Orientation. The addition of vinyl monomer to a growing PVC chain can be considered to add in a head-to-tail fashion, resulting in a chlorine atom on every other carbon atom, ie,... 

Dechlorination of head-to-head, tail-to-tail stmcture can be expected to go to 100% completion. If dechlorination of head-to-tail stmcture starts at random positions, then 13.5% of the chlorine should remain at the end of reaction. Dilute solutions of PVC treated with zinc removes 87% of the chlorine, proving the head-to-tail stmcture of PVC (71). 

Microstructure. Whereas the predominate stmcture of polychloroprene is the head to tail /n7 j -l,4-chloroprene unit (1), other stmctural units (2,3,4) are also present. The effects of these various stmctural units on the chemical and physical properties of the polymer have been determined. The high concentration of stmcture (1) is responsible for crystallization of polychloroprene and for the abiUty of the material to crystallize under stress. Stmcture (3) is quite important in providing a cure site for vulcanization, but on the other hand reduces the thermal stabiUty of the polymer. Stmctures (3),(4), and especially (2) limit crystallization of the polymer. 

Epichlorohydrin Elastomers without AGE. ECH homopolymer, polyepichlorohydrin [24969-06-0] (1), and ECH—EO copolymer, poly(epichlorohydrin- (9-ethylene oxide) [24969-10-6] (2), are linear and amorphous. Because it is unsymmetrical, ECH monomer can polymerize in the head-to-head, tail-to-tail, or head-to-tail fashion. The commercial polymer is 97—99% head-to-tail, and has been shown to be stereorandom and atactic (15—17). Only low degrees of crystallinity are present in commercial ECH homopolymers the amorphous product is preferred. 

A-Substituted polypyrazoles can also be obtained by using A-alkylhydrazines, and it should be noted that these polymers consist of a random mixture of head-to-head and head-to-tail structures. Other syntheses of polypyrazoles have been described in the literature. Thus polyphenylene pyrazoles (742) and (743) occurred when m- or p-diethynyl-benzene (DEB) reacted with 1,3-dipoles such as sydnones or bis(nitrilimines) (Scheme 64). 

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