Acyclic alkanes structure

Despite these structural features, the adamantane ring system is not strain-free 142). Comparison of an estimated heat of formation for adamantane based on group increments derived from acyclic alkanes in completely skew-free conformations 142> with the experimentally determined value 143 indicates that adamantane is strained to the extent of 6.48 kcal/mole. 

The source of the strain in adamantane is not readily apparent but appears to be due to features present in the rigid, cage structure of the molecule. Less rigid molecules, e.g. acyclic alkanes, cyclohexane and /nms-decalin, are free to relax and to adopt conformations in which the best balance between angle, nonbonded and torsional strain is achieved. Thus, C-C-C bond angles of 112.4... 

Structures for the two simplest acyclic alkanes were given in Chapter 1. Methane, CH4, has a single carbon atom, and ethane, CH3CH3, has two. All C atoms in an alkane are surrounded by four groups, making them sp hybridized and tetrahedral, and all bond angles are 109.5°. 

We can use these same principles to determine conformations and relative energies for any acyclic alkane. Because the lowest energy conformation has aU bonds staggered and all large groups anti, alkanes are often drawn in zigzag skeletal structures to indicate this. 

The mesh structure formed by humic substances is capable of trapping smaller chemical species. For example, minor amounts of acyclic alkanes are found in most samples of humic and fulvic acids, and some of the fatty acids associated with humics may be similarly trapped components rather than bonded to the macromolecular backbone. Humic substances also usually contain a variety of metals, which are incorporated into the macromolecular structure. Metal ions can be surrounded by and bonded to suitable chelating groups, chiefly carboxylic acids, on humic molecules that stabilize the ions and allow them to be transported with the organic material. This important property of humic substances is examined again later (Section 7.6.5) in relation to the environmental fate of heavy metals. 

The UV spectrum of cyclopropane is quite different from those of the alkanes we have examined so far. Whereas acyclic alkanes and cyclic alkanes other than cyclopropane and cyclobutane have only Rydberg bands in the lower frequency (and photo chemically important) part of the spectrum, for cyclopropane the lowest Rydberg bands intermingle with bands due to valence-shell transitions. Two structured bands at 63000 and 78000 cm (159 and 128 nm) have been assigned to the 3e a,3p) and (3e o,4p) transitions on the grounds of the similarity of their vibrational fine structure with that of the respective photoelectron bands and their term values. Two other bands, near 70000 cm (143 nm) and 83000 cm (120 nm)... 

Since 1971 Maxwell s group at Bristol used these methods to separate and identify the stereochemistry of isoprenoidic acyclic alkanes . Whereas most acyclic isoprenoids in nature appear functionalized (alcohols, acids, alkenes, etc.), they are found in the geoenvironment as fossil alkanes. Hence it is important not only to analyse the structural isomers, but also to determine the stereochemistry. 

Sidedne and Handedness. You have the broad outlines of structure under control now—acyclic alkanes, alkenes, and alkynes have appeared, as have rings. Now we come to the details, to stereochemistry. Sidedness"—cis/trans isomerism—is augmented by questions of chirality— handedness. "Learning to see one level deeper into three-dimensionality is the next critical skiU. 

What kind of structure would you predict for benzene if you knew only what the early chemists knew The molecular formula (CsH ) tells us that benzene has eight fewer hydrogens than an acyclic alkane with six carbons (C H2 +2 = C6H14). Benzene, therefore, has a degree of unsaturation of four. In other words, the total number of rings and TT bonds in benzene is four (Section 5.1). 

Bartell and coworkers investigated the structures of a series of noncyclic alkanes by means of gas electron diffraction (14, 44, 45) and invoked for the interpretation of their results a simple force field which contained to a high extent vibrational spectroscopic constants of Snyder and Schachtschneider. This force field reproduces bond lengths and bond angles of acyclic hydrocarbons well, energies of isomerisation satisfactorily. As an example, Fig. 8 shows geometry parameters of tri-t-butylmethane as observed by electron diffraction and calculated with this force field (14). 

As with alkanes, the boiling points and melting points of alkenes decrease with increasing molecular weight, but show some variations that depend on the shape of the molecule. Alkenes with the same molecular formula are isomers of one another if the position and the stereochemistry of the double bond differ. For example, there are four different acyclic structures that can be drawn for butene (C4H8). They have different b.p. and m.p. as follows. 

Alkanes and cycloalkanes. Obviously a variety of acyclic and cyclic hydrocarbon structures can be synthesized from the appropriate alkyl- or aryl-thiophenes (Scheme 46). The reaction is especially useful for the construction of macrocycles (Scheme 47). One of the most interesting applications of this reaction is in the synthesis of the chiral hydrocarbon butylethylmethylpropylmethane (210) (80JOC2754). The chiral acid (209) was the precursor, in which the thiophene was the potential n-butyl group. Raney nickel desulfurization, followed by standard manipulations to convert the acetic acid unit into an ethyl group, gave the hydrocarbon (210) (Scheme 48) this had [a]578 = -0.198°. It was established that... 

Acyclic Hydrocarbons, A knowledge of the structural features of hydrocarbon skeletons is basic to the understanding of organic chemical nomenclature. The generic name of saturated acyclic hydrocarbons, branched or unbranched, is alkane. The term saturated is applied to hydrocarbons containing no double or triple bonds. 

In this article (Part I) we have comprehensively reviewed the structural implications of the vibrational spectroscopic results from the adsorption of ethene and the higher alkenes on different metal surfaces. Alkenes were chosen for first review because the spectra of their adsorbed species have been investigated in most detail. It was to be expected that principles elucidated during their analysis would be applicable elsewhere. The emphasis has been on an exploration of the structures of the temperature-dependent chemisorbed species on different metal surfaces. Particular attention has been directed to the spectra obtained on finely divided (oxide-supported) metal catalysts as these have not been the subject of review for a long time. An opportunity has, however, also been taken to update an earlier review of the single-crystal results from adsorbed hydrocarbons by one of us (N.S.) (7 7). Similar reviews of the fewer spectra from other families of adsorbed hydrocarbons, i.e., the alkynes, the alkanes (acyclic and cyclic), and aromatic hydrocarbons, will be presented in Part II. 

Zirconium tribenzyl complex 212 incorporating diisopropyltriazacyclonone, a type of mono-anionic, tridentate diamino-amido [/V,iV ,./V] ligand, was obtained via alkane elimination (Scheme 42) alternatively, this complex can also be prepared via either salt metathesis or amine elimination approaches.179 The crystal structure of 212 reveals a monomeric form on heating at 80 °C in benzene solution for 24 h, this complex undergoes elimination of 1 equiv. of toluene, affording complex 213 bearing a dianionic, acyclic, diamido-amino [/V-,iV,./V-] moiety. 

It is easy to list the various chemical and biological events influenced by flexibility, but unfortunately efforts to quantitate this structural attribute have been few. Mann analyzed the conformation of alkanes by modifying the number of gauche arrangements with Pitzer s steric partition function. Luisiranked alkanes on a. scale of conformational rigidity based on three-states rotational isomerism. Unfortunately, these schemes are designed for acyclic hydrocarbons and have no inherent capability to be adapted to heteroatomic molecules. 

The effect of introducing /j -hybridized atoms into acyclic molecules was discussed in Section 2.2.1, and it was noted that torsional barriers in 1-alkenes and aldehydes are somewhat smaller than in alkanes. Similar effects are seen when sp centers are incorporated into six-membered rings. Whereas the energy barrier for ring inversion in cyclohexane is 10.3 kcal/mol, it is reduced to 7.7 kcal/mol in methylenecy-clohexane ° and to 4.9 kcal/mol in cyclohexanone. The conformation of cyclohexene is described as a half-chair. Structural parameters determined on the basis of electron diffraction and microwave spectroscopy reveal that the double bond can be accommodated into the ring without serious distortion. The C(l)—C(2) bond length is 1.335 A, and the C(l)-C(2)-C(3) bond angle is 123°. The substituents at C(3) and C(6) are tilted from the usual axial and equatorial directions and are referred to as pseudoaxial and pseudoequatorial. 

Saturated aliphatic amines are similar to the alkanes, the main differences being the short C—N bond (C—C 1.523 A, C—N 1.450 A) and the replacement of one substituent by a lone pair of electrons (LP). The pyramidal structure of acyclic amines is quite labile, with barriers to inversion in the range 4-8 kcal mol (Rauk et al, 1971). The problem of the steric requirements of the lone pair has been much discussed in relation to conformational equilibria in saturated six-membered N-heterocycIes (Lambert and Feather-man, 1975 Blackburne et al, 1975). 

Now that we have a code to canonize acyclic structures we can use an orderly algorithm. Next, we illustrate how the -tuple code enumerates alkanes up to n carbon atoms with a McKay-type orderly generation (Scheme III). For simplicity, all hydrogen atoms are ignored, and carbon atoms may thus have a number of bonds ranging between 1 and 4. As depicted in Figure 12, the initial graph contains one atom and no bond, so its canonical -tuple is (0). 

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