The Structure of Alkanes

Three-dimensional models of ethane, propane, and butane. The ball-and-stick models at the left show the way in which the atoms are connected and depict the correct bond angles. The spacefilling models at the right are constructed to scale and give a better idea of the molecular shape, though some of the hydrogens may appear hidden. 

Chapter 2 Alkanes and Cycloalkanes Conformational and Geometric Isomerism 

Name Number of carbons Molecular formula Number of Structural formula structural isomers  

Unbranched alkanes are called normal alkanes, or n-alkanes. 

What is the molecular formula of an alkane with six carbon atoms  

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The structure of alkanes can be difficult to establish as their KMR spectra are difficult to interpret. The stereochemistry as well as the structure of (34) was in doubt, but synthesis by three different routes established both. 

The pattern of bond lengths obtained by quantum chemical (QC) methods is almost identical for all of the alkanes and alkyl moieties given. The structures of alkanes are always easily predicted (see CH4 above) i.e. the vertical spread of points in the diagrams is always less than for the X-X... 

Describe how the molecular structures of alkenes and alkynes differ from the structure of alkanes. 

Bearing this shape in mind, it is possible to write the structural formula in an even simpler form. The figure below shows the structures of alkanes, their simplified structural formulas as well as their condensed structural formulas. 

Looking at the structures of alkanes in the figure we can see that while the molecules with an odd number of carbon atoms appear symmetrical (both terminal carbon-carbon bonds being oriented upwards - black lines), where the structures with an even number are asymmetrical in the sense that the terminal carbon-carbon bonds are oriented upwards on the left end of the chain, but downwards on the right end (blue lines). Although the correct explanation is not simple, this example demonstrates how macroscopic properties correlate with microscopic structures. We can assume that odd molecules in the condensed state shall be packed differently from even molecules. 

It is important to know the various methods used for drawing the structures of alkanes. Initially, focus on alkanes that have linear chains. This means that all of the carbon atoms are connected in a chain, with no branches. The alkane with a linear chain of four carbon atoms has the butane (see Section 4.3), and it... 

The Structure of Alkanes Constitutional Isomerism in Alkanes Nomenclature of Alkanes and the lUPAC System Cycloalkanes... 

The structures of alkane thiol monolayers and submonolayers on gold [75] and silver [76] have been studied using STM. In general, the stracture and mobility of thiol molecules on gold and silver changes at threshold surface coverages. 

Properties for some straight-chain alkanes are listed in Figure 2.4. The trends in these properties can be explained by examining the structure of alkanes. The carbon-hydrogen bonds of alkanes are nonpolar. The only forces of attraction between nonpolar molecules are weak intermolecular forces, or London dispersion forces. The strength of London dispersion forces increases as the mass of a molecule increases. 

Based on a large number of reported studies, an accepted model has been developed for the structure of alkane thiols on the Au(lll) surface (see Figure 6.5). The various spectroscopies and diffraction methods used to help establish these structures are shown on the figure. Sulfur is chemisorbed in a next-nearest-neighbor threefold hollow site within a trimetallic Au cluster. The... 

Sen S, Cohen J M, McCoy J D and Curro J G 1994 The structure of a rotational isomeric state alkane melt near a hard wall J Chem. Phys. 101 9010... 

In order to develop a quantitative interpretation of the effects contributing to heats of atomization, we will introduce other schemes that have been advocated for estimating heats of formation and heats of atomization. We will discuss two schemes and illustrate them with the example of alkanes. Laidler [11] modified a bond additivity scheme by using different bond contributions for C-H bonds, depending on whether hydrogen is bonded to a primary (F(C-H)p), secondary ( (C-H)g), or tertiary ( (C-H)t) carbon atom. Thus, in effect, Laidler also used four different kinds of structure elements to estimate heats of formation of alkanes, in agreement with the four different groups used by Benson. 

Fig. 8.22 Schetnatic structure of the zeolite silicalite showing the straight and zig-zag chaimels. (Figure adapted fron Smit B and JI Siepmann 2994. Simulating the Adsorption of Alkanes in Zeolites. Science 264 1118-1120.)...

Boiling Point When describing the effect of alkane structure on boiling point m Sec tion 2 17 we pointed out that van der Waals attractive forces between neutral molecules are of three types The first two involve induced dipoles and are often referred to as dis persion forces or London forces... 

Typical carbon-oxygen bond distances m ethers are similar to those of alcohols (—142 pm) and are shorter than carbon-carbon bond distances m alkanes (—153 pm) An ether oxygen affects the conformation of a molecule m much the same way that a CH2 unit does The most stable conformation of diethyl ether is the all staggered anti conformation Tetrahydropyran is most stable m the chair conformation—a fact that has an important bearing on the structures of many carbohydrates... 

Cation (Section 1 2) Positively charged ion Cellobiose (Section 25 14) A disacchande in which two glu cose units are joined by a 3(1 4) linkage Cellobiose is oh tamed by the hydrolysis of cellulose Cellulose (Section 25 15) A polysaccharide in which thou sands of glucose units are joined by 3(1 4) linkages Center of symmetry (Section 7 3) A point in the center of a structure located so that a line drawn from it to any element of the structure when extended an equal distance in the op posite direction encounters an identical element Benzene for example has a center of symmetry Cham reaction (Section 4 17) Reaction mechanism m which a sequence of individual steps repeats itself many times usu ally because a reactive intermediate consumed m one step is regenerated m a subsequent step The halogenation of alkanes is a chain reaction proceeding via free radical intermediates... 

Draw structures of alkanes that meet the following descriptions ... 

Let s assume that we want to find the structure of an unknown hydrocarbon. A molecular weight determination on the unknown yields a value of 82, which corresponds to a molecular formula of CfcHjo Since the saturated Q alkane (hexane) has the formula C61-114, the unknown compound has two fewer pairs of hydrogens (H]4 - H l() = H4 = 2 H2), and its degree of unsaturation is two. The unknown therefore contains two double bonds, one ring and one double bond, two rings, or one triple bond. There s still a long way to go to establish structure, but the simple calculation has told us a lot about the molecule. 

Petroleum contains hydrocarbons other than the open-chain alkanes considered to this point. These include cycloalkanes in which 3 to 30 CH2 groups are bonded into closed rings. The structures of the two most common hydrocarbons of this type are shown in Figure 22.5 (p. 585). Cyclopentane and cyclohexane, where the bond angles are close to the ideal tetrahedral angle of 109.5°, are stable liquids with boiling points of 49°C and 81°C, respectively. 

The number (nj) of the cross ct conjugations of the trios of a C-C bond and two antiperiplanar C-H bonds is important for the stabilities of alkanes. The cross conjugation number (nj) of an alkane is defined as that of the conformer where the longest C-C chain has trans a zigzag structure. For example, there are three cross conjugations (n = 3) in isobutene and none in n-butane (n = 0) (Scheme 27). Isobutane is more stable than n-butane [34, 35]. 

Carbon forms a huge number of binary compounds with hydrogen. Three major categories of these compounds are alkanes, alkenes, and alkynes. An alkane has only single bonds between carbon atoms. The four simplest alkanes, which are shown in Figure 3-7. are methane, ethane, propane, and butane. An alkene, on the other hand, contains one or more double bonds between carbons, and an alkyne has one or more triple bonds between carbon atoms. Figure shows the structures of ethylene, the simplest alkene, and acetylene, the simplest alkyne. 

As the number of carbon atoms in the alkane increases, so does the number of possible stractural isomers. Thousands of different alkanes exist, because there are no limits on the length of the carbon chain. Regardless of the number of the chain length, alkanes have tetrahedral geometry around all of their carbon atoms. The structure of decane, Cio H22, shown in Figure 9-15. illustrates this feature. Notice that the carbon backbone of decane has a zigzag pattern because of the 109.5° bond angles that characterize the tetrahedron. 

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