Cycloalkanes :

Cycloalkanes have molecular formula C H2 and contain carbon atoms arranged in a ring. 

Think of a cycloalkane as being formed by removing two H atoms from the end carbons of a chain, and then bonding the two carbons together. Simple cycloalkanes are named by adding the prefix cyclo- to the name of the acyclic alkane having the same number of carbons. 

Cycloalkanes having three to six carbon atoms are shown in the accompanying figure. They are most often drawn in skeletal representations. 

The series of homologous cycloalkanes ould perhaps already begin with 

No evidence of any transition is shown below the melting temperature. In additirai, the measured entropy of fusion, ASj, is that expected for full orientational and positional disordering (see Table 3.1). It is thus clear fltat the torsional libration about the C3 axis must lead to occasional jumps into nei boiing symmetry positions that reach the NMR frequency of 10-100 kHz at about 115 K, M K below the melting temperature. Such rather large-scale jump-motion does not disturb the crystal symme- 

Cyclobutane is now the first member of the cycloalkanes that shows a plastk crystalline state at atmospheric pressure. The molecule deviate by about 15° from planarity, i.e. the molecular symmetry is instead of 64,65) 145 tjjg 

Detailed information on structure and dynamics is also available for cyclopentane. Proton NMR studies show in this case coincidence of the linewidth narrowing from a second moment of 26.2 1.5 G expected for a rigid crystal (calculated 28 + 2 G ) with a sharp thermodynamic transition at T, = 122.4 K. The re- 

Nuclear magnetic resonance data on cyclohexane are reproduced together with heat capacity information in Fig. 3.2. The transition parameters are listed in Table 3.1. Below 150 K the experimental proton NMR second moment of 26.0 + 0.5 G corresponds to that calculated for a crystal of rigid molecules of Djj dymmetry in the chair conformation. The decrease in secoixi moment from 155 to 180 K is caused by jump-reorientation about the triad axis with a 46 kJ/mol activation energy. The experimental second moment somewhat below T of 6.4 G corresponds to the calculated value of 6.1 l.OG for such motion. At the transition the ond moment drops to 1.4 G which is in line with additional reorientation about aU other axes (1.3 to 1.1 G calculated for different assumptions). Above 240 K, 

Many organic compounds are cyclic They contain tings of atoms. The carbohydrates we eat are cyclic, the nucleotides that make up our DNA and RNA are cyclic, and the antibiotics we use to treat diseases are cyclic. In this chapter, we use the cycloalkanes to illustrate the properties and stability of cyclic compounds. 

Most cycloalkanes resemble the acyclic (noncyclic), open-chain alkanes in their physical properties and in their chemistry. They are nonpolar, relatively inert compounds with boiling points and melting points that depend on their molecular weights. The cycloalkanes are held in a more compact cyclic shape, so their physical properties are similar to those of the compact, branched alkanes. The physical properties of some common cycloalkanes are listed in Table 3-4. 

Cyclopropane was once used as general anesthetic because its vapors, ike those of other simple alkanes and cycloalkanes, cause sleepiness and a loss of consciousness. After inhalation into the lungs, cyclopropane goes into the blood Due to its nonpolar nature, it rapidly leaves the blood and passes through the nonpolar membranes surrounding the central nervous system, where it produces anesthesia Cyclopropane is no longer used as an anesthetic because it is highly flammable (like ether) and can cause explosions when mixed with air. 

TABLE 3-4 Physical Properties of Some Simple Cycloalkanes  

Cycloalkane Formula Boiling Point (°C) Melting Point (°C) Density 

The results are reported in Table 13.1. The zero-point plus heat content energies, abbreviated as ZPE + A//, are those described in Chapter 9, Eq. (9.9). The shifts are from Refs. 166, 169, and 243. 

The fine-mning of molecular AE energies, responsible for the differences between strucmral isomers, rests entirely with small but extremely important modifications of charge distributions affecting Because the sum of 

This section is about alkylcyclohexanes and related polycyclic molecules consisting of chair six-membered rings. Our work must thus accommodate conformational features such as those commonly described as hutanc-gauche interactions. 

The latter are, indeed, of considerable interest. They have a long history in conformational chemistry [258,259] and deserve attention for the major role they play in the discussion and prediction of stmcmral feamres. Typically, we refer here to gauche interactions exemplified by one of the methyl protons of the axial methylcyclohexane (for instance) interacting with the axial protons at C-3 and C-5 of the ring, or to the three gauche interactions occurring in cw-decalin.  

The occurrence of these interactions is not under dispute. The question lies with the interpretation of gauche interactions—are they somehow related to the vibrational energy content of the molecule, or should they rather be traced back to a particularity in the chemical binding in the vibrationless state The answer is given in Chapter 9. 

The large CH coupling constants Jcll of cyclopropane and - to a smaller extent of cyclobutane are attributed to a higher s character of the carbon bond orbitals. 

Carbon-13 shifts of some methylcycloalkanes are given in Table 4.5. Methyl substitution increments have been derived for methylated cyclopentanes (Table 4.6 [210]) and cyclohexanes (Table 4.7 [87]) in order to predict carbon shift values. While y effects in methylated cyclopentanes are small (Table 4.6), shieldings of carbon atoms in y position 

Ring inversion of methylcyclohexane, which exchanges equatorial and axial methyl groups at room temperature, is frozen at 100 °C [213]. In this situation, the methyl and 

5 carbons in (a) are shielded by about 6 ppm relative to (e). Similar shift differences have been obtained by low-temperature 13C NMR of 1,2- and 1,4-dimethylcyclohexane [112] (Fig. 3.13). 

Exercise 3-1 Draw structural formulas corresponding to the following names  

Exercise 3-2 Give the IUPAC name for each of the following structures a. (CH3)2CHCH(CH3)CH2CH(CH3)2 c. CH3CH2CHCH2CH(CH3)2 

Exercise 3-3 The following are improper IUPAC names. Determine what is incorrect or ambiguous about the name and give the correct name. 

4-dimethylcyclohexane [not 3,6-dimethylcyclohexane because of lowest number rule] 1-ethyl-3-methylcyclopentane [not 1-ethyl-4-methyl-cyclopentane because of lowest number rule, and not 1-methyl-3-ethylcyclopentane because of alphabetical order rule] 

The substituent groups derived from cycloalkanes by removing one hydrogen are named by replacing the ending -ane of the hydrocarbon with -yl 

Attractive van der Waals forces are caused by temporary dipoles in molecules, as shown in these space filling models of pentane. 

WeVe discussed only open-chain alkanes up to this point, but chemists have known for over a century that compounds with rings of carbon atoms also exist. Such compounds are called cycloalkanes, or alicyclic compounds (aliphatic cyclic). Since cycloalkanes consist of rings of -CH2- units, they hav e the general formula or C R-in, and are represented by polygons 

Alicyclic compounds with many different ring sizes abound in nature. For example, chrysanthemic acid contains a three-membered (cyclopropane) ring. Various esters of chrysanthemic acid occur naturally as the active insecticidal constituents of chrysanthemum flowers. 

Prostaglandins, such as PGEi, contain a five-membered fcydopentane) ring. Prostaglandins are potent hormones that control a wide variety of phys- 

Steroids, such as cortisone, contain four rings joined together—three of them six-membered (cyclohexane) and one five-membered (cyclopentane). We ll discuss steroids in more detail in Sections 27.7 and 27.8. 

Melting points and boiling points for cycloalkanes, cyclo-(CHj)n. 

Give the lUPAC name for each of the following hydrocarbons.  

The longest continuous carbon chain consists of 6 carbon atoms, and one methyl group is attached to each of the two terminal carbons. The lUPAC name is thus 2,5-dimethylhexane, 

The longest chain contains 9 carbon atoms. A methyl group is attached to the 7th carbon 2 methyl groups are attached to the 5th carbon and an ethyl group is bonded to the third carbon. The name of this compound is thus 3-ethyl-5,5,7-trimethylnonane. 

The longest chain has 7 carbon atoms. The 3rd carbon has a methyl group attached to it and the 4th carbon an ethyl group. The name given to this compound is 4-ethyl-3-methylheptane (alphabetical order has precedence over numerical order in side chains). 

Using the same principles illustrated above, this compound is named 3-ethyl-6-isopropyl-3-methyloctane. 

Cyclohexene has sometimes been used for hydrogenation of organic molecules in place of molecular hydrogen. The situation on the surface in the presence of hydrogen is therefore one of considerable complexity in what follows we focus on the dehydrogenation of cyclohexane. 

BUILD MODELS 1,2-DIMETHYLCYCLOPENTANE PART A C/S AND TRANS RINGS 

Use model(s) of 1,2-dimethylcyclopentane to determine if the molecule in the left box is the same as the molecule in the right box. Recall that... 

Two molecules are the same if models of each can be inter converted without breaking bonds. 

Memorization Task 7.1 Determining cisitrans for rings that lie in the plane of the paper 

To determine cis/trans for rings use the following algorithm For two groups on a ring... 

Give a name for the following molecules, according to lUPAC rules  

Carbon Atoms Range (°C) Common Name Uses, Comments 

5-9 40-100 Ligroin, naphtha Ligroin is primarily used as a solvent in chemical manufacture and contains 55 % alkanes arxf 30 % cycloalkanes. Sometimes referred to as petroleum ether, but it is NOT an ether. Naphtha should not be confused with the solid aromatic compound naphthalene. 

5-12 40-205 Gasoline Automobile fuel. Exaa composition varies internationally and with climate. 

10-18 150-300 Kerosene, paraffin Fuel for Jet engines and tractors. Some uses as heating oil. Contains significant amounts of aromatic hydrocarbons. Paraffin Is the term used in the United Kingdom and Ireland. 

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Paraffin waxes have a macrocrystalline structure and consist, largely, of -alkanes of formulae C20H42 nd upwards with some iso-and cycloalkanes. 

Microcrystalline waxes, produced from heavy lubricating oil residues, have a micro-crystalline structure and consist largely of iso-and cycloalkanes with some aromatics. 

Paraffins consist mainly of straight chain alkanes, with a very small proportion of isoalkanes and cycloalkanes. Their freezing point is generally between 30°C and 70°C, the average molecular weight being around 350. When present, aromatics appear only in trace quantities. 

Waxes are less well defined aliphatic mixtures of n-alkanes, isoalkanes and cycloalkanes in various proportions. Their average molecular weights are higher than those of the paraffins from 600 to 800. 

The napthanes (C H2n), or cycloalkanes, are ring or cyclic saturated structures, such as cyclo-hexane (CgH 2) though rings of other sizes are also possible. An important series of cyclic structures is the arenes (or aromatics, so called because of their commonly fragrant odours), which contain carbon-carbon double bonds and are based on the benzene molecule. 

There are a total of eighteen different hydrocarbon series, of which the most common constituents of crude oil have been presented - the alkanes, cycloalkanes, and the arenes. The more recent classifications of hydrocarbons are based on a division of the hydrocarbons in three main groups alkanes, naphthanes and aromatics, along with the organic compounds containing the non-hydrocarbon atoms of sulphur, nitrogen and oxygen. 

Cycloalkanes are alkanes that contain a ring of three or more carbons They are fre quently encountered m organic chemistry and are characterized by the molecular formula C H2 Some examples include... 

As you can see cycloalkanes are named under the lUPAC system by adding the prefix cyclo to the name of the unbranched alkane with the same number of carbons as the ring Substituent groups are identified m fhe usual way Their posifions are specified by numbering fhe carbon atoms of fhe ring m fhe direction fhaf gives fhe lowesf num ber to fhe subsfifuenfs af fhe firsf pomf of difference... 

Cycloalkanes are one class of alicyclic (a//phatic cyclic) hy drocarbons... 

Cyclopentane and cyclohexane are present m petroleum but as a rule unsubsti tuted cycloalkanes are rarely found m natural sources Compounds that contain rings of various types however are quite abundant... 

A single alkane may have several different names a name may be a common name or it may be a systematic name developed by a well defined set of rules The most widely used system is lUPAC nomencla ture Table 2 6 summarizes the rules for alkanes and cycloalkanes Table 2 7 gives the rules for naming alkyl groups... 

Summary of lUPAC Nomenclature of Alkanes and Cycloalkanes (Continued)... 

Count the number of carbons in the ring and assign a basis name to the cycloalkane corre spending to the lUPAC name of the unbranched cycloalkane having the same number of carbons... 

Name the alkyl group and append it as a prefix to the cycloalkane No locant is needed if the com pound IS a monosubstituted cycloalkane It is understood that the alkyl group is attached to C 1... 

Alkanes and cycloalkanes are nonpolar and insoluble m water The forces of attraction between alkane molecules are induced dipole/induced dipole attractive forces The boiling points of alkanes increase as the number of carbon atoms increases Branched alkanes have lower boiling points than their unbranched isomers There is a limit to how closely two molecules can approach each other which is given by the sum of their van der Waals radii... 

Alkanes and cycloalkanes burn m air to give carbon dioxide water and heat This process is called combustion... 

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