Carbon rings of atoms

Functional Group — an atom or group of atoms, other than hydrogen, bonded to the chain or ring of carbon atoms (e.g., the -OH group of alcohols, the -COOH... 

Cycloalkane (Section 2.15) An alkane in which a ring of carbon atoms is present. 

Alkenes, alkynes, and arenes (aromatic compounds) all contain carbon-carbon multiple bonds. Alkenes have a double bond, alkynes have a triple bond, and cneues have alternating double and single bonds in a six-membered ring of carbon atoms. Because of their structural similarities, these compounds also have chemical similarities. 

We ve discussed only open-chain compounds up to this point, but most organic compounds contain rings of carbon atoms. Chcysanthemic acid, for instance, whose esters occur naturally as the active insecticidal constituents of chrysanthemum flowers, contains a three-membered (cyclopropane) ring. 

The cyclopentadienide ion, C H , is a common organic anion that forms very stable complexes with metal cations. The anion is derived by removing a proton from cyclopentadiene, QH, with strong base. The molecule has a five-memhered ring of carbon atoms, with four carbon atoms attached to only one proton and one carbon atom bonded to two. Draw the Lewis... 

The framework for the tropylium cation, C7H7f, is a seven-membered ring of carbon atoms with a hydrogen atom... 

Cyclopropane, C.H, is a hydrocarbon composed of a three-membered ring of carbon atoms, (a) Determine the hybridization of the carbon atoms, (b) Predict the CCC and HCH bond angles at each carbon atom on the basis of your answer to part (a), (c) What must the real CCC bond angles in cyclopropane be (d) What is the defining characteristic of a cr-bond compared with a ir-bond, for example (e) How do the C—C cr-bonds in cyclopropane extend the definition of conventional o-bonds (f) Draw a picture depicting the molecular orbitals to illustrate your answer. 

One common indicator is phenolphthalein (Fig. 11.10). The acid form ol this large molecule (3) is colorless its conjugate base form (4) is pink. The structure of the base form of phenolphthalein allows electrons to be delocalized across all three of the benzenelike rings of carbon atoms, and the increase in delocalization is part of the reason for the change in color. The pFCIn of phenolphthalein is 9.4, and so the end point occurs in slightly basic solution. Litmus, another well-known indicator, has pkln = 6.5 it is red for pH < 5 and blue for pH > 8. 

To name an alkane in which the carbon atoms form a single chain, we combine a prefix denoting the number of carbon atoms with the suffix -ane (Table 18.1). For example, CH,—CH, (more simply, CH,CH,) is ethane and CH,—CH2—CH, (that is, CH,CH2CH,) is propane. Cyclopropane, C,H6 (15), and cyclohexane, C6H12 (16), are cycloalkanes, alkanes that contain rings of carbon atoms. 

The aromatic hydrocarbons are characterized by molecules containing six-membered rings of carbon atoms with each carbon atom attached to a maximum of one hydrogen atom. The simplest member of the series is benzene, ChH6. Using the total bond order rules discussed above, the structural formula of benzene can be written as follows ... 

Because of its great stability, the six-membered ring of carbon atoms persists in most reactions. For simplicity, the ring is sometimes represented as a hexagon, each corner of which is assumed to be occupied by a carbon atom with a hydrogen atom attached (unless some other atom is explicitly indicated at that point). The delocalized electrons are indicated by a circle within the hexagon. The following representations illustrate these rules ... 

Equatorial hydrogens the hydrogen atoms lie around the perimeter of the ring of carbon atoms. 

Cycloalkanes or cycloparaffins (also called naphthenes). These constituents are characterized by the presence of simple closed rings of carbon atoms (such as the cyclopentane ring or the cyclohexane ring). Naphthenes are generally stable and relatively insoluble in water. 

Each graphene sheet is composed of rings of carbon atoms arranged on a hexagonal 2D tiling. This form of carbon has n bonds in addition to the a bonds, as shown in Fig. 4.6, leading to a bond order of 1.33. 

Figure 2.5 Artist s depiction of Friedrich August von Kekule s dream of a snake eating its own taii, a dream that aided von Kekule in soivingthe probiem of the structure for the organic compound benzene, composed of a ring of carbon atoms.

Up to this point, this chapter has discussed aromatic systems composed exclusively of rings of carbon atoms. But aromatic systems can contain heteroatoms, which in this case means any atoms in the ring other than carbon. 

Students will be familiar with carbocyclic compounds, such as cyclo-hexane and benzene, that are built from rings of carbon atoms. If one or more of the carbon atoms is replaced by another element, the product is a heterocycle. Multiple replacements are commonplace, and the elements involved need not be the same. The most common are oxygen. 

In this molecule ethylene lies in the same plane as the other ligands, unlike the case in Zeise s salt where planarity is sterically prevented. By analogy to a three-membered ring of carbon atoms, this compound may be viewed as a mctallacyclopropane. 

An alicydic compound is an organic compound containing a saturated ring of carbon atoms, such as a cycloparaffin or other hydroaro-maiic compound. 

INORGANIC CHEMISTRY. A major branch or chemistry that is generally considered to embrace all substances except hydrocarbons and their derivatives, or substances that are not compounds of carbon, with the exception of carbon oxides and carbon disulfide. The chemical compounds, which are based upon chains or rings of carbon atoms, which are termed organic compounds, are studied under the separate heading of organic chemistry. See also Organic Chemistry. 

Formation of a four-membered ring of carbon atoms can be achieved only with substantial distortion of the normal valence angles of carbon, regardless of whether the ring is planar or nonplanar. In cyclobutane, for example, if the valence bonds are assumed to lie along straight lines drawn between the carbon nuclei, each C-C-C bond angle will be 19.5° smaller than the 109.5° tetrahedral value ... 

Exercise 21-11 Graphite crystals consist of a network of planar hexagonal rings of carbon atoms arranged in parallel layers. The distance between the layer planes is 3.35 A and all the C-C bonds within the hexagonal network are equal to 1.421 A. 

Explain why cyclohexane, a substance that contains a six-membered ring of carbon atoms, is not flat but instead has a puckered, nonplanar shape. Predict the values of the C-C-C bond angles. 

The compounds we ve been dealing with thus far have all been open-chain, or acyclic, alkanes. Cycloalkanes, which contain rings of carbon atoms, are also well known and are widespread throughout nature. Compounds of all ring sizes from 3 through 30 carbons and beyond have been prepared. The four simplest cycloalkanes having three carbons (cyclopropane), four carbons (cyclobutane), five carbons (cyclopentane), and six carbons (cyclohexane) are shown at the top of the next page. 

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