The C1, and C2, hydrocarbons




Hydrocarbons are compounds that contain only carbon and hydrogen. We shall consider in this chapter the four simplest known hydrocarbonsthose with the lowest molecular weights-and we shall see that they represent three classes of compounds: the alkanes, in which each carbon atom has four single bonds; the alkenes, in which two carbon atoms are joined by a double bond (two electron pairs); and the alkynes, in which two carbon atoms are joined by a triple bond (three electron pairs). We shall also see that these classes of compounds are physically similar but chemically rather different.

There are four stable hydrocarbons of molecular weight 30 or less. They are all gases at room temperature, and analyses for carbon and hydrogen content coupled with determinations of their molecular weights show them to have the formulas CH4, , C2H6, C2H2,, and C2H2 . The first of these is methane, CH4,, whose physical properties and molecular shape were discussed in Chapter 1. The other three are all C2, compounds and are called, respectively, ethane, ethene, and ethyne (ethyne rhymes with brine). These are the systematic names approved by the International Union of Pure and Applied Chemistry, IUPAC1.' However, ethene is often called ethylene and ethyne called acetylene. It is to be hoped that both of these older names will pass out of use in time.

If the carbon atoms in each of the three C, compounds are tetravalent, then there is only one possible way to bond the atoms together in each case:

The C1, and C2, hydrocarbons

The tendency of carbon to form bonds at the tetrahedral angle results in compounds such as methane and ethane being nonplanar. The two-dimensional representation of ethane, above, is thus misleading and it is just as informative (and quicker) to write the formula as CH3-CH3, . (Or, indeed, as C2H6, since there is only one stable compound known with this formula. We shall see that with some C3 hydrocarbons and with all hydrocarbons having four or more carbons, some indication of structure is necessary because a designation such as C4,H8, is ambiguous, there being five known compounds with this formula.)

The C1, and C2, hydrocarbons

Because of the importance of molecular structure in organic chemistry, we shall consider the three-dimensional shapes of these compounds in the next section.

molecular shape of CH4,, C2H6, C2H4, and C2H2

You can illustrate the shape of a tetrahedral molecule such as methane with ball-and-stick models (Figure 2.1)

With ethene and ethyne, the model's carbon-to-carbon bonds are constructed from stiff metal springs or flexible or curved plastic connectors because more than one bond exists between the carbon atoms (Figure 2-2).

These simple mechanical models are surprisingly good for predicting the shapes of molecules and, indeed, their reactivity. Ethene is known from spectroscopic measurements to be planar, and this is the shape the model naturally takes. The electronic analogy here is that the orbitals for each pair of electrons extend as far away from one another as possible. Ethyne, likewise, is known to be linear. The strain involved in making "bent bonds " for these models is reflected in a higher degree of chemical reactivity for these compounds than for ethane.

The C1, and C2, hydrocarbons

The C1, and C2, hydrocarbons

The arrangement of the linkages in the ethene model suggests that dne CH2, group cannot twist with respect to the other CH2, group without gross distortion from the favored geometry. We shall see that this conclusion, too, is borne out by chemical evidence (Section 2.6B). By contrast, the model of the saturated compound, ethane, suggests that free rotation should be possible about the single bond joining the two carbon atoms if the sticks representing the bonds are allowed to rotate in the holes of the balls representing the atoms. Such rotation is considered in more detail in the next section.



Frequently Asked Questions

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Ans: Lithium and beryllium are able to form positive ions by loss of one or two electrons, respectively. Boron is in an intermediate position and its somewhat unusual bonding properties are considered later in the book (Section 19.5). view more..
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Ans: Why is carbon unique? What accounts for the apparently limitless number of carbon compounds that can be prepared? The answer is that bonds between carbon atoms are stable, allowing chains of carbon atoms to be formed, with each carbon atom of a chain being capable of joining to other atoms such as hydrogen, oxygen, sulfur, nitrogen, and the halogens. view more..
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Ans: Hydrocarbons are compounds that contain only carbon and hydrogen. We shall consider in this chapter the four simplest known hydrocarbonsthose with the lowest molecular weights-and we shall see that they represent three classes of compounds: the alkanes, in which each carbon atom has four single bonds; the alkenes, in which two carbon atoms are joined by a double bond (two electron pairs); and the alkynes, in which two carbon atoms are joined by a triple bond (three electron pairs). view more..
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Ans: In organic chemistry, the word structure has a specific meaning; It designates the order in which the atoms are joined to each other. A structure does not necessarily specify the exact shape of a molecule because rotation about single bonds could lead, even for a molecule as simple as ethane, to an infinite number of different arrangements of the atoms in space. view more..
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Ans: The two simplest unsaturated compounds (those containing a multiple bond) are ethene (CH,=CH,) and ethyne (HCzCH). The generally lower stability of multiply bonded compounds arises from the restriction that only one electron pair can occupy a given orbital view more..
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Ans: In the previous two chapters we have studied in some detail the properties of the two simplest saturated hydrocarbons, methane and ethane, and have shown how their simple derivatives are named using the rules of the International Union of Pure and Applied Chemistry (IUPAC rules). view more..
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Ans: The series of continuous-chain alkanes, CH,(CH,),-,CH, , shows a remarkably smooth gradation of physical properties (see Table 3.3 and Figure 3-2). As you go up the series, each additional CH, group contributes a fairly constant increment to the boiling point and density and, to a lesser extent, to the melting point. view more..
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Ans: As a class, alkanes are singularly unreactive. The name saturated hydrocarbon (or " paraffin," which literally means " little affinity " [L. par(um), little, + afins, affinity1)arises because their chemical affinity for most common reagents may be regarded as saturated or satisfied. view more..
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Ans: An important and interesting group of hydrocarbons, known as cycloalkanes, contain rings of carbon atoms linked together by single bonds. The simple unsubstituted cycloalkanes of the formula (CH,), make up a particularly important homologous series in which the chemical properties change in a much more striking way than do the properties of the open-chain hydrocarbons, CH3(CH2),-,CH3. view more..
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Ans: In the early days of organic chemistry, when it was found that the alkenes, but not the alkanes, readily undergo addition reactions with substances such as halogens, hydrogen halides, sulfuric acid, and oxidizing agents, the chemical affinity of alkanes was said to be " saturated" while that of the alkenes was said to be " unsaturated." Now, even though we recognize that no chemical entity (even the noble gases such as helium and xenon) can surely be classified as saturated, the description of alkanes and alkenes as saturated and unsaturated is still commonly used. view more..
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Ans: In the homologous series of alkanes, isomerism first appears at the C, level, two compounds of formula C4H,, being known. These are structural isomers: view more..
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Ans: By convention, the configuration of complex alkenes is taken to correspond to the configuration of the longest continuous chain as it passes through the double bond. Thus the following compound is 4-ethyl-3-methyl-trans-3- heptene, despite the fact that two identical groups are cis with respect to each view more..
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Ans: We have previously examined briefly two addition reactions of ethene, the first member of the homologous series of alkenes. These were addition of hydrogen, catalyzed by surfaces of finely divided metals such as nickel, and the addition of bromine. view more..




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