addition reactions of unsaturated hydrocarbons
The two simplest unsaturated compounds (those containing a multiple bond) are ethene (CH2,=CH2,) 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. Because the most effective region for interaction between an electron pair and the two nuclei it links is along the bond axis, we expect to find this to be the region of highest electron density in a single bond. Bonds that run symmetrically along the bond axis are called sigma bonds (o bonds).
In a molecule such as ethene, though, the two carbon atoms are linked by two electron pairs and it is quite clear that only one pair can occupy the prime space along the bond axis. There are two ways of looking at the electronic arrangement of ethene. The first, and simplest, is to consider the two orbitals used to link the carbons together as being identical, both being bent, somewhat like the arrangement taken up by springs in the ball-and-stick model of ethene (Figure 2.7). This siinple view of the bonding of ethene accounts for its molecular geometry (repulsion between the electron pairs produces a planar molecule) and its chemical reactivity (the electron pairs are not bound as tightly to the nuclei as in the case of ethane where space along the bond axes can be used).
The alternative way of looking at the bonding of ethene is to consider the double bond as being made up of an ordinary single-type bond along the bond axis, a o bond, and a second bond occupying the regions of space above and below the plane of the molecule, a 7c bond (pi bond). (A n: bond is not cylindrically symmetrical along the bond axis.) The shaded regions above and below the plane of the molecule (see Figure 2.8) represent just one bonding orbital:
that which corresponds to the 71 bond. This model also accounts for the geometry of ethene and for its high chemical reactivity. Despite being less simple than the bent-bond model, it is extensively used by chemists to describe the bonding in unsaturated systems. In the case of ethyne, two 71 bonds and one a bond can be said to link the two carbons together.
Neither model can be called correct or incorrect. And, indeed, refined theory suggests that they represent equivalent approximations. Each is useful according to the degree of clarity it gives the user and according to its ability to predict molecular behavior. The language of chemistry (particularly the theory of spectroscopy and bonding) is based to a great extent on the o, n model, and it must be considered by anyone wishing to explore organic chemistry in depth. Paradoxically, it is only recently that the simple bentbond model has received much attention from theorists.
Both models account for the shortening of the carbon-carbon bond distance as the number of bonds between the carbons increases-the greater the forces, the shorter the bond distance. We shall see that the analogy of springs and bonds also accounts for the vibrational energies of these compounds. A greater amount of energy is required to increase the vibration in the strong triple bond of ethyne than in the double bond in ethene or the single bond in ethane.
A. HYDROGENATION OF MULTIPLE BONDS
The reaction of hydrogen with ethyne is highly exothermic:
However, mixtures of either compound with hydrogen are indefinitely stable under ordinary conditions, and this again reminds us that reaction rates cannot be deduced from heats of reaction. Both ethyne and ethene, however, react rapidly and completely with hydrogen at low temperatures and pressures in the presence of metals such as nickel, platinum, and palladium. For maximum catalytic effect, the metal is usually obtained in a finely divided state. This is achieved for platinum and palladium by reducing the metal oxides with hydrogen before hydrogenating the alkene or alkyne. A specially active form of nickel (" Raney nickel ") is prepared from a nickel-aluminum alloy; sodium hydroxide is added to dissolve the aluminum, and the nickel remains as a black, pyrophoric powder.
Highly active platinum, palladium, and nickel catalysts can also be prepared by reducing metal salts with sodium borohydride.
Besides having synthetic applications, catalytic hydrogenation is useful for analytical and thermochemical purposes. The analysis of a compound for the number of double bonds is carried out by measuring the uptake of hydrogen for a given amount of sample.
The reaction occurs on the surface of the catalyst to which the reacting substances may be held loosely by van der Waals forces or, more tightly, by chemical bonds. The relatively loosely held electrons in a double or triple bond participate in forming carbon-metal bonds to the surface while hydrogen combines with the surface to give metal-to-hydrogen bonds (see Figure 2.9). The new bonds are much more reactive than the old ones and allow combination to occur readily. The hydrogenated compound is then replaced on the surface by a fresh molecule of unsaturated compound, which has a stronger attraction for the surface.
This is an example of heterogeneous catalysis-a type of reaction which involves adsorption on a surface of a solid or liquid and is often hard to describe in precise terms because the chemical nature of a surface is hard to define in precise terms. Homogeneous catalysis occurs in solution or in the vapor state. For such reactions, it is usually easier to trace the path from reactants to products in terms of intermediates with discrete structures. The light-induced chlorination of methane is an example of a homogeneous reaction.
Ethene will add one mole of hydrogen while ethyne can add either one or
two. Special catalysts are available which will convert ethyne to ethene much faster than they convert ethene to ethane.
B. ADDITION OF BROMINE TO MULTIPLE BONDS
When ethene is treated with bromine (or with chlorine), a rapid addition occurs to give the 1,Zdihaloethane. The name given to the reaction product can be
understood as follows. First, it is saturated (no double or triple bonds) and contains two carbon atoms and therefore is a derivative of ethane. The positions of two of the hydrogen atoms of ethane have been taken by two bromine atoms; hence it is a dibromoethane. The two bromines are located on different carbon atoms. We call one of the carbon atoms number 1 and the other number 2, hence the name 1,2-dibromoethane. This naming system can be used to name the vast majority of organic compounds. In using it one should remember these points.
er these points. 1. Pick out the parent hydrocarbon. Here it is ethane, not ethene, because we are interested in designating the structure, not the way the compound is formed in any particular reaction.
2. Pick out those groups or atoms that replace any of the hydrogens of the parent compound and join their names to the front of the name of the parent hydrocarbon. If there are two such groups, as in the case we are working with, designate them with the prefix di; three such groups, tri; and so on. The name at this stage should be all one word-for example, dibromoethane.
3. Locate the substituent groups by counting the carbon atoms from the end of the carbon chain. One can start numbering at either end in the example above, but this will not be generally true. The numbers designating the carbon atoms that bear substituents are separated from one another by commas and from the rest of the name by a hyphen-for example, 1,2-dibromoethane.
Remember that the number of substituents is designated by the prefixes di, tri, and so on, but that their locations are designated by the numerals that identify particular carbon atoms.
We shall examine nomenclature further in the next chapter, which deals with alkanes.
The rapidity of the reaction of ethene with bromine illustrates the high reactivity of carbon-carbon double bonds. It would be natural to suppose that this reaction occurs by a simple simultaneous addition of both atoms of bromine to the double bond:
It will be recalled from the discussion of the methane-chlorine reaction, how-ever, that four-center reactions are rare, and in fact the addition of bromine to alkenes is known to occur in a stepwise manner, usually (but not always) to involve ionic intermediates, and usually to proceed by addition of the two bromines from opposite sides of the double bond. The evidence for this course of reaction is described in detail in Chapter 4 on alkenes. Suffice it to say here that an examination of the product of the ethene-bromine addition reaction will not tell us whether, addition occurred from the same or the opposite side of the double bond, since rotation about the C-C bond converts one product into the other and thus obliterates the distinction between the two possible products (Equation 2.1). Both products are different conformations of the same compound, 1,2-dibromoethane.
Addition of bromine to ethyne also occurs readily to give the compound 1,2-dibromoethene. (Note that the parent compound is now etkene, not ethyne.) The product of this reaction is a liquid, bp log0, mp -6.5". It is
immiscible with water and easily can be shown to possess no dipole moment. However, there is another known compound that has the same structure -that is, possesses two carbon atoms joined by a double bond and has a bromine and hydrogen atom on each carbon. This second compound is a liquid, bp 11Ou, mp -53". It is also immiscible with water but has a large dipole moment. The two isomers arise because of a lack of rotation about the double bond. The molecule with the bromine atoms on the opposite side is called the trans isomer, and the other the cis isomer.
These two compounds are said to have different configurations. It is worth reviewing here the meanings of the terms structure, conformation, and configuration. Structure designates the atoms that are linked together and the bonds that do this. Compounds with the same formula, C2,H4,Br2, for example, but different structures, such as Br2,CH-CH3, and BrCH2-CH2Br, are called structural isomers. If the compound contains one or more carbon- carbon single bonds, rotation can usually occur freely about these and give rise to different conformations. If the compound contains double bonds (or a ring of atoms), rotation is prevented and different configurations may then be possible. Compounds with different configurations are called stereoisomers -for example, cis- and trans-l,2-dibromoethene-and they can only be interconverted by the rupture of chemical bonds. We shall encounter later in the book (Chapter 14) a more subtle form of stereoisomerism. The form of stereoisomerism we are discussing here is called either cis-trans isomerism or geometrical isomerism and, harking back to our ball-and-stick models, it is easy to rationalize why interconversion between geometrical isomers does not take place readily (Equation 2.2). For rotation to occur, one of the C-C
bonds of the double bond must be broken. For this, the necessary energy input would be roughly equal to the difference in energy between a double and a single bond-63 kcal/mole (see Table 2.1). Such an amount of energy is not available from molecular collision at ordinary temperatures.
It is a simple matter to assign configurations to the two geometrical isomers of BrCHxCHBr. The one formed by the addition of bromine to ethyne has no dipole moment and hence must be the trans isomer. The boiling points of these compounds are nearly the same, but the melting points are vastly
different. Trans compounds often have somewhat higher melting points than the corresponding cis isomers, reflecting greater ease of crystal packing of their somewhat more symmetrical molecules. Dihaloethenes are exceptional in that the cis isomers tend to be slightly more stable than the trans. This is because the distances between the halogens in these compounds (but not usually between other substituents) are just right for operation of favorable van der Waals attractive forces. With most other substituents, particularly if they are bulky, the trans arrangement is preferred. Where three or four different groups are attached to the double bond, you must define what you mean by the terms cis and trans, and the generalizations given here about melting points and stabilities do not apply.
Will I, 1-dibromoethene (CH,=CBr,), which is a structural isomer of the above compounds, also exist in cis and trans forms? Clearly not, because interchange of the two bromines on one carbon or interchange of the two hydrogens on the other produces the same molecule. The requirement for the existence of geometrical isomers of an alkene is that the two groups on one end of the double bond be different from each other and the two groups on the other end be different from each other.
Geometrical isomerism does not arise with triply bonded compounds, because the -CrC- bonds in these molecules are linear.