Alkynes and Dienes

Chapter 8

ALKYNES AND DIENES

Higher alkynes. Nomenclature

• Like the alkanes and alkenes, the alkynes form a homologous series, the increment again being CH2. The alkynes are named according to two systems. In one, they are considered to be derived from acetylene by replacement of one or both hydrogen atoms by alkyl groups. H-C-C-C2H5 CH3 GsC-CH3 CH3-O-C CH(CH3)2 Ethyl acetylene Dimethyl acetylene Methylisopropylacetylene 1-Butyne 2-Butyne 4-Methyl-2-pentyne for more complicated alkynes the IUPAC names are used. The rules are exactly the same as for the naming of alkenes, except that the ending -yen replace.

• -Ene. The parent structure is the longest continuous chain that contains the triple bond, and the positions both of substituents and of the triple bond are indicated by numbers. The triple bond is given the number of Ihe first triply bonded carbon encountered, starting from the end of the chain nearest the triple.

Physical properties of alkynes

• Being compounds of low polarity, the alkynes have physical properties that are essentially the same as those of the alkanes and alkenes. They are insoluble in water but quite soluble in the usual organic solvents of low polarity: ligroin, ether, benzene, carbon tetrachloride. They are less dense than water. Their boiling points (Table 8.1) show the usual increase with increasing carbon number.

• and the usual effects of chain-branching; they are very nearly the same as the boiling points of alkanes or alkenes with the same carbon skeletons.

Industrial source of acetylene

• The alkyne of chief industrial importance is the simplest member of the family, acetylene. It can be prepared by the action of water on calcium carbide, CaC2 , which itself is prepared by the reaction between calcium oxide and coke at the very high temperatures of the electric furnace. The calcium oxide and coke are in turn obtained from limestone and coal, respectively. Acetylene is thus obtained by a few steps from three abundant, cheap raw materials: water, coal, limestone.

• Enormous quantities of acetylene are consumed each year. Dissolved under pressure in acetone contained in tanks, - it is sold to be used as fuel for the oxyacetylene torch. It is the organic starting material for the large-scale synthesis of important organic compounds, including acetic acid and a number of unsaturated compounds that are used to make plastics and synthetic rubber. Many of the synthetic uses of acetylene have grown out of work done in Germany before and during World War II by W. Reppe (at the I, G. Farbenindustrie). Aimed at replacing petroleum (scarce in Germany) by the more abundant coal as the primary organic source, this work has revolutionized the industrial chemistry of acetylene. 

Preparation of alkynes

A carbon-carbon triple bond is formed in the same way as a double bond: elimination of atoms or groups from two adjacent carbons. The groups eliminated .

Reactions of alkynes

• Just as alkene chemistry is the chemistry of the carbon-carbon double bond, so alkyne chemistry is the chemistry of the carbon-carbon triple bond. Like alkenes, alkynes undergo electrophilic addition, and for the same reason: availability of the loosely held n electrons. For reasons that are not understood, the carbon-carbon triple bond is less reactive than the carbon-carbon double bond toward electrophilic reagents. Reasonably enough, the triple bond is more reactive than the double bond toward reagents that are themselves electron rich. Thus, alkynes undergo a set of reactions, nucleophilic addition, that are virtually unknown for simple alkenes. Although time does not permit us to go into these particular reactions here, we shall take up nucleophilic addition later in connection with other kinds of compounds (Chaps. 19 and 27). Besides addition, alkynes undergo certain reactions that are due to the acidity of a hydrogen atom held by triply bonded carbon.

Addition reactions of alkyne

• Addition of hydrogen, halogens, and hydrogen halides to alkynes is very much like addition' to alkenes, except that here two molecules of reagent can be consumed for each triple bond. As shown, it is generally possible, by proper selection of conditions, to limit reaction to the first stage of addition, formation of alkenes. In some cases, at least, this is made simpler because of the way that the atoms introduced in the first stage affect the second stage. Problem 8.2 (a) Write the equation for the two-stage addition of bromine to 2-butyne. (b) How will the first two bromine atoms affect the reactivity of the double bond? (c) How will this influence the competition for halogen between 2-butyne and 2,3-dibromo-2-butene? (d) In what proportions would you mix the reagents to help limit reaction to the first stage? (e) Would you bubble 2-butyne into a solution of Br2 in CC14, or drip the bromine solution into a solution of 2-butyne?

Reduction to alkenes

• Reduction of an alkyne to the double-bond stage can unless the triple bond is* at the end of a chain yield either a so-alkene or a f/mms-alkene. Just which isomer predominates depends upon the choice of reducing agent. Predominantly flaws-alkene is obtained by reduction of alkynes with sodium or lithium in liquid ammonia. Almost entirely so-alkene (as high as 98%) is obtained by hydrogenation of alkynes with several different catalysts: a special repaired palladium called Lindler's catalyst \ or a nickel boride called P-2 catalyst reported by H. C. Brown (see p. 507) and his son, C. A.

Acidity of alkynes. Very weak acids

• In our earlier consideration of acids (in the Lowry-Bronsted sense, Sec. 1.22), we took acidity to be a measure of the tendency of a compound to lose a hydrogen ion. Appreciable acidity is generally shown by compounds in which hydrogen is attached to a rather electronegative atom (e.g., N, O, S, X). The bond holding the hydrogen is polar, and the relatively positive hydrogen can separate as the positive ion; considered from another viewpoint, an electronegative element can better accommodate the pair of electrons left behind. In view of the electronegativity series, F > O > N > C, it is not surprising to find that HF is a fairly strong acid, H2O a comparatively weak one, NH3 still weaker, and CH4 so weak that we would not ordinarily consider it an acid at all Just how strong an acid is acetylene? Let us compare it with two familiar compounds, ammonia and water.

• pair of electrons occupies an spa orbital; in the ethine anion the unshared pair of electrons occupies an spa 3 orbital. The availability of this pair for sharing with acids determines the basicity of the anion. Now, compared with an spa 3 orbital, 

Formation of heavy metal acetylides

• The acidic acetylenes react with certain heavy metal ions, chiefly Ag* and Cu + , to form insoluble acetylides. Formation of a precipitate upon addition of an alkyne to a solution of AgNO3 in alcohol, for example, is an indication of hydrogen attached to triply bonded carbon. This reaction can be used to differentiate terminal alkynes (those with the triple bond at the end of the chain) from nonterminal alkynes.

Reaction of sodium acetylides with alkyl halides. Substitution vs. elimination

Hydration of alkynes. Tautomerism

• Addition of water to acetylene to form acetaldehyde, which can then be oxidized to acetic acid, is an extremely important industrial process. From the structure of acetaldehyde, it at first appears that this reaction follows a different pattern from the others, in which two groups attach themselves to the two triply bonded carbons. Actually, however, the product can be accounted for in a rather simple way.

Structure and nomenclature of 

• Dienes are simply alkenes that contain two carbon-carbon double bonds. They therefore have essentially the same properties as the alkenes we have already studied. For certain of the dienes, these alkene properties are modified in important ways; we shall focus our attention on these modifications. Although we shall consider chiefly dienes in this section, what we shall say applies equally well to compounds with more than two double bonds. Dienes are named by the ILJPAC system in the same way as alkenes, except that the ending -diene is used, with two numbers to indicate the positions of the two double bonds. This system is easily extended to compounds containing any number of double bonds. CH2=CH CH=CH2 CH2=CH CH2 CH=CH2 CH2=CH CH=CH CH

• A third class of dienes of increasing interest to organic chemists, contain cumulated double bonds; these compounds are known as allenes.

Preparation and properties of diene

• The chemical properties of a diene depend upon the arrangement of its double bonds. Isolated double bonds exert little effect on each other, and hence each reacts as though it were the only double bond in the molecule. Except for the consumption of larger amounts of reagents, then, the chemical properties of the non-conjugated dienes are identical with those of the simple alkenes. Conjugated dienes differ from simple alkenes in three ways: (a) they are more stable, (b) they undergo 1,4-addition, and (c) toward free radical addition, they are more reactive.

Stability of conjugated diene

• If we look closely at Table 6.1 (p. 183), we find that the heats of hydrogenation of alkenes having similar structures are remarkably constant. For monosubstituted alkenes (RCH CH2) the values are very close to 30 kcal/mole; for disubstituted alkenes (R2C CH2 or RCH-CHR), 28 kcal/mole; and for trisubstituted alkenes (R2C ^CHR), 27 kcal/mole. For a compound containing more than one double bond we might expect a heat of hydrogenation that is the sum of the heats of hydrogenation of the individual double bonds. For non-conjugated dienes this additive relationship is found to hold. As shown in Table 8.2, 1,4-pentadiene and 1,5-hexadiene, for example, have heats of hydrogenation very close to 2 x 30 kcal, or 60 kcal/mole.

• For conjugated dienes, however, the measured values are slightly lower than expected. For 1,3-butadiene we might expect 2 x 30, or 60 kcal: the actual value, 57 kcal, is 3 kcal lower. In the same way the values for 1 ,3-pentadiene and 2,3- dimethyl- 1,3-butadiene are also below the expected values by 2-4 kcal. Heats of Hydrogenation CH2==CH-CH==CH2 CH3 CH=CH CH=CH2 Expected: 30 + 30 = 60 kcal Expected: 28 4- 30 = 58 kcal Observed: 57 Observed: 54 CH3 CH3 CH2=C C=CH2 Expected: 28 + 28 = 56.

Resonance in conjugated dienes

• Let us focus our attention on the four key carbon atoms of any conjugated diene system. We ordinarily write the?! C2 and C3 C4 bonds as double, and the C2 C3 bond as single: 1234 ~M i r This would correspond to an orbital picture of the molecule (see Fig. 8.50), in which IT bonds are formed by overlap of the p orbitals of C\ and C2 , and overlap of the p orbitals of C3 and C4 . In the allyl radical we saw that resonance resulted from the overlap of the p orbital of a carbon atom with p orbitals on both sides. We might expect 

Resonance in alkenes. Hyperconjugation

• Heats of hydrogenation showed us (Sec. 6.4) that alkenes are stabilized not only by conjugation but also by the presence of alkyl groups: the greater the number of alkyl groups attached to the doubly bonded carbon atoms, the more stable the alkene. To take the simplest example, the heat of hydrogenation of propylene is 2.7 kcal lower than that of ethylene, indicating that (relative to the corresponding alkane) propylene is 2.7 kcal more stable than ethylene.

• Consistent with partial double-bond character, the carbon-carbon "single" bond in propylene is 1.50 A long, as compared to 1.53 A for a pure single bond. The greater the number of alkyl groups attached to the doubly bonded carbons, the greater the number of contributing structures like II, the greater the delocalization of electrons, and the more stable them.

Stability of dienes and alkenes: an alternative interpretation

• We have seen that the carbon-hydrogen bond length decreases as we proceed along the series ethane, ethylene, acetylene, and we attributed this to changes in hybridization of carbon (see Table 8.3). As the/? character of the bonding orbit In a similar way, the unusual stability of conjugated dienes is attributed, not to delocalization of the -n electrons, but to the fact that &p 2 -sp 2 hybridization makes the C2 C3 bond short (1.48 A) and strong. There is little doubt that both factors, delocalization of n electrons and change in a bond, are at work. The question is: what is the relative importance of each? The answer may well turn out to be: both are important. In the case of molecules like the allyl radical, where clearly no single structure is acceptable, Dewar has not questioned the importance of ^-electron delocalization.

Electrophilic adobe to conjugated dienes. 1,4-Addition

Allyl cations. Delocalization in carbonium ion

• How can we account for the products obtained? We have seen (Sees. 6.10 and 6.1 1) that electrophilic addition is a two-step process, and that the first steaked place in the way that yields the more stable carbonium ion. Let us apply this principle to the addition, for example, of HC1 to 2,4-hexadiene, which yields 4-chloro-2-hexene and 2-chloro-3-hexene:

1,2- vs. 1,4-Addition. Rate vs. equilibrium

• The fact that either compound is converted into the same mixture by heating indicates that this mixture is the result of equilibrium between the two compounds. The fact that the 1,4-compound predominates in the equilibrium mixture- indicates that it is the more stable of the two. The fact that more 1,2- than 1,4-product is obtained at 80 indicates that the 1,2-product is formed faster than the 1,4-product; since each compound remains unchanged at 80, the proportions in which they are isolated show the proportions in which they were initially formed. As the reaction temperature is raised, the proportions in which the products are initially formed may remain the same, but there is faster conversion of the initially formed products into the equilibrium mixture. The proportions of products actually isolated from the low-temperature addition are determined by the rates of addition, whereas for the high-temperature addition they are determined by the equilibrium between the two isomers. Let us examine the matter of 1,2- and 1,4-addition more closely by drawing a potential energy curve for the reactions involved (Fig. 8.8). The carbonium ion initially formed reacts to yield the 1,2-product faster than the 1,4-product; consequently, the energy of activation leading to the 1 ,2-product must be 

• These facts illustrate two important points. First, we must be cautious wh'en we interpret product composition in terms of rates of reaction; we must be sure that one product is not converted into the other after its formation. Second, the more stable product is by no means always formed faster. On the basis of much evidence, we have concluded that generally the more stable a carbonium ion or free radical, the faster it is formed; a consideration of the transition states for the various reactions has shown (Sees. 3.26, 5.21, and 6.11) that this is reasonable. We must not, however, extend this principle to other reactions unless the evidence warrants it.

Free-radical addition to conjugated dienes: orientation

• Like other alkenes, conjugated dienes undergo addition not only by electrophilic reagents but also by free radicals. In free-radical addition, conjugated dienes show two special features: they undergo 1,4-addition as we Jl as 1,2-addition, and they are much more reactive than ordinary alkenes. We can account for both features orientation and reactivity by examining the structure of the intermediate free radical. Let us take, as an example, addition fabric 3 to 1,3-butadiene in the presence of a peroxide. As we have seen (Sec. 6.18), the peroxide decomposes (step 1) to yield a free radical, which abstracts bromine from Brickl} (step 2) to generate a CC13 radical.

Free-radical addition to conjugated dienes: reactivity

• On the other hand, we have just seen (Sec. 8.16) that conjugated dienes are more stable than simple alkenes. On this basis alone, we might expect addition to conjugated dienes to occur more slowly than to simple alkenes. The relative rates of the two reactions depend chiefly upon the ac t's- Stabilization of the incipient allyl free radical lowers the energy level of the transition state; stabilization of the diene lowers the energy of the reactants. Whether the net oat is larger or smaller than for addition to a simple alkene depends upon which is stabilized more.

Free-radical polymerization of dienes. Rubber and rubber substitutes

• Like substituted ethylene's, conjugated dienes, too, undergo free-radical polymerization. From 1,3-butadiene, for example, there is obtained a polymer.

 Isoprene and the isoprene rule

• The isoprene unit is one of nature's favorite building blocks. It occurs not only in rubber, but in a wide variety of compounds isolated from plant and animal sources. For example, nearly all the terpenes (found in the essential oils of many plants) have carbon skeletons made up of isoprene units joined in a regular, head to-tail way. Recognition of this fact the so-called isoprene rule has been of great help in working out structures of terpenes.

Analysis of alkynes and dienes

• Alkynes and dienes respond to characterization tests in the same way as alkenes: they decolorize bromine in carbon tetrachloride without evolution of hydrogen bromide, and they decolorize cold, neutral, dilute permanganate; they are not oxidized by chromic anhydride. They are, however, more unsaturated than alkenes. This property can be detected by determination of their molecular formulas (CnH2n -2) an^ by a quantitative hydrogenation (two moles of hydrogen are taken up per mole of hydrocarbon). Proof of structure is best accomplished by the same degradative methods that are used in studying alkenes. Upon ozonolysis alkynes yield carboxylic acids, whereas alkenes yield aldehydes and ketones. For example: CH3CH2C==Chj 2-Pentyne. CH3CH2COOH + Houch.