A parallel and independent method of characterizing organic reactions is by oxidation-reduction terminology. Carbon atoms may have any oxidation state from –4 (e.g. CH4 ) to +4 (e.g. CO2 ), depending upon their substituents. Fortunately, we need not determine the absolute oxidation state of each carbon atom in a molecule, but only the change in oxidation state of those carbons involved in a chemical transformation. To determine whether a carbon atom has undergone a redox change during a reaction we simply note any changes in the number of bonds to hydrogen and the number of bonds to more electronegative atoms such as O, N, F, Cl, Br, I, & S that has occurred. Bonds to other carbon atoms are ignored. This count should be conducted for each carbon atom undergoing any change during a reaction.
1. If the number of hydrogen atoms bonded to a carbon increases, and/or if the number of bonds to more electronegative atoms decreases, the carbon in question has been reduced (i.e. it is in a lower oxidation state).
2. If the number of hydrogen atoms bonded to a carbon decreases, and/or if the number of bonds to more electronegative atoms increases, the carbon in question has been oxidized (i.e. it is in a higher oxidation state).
3. If there has been no change in the number of such bonds, then the carbon in question has not changed its oxidation state. In the hydrolysis reaction of a nitrile, shown below, the light-blue colored carbon has not changed its oxidation state.
These rules are illustrated by the following four addition reactions involving the same starting material, cyclohexene. Carbon atoms colored green are reduced, and those colored red are oxidized. In the addition of hydrogen both carbon atoms are reduced, and the overall reaction is termed a reduction. Addition of bromine or HOCl oxidizes both carbon atoms, so these may be termed oxidation reactions. Addition of HBr reduces one of the double bond carbon atoms and oxidizes the other; consequently, there is no overall redox change in the substrate molecule.
For a discussion of how oxidation state numbers may be assigned to carbon atoms Click Here.
Since metals such as lithium and magnesium are less electronegative than hydrogen, their covalent bonds to carbon are polarized so that the carbon is negative (reduced) and the metal is positive (oxidized). Thus, Grignard reagent formation from an alkyl halide reduces the substituted carbon atom. In the following equation and half-reactions the carbon atom (blue) is reduced and the magnesium (magenta) is oxidized.
1. Hydroxylation Dihydroxylated products (glycols) are obtained by reaction with aqueous potassium permanganate (pH > 8) or osmium tetroxide in pyridine solution. Both reactions appear to proceed by the same mechanism (shown below); the metallocyclic intermediate may be isolated in the osmium reaction. In basic solution the purple permanganate anion is reduced to the green manganate ion, providing a nice color test for the double bond functional group. From the mechanism shown here we would expect syn-stereoselectivity in the bonding to oxygen, and regioselectivity is not an issue. When viewed in context with the previously discussed addition reactions, the hydroxylation reaction might seem implausible. Permanganate and osmium tetroxide have similar configurations, in which the metal atom occupies the center of a tetrahedral grouping of negatively charged oxygen atoms. How, then, would such a species interact with the nucleophilic pi-electrons of a double bond? A possible explanation is that an empty d-orbital of the electrophilic metal atom extends well beyond the surrounding oxygen atoms and initiates electron transfer from the double bond to the metal. Back-bonding of the nucleophilic oxygens to the antibonding pi orbital completes this interaction. The result is formation of a metallocyclic intermediate, as shown.
2. Epoxidation Some oxidation reactions of alkenes give cyclic ethers in which both carbons of a double bond become bonded to the same oxygen atom. These products are called epoxides or oxiranes. An important method for preparing epoxides is by reaction with peracids, RCO3H. The oxygen-oxygen bond of such peroxide derivatives is not only weak (ca. 35 kcal/mole), but in this case is polarized so that the acyloxy group is negative and the hydroxyl group is positive (recall that the acidity of water is about ten powers of ten weaker than that of a carboxylic acid). If we assume electrophilic character for the OH moiety, the following equation may be written.
It is unlikely that a dipolar intermediate, as shown above, is actually formed. The epoxidation reaction is believed to occur in a single step with a transition state incorporating all of the bonding events shown in the equation. Consequently, epoxidations by peracids always have syn-stereoselectivity, and seldom give structural rearrangement. You may see the transition state by clicking the Change Equation button. Presumably the electron shifts indicated by the blue arrows induce a charge separation that is immediately neutralized by the green arrow electron shifts.
Vicinal halohydrins provide an alternative route for the epoxidation of alkenes over that of reaction with peracids. As illustrated in the following diagram, a base induced intramolecular substitution reaction forms a three-membered cyclic ether called an epoxide. Both the halohydrin formation and halide displacement reactions are stereospecific, so stereoisomerism in the alkene will be reflected in the epoxide product (i.e. trans-2-butene forms a trans-disubstituted epoxide).
The previous few reactions have been classified as reductions or oxidations, depending on the change in oxidation state of the functional carbons. It is important to remember that whenever an atom or group is reduced, some other atom or group is oxidized, and a balanced equation must balance the electron gain in the reduced species with the electron loss in the oxidized moiety, as well as numbers and kinds of atoms. Starting from an alkene (drawn in the box), the following diagram shows a hydrogenation reaction on the left (the catalyst is not shown) and an epoxidation reaction on the right. Examine these reactions, and for each identify which atoms are reduced and which are oxidized.
Epoxides may be cleaved by aqueous acid to give glycols that are often diastereomeric with those prepared by the syn-hydroxylation reaction described above. Proton transfer from the acid catalyst generates the conjugate acid of the epoxide, which is attacked by nucleophiles such as water in the same way that the cyclic bromonium ion described above undergoes reaction. The result is anti-hydroxylation of the double bond, in contrast to the syn-stereoselectivity of the earlier method. In the following equation this procedure is illustrated for a cis-disubstituted epoxide, which, of course, could be prepared from the corresponding cis-alkene. This hydration of an epoxide does not change the oxidation state of any atoms or groups.
1. Ozonolysis In determining the structural formula of an alkene, it is often necessary to find the location of the double bond within a given carbon framework. One way of accomplishing this would be to selectively break the double bond and mark the carbon atoms that originally formed that bond. For example, there are three isomeric alkenes that all give 2-methylbutane on catalytic hydrogenation. These are 2-methyl-2-butene (compound A), 3-methyl-1-butene (compound B) and 2-methyl-1-butene (compound C), shown in the following diagram. If the double bond is cleaved and the fragments marked at the cleavage sites, the location of the double bond is clearly determined for each case. A reaction that accomplishes this useful transformation is known. It is called ozonolysis, and its application to each of these examples may be seen by clicking the "Show Reaction" button.
Ozone, O3, is an allotrope of oxygen that adds rapidly to carbon-carbon double bonds. Since the overall change in ozonolysis is more complex than a simple addition reaction, its mechanism has been extensively studied. Reactive intermediates called ozonides have been isolated from the interaction of ozone with alkenes, and these unstable compounds may be converted to stable products by either a reductive workup (Zn dust in water or alcohol) or an oxidative workup (hydrogen peroxide). The results of an oxidative workup may be seen by clicking the "Show Reaction" button a second time. Continued clicking of this button repeats the cycle. The chief difference in these conditions is that reductive workup gives an aldehyde product when hydrogen is present on a double bond carbon atom, whereas oxidative workup gives a carboxylic acid or carbon dioxide in such cases. The following equations illustrate ozonide formation, a process that is believed to involve initial syn-addition of ozone, followed by rearrangement of the extremely unstable molozonide addition product. They also show the decomposition of the final ozonide to carbonyl products by either a reductive or oxidative workup.
From this analysis and the examples given here, you should be able to deduce structural formulas for the alkenes that give the following ozonolysis products.
2. Glycol Cleavage The vicinal glycols prepared by alkene hydroxylation (reaction with osmium tetroxide or permanganate) are cleaved to aldehydes and ketones in high yield by the action of lead tetraacetate (Pb(OAc)4) or periodic acid (HIO4). This oxidative cleavage of a carbon-carbon single bond provides a two-step, high-yield alternative to ozonolysis, that is often preferred for small scale work involving precious compounds. A general equation for these oxidations is shown below. As a rule, cis-glycols react more rapidly than trans-glycols, and there is evidence for the intermediacy of heterocyclic intermediates (as shown), although their formation is not necessary for reaction to occur.
1. Addition of Radicals to Alkenes Protons and other electrophiles are not the only reactive species that initiate addition reactions to carbon-carbon double bonds. Curiously, this first became evident as a result of conflicting reports concerning the regioselectivity of HBr additions. As noted earlier, the acid-induced addition of HBr to 1-butene gave predominantly 2-bromobutane, the Markovnikov Rule product. However, in some early experiments in which peroxide contaminated reactants were used, 1-bromobutane was the chief product. Further study showed that an alternative radical chain-reaction, initiated by peroxides, was responsible for the anti-Markovnikov product. This is shown by the following equations.
The weak O–O bond of a peroxide initiator is broken homolytically by thermal or hight energy. The resulting alkoxy radical then abstracts a hydrogen atom from HBr in a strongly exothermic reaction. Once a bromine atom is formed it adds to the π-bond of the alkene in the first step of a chain reaction. This addition is regioselective, giving the more stable carbon radical as an intermediate. The second step is carbon radical abstraction of another hydrogen from HBr, generating the anti-Markovnikov alkyl bromide and a new bromine atom. Each of the steps in this chain reaction is exothermic, so once started the process continues until radicals are lost to termination events. This free radical chain addition competes very favorably with the slower ionic addition of HBr described earlier, especially in non-polar solvents. It is important to note, however, that HBr is unique in this respect. The radical addition process is unfavorable for HCl and HI because one of the chain steps becomes endothermic (the second for HCl & the first for HI). Other radical addition reactions to alkenes have been observed, one example being the peroxide induced addition of carbon tetrachloride shown in the following equation
The best known and most important use of free radical addition to alkenes is probably polymerization. Since the addition of carbon radicals to double bonds is energetically favorable, concentrated solutions of alkenes are prone to radical-initiated polymerization, as illustrated for propene by the following equation. The blue colored R-group represents an initiating radical species or a growing polymer chain; the propene monomers are colored maroon. The addition always occurs so that the more stable radical intermediate is formed.
2. Allylic Substitution We noted earlier that benzylic and allylic sites are exceptionally reactive in free radical halogenation reactions. Since carbon-carbon double bonds add chlorine and bromine in liquid phase solutions, radical substitution reactions by these halogens are often carried out at elevated tempreature in the gas phase (first equation below). Formation of the ionic π-complexes that are intermediates in halogen addition is unfavorable in the absence of polar solvents, and entropy generally favors substitution over addition. The brominating reagent, N-bromosuccinimide (NBS), has proven useful for achieving allylic or benzylic substitution in CCl4 solution at temperatures below its boiling point (77 ºC). One such application is shown in the second equation.
The predominance of allylic substitution over addition in the NBS reaction is interesting. The N–Br bond is undoubtedly weak (probably less than 50 kcal/mol) so bromine atom abstraction by radicals should be very favorable. The resulting succinimyl radical might then establish a chain reaction by removing an allylic hydrogen from the alkene. One problem with this mechanism is that NBS is very insoluble in CCl4, about 0.006 mole / liter at reflux. Although it is possible that the allylic bromination occurs at a solid-liquid interface, evidence for another pathway has been obtained. In the non-polar solvent used for these reactions, very low concentrations of bromine may be generated from NBS. This would serve as a source of bromine atoms, which would abstract allylic hydrogens irreversibly (an exothermic reaction) in competition with reversible addition to the double bond. The HBr produced in this way is known to react with NBS, giving a new bromine molecule and succinimide, as shown here. Ionic addition of bromine to the double bond would be very slow in these circumstances.
This mechanism is essentially the same as that for the free radical halogenation of alkanes, with NBS serving as a source of very low concentrations of bromine. Thanks to resonance delocalization, unsymmetrical allylic radicals will react to give two regioisomers. Thus, 1-octene on bromination with NBS yields a mixture of 3-bromo-1-octene (ca. 18%) and 1-bromo-2-octene (82%) - both cis and trans isomers.
The following problems review many aspects of alkene chemistry. Finally, a random quiz may be examined.
A carbon-carbon triple bond may be located at any unbranched site within a carbon chain or at the end of a chain, in which case it is called terminal. Because of its linear configuration ( the bond angle of a sp-hybridized carbon is 180º ), a ten-membered carbon ring is the smallest that can accomodate this function without excessive strain. Since the most common chemical transformation of a carbon-carbon double bond is an addition reaction, we might expect the same to be true for carbon-carbon triple bonds. Indeed, most of the alkene addition reactions discussed earlier also take place with alkynes, and with similar regio- and stereoselectivity.
1. Catalytic Hydrogenation The catalytic addition of hydrogen to 2-butyne not only serves as an example of such an addition reaction, but also provides heat of reaction data that reflect the relative thermodynamic stabilities of these hydrocarbons, as shown in the diagram to the right. From the heats of hydrogenation, shown in blue in units of kcal/mole, it would appear that alkynes are thermodynamically less stable than alkenes to a greater degree than alkenes are less stable than alkanes. The standard bond energies for carbon-carbon bonds confirm this conclusion. Thus, a double bond is stronger than a single bond, but not twice as strong. The difference ( 63 kcal/mole ) may be regarded as the strength of the π-bond component. Similarly, a triple bond is stronger than a double bond, but not 50% stronger. Here the difference ( 54 kcal/mole ) may be taken as the strength of the second π-bond. The 9 kcal/mole weakening of this second π-bond is reflected in the heat of hydrogenation numbers ( 36.7 - 28.3 = 8.4 ). Since alkynes are thermodynamically less stable than alkenes, we might expect addition reactions of the former to be more exothermic and relatively faster than equivalent reactions of the latter. In the case of catalytic hydrogenation, the usual Pt and Pd hydrogenation catalysts are so effective in promoting addition of hydrogen to both double and triple carbon-carbon bonds that the alkene intermediate formed by hydrogen addition to an alkyne cannot be isolated. A less efficient catalyst, Lindlar's catalyst, prepared by deactivating (or poisoning) a conventional palladium catalyst by treating it with lead acetate and quinoline, permits alkynes to be converted to alkenes without further reduction to an alkane. The addition of hydrogen is stereoselectively syn (e.g. 2-butyne gives cis-2-butene). A complementary stereoselective reduction in the anti mode may be accomplished by a solution of sodium in liquid ammonia. This reaction will be discussed later in this section.
Alkenes and alkynes show a curious difference in behavior toward catalytic hydrogenation. Independent studies of hydrogenation rates for each class indicate that alkenes react more rapidly than alkynes. However, careful hydrogenation of an alkyne proceeds exclusively to the alkene until the former is consumed, at which point the product alkene is very rapidly hydrogenated to an alkane. This behavior is nicely explained by differences in the stages of the hydrogenation reaction. Before hydrogen can add to a multiple bond the alkene or alkyne must be adsorbed on the catalyst surface. Alkynes adsorb more strongly than alkenes, and preferentially occupy reactive sites on the catalyst. Subsequent transfer of hydrogen to the adsorbed alkyne proceeds slowly, relative to the corresponding hydrogen transfer to an adsorbed alkene molecule. Consequently, reduction of triple bonds occurs selectively at a moderate rate, followed by rapid addition of hydrogen to the alkene product. The Lindlar catalyst permits adsorbtion and reduction of alkynes, but does not adsorb alkenes sufficiently to allow their reduction.
2. Addition by Electrophilic Reagents When the addition reactions of electrophilic reagents, such as strong Brønsted acids and halogens, to alkynes are studied we find a curious paradox. The reactions are even more exothermic than the additions to alkenes, and yet the rate of addition to alkynes is slower by a factor of 100 to 1000 than addition to equivalently substituted alkenes. The reaction of one equivalent of bromine with 1-penten-4-yne, for example, gave 4,5-dibromo-1-pentyne as the chief product.
Although electrophilic additions to alkynes are sluggish, they do take place and generally display Markovnikov Rule regioselectivity and anti-stereoselectivity. One problem, of course, is that the products of these additions are themselves substituted alkenes and can therefore undergo further addition. Because of their high electronegativity, halogen substituents on a double bond act to reduce its nucleophilicity, and thereby decrease the rate of electrophilic addition reactions. Consequently, there is a delicate balance as to whether the product of an initial addition to an alkyne will suffer further addition to a saturated product. Although the initial alkene products can often be isolated and identified, they are commonly present in mixtures of products and may not be obtained in high yield. The following reactions illustrate many of these features. In the last example, 1,2-diodoethene does not suffer further addition inasmuch as vicinal-diiodoalkanes are relatively unstable.
Why are the reactions of alkynes with electrophilic reagents more sluggish than the corresponding reactions of alkenes? After all, addition reactions to alkynes are generally more exothermic than additions to alkenes, and there would seem to be a higher π-electron density about the triple bond ( two π-bonds versus one ). Two factors are significant in explaining this apparent paradox. First, although there are more π-electrons associated with the triple bond, the sp-hybridized carbons exert a strong attraction for these π-electrons, which are consequently bound more tightly to the functional group than are the π-electrons of a double bond. This is seen in the ionization potentials of ethylene and acetylene.
As defined by the preceding equations, an ionization potential is the minimum energy required to remove an electron from a molecule of a compound. Since pi-electrons are less tightly held than sigma-electrons, we expect the ionization potentials of ethylene and acetylene to be lower than that of ethane, as is the case. Gas-phase proton affinities show the same order, with ethylene being more basic than acetylene, and ethane being less basic than either. Since the initial interaction between an electrophile and an alkene or alkyne is the formation of a pi-complex, in which the electrophile accepts electrons from and becomes weakly bonded to the multiple bond, the relatively slower reactions of alkynes becomes understandable. The second factor is the stability of the carbocation intermediate generated by sigma-bonding of a proton or other electrophile to one of the triple bond carbon atoms. This intermediate has its positive charge localized on an unsaturated carbon, and such vinyl cations are less stable than their saturated analogs. Indeed, we can modify our earlier ordering of carbocation stability to include these vinyl cations.
Application of the Hammond postulate then suggests that the activation energy for the generation of such an intermediate would be higher than that for a lower energy intermediate. This is illustrated by the following energy diagrams.
Despite these impediments, electrophilic additions to alkynes have emerged as exceptionally useful synthetic transforms. For example, addition of HCl, acetic acid and hydrocyanic acid to acetylene give respectively the useful monomers vinyl chloride, vinyl acetate and acrylonitrile, as shown in the following equations. Note that in these and many other similar reactions transition metals, such as copper and mercury salts, are effective catalysts.
3. Hydration of Alkynes and Tautomerism As with alkenes, the addition of water to alkynes requires a strong acid, usually sulfuric acid, and is facilitated by mercuric sulfate. However, unlike the additions to double bonds which give alcohol products, addition of water to alkynes gives ketone products ( except for acetylene which yields acetaldehyde ). The explanation for this deviation lies in enol-keto tautomerization, illustrated by the following equation. The initial product from the addition of water to an alkyne is an enol (a compound having a hydroxyl substituent attached to a double-bond), and this immediately rearranges to the more stable keto tautomer.
Tautomers are defined as rapidly interconverted constitutional isomers, usually distinguished by a different bonding location for a labile hydrogen atom (colored red here) and a differently located double bond. The equilibrium between tautomers is not only rapid under normal conditions, but it often strongly favors one of the isomers ( acetone, for example, is 99.999% keto tautomer ). Even in such one-sided equilibria, evidence for the presence of the minor tautomer comes from the chemical behavior of the compound. Tautomeric equilibria are catalyzed by traces of acids or bases that are generally present in most chemical samples. The three examples shown below illustrate these reactions for different substitutions of the triple-bond. The tautomerization step is indicated by a red arrow. For terminal alkynes the addition of water follows the Markovnikov rule, as in the second example below, and the final product ia a methyl ketone ( except for acetylene, shown in the first example ). For internal alkynes ( the triple-bond is within a longer chain ) the addition of water is not regioselective. If the triple-bond is not symmetrically located ( i.e. if R & R' in the third equation are not the same ) two isomeric ketones will be formed.
Two factors have an important influence on the enol-keto tautomerizations described here. The first is the potential energy difference between the tautomeric isomers. This factor determines the position of the equilibrium state. The second factor is the activation energy for the interconversion of one tautomer to the other. This factor determines the rate of rearrangement. Since the potential energy or stability of a compound is in large part a function of its covalent bond energies, we can estimate the relative energy of keto and enol tautomers by considering the bonds that are changed in the rearrangement. From the following diagram, we see that only three significant changes occur, and the standard bond energies for those changes are given to the right of the equation. The keto tautomer has a 17.5 kcal/mole advantage in bond energy, so its predominance at equilibrium is expected.
The rapidity with which enol-keto tautomerization occurs suggests that the activation energy for this process is low. We have noted that the rearrangement is acid & base catalyzed, and very careful experiments have shown that interconversion of tautomers is much slower if such catalysts are absent. A striking example of the influence of activation energy on such transformations may be seen in the following hypothetical rearrangement. Here we have substituted a methyl group (colored maroon) for the proton of a conventional tautomerism, and shown the methyl shifting from oxygen to carbon, just as the proton does in going from an enol to a ketone.
H2C=CH-O-CH3 –X–> CH3-CH2-CH=O
The potential energy change for this rearrangement is even more advantageous than for enol-keto tautomerism, being estimated at over 25 kcal/mole from bond energy changes. Despite this thermodynamic driving force, the enol ether described above is completely stable to base treatment, and undergoes rapid acid-catalyzed hydrolysis with loss of methanol, rather than rearrangement. The controlling difference in this case must be a prohibitively high activation energy for the described tautomeric rearrangement, combined with lower energy alternative reaction paths.
4. Hydroboration Reactions Diborane reacts readily with alkynes, but the formation of substituted alkene products leaves open the possibility of a second addition reaction. A clever technique for avoiding this event takes advantage of the fact that alkynes do not generally suffer from steric hindrance near the triple-bond (the configuration of this functional group is linear). Consequently, large or bulky electrophilic reagents add easily to the triple-bond, but the resulting alkene is necessarily more crowded or sterically hindered and resists further additions. The bulky hydroboration reagent needed for this strategy is prepared by reaction of diborane with 2-methyl-2-butene, a highly branched alkene. Because of the alkyl branching, only two alkenes add to a BH3 moiety (steric hindrance again), leaving one B-H covalent bond available for reaction with an alkyne, as shown below. The resulting dialkyl borane is called disiamylborane, a contraction of di-secondary-isoamylborane (amyl is an old name for pentyl).
An important application of disiamylborane is its addition reaction to terminal alkynes. As with alkenes, the B-H reagent group adds in an apparently anti-Markovnikov manner, due to the fact that the boron is the electrophile, not the hydrogen. Further addition to the resulting boron-substituted alkene does not occur, and the usual oxidative removal of boron by alkaline hydrogen peroxide gives an enol which rapidly rearranges to the aldehyde tautomer (red arrow). Thus, by the proper choice of reagents, terminal alkynes may be converted either to methyl ketones (mercuric ion catalyzed hydration) or aldehydes (hydroboration followed by oxidation).
Hydroboration of internal alkynes is not a particularly useful procedure because a mixture of products will often be obtained, unless the triple-bond is symmetrically substituted. Mercuric ion catalyzed hydration gives similar results.
5. Oxidative Cleavage Reactions of alkynes with oxidizing agents such as potassium permanganate and ozone usually result in cleavage of the triple-bond to give carboxylic acid products. A general equation for this kind of transformation follows. The symbol [O] is often used in a general way to denote an oxidation.
The sp-hybrid carbon atoms of the triple-bond render alkynes more electrophilic than similarly substituted alkenes. As a result, alkynes sometimes undergo addition reactions initiated by bonding to a nucleophile. This mode of reaction, illustrated below, is generally not displayed by alkenes, unless the double-bond is activated by electronegative substituents, e.g. F2C=CF2, or by conjugation with an electron withdrawing group.
HC≡CH + HCN + NaCN (catalytic) ——> H2C=CH-CN
The smallest and most reactive nucleophilic species is probably an electron. Electron addition to a functional group is by definition a reduction, and we noted earlier that alkynes are reduced by solutions of sodium in liquid ammonia to trans-alkenes. To understand how this reduction occurs we first need to identify two distinct reactions of sodium with liquid ammonia (boiling point -78 ºC). In the first, sodium dissolves in the pure liquid to give a deep blue solution consisting of very mobile and loosely bound electrons together with solvated sodium cations (first equation below). For practical purposes, we can consider such solutions to be a source of "free electrons" which may be used as powerful reducing agents. In the second case, ferric salts catalyze the reaction of sodium with ammonia, liberating hydrogen and forming the colorless salt sodium amide (second equation). This is analogous to the reaction of sodium with water to give sodium hydroxide, but since ammonia is 1018 times weaker an acid than water, the reaction is less violent. The usefulness of this reaction is that sodium amide, NaNH2, is an exceedingly strong base (18 powers of ten stronger than sodium hydroxide), which may be used to convert very weak acids into their conjugate bases.
Returning to the reducing capability of the blue electron solutions, we can write a plausible mechanism for the reduction of alkynes to trans-alkenes, as shown below. Isolated carbon double-bonds are not reduced by sodium in liquid ammonia, confirming the electronegativity difference between sp and sp2 hybridized carbons.
Alkanes are undoubtedly the weakest Brønsted acids commonly encountered in organic chemistry. It is difficult to measure such weak acids, but estimates put the pKa of ethane at about 48. Hybridizing the carbon so as to increase the s-character of the C-H increases the acidity, with the greatest change occurring for the sp-C-H groups found in terminal alkynes. Thus, the pKa of ethene is estimated at 44, and the pKa of ethyne (acetylene) is found to be 25, making it 1023 times stronger an acid than ethane. This increase in acidity permits the isolation of insoluble silver and copper salts of such compounds.
Despite the dramatic increase in acidity of terminal alkynes relative to other hydrocarbons, they are still very weak acids, especially when compared with water, which is roughly a billion times more acidic. If we wish to prepare nucleophilic salts of terminal alkynes for use in synthesis, it will therefore be necessary to use a much stronger base than hydroxide (or ethoxide) anion. Such a base is sodium amide (NaNH2), discussed above, and its reactions with terminal alkynes may be conducted in liquid ammonia or ether as solvents. The products of this acid-base reaction are ammonia and a sodium acetylide salt. Because the acetylide anion is a powerful nucleophile it may displace halide ions from 1º-alkyl halides to give a more highly substituted alkyne as a product (SN2 reaction). This synthesis application is described in the following equations. The first two equations show how acetylene can be converted to propyne; the last two equations present a synthesis of 2-pentyne from propyne.
The use of acetylide nucleophiles in organic synthesis will be noted again in the alkyl halide chapter.
The following problems review the chemistry of alkynes. The first question requires you to select one of two starting alkynes and one of nine possible reagent systems to prepare a designated product. The second question asks you to draw the products from various alkyne + reagent combinations. Finally, a large assortment of multiple choice questions concerning aliphatic hydrocarbons is presented.