Unlike alkanes, alkynes are
unstable and very reactive. This gives rise to the
intense heat (> 3000 °C) of the acetylene flame used in welding.
Alkynes
Synthesis, Structure & Reactions
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| Unit 14
Alkynes Synthesis, Structure & Reactions |
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Nomenclature
The alkynes form another homologous series, with the incremental unit again being CH2. They are named according to two systems. In one, they are considered to be derived form acetylene by replacement of one or both H atoms by alkyl groups (e.g. methylacetylene).
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 -yne replaces -ene. The parent structure is the longest continuous chain that contains the triple bond. The positions of both the substituents and the triple bond are identified numerically.
Physical Properties
Due to their 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, and carbon tetrachloride. They are less dense than water, with relative densities ranging from 0.67 to 0.77. Their boiling points show the usual increase with increasing carbon number, and the usual effects of chain-branching (lower melting and boiling points due to increased surface area).
Acetylene: Industrial Source / Uses
The alkyne of chief industrial importance is the simplest member of the family: acetylene.
In addition, research by European aerospace firms into using light hydrocarbon compounds with liquid oxygen as a relatively high performing propellant combination showed that methylacetylene (propyne) would be highly advantageous as rocket fuel for craft intended for low Earth orbital operations. This conclusion was reached based upon a high density and energy/volume ratio. Another advantage is the moderate boiling point, which causes the chemical to present fewer problems in storage than a fuel that needs to be kept at extremely low temperatures.
The principal raw materials for acetylene manufacture are calcium carbonate (limestone) and coal. The calcium carbonate is first converted into calcium oxide and the coal into coke. Then these two are reacted with each other to form calcium carbide and carbon monoxide:
Calcium carbide (or calcium acetylide) and water are then reacted by any of several methods to produce acetylene and calcium hydroxide.
Calcium carbide synthesis requires an extremely high temperature (~ 2000 °C) so the reaction is performed in an electric arc furnace. This reaction was an important part of the industrial revolution in chemistry that occurred as a product of massive amounts of cheap hydroelectric power liberated from Niagara Falls before the turn of the (19th)century.
Acetylene can also be manufactured by the cracking of hydrocarbons, as well as by the controlled, high-temperature (~ 1500 °C) partial oxidation of methane.
6 CH4 + O2 --> 2 C2H2 + 2 CO + 10 H2
Using acetylene, Bertholet was the first to show that an aliphatic compound could be used to form an aromatic compound. He did this by heating acetylene in a glass tube to produce benzene and toluene. He also oxidized acetylene to yield acetic acid and oxalic acid, as well as reducing acetylene to form ethylene and ethane.
Structure of the Triple Bond
The C-C triple bond is the unique functional group of the chemical family called alkynes. Like the double bond it is unsaturated and highly reactive and plays a special role - one of increasing importance - in organic chemistry.
The simplest member of the alkyne family is acetylene, C2H2.
Using the methods we applied to the structure of ethylene, we arrive at a structure in which the carbon atoms share three pairs of electrons. The C-C triple bond is the distinguishing feature of the alkyne structure.
In order to form bonds with two other atoms , carbon makes use of two equivalent sp hybrid orbitals. These sp orbitals lie along a straight line that passes through the carbon nucleus. The bond angle between the two orbitals is thus 180 degrees. This linear arrangement permits the hybrid orbitals to be as far apart as possible.
If we arrange the two C atoms and the two H atoms of acetylene to permit a maximum overlap of orbitals, we obtain the linear structure. Acetylene is a linear molecule with all 4 atoms lying along a single straight line. Both the C-H bonds and the C-C bonds are cylindrically symmetrical about a line joining the nuclei, and are therefore sigma bonds.
In forming the sp orbitals already described, each carbon atom has used only one of its three p orbitals. Each of these consist of two equal lobes whose axis lies at right angles both to the axis of the other p orbital and to the line of the sp orbitals. Each p orbital is occupied by a single electron. But the sum of the two perpendicular p orbitals is not four spherical lobes, but a single doughnut-shaped cloud.
Overlap of the p orbitals on one carbon with the p orbitals on the other carbon permits pairing of electrons. The two pi bonds are formed, which together compose a continuous cylindrical sheath about the line joining the nuclei.
The triple bond is thus made up of one strong sigma bond (198 kcal) and two weaker pi bonds. It is stronger than the C-C double bond of ethylene (163 kcal) or the C-C single bond of ethane (88 kcal) and therefore is shorter (1.21 angstroms) than either ethylene (1.34 angstroms) or ethane (1.53 angstroms). The C-H distance in acetylene is 1.08 angstroms, even shorter than in ethylene (1.10 angstroms).
Interestingly, the same sp-hybridization which almost certainly makes cleavage of the C-H bond to form free radicals (homolysis) more difficult, makes cleavage to form ions (heterolysis) easier.
Thus, linear sp geometries characterize the units of both terminal and and internal triple bonds. This linear geometry is responsible for the relatively small number of known cycloalkynes. It is useful to compare some structural features of alkanes, alkenes, and alkynes. We find as we progress through a series in the order of:
ethane --> ethylene --> acetylene
1) The geometry at the C atom changes from:
tetrahedral --> trigonal planar --> linear
2) The C-C and C-H bonds become shorter and stronger
3) The acidity of the C-H bond increases.
The bond distances, bond strengths, and acidities are related to the s character in the orbitals used for bonding. This character results directly form the percentaqe of the hybrid orbital contributed by an s orbital. Thus, an sp3 orbital has one-quarter s character and three-quarters p, an sp2 orbital has one-third s and two-thirds p, and an sp orbital has one-half s and one-half p.
One simple method f gauging the effect of the s character of carbon is to associate it with electronegativity. As the s character of carbon increases, so does its electronegativity. This is due to the fact that the electrons in the bond involving that orbital are closer to the carbon atom. Thus, the H atoms in the bonds behave as if they are attached to an increasingly more electronegative carbon in the series ethane --> ethylene --> acetylene.
Alkynes as Very Weak Acids
In our earliest considerations of acids (Lowry-Bronsted theory) acidity is defined as a measure of the tendency of a compound to lose a hydrogen ion. Appreciable acidity is evidenced by compounds in an H atom is attached to a relatively electronegative atom (e.g. N, O, S. X). The bond holding the H atom is polar, and the relatively (+) H atom can separate as the cation.
Alternatively, 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 normally consider it an acid at all.
In organic chemistry, we are frequently concerned with the acidities of compounds that do not turn litmus red or neutralize aqueous bases - yet they have a small tendency to lose a hydrogen ion. For example, a triply bonded carbon acts as thought it were an entirely different element -- a more electronegative one -- form a carbon having single or even double bonds. Thus, an H atom attached to a triply bonded carbon (terminal or not) shows appreciable acidity.
For example, sodium reacts with acetylene to liberate hydrogen gas and form the compound sodium acetylide.
H - C = C - H + Na --> [ H - C = C: - , Na+ ] + H2
Thus, as one might suspect, the property that most distinguishes acetylene (or ethyne) from ethane and ethylene is its acidity. In general, the C-H bonds of hydrocarbons show little tendency to ionize, and alkanes, alkenes, and alkynes are all very weak acids. Even so, the rarely seen conjugate base of a hydrocarbon is called a carbanion. It is an anion in which the negative charge is borne by carbon. Because it is derived form a very weak acid, a carbanion such as :CH3- is an exceptionally strong base -- or nucleophile.
As stated previously, the ability of an atom to bear a negative charge is directly related to its electronegativity. Both the electronegativity of an atom X and the acidity of compound increase across a row in the periodic table.
CH4 < NH3 < H2O < HF
Using the fact that the effective electronegativity of carbon in a C-H bond increases with its s character (sp3 < sp2 < sp), the order of hydrocarbon acidity behaves quite similarly.
alkane < alkene < alkyne
Thus, ionization of acetylene gives an anion in which the unshared electron pair occupies an orbital with 50% s character. Let us now compare the relative acidity of ethylene (acetylene) with a number of familiar inorganic and organic compounds.
Acetylene is a weaker acid than water. Thus, when water is added to an acetylide, hydroxide ion is formed and acetylene is liberated.
H - OH + [ H - C = C - , Li + ] --> H - C = C - H + [ Li + , OH - ]
Or, noting the stronger arrow for the reverse reaction:
How can we account for the fact that hydrogen attached to a triply bonded carbon is especially acidic - or the fact that acetylene is a stronger acid than, say ethane ? One possible explanation can be found in the electronic configurations of the anions. For example, in the corresponding ionizations of ethylene and ethane, the unshared pair occupies an orbital with 33% (sp2) and 25% (sp3) character, respectively. Terminal alkynes (RC=CH) resemble acetylene in acidity.
If acetylene is a stronger acid than ethane, then the acetylide ion must be a weaker base than the ethide ion, C2H5 -. In the acetylide anion, the unshared pair of electrons occupies
an sp orbital. In the ethide ion, the unshared pair of electrons occupies an sp3 orbital. The availability of this pair for sharing with acids determines the basicity of the anion. Now, compared with an sp3 orbital, an sp orbital has less p character and more s character. An electron in a orbital is at some distance from the nucleus and is held relatively loosely. An electron in an s orbital, however, is close to the nucleus and is held more tightly. The acetylide ion is the weaker base since its pair of electrons is held more tightly - in an sp orbital.
Although acetylene and terminal alkynes are far stronger acids than other hydrocarbons, we must remember that they are, nevertheless, very weak acids. They are much weaker, for example, than water and alcohols. The hydroxide ion is too weak a base to convert acetylene to its anion in meaningful quantities. The position of the equilibrium described by the following equation lies overwhelmingly to the left:
Because acetylene is a far weaker acid than water and alcohols, these substances are not suitable solvents for reactions involving acetylide ions. Acetylide is instantly converted to acetylene by proton transfer from compounds that contain OH groups.
However, acetylene is stronger acid than ammonia. For example, lithium metal reacts with ammonia NH3 to form lithium amide LiNH2. This is the salt of the weak acid H-NH2 (or NH3, ammonia).
NH3 + Li --> [ Li + , NH3 - ] + H2
Addition of acetylene to lithium amide dissolved in ether produces ammonia and lithium acetylide. Thus, the amide ion is a much stronger base than acetylide ion and converts acetylene to its conjugate base quantitatively:
Solutions of sodium acetylide (HC=CNa) may be prepared by adding sodium amide (NaNH2) to acetylene in liquid ammonia as the solvent. Terminal alkynes react similarly to give species of the type RC=CNa.
We can now insert acetylene into our sequence of relative acidity and basicity. Other alkynes containing an H atom attached to a triply bonded carbon (terminal alkynes) show comparable trends.
Relative Acidities
H2O > ROH > HC=CH > NH3 > RH
Relative Basicities
OH- < OR - < HC=C- < NH2 - < R -
Thus, for example, acetylene should be a stronger acid than an alkane RH. This is quite true, and the difference in acidity is of considerable use synthesis. If a terminal acetylene is treated with alkylmagnesium halide (or an alkyl lithium), the alkane is displaced from its salt, and the metal acetylide is obtained. For example:
CH3=CH + C2H5MgBr --> C2H6 + CH3C=C-MgBr
Such reactions provide the best route to this class of important organometallic compounds.
Anions of acetylene and terminal alkynes are nucleophilic and react with methyl and primary alkyl halides to form C-C bonds by nucleophilic substitution. Useful applications of this reaction will follow.
Preparation of Alkynes: Elimination
A C=C triple bond is typically formed in the same way as a double bond: by elimination of atoms or groups from two adjacent C atoms. The eliminated groups and the reagents used in the process are essentially the same as in the preparation of alkenes.
Dehydrohalogenation of Dihalides
This method is particularly useful since the dihalides themselves are readily obtained form the corresponding alkenes by the addition of halogen. This amounts to conversion -- by several steps -- of a double bond into a triple bond. Dehydrohalogenation can generally be carried out in two stages as shown.
The method can be implemented by utilizing vicinal dihalides, in which the halogens are on adjacent carbons, as follows:
Alternatively, geminal dihalides (both halogens on the same carbon) may be used .
Carried through only the first stage, it is a valuable method for preparing unsaturated vinylic halides, which we already know to be highly unreactive (see Unit 13: Conjugated Systems). Thus, vigorous conditions (use of a stronger base, e.g. NaNH2) are required for alkyne formation.
The most frequent applications of these procedures lie in the preparation of terminal alkynes. Because the terminal alkyne is sufficiently acidic to transfer a proton to the amide ion, one equivalent of base in required in addition to the two equivalents needed for double dehydrohalogenation. Adding water or acid after the reaction will convert the sodium salt (e.g. CH3CC=C- , Na+) to the corresponding alkyne (e.g. CH3CC=CH).
This same reaction can be carried out by heating geminal and vicinal dihalides with potassium tert-butoxide in dimethyl sulfoxide.
Also, because vicinal dihalides are prepared by addition of halogens to alkenes (see Unit 11: Alkenes II), alkenes - esp. terminal alkenes - can serve as precursors for terminal alkynes.
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Alternatively, conversion of smaller alkynes into larger ones is accomplished by use of metal acetylides. As we shall see, these are particularly easy to generate because of a special property of certain alkynes and, once formed, are highly versatile reagents.
Reactions of the Triple Bond
Just as alkene chemistry is the chemistry of the C-C double bond, so alkyne chemistry is the chemistry of the C-C triple bond. Like alkenes, alkynes undergo electrophilic addition. For the same reason, they are characterized by the availability of loosely bound pi electrons.
Addition of hydrogen, halogens, and hydrogen halides to alkynes is very much like addition to alkenes. The difference is that in alkynes, two molecules of reagent can be consumed for each triple bond.
As shown here, it is generally possible whenever desired to limit the reaction to the first stage of addition -- the formation of alkenes.
Addition of Hydrogen
The conditions for hydrogenation of alkynes are similar to those employed for alkenes. In the presence of finely divided platinum, palladium, nickel, or rhodium, two molar equivalents of hydrogen add to the triple bond to yield an alkane. Thus, The end result of the unlimited addition of hydrogen to an alkyne would be a completely saturated compound -- or alkane.
The heat of hydrogenation of an alkyne is greater than twice the heat of hydrogenation of an alkene. When two moles of hydrogen add to an alkyne, addition of the first mole (triple bond --> double bond) is more exothermic than then the second (double bond --> single bond).
Substituents affect the heats of hydrogenation in the same way they affect alkenes. E.G. Both 1-butyne and 2-butyne produce butane when combined with 2 moles of H2. But the heat of hydrogenation of 1-butyne (CH3CH2C=CH) is greater than that of 2-butyne (CH3C=CCH3) by ~ 20 kJ/mol. The internal triple bond of 2-butyne is stabilized relative to the terminal triple bond of 1-butyne. Alkyl groups release electrons to sp-hybridized carbon, stabilizing the alkyne and decreasing the heat of hydrogenation.
Stereoisomers. It is generally possible whenever desired to limit the reaction to the first stage of addition -- the formation of alkenes. Reduction of alkynes to the double-bond stage can yield either a cis or a trans alkene. The predominant isomer depends upon the choice of reducing agent.
Each of these reactions is thus highly stereoselective. The stereoselectivity in the syn-reduction of alkynes is attributed to the attachment of two H atoms to the same side of an alkyne sitting on the catalyst surface. Presumably this same stereoselectivity holds for the hydrogenation of terminal alkynes, RC=CH, which cannot yield cis and trans alkenes.
Thus, hydrogenation of alkynes using the Lindlar catalyst is attractive because it sidesteps the regioselectivity and stereoselectivity issues that accompany the dehydration of alcohols and the dehydrohalogenation of alkyl halides. The position of the double bond is never in doubt. It appears in the carbon chain at exactly the same location where the triple bond was located. In terms of stereoselectivity, only the cis-alkene forms.
Electrophilic Addition Reactions
Addition of acids like halogens or hydrogen halides is electrophilic addition -- and it appears to follow the same mechanism with alkynes as with alkenes. This involves an intermediary carbocation stage. But in this case, the intermediary is a vinylic cation.
We learned previously that -- relative to the substrates for heterolysis -- vinyl cations are even less stable than primary alkyl cations. We also saw that -- by heterolysis -- they are formed comparatively slowly and can be generated only by the departure of "super" leaving groups.
Now, in electrophilic addition to alkenes, we saw that reactivity depends upon the stability of the intermediate carbocation. The more stable the carbocation, the faster it is formed. And yet experiments have shown that addition to alkynes is not very much slower that to alkenes. Why not ?
Recall by definition that the stability of a carbocation is relative to the substrate from which it is generated. Thus, relative to substrates for heterolysis, vinylic cations are unstable. We have attributed thus to the unusually strong bond holding the leaving group in vinylic substrates -- not to any inherent instability in the cations themselves. And by heterolysis, vinylic cations are slow to form.
But in addition reactions, the substrates are alkenes and alkynes - and these compounds must be the standards for comparison of carbocation stability: an alkene for a saturated carbocation, and an alkyne of a vinylic cation. Relative to these respective substrates, the two are of comparable stability. I.E. the energy difference between an alkyne and a vinylic cation is approximately equal to that between an alkene and a saturated carbocation.
I. Addition of Halogens
Alkynes react with halogens to yield tetrahaloalkanes. Two molecules of the halogen (chlorine or bromine) add to the triple bond as follows:
A dihaloalkane is an intermediate and is the isolated product when the alkyne and the halogen are present in equimolar amounts. Toward the addition of of halogens, alkynes are considerably less reactive than alkenes. For alkenes, as we have seen, this reaction involves the the initial formation of a cyclic halonium ion. The lower reactivity of alkynes has been attributed to the greater difficulty of forming such cyclic intermediates.
II. Addition of Hydrogen Halides
Alkynes react with many of the same electrophilic reagents that add to the double bond of alkanes. E.G. Hydrogen halides add to the alkynes to form alkenyl halides.
The regioselectivity of addition follows Markovnikov's rule. A proton adds to the carbon that has the greatest number of hydrogens attached to it, and a halide adds to the C atom with the fewer H atoms attached to it. For example:
To explain this, we could propose a process analogous to that of electrophilic addition to alkenes -- in which the first step is the formation of a carbocation and is rate-determining. Then the second step would be nucleophilic capture of the carbocation by a halide ion as follows:
Evidence indicates, however, that alkenyl cations (aka vinylic cations) are far less stable than simple cations, and their involvement in these additions has been questioned. E.G. Although electrophilic addition of hydrogen sulfides to alkynes occurs more slowly than the corresponding additions to alkenes. The difference is not nearly as great as the difference in carbocation stabilities would suggest.
In addition, kinetic studies suggest that electrophilic addition of hydrogen halides to alkynes follows a rate law that is third-order overall and second order in hydrogen halide.
The third order rate dependence suggests a transition state involving two molecules of the hydrogen halide and one of the alkyne. The following figure depicts a one-step termolecular process using curved arrows to show the follow of electrons.
Dashed lines are to indicate the bonds being made and broken at the transition state.
This reaction mechanism, called AdE3 for addition-electrophilic-termolecular, avoids the formation of a very unstable alkenyl cation intermediate by invoking nucleophilic participation by the halogen at an early stage. Nevertheless, because Markovnikov's rule is observed, it seems likely that some degree of positive character develops at the C atom and controls the regioselectivity of addition.
In the presence of excess hydrogen halide, geminal dihalides are formed by sequential addition of two molecules of hydrogen halide to the C-C triple bond.
The second mole of hydrogen halide adds to the initially formed alkenyl halide in accordance with Markovnikov's rule. Both protons become bonded to the same carbon and both halogens to the adjacent carbon.
III. Addition of Water (Hydration)
By analogy to the hydration of alkenes, hydration of an alkyne is expected to produce an alcohol. The alcohol, however, would have to be one in which the hydroxyl group is a substituent on a C=C double bond. This type of alcohol is called an -enol (the double bond suffix -ene plus the alcohol suffix -ol). An important property of enols is their rapid isomerization to aldehydes or ketones under the conditions of their formation.
The aldehyde or ketone is called the keto form, and the keto = enol equilibration is referred to as keto-enol isomerism or keto-enol tautomerism.
Tautomers are constitutional isomers that equilibrate by migration of an atom or group. Their equilibration is called tautomerism.
Keto-enol isomerism involves the sequence of proton transfers shown in the reaction mechanism described here for the formation of a ketone from an alcohol.
Step 1: Protonation of the double bond of the enol (analogous to the protonation of the double bond of an alkene).
It takes place more readily, however, because the carbocation formed in this step is stabilized by resonance involving delocalization of a lone pair of electrons on the oxygen.
Of the two, carbocation A has only 6 electrons around its positively charged C atom -- whereas carbocation B satisfies the octet rule for both carbon and oxygen. Thus carbocation B is more stable than carbocation A. (It is also more stable than a carbocation formed by protonation of a typical alkene).
Step 2: Proton transfer from a carbocation oxygen to a water molecule.
In general, ketones are more stable than their enol precursors and are the products actually isolated when alkynes undergo acid-catalyzed hydration. The standard method for alkyne hydration employs sulfuric acid as the reaction medium and mercury(II) sulfate or mercury(II) oxide as a catalyst.
I. Formation of Metal Acetylides
Besides addition, alkynes undergo certain reactions which are due to the acidity of an H atom held by a triply bonded C atom.
