Vollhardt 6e Lecture PowerPoints - Chapter 13

5/21/2018 Vollhardt 6e Lecture PowerPoints - Chapter 13

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CHAPTER 13

Alkynes:

The Carbon-Carbon Triple Bond


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Naming the Alkynes13-1

The general formula for the alkynes is CnH2n-2, the same as forcycloalkenes.

The common name for the smallest alkyne, C2H2, is acetylene.Common names of other alkynes are treated as its derivatives for instance, the alkylacetylenes.


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The IUPAC rules for naming alkenes also apply to alkynes with theending yne replacing ene. A number indicates the position ofthe triple bond in the main chain.

Alkynes having the structure, RCCH are terminal; those with thestructure RCCRare internal.

Substituents bearing a triple bond are alkynl groups: -CCH is

named ethynyl; -CH2CCH is 2-propynyl (propargyl).


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In IUPAC nomenclature, a hydrocarbon containing both a doubleand a triple bond is called an alkenyne.

The chain is numbered starting at the end closest to eitherfunctional group. In the case of a tie, the double bond is given thelower number.

Alkynes containing a hydroxyl group are named alkynols. In thiscase, the OH takes precedence over both double and triple

bonds in numbering the chain.


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Properties and Bonding in the Alkynes13-2

Alkynes are relatively nonpolar.

Corresponding alkynes, alkenes and alkanes have very similarboiling points.

Ethyne: sublimes at -84oC

Propyne: b.p. -23.2oC

1-Butyne: b.p. 8.1oC

2-Butyne: b.p. 27oC

Medium sized alkanes: distillable liquids

Alkynes polymerize easily, frequently with violence.


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Ethyne is linear and has strong, short bonds.

The two carbons in ethyne are sp2hybridized.

The

bonds are diffuse and resemble a cylindrical cloud:


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The strength of a C-C triplebond is about 229 kcal mol-1.

As with alkenes, the strengthof the bonds is much weakerthan that of the bond whichgives rise to much of thechemical reactivity of thealkynes.

The CC bond length is 1.20 (C=C is 1.33 ).

The C-H bond lengths are also shorter than in ethene due to the largerdegree of s character in the sp hybrid bonds (as compared to sp2).


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Alkynes are high-energy compounds.

Alkynes often react with the release of considerable amounts ofenergy (prone to explosive decomposition).

Ethyne has a heat of combustion of 311 kcal mol-1which iscapable of generating a flame temperature >2500oC, sufficientfor use in welding torches.


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The heats of hydrogenation of alkyne isomers can be used todetermine their relative stabilities:

The greater relative stability of internal alkynes is due tohyperconjugation.


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Terminal alkynes are remarkably acidic.

The electronegativity of a carbon atom depend upon itshybridization. The more s character in its hybrid orbitals, the

greater the electronegativity.The acidity of a C-H bond is directly related to theelectronegativity of the carbon atom:


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Strong bases such as sodium amide in liquid ammonia,alkyllithiums, and Grignard reagents can deprotonate terminalalkynes to the corresponding alkynyl anions.

Alkynyl anions can react as bases and nucleophiles, much likeother carbanions.


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Spectroscopy of the Alkynes13-3

The NMR absorptions of alkyne hydrogens show acharacteristic shielding.

The 1H NMR absorptions of alkynyl hydrogens occur at = 1.7 3.1 ppm, much more shielded than alkenyl hydrogens at =4.6 5.7 ppm.


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The shielding of an alkynyl hydrogen is due to the circularmotions of the electrons.

Unlike in an alkene, the electrons in an alkyne are found in a

cylindrical distribution about the C-C bond. The motions of theseelectrons generate a local magnetic field which opposes Hoin thevicinity of the alkyne hydrogen.

The result is a strong shielding effect that cancels the deshieldingtendency of the electron-withdrawing sp-hybridized carbon.


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The triple bond transmits spin-spin coupling.

The triple bond in an alkyne transmits long-range couplingbetween the alkynyl hydrogen and the hydrogens located 3

carbon atoms away (J = 2-4 Hz).


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The signal for the C1 proton at 1.94 ppm is split into a triplet bythe protons on C3.

The signal for the C3 protons at 2.16 ppm are split into a doublet

of triplets by the protons on C1 and C4.


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The 13C NMR chemical shifts of alkyne carbons aredistinct from those of the alkanes and alkenes.

The 13C absorbances of alkyne, alkene, and alkane carbons are all

sufficiently different to unambiguous assignments in a 13C NMRspectrum.

Alkyne C: = 65-95 ppm

Alkene C: = 100-150 ppm

Alkane C: = 5-45 ppm


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Terminal alkynes give rise to two characteristicinfrared absorptions.

Terminal alkynes exhibit characteristic stretching bands for the

alkynyl hydrogen at 3260-3330 cm-1and for the CC at 21002260 cm-1(often weak for internal alkynes).


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Preparation of Alkynes by Double Elimination13-4

Alkynes are prepared from dihaloalkanes byelimination.

Treatment of vicinal dihaloalkanes with two equivalents of strongbase results in a double elimination reaction to form a triple bond.


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In the case of a terminal alkyne, three equivalents of base arerequired due to the immediate deprotonation of each alkyneformed by an equivalent of base.

A synthetic sequence called halogenation-doubledehydrohalogenation can be used to convert alkenes into thecorresponding alkynes.


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Haloalkenes are intermediates in alkyne synthesisby elimination.

An intermediate product in the dehydrohalogenation of a

dihaloalkane is a haloalkene or alkenyl halide.With diastereomerically pure vicinal dihaloalkones, a singlehaloalkene product is formed due to the anti elimination reactionmechanism.


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Preparation of Alkynes from Alkynyl Anions13-5

Terminal alkynyl anions will react with alkylating agents such asprimary haloalkanes, oxacyclopropanes, and aldehydes or

ketones.Unlike ordinary alkyl organometallic compounds, the reactionalkynyl anions with primary haloalkanes results in C-C bondformation.

Reaction with secondary and tertiary halides leads to E2 products.


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Other reactions of alkynyl anions:


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Reduction of Alkynes: The Relative Reactivity ofthe Two Bonds

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Alkynes can undergo addition reactions, such as hydrogenationand electrophilic attack.


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Cis alkenes can be synthesized by catalytichydrogenation.

Catalytic hydrogenation of alkynes using hydrogen and a platinumor palladium on charcoal catalyst results in complete saturation.


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Catalytic hydrogenation using a Lindlarcatalyst (palladium precipitated on CaCO3,and treated with lead acetate and quinoline)

adds only one equivalent of hydrogen in asyn process:

This method affords a stereoselective synthesis of cis alkenesfrom alkynes.


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Sequential one-electron reductions of alkynesproduce trans alkenes.

Reduction of alkynes using metallic sodium dissolved in liquid

ammonia (dissolving-metal reduction) produces trans alkenes.


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The final alkene is stableto further reduction by thisreagent.

The second electrontransfer takes place faster

than any cis/transequilibrium of the alkenylradical.


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Electrophilic Addition Reactions of Alkynes13-7

Addition of hydrogen halides forms haloalkenes andgeminal dihaloalkanes.

In an analogous reaction with alkenes, hydrogen halides addacross alkyne triple bonds.

The stereochemistry of this reaction is typically anti, particularlywhen excess halide ion is used.


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A second molecule of hydrogen halide may also add, followingMarkovnikovs rule, producing a geminal dihaloalkane.

Terminal alkynes also react with hydrogen halide, again followingMarkovnikovs rule, although it is difficult to limit the reaction to asingle molecule of hydrogen halide.


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Halogenation also takes place once or twice.

Halogenation of alkynes proceeds through an isolatableintermediate vicinal dihaloalkene, to the tetrahaloalkane. The two

additions are anti.


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Mercuric ion-catalyzed hydration of alkynesfurnishes ketones.

In a reaction catalyzed by mercuric ion, water can be added to

alkynes to give enols, which then tautomerize to give ketones.


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Hydration follows Markovnikovs ruleterminal alkynes givemethyl ketones:


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Symmetrical internal alkynes give a single carbonyl compound;unsymmetrical systems give a mixture of products:

A ti M k ik Additi t T i l B d13 8


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Anti-Markovnikov Additions to Triple Bonds13-8

Radical addition of HBr gives 1-bromoalkenes.

In the presence of light or other radical initiators, HBr can addto an alkyne by a radical mechanism in an anti-Markovnikovfashion. Both syn and anti additions are observed.


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Aldehydes result from hydroboration-oxidation ofterminal alkynes.

Hydroboration of terminal alkynes occurs in an anti-Markovnikov

fashion: The less hindered carbon is attacked by the boron.In the case of borane, BH3, both bonds react.

To stop at the alkenyl-borane stage, a bulky borane reagent mustbe used:

Ch i t f Alk l H lid13 9


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Chemistry of Alkenyl Halides13-9

Alkenyl halides do not undergo SN2 or SN1reactions.

Alkenyl halides are relatively unreactive towards nucleophiles.

They undergo elimination reactions with strong bases to givealkynes, however, they do not react with weak bases andrelatively nonbasic nucleophiles, such as iodide.

In addition, SN1 reactions of alkenes do not generally take placebecause the intermediate alkenyl cations are of too high anenergy.


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Organometallic derivatives of alkenyl halides can react to producea variety of specifically substituted alkenes:

Ethyne as an Industrial Starting Material13 10


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Ethyne as an Industrial Starting Material13-10

Production of ethyne from coal requires hightemperatures.

Several thousand degrees Celsius are required to convert coal andhydrogen gas into ethyne:

In an alternate reaction, coke and limestone are heated to formcalcium carbide, which is then treated with water to produceethyne.


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Ethyne is a source of valuable monomers forindustry.

Ethyne is involved in a wide variety of industrial reactions.

Propenoic acid can be polymerized into polyacrylates which havereplaced natural rubber in many applications.


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2-Butyne-1,4-diol is the precursor for the production ofoxacyclopentane (tetrahydrofuran):


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Polymers of acrylonitrile (acrylic fibers) include clothing (Orlon),

carpets, and insulation.Copolymers of acrylonitrile and 1015% vinyl chloride are used inchildrens sleepwear due to their fire resistant properties.

Naturally Occurring and Physiologically13 11


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Naturally Occurring and PhysiologicallyActive Alkynes

13-11

Chamomile flower

Chrysanthemum


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Lower Amazon Indianarrowhead poison

Poison Arrow Frog toxin


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Nonprescription hypnotic Naturally occurring antibiotic-antitumor agents

Important Concepts13


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1. Naming Alkynes same as for alkenes When both double and triple bonds are present, the

name alkenyne is used with the double bondreceiving the lower number if both are at equivalentpositions.

Hydroxyl groups are given precedence when

numbering alkynl alcohols (alkynols).2. Triple Bond Electronic Structuretwo

perpendicular bonds and a bond (formed fromoverlapping sp orbitals)

Linear Structure CC bond energy: 229 kcal/mol C-H bond energy: 131 kcal/mol CC bond length: 1.20

C-H bond length: 1.06



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3. C1 Bound Hydrogen Is Relatively AcidicpKa~ 25.

4. NMR

Alkynl hydrogen shift (= 1.7-3.1 ppm) low

compared to alkenyl hydrogen Triple bond allows for long-range coupling IR terminal alkynes

CC 2100-2260 cm-1 C-H 3260-3330 cm-1

5. Vicinal Dihaloalkanes Elimination reaction

proceeds regioselectively and stereospecifically to givealkenyl halides.



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6. Selective Dihydrogenation of Alkynes Syn dihydrogenation using a Lindlar catalyst (does not

hydrogenate alkenes) Anti hydrogenation using sodium metal dissolved in liquid

ammonia (simple alkenes cannot be reduced by one electrontransfer)

7. Addition Reactions Alkynes undergo the samereactions as alkenes. Reactions may occur twice insuccession.

Hydration of alkynes is unusual. It requires a Hg(II) catalyst andthe enol formed tautomerizes to a ketone.

8. Hydroboration

Dialkylboranes(dicyclohexylborane) are used to stop thehydroboration of terminal alkynes at the alkenylboronstage. Oxidation of the resulting alkenylboranesproduces enols, which tautomerize to aldehydes.

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Vollhardt 6e Lecture PowerPoints - Chapter 13