Chapter 5Chapter 5 ALKYNESALKYNES

OutlinesOutlines

Nomenclature Nomenclature Preparation Preparation StructureStructureChemical reactionsChemical reactions

2010

IntroductionIntroduction

AlkynesAlkynes are hydrocarbons that have at are hydrocarbons that have at least one least one triple bondtriple bond, , CC≡≡CC with the with the formula formula CCnnHH2n-22n-2. .

Disubstituted alkynes, R-Disubstituted alkynes, R-CC≡≡CC-R', are -R', are described as "described as "internalinternal" alkynes;" alkynes;

Monosubstituted alkynes, R-Monosubstituted alkynes, R-CC≡≡CC-H, -H, are described as "are described as "terminalterminal" alkynes. " alkynes.

Because of its linear configuration, Because of its linear configuration, ten-ten-membered carbon ringmembered carbon ring is the smallest that is the smallest that can contain the alkyne function without can contain the alkyne function without excessive strain. excessive strain.

ethyne (or acetylene) propyne 1-butyne

H-C C-H H-C C- CH3 H-C C-CH2-CH3

2-butyne

Terminal

Internal

CH3-C C-CH3

QuizzQuizz Classify each of the following as an internal or Classify each of the following as an internal or

a terminal alkyne:a terminal alkyne:

a) 1-hexyne  b) 3-octynea) 1-hexyne  b) 3-octyne

c) cyclooctyne   d) propyne c) cyclooctyne   d) propyne 

NomenclatureNomenclature

Akynes are named by general rules similar Akynes are named by general rules similar to those used for alkanes and alkenes.to those used for alkanes and alkenes.

The suffix The suffix -yne-yne is used in the paren is used in the paren hydrocarbon name to denote an alkyne, hydrocarbon name to denote an alkyne, and the position of the triple bond is and the position of the triple bond is indicated by its number in the chain.indicated by its number in the chain.

Numbering begin at the chain end near the Numbering begin at the chain end near the triple bond so that the triple bond as low a triple bond so that the triple bond as low a number as possible.number as possible.

Compounds containing both double and Compounds containing both double and triple bonds are called triple bonds are called enynesenynes..

CH3CH2CHCH2C CCH2CH3

CH3

Begin numbering atthe end nearer the triple bond

6-Methyl-3-octyne

Numbering of the hydrocarbon chain starts Numbering of the hydrocarbon chain starts from the end nearer the first multiple from the end nearer the first multiple bond, whether double or triple. bond, whether double or triple.

If there is a choice in numbering, If there is a choice in numbering, doubledouble bondsbonds receive lowerreceive lower numbers than triple numbers than triple bonds. For example,bonds. For example,

HC CCH 2CH2CH2CH CH2

CH3

1-Hepten-6-yne

167HC CCH 2CHCH2CH2CH CHCH3

4-Methyl-7-nonen-1-yne

1 6 7 9

Preparation of AlkynesPreparation of Alkynes

Alkynes are generally prepared by Alkynes are generally prepared by

** DDehydrohalogenation of ehydrohalogenation of either either geminal geminal (1,1-) or (1,1-) or vicinalvicinal (1,2-) (1,2-)alkyl dihalides alkyl dihalides (usually Br or Cl), using a strong base, (usually Br or Cl), using a strong base, usually NaNHusually NaNH22. .

These reactions are typically E2 reactions These reactions are typically E2 reactions and occur via an and occur via an alkenyl halidealkenyl halide. .

** TThe reaction of metal acetylides with he reaction of metal acetylides with primary alkyl halides. primary alkyl halides.

C C Na + R- X C C R

QuizzQuizz

What is the alkyne product from the reactions What is the alkyne product from the reactions of the following with NaNHof the following with NaNH22::

a) 2,2-dibromopropane a) 2,2-dibromopropane

b) 1,2-dibromohexaneb) 1,2-dibromohexane

c) 1,1-dibromooctanec) 1,1-dibromooctane

d) 2,3-dibromohexaned) 2,3-dibromohexane

Structure and ReactivityStructure and Reactivity

The The alkyne functional groupalkyne functional group consists of two consists of two spsp hybridised C atoms bonded to each other hybridised C atoms bonded to each other via one via one σσ and two and two bonds bonds. .

The 2The 2 bonds are produced by the bonds are produced by the side-to-side-to-side overlapside overlap of the two pairs of of the two pairs of pp-orbitals -orbitals not utilised in the hybridsnot utilised in the hybrids..

The diagram shows the heats of hydrogenation The diagram shows the heats of hydrogenation of 2-butyneof 2-butyne ((in the slide: blue in units of kcal/molein the slide: blue in units of kcal/mole))

It indicates that alkynes are thermodynamically It indicates that alkynes are thermodynamically less stable than alkenes less stable than alkenes to a greater degree than to a greater degree than alkenes are less stable alkenes are less stable than alkanesthan alkanes. .

The "extra" The "extra" bond in an alkyne is weaker that an bond in an alkyne is weaker that an alkene alkene bond: bond:

2 x 2 x 28.328.3 (C=C) – (C=C) – 6565 (C (C≡≡C)= C)= 8.48.4 kcal kcal

The 2 C of the The 2 C of the CC≡≡CC and the 2 atoms and the 2 atoms attached directly to the attached directly to the CC≡≡CC are are linear,linear, so so they they cannot cannot exist as exist as cis-cis-//transtrans- - isomers.isomers.

Two separate perpendicular Two separate perpendicular pp molecular orbitals molecular orbitals

σ

σ

The The bonds are a region of bonds are a region of high electron high electron densitydensity; so it ; so it allows an acetylinic carbon allows an acetylinic carbon to have a greater amount of to have a greater amount of electronegative character. electronegative character.

As a result, As a result, alkynes are typically alkynes are typically nucleophilesnucleophiles. .

Terminal alkynes, R-Terminal alkynes, R-CCCC--HH, are quite , are quite acidicacidic (pKa = 26). (pKa = 26).

ReactingReacting with a strong base such as with a strong base such as sodium, sodium amide, sodium, sodium amide, nn-butyllithium or a -butyllithium or a Grignard reagent, a terminal alkyne gives Grignard reagent, a terminal alkyne gives the the anionanion of the terminal alkyne (a of the terminal alkyne (a metal metal acetylideacetylide):):

2 RC 2 RC ≡ ≡ CH + 2 Na CH + 2 Na →→ 2 RC 2 RC ≡ ≡ CNa + HCNa + H22

The acetylide ion is a good nucleophile and The acetylide ion is a good nucleophile and can be alkylated to give higher alkynes. can be alkylated to give higher alkynes.

Like alkenes (Like alkenes (C=CC=C), the alkyne), the alkyne C C≡≡CC undergoes a variety of addition reactions in undergoes a variety of addition reactions in which one or both of the which one or both of the bonds are bonds are converted to new converted to new σσ bonds. bonds.

Alkylation of AlkynesAlkylation of Alkynes

The acetylide carbanion, The acetylide carbanion, RCRC≡≡CC--, is a good , is a good CC-nucleophile and can undergo -nucleophile and can undergo nucleophilic substitution reactions (usually nucleophilic substitution reactions (usually SNSN22) with 1) with 1oo or 2 or 2oo alkyl halides (Cl, Br or I) alkyl halides (Cl, Br or I)

to produce an to produce an internalinternal alkyne. alkyne.

One or both of the terminal H atoms in One or both of the terminal H atoms in ethyne (acetylene) H-ethyne (acetylene) H-CC≡≡CC-H can be -H can be susbtituted providing monosubstituted (R-susbtituted providing monosubstituted (R-CC≡≡CC-H) and -H) and symmetrical symmetrical (R=R') or (R=R') or unsymmetricalunsymmetrical (R (R ≠ ≠ R') disubstituted R') disubstituted alkynes R-alkynes R-C C ≡≡ C C-R' -R'

Mechanism for Alkylation of alkynesMechanism for Alkylation of alkynes

Step 1Step 1. . The amide ion acts as a The amide ion acts as a base removing the acidic terminalbase removing the acidic terminal H to generate the acetylide ion, H to generate the acetylide ion, a carbon nucleophile.a carbon nucleophile.

Step 2Step 2. . The carbanion reactsThe carbanion reacts with the electrophilic carbon in thewith the electrophilic carbon in the alkyl halide with loss of the leaving alkyl halide with loss of the leaving group, forming a new group, forming a new C-CC-C bond. bond.

Practice ProblemsPractice Problems

What is the product of the reactions of What is the product of the reactions of CHCH33-C-C≡≡CC-- with each of the following:with each of the following:

a) 2-bromopropane (d)  ethanola) 2-bromopropane (d)  ethanol b) 1-iodooctane e)  ethyl b) 1-iodooctane e)  ethyl

tosylatetosylate c) (R)-2-bromohexane f)   c) (R)-2-bromohexane f)  

bromobenzenebromobenzene

Addition Reactions of AlkynesAddition Reactions of Alkynes

AAlkynes undergo addition reactions in an lkynes undergo addition reactions in an analogous fashion to those of alkenes. analogous fashion to those of alkenes.

Two factors influence the relative Two factors influence the relative reactivity of alkynes compared to alkenes: reactivity of alkynes compared to alkenes:

increased nucleophilicity of the starting increased nucleophilicity of the starting system (system (CC≡≡CC  vs    vs  C=CC=C), and ), and

stability of any intermediates stability of any intermediates (for example, carbocations). (for example, carbocations).

Since alkynes are thermodynamically less Since alkynes are thermodynamically less stable than alkenes, we might expect stable than alkenes, we might expect addition reactions of alkynes to be addition reactions of alkynes to be more more exothermicexothermic and relatively and relatively fasterfaster than than equivalent reactions of the alkenes.equivalent reactions of the alkenes.

1. Hydrogenation of Alkynes1. Hydrogenation of Alkynes

Alkynes can be partially reduced to Alkynes can be partially reduced to ciscis--alkenesalkenes with H with H22 in the presence of poisoned in the presence of poisoned catalysts, catalysts,

eeg.g. Pd / CaCO3/ quinoline Pd / CaCO3/ quinoline which is also which is also known as known as LindlarLindlar's's catalyst. catalyst.

Alkynes can be reduced to alkanes with HAlkynes can be reduced to alkanes with H22 in the presence of catalysts (in the presence of catalysts (Pt, Pd, NiPt, Pd, Ni etc.) etc.)

The reaction is The reaction is stereospecificstereospecific giving only thegiving only the syn syn additionaddition product since the new C-H product since the new C-H σσ bonds is formed simultaneously bonds is formed simultaneously from H atoms absorbed onto the from H atoms absorbed onto the metal surface. metal surface.

2. Dissolving Metal Reduction of 2. Dissolving Metal Reduction of AlkynesAlkynes

The reaction of Na in NHThe reaction of Na in NH33(l) with alkynes (l) with alkynes occurs occurs stereospecificlystereospecificly giving only the giving only the transtrans--alkene via an alkene via an antianti-addition-addition. .

The reaction proceeds via single The reaction proceeds via single electron transfer from the Na withelectron transfer from the Na with

H coming from the NHH coming from the NH33. .

These reaction conditions do These reaction conditions do notnot reduce alkenes. reduce alkenes.

3. Reaction of Alkynes with HX3. Reaction of Alkynes with HX

When treated with HX alkynes form vinyl When treated with HX alkynes form vinyl halides. halides.

Hydrogen halide reactivity order : Hydrogen halide reactivity order : HI > HBr > HCl > HFHI > HBr > HCl > HF

((paralleling acidity orderparalleling acidity order).).

In the presence of excess HX, a second In the presence of excess HX, a second addition can occur to the product alkene addition can occur to the product alkene giving a geminal dihalide. giving a geminal dihalide.

Regioselectivity predicted byRegioselectivity predicted by Markovnikov's ruleMarkovnikov's rule with the H adding to with the H adding to the C with the most H already present.the C with the most H already present.

Reaction proceeds via protonation to give Reaction proceeds via protonation to give the the more stable carbocationmore stable carbocation intermediate. intermediate.

Not stereoselective since reaction Not stereoselective since reaction proceeds via planar carbocation proceeds via planar carbocation ((CHCH33CC++=CH=CH22 & & CHCH33CBrCBr++CHCH33). ).

QuizzQuizz

What would be the product from the What would be the product from the reaction of 2-butyne with excess HBr ?  reaction of 2-butyne with excess HBr ? 

For For HBrHBr, in the presence of radicals , in the presence of radicals (compounds such as peroxides) the (compounds such as peroxides) the radical addition occurs with radical addition occurs with opposite opposite regiochemistryregiochemistry: :

Quizz. Quizz. Why does this reaction have the Why does this reaction have the opposite regiochemistry ? opposite regiochemistry ?

Carbocation StabilityCarbocation Stability

CHCH33(+)(+) ≈≈ RCH=CHRCH=CH(+) (+) << RCH RCH22

(+) (+) ≈≈ RCH=CRRCH=CR(+) (+)

Methyl 1°-Vinyl 1° Methyl 1°-Vinyl 1° 2°-Vinyl2°-Vinyl

< < RR22CHCH(+) (+) ≈ ≈ CHCH22=CHCH=CHCH22(+) (+) < < CC66HH55CHCH22

(+)(+) ≈ ≈ RR33CC(+)(+)

2° 1°-Allyl 1°-Benzyl2° 1°-Allyl 1°-Benzyl 3°3°

4. Hydration of Alkynes 4. Hydration of Alkynes

Alkynes can be hydrated to form enols that Alkynes can be hydrated to form enols that immediately immediately tautomerisetautomerise to ketones. to ketones.

ReagentsReagents:  aq. acid, most commonly :  aq. acid, most commonly HH22SOSO44, with a mercury salt. , with a mercury salt.

Regioselectivity predicted by Regioselectivity predicted by Markovnikov's ruleMarkovnikov's rule..

Reaction proceeds via protonation to give Reaction proceeds via protonation to give the more stable carbocation intermediate.the more stable carbocation intermediate.

CHCH33-C≡CH + H-C≡CH + H++ CH CH33-C-C++= CH= CH22

NotNot stereoselective since reactions proceeds stereoselective since reactions proceeds via planar carbocation. via planar carbocation.

5 . Halogenation of Alkynes5 . Halogenation of Alkynes

Overall transformation:  Overall transformation:  C≡ CC≡ C to to XX--C=CC=C--XX (and potentially(and potentially toto XX22C-CXC-CX22).).

ReagenReagent: normally the halogen (t: normally the halogen (e.g.e.g. Br Br22) in an ) in an inert solvent like methylene chloride, CHinert solvent like methylene chloride, CH22ClCl22. .

Reaction proceeds via cyclic Reaction proceeds via cyclic halonium ion.halonium ion.

Stereoselectivity : Stereoselectivity : antianti since since the two the two C-XC-X bonds form in bonds form in

separate steps one from Xseparate steps one from X22,, the other Xthe other X--. .

6. Ozonolysis of Alkynes6. Ozonolysis of Alkynes

Overall transformation :  Overall transformation :  CC≡≡CC to 2 x to 2 x COCO22

Ozonolysis implies that ozone causes the Ozonolysis implies that ozone causes the alkyne to break (-alkyne to break (-lysislysis))

ReagentsReagents : ozone followed by aqueous : ozone followed by aqueous work-up. work-up.

Note that each of the Note that each of the CCCC bonds in the bonds in the CºC CºC becomes a becomes a C=OC=O bond. bond.

Problems at homeProblems at home Classify each of the following as an internal or a terminal Classify each of the following as an internal or a terminal

alkyne alkyne (from the slide 4)(from the slide 4)

a) 1-hexyne  b) 3-octynea) 1-hexyne  b) 3-octyne c) cyclooctyne   d) propyne c) cyclooctyne   d) propyne 

What is the alkyne product from the reactions of the What is the alkyne product from the reactions of the following with NaNHfollowing with NaNH22: (from the slide 10): (from the slide 10)

a) 2,2-dibromopropane a) 2,2-dibromopropane b) 1,2-dibromohexaneb) 1,2-dibromohexane c) 1,1-dibromooctanec) 1,1-dibromooctane d) 2,3-dibromohexaned) 2,3-dibromohexane

What is the product of the reactions of CHWhat is the product of the reactions of CH33--C≡CC≡C-- with each of the followingwith each of the following: : (from the (from the slide 20)slide 20)

a) 2-bromopropane (d)  ethanola) 2-bromopropane (d)  ethanol

b) 1-iodooctane e)  ethyl tosylateb) 1-iodooctane e)  ethyl tosylate

c) (R)-2-bromohexane f)   bromobenzenec) (R)-2-bromohexane f)   bromobenzene

What would be the products of the What would be the products of the ozonolysis reactions ofozonolysis reactions of:  : 

(a) ethyne ?   (b) 1-butyne ?  (a) ethyne ?   (b) 1-butyne ?  (c) 2-butyne ?   (d) cyclooctyne ?(c) 2-butyne ?   (d) cyclooctyne ?

The End of The End of AlkynesAlkynes