CHAPTER 7

ALKYNES

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Nomenclature

• IUPAC: use the infix -ynyn- to show the presence of a carbon-carbon triple bond.

• Common names: prefix the substituents on the triple bond to the word “acetylene”. (Acetylene is an IUPAC accepted name for ethyne.)

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Cycloalkynes

• Cyclooctyne is the smallest cycloalkyne isolated. (unstable; 155°bond angle)

• Cyclononyne has high degree angle strain.• It is stable at room temp.• The C-C-C bond angle about the triple bond is approximately

160° (rather than the optimal angle of 180°), indicating high angle strain.

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Physical Properties

• Similar to alkanes and alkenes of comparable molecular weight and carbon skeleton.

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Acidity

• The pKa of acetylene and terminal alkynes is approximately 25, which makes them stronger acids than ammonia (pka 38) but weaker acids than alcohols (pka ~16)(Section 4.3).• H bonded to triple bonded C acidic enough to be removed

only by very strong base• Terminal alkynes react with sodium amide to form alkyne

anions.

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Acidity

• Terminal alkynes can also be converted to alkyne anions by reaction with sodium hydride or lithium diisopropylamide (LDA).

• Because water is a stronger acid than terminal alkynes, hydroxide ion is not a strong enough base to convert a terminal alkyne to an alkyne anion.

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Alkylation of Alkyne Anions

• Alkyne anions are both strong bases and good nucleophiles.

• They participate in nucleophilic substitutionnucleophilic substitution reactions with alkyl halides to form new C-C bonds to alkyl groups; they undergo alkylationalkylation.• Because alkyne anions are also strong bases, alkylation is

practical only with methyl and 1° halides

• With 2° & 3° halides, elimination is the major reaction.

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Alkylation of Alkyne Anions

• Alkylation of alkyne anions is the most convenient method for the synthesis of terminal alkynes.

• Alkylation can be repeated and a terminal alkyne in turn converted to an internal alkyne.

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Alkylation of Alkyne Anions

• What is the product of the following alkylation?

CH3CH2CH2CH2C≡CH 1) NaNH2, NH3

2) CH3CH2CH2CH2Br

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Alkylation of Alkyne Anions

• Using NaNH2, show the starting alkyne and alkyl halide needed to make

• A) CH3CH2CH2C≡CCH3

• B)

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Preparation of Alkynes from Alkenes

• Treatment of a vicinal dibromoalkane with two moles of base, most commonly sodium amide, results in two successive dehydrohalogenationdehydrohalogenation reactions (removal of H and X from adjacent carbons) and formation of an alkyne.

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Alkylation of Alkyne Anions

• Draw the intermediate and the major product of the following:

• ?

NaNH2, NH3Cl2

DCM? ?

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Preparation from Alkenes

• For a terminal alkene to a terminal alkyne, 3 moles of base are required.

• A side product of this synthetic scheme may be an alleneallene, a compound containing adjacent carbon-carbon double bonds, C=C=C.

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Allene

• AlleneAllene:: A compound containing a C=C=C group• the simplest allene is 1,2-propadiene, commonly named

allene

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Allenes

• Most allenes are less stable than their isomeric alkynes, and are generally only minor products in alkyne-forming dehydrohalogenation reactions.

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Addition of X2 (electrophilic addition)

• Alkynes add one mole of bromine to give a dibromoalkene.• Addition shows anti stereoselectivity.• Addition of chlorine similar, but less stereoselective.

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Addition of X2

• Addition of bromine to an alkyne follows much the same mechanism as that for the bromination of an alkene, namely formation of a bridged bromonium ion intermediate, which is then attacked by bromide ion from the face opposite that occupied by the positively charged bromine atom.

• Addition of a second mole bromine gives tetrabromoalkane.

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Addition of X2

• What is the major product of the following?

• A) CH3CH2C≡CH + Br2

• B) CH3CH2CH2C≡CH + 2 Cl2

CH2Cl2

CH2Cl2

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Electrophilic Addition of HX

• Alkynes undergo regioselective (follows Markovnikov’s rule) addition of either 1 or 2 moles of HX, depending on the ratios in which the alkyne and halogen acid are mixed.

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Electrophilic Addition of HX

• Step 1: Make a new bond between a nucleophile (a bond) and an electrophile—add a proton.

• Step 2: Make a new bond between a nucleophile and an electrophile.

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Electrophilic Addition of HX

• In the addition of the second mole of HX,• Step 1 is reaction of the electron pair of the remaining pi

bond with HBr to form a carbocation.• Of the two possible carbocations, the resonance-stabilized

2° carbocation is favored.

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Addition of HX

• What is the major product of the following?

• A) + HBr

• B) CH3CH2CH2CH2C≡CH + HBr

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Hydroboration

• Addition of borane to an internal alkyne gives a trialkenylborane.• Addition is syn stereoselective.

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Hydroboration

• To prevent dihydroboration with terminal alkynes, it is necessary to use a sterically hindered dialkylborane, such as (sia)2BH.

• Treatment of a terminal alkyne with (sia)2BH results in stereoselective and regioselective hydroboration.

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Hydroboration

• Treating an alkenylborane with H2O2 in aqueous NaOH gives an enol.

• EnolEnol:: A compound containing an -OH group on one carbon of a carbon-carbon double bond.

• An enol is in equilibrium with a keto form by migration of a hydrogen from oxygen to carbon and migration of the double bond from C=C to C=O.

• Keto forms generally predominate at equilibrium.• Keto and enol forms are tautomerstautomers and their interconversion

is called tautomerismtautomerism.

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Hydroboration/Oxidation; hydration

• Hydroboration/oxidation of an internal alkyne gives a ketone.

• Hydroboration/oxidation of a terminal alkyne gives an aldehyde.

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Hydroboration/oxidation

• What alkyne and reagents would give the following?

• A)

• B)

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Addition of H2O: hydration

• In the presence of concentrated sulfuric acid and Hg(II) salts, alkynes undergo addition of water.

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Addition of H2O: hydration

• Step 1: Make a new bond between a nucleophile ( bond) and an electrophile.

• Step 2: Make a new bond between a nucleophile and an electrophile. (attack of water opens ring)

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Addition of H2O: hydration

• Step 3: Take a proton away. Proton transfer to solvent gives an organomercury enol.

• Step 4: Keto-enol tautomerism of the enol gives the keto form.

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Addition of H2O: hydration

• Step 5: Add a proton. Proton transfer to the carbonyl oxygen gives an oxonium ion.

• Steps 6 and 7: Break a bond to give stable molecules or ions followed by keto-enol tautomerism.

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Acid catalyzed hydration

• What is the major product of the following?

• A) CH3CH2CH2C≡CCH2CH2CH3

• B) (CH3)2CHCH2C≡CCH2CH2CH3

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Reduction

• Treatment of an alkyne with H2 in the presence of a transition metal catalyst, most commonly Pd, Pt, or Ni, converts the alkyne to an alkane.

• With the Lindlar catalyst, the reduction stops at syn-stereoselective addition of one mole of H2.

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Hydroboration - Protonolysis

• Syn-stereoselective addition of borane to an internal alkyne gives a trialkenylborane.

• Treatment of a trialkenylborane with acetic acid results in stereoselective replacement of B by H.

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Dissolving Metal Reduction

• Reduction of an alkyne with Na or Li in liquid ammonia converts an alkyne to an alkene with anti stereoselectivity.

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Dissolving Metal Reduction

• Step 1: A one-electron reduction of the alkyne gives a radical anion intermediate.

• Step 2: Add a proton. The alkenyl radical anion (a very strong base) abstracts a proton from ammonia (a very weak acid).

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Dissolving Metal Reduction

• Step 3: A second one-electron reduction gives an alkenyl anion.

• This step establishes the configuration of the alkene• A trans alkenyl anion is more stable than its cis isomer.

• Step 4: Add a proton. A second acid-base reaction gives the trans alkene.

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Organic Synthesis

• A successful synthesis must• provide the desired product in maximum yield.• have the maximum control of stereochemistry and

regiochemistry.• do minimum damage to the environment (it must be a

“green” synthesis).

• Our strategy will be to work backwards from the target molecule.

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Organic Synthesis

• We analyze a target molecule in the following ways:• The carbon skeleton: how can we put it together? Our only

method to date for forming a new C-C bond is the alkylation of alkyne anions with 1° haloalkanes.

• The functional groups: what are they, how can they be used in forming the carbon-skeleton of the target molecule, and will they be changed by the reaction conditions to give the functional groups of the target molecule?

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Organic Synthesis

• We use a method called a retrosynthesis and an open arrow to symbolize a step in a retrosynthesis.

• RetrosynthesisRetrosynthesis:: A process of reasoning backwards from a target molecule to a set of suitable starting materials.

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Organic Synthesis

• Target molecule: 2-heptanone; starting with acetylene and haloalkanes.

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Organic Synthesis

• Starting materials are acetylene and 1-bromopentane.