1

Chapter 9

Alkynes

2

9.1 Introduction to Alkynes• Alkynes are molecules that possess a CºC triple bond.• Given the presence of p bonds, alkynes are similar to alkenes in their ability to act as

a nucleophile.• Many of the addition reactions of alkenes also work on alkynes.

3

• Alkynes are named using the same procedure we used in Chapter 4 to name alkanes with minor modifications:1. Identify the parent chain, which should include the CºC triple bond.2. Identify and Name the substituents.3. Assign a locant (and prefix if necessary) to each substituent giving the CºC

triple bond the lowest number possible.4. List the numbered substituents before the parent name in alphabetical order.

Ignore prefixes (except iso) when ordering alphabetically.5. The CºC triple bond locant is placed either just before the parent name or

just before the -yne suffix.

9.2 Nomenclature of Alkynes

The locant is ONE number, NOT two. Although the triple bond bridges carbons 2 and 3, the locant is the lower of those two numbers.

4

• Common names derived from acetylene are often used as well.

• Alkynes are also classified as terminal alkyne or internal alkyne.

SBP. 9.1: Provide a systematic name for the following compound.

4-Ethyl-5-methyl-3-propyl-1-heptyne

Try P. 9.1.

5

• Recall that terminal alkynes have a lower pKa (i.e. more acidic) than other hydrocarbons.

• Acetylene is 19 orders of magnitude (1019) more acidic than ethylene.

9.3 Acidity of Terminal Alkynes

• Acetylene can be deprotonated by a strong based to form the conjugate base (acetylide ion).

• Recall ARIO to explain why acetylene is a stronger acid than ethylene which is stronger than ethane.

• The acetylide ion is more stable because the lone pair occupies a sp orbital.

More stableLess stable

6

• A bases conjugate acid pKa must be greater than 25 for it to be able to deprotonate a terminal alkyne.

P. 9.5: In each of the following cases, determine if the base is sufficiently strong to deprotonate the terminal alkyne.

7

• Just as alkenes can be prepared from alkyl halides, alkynes can be prepared from alkyl dihalides.

9.4 Preparation of Alkynes

• Such eliminations usually occur via an E2 mechanism.

• Geminal or vicinal dihalides can be used.

Geminal dihalide Vicinal dihalide

• Excess equivalents of NaNH2 are used to shift the equilibrium toward the elimination products.

• Aqueous workup is then needed to produce the neutral alkyne:

Try P. 9.7 & 9.8.

8

Catalytic hydrogenation: alkyne is concerted to an alkane by addition of two equivalents of H2.

• The first addition produces a cis alkene (via syn addition) which then undergoes addition to yield the alkane.

9.5 Reduction of Alkynes

• A deactivated or poisoned catalyst can be used to stop the reaction at the cis alkene, without further reduction:

• Lindlar’s catalyst and P-2 (Ni2B complex) are common examples of a poisoned catalysts.

9

Dissolving metal reduction: reduces an alkyne to a trans alkene using sodium metal and ammonia.• This reaction is stereoselective for anti addition of H and H.

P. 9.9: Draw the major product expected from each of the following reactions:

10

Mechanism: Step 1• Na atom transfer an electron to the alkyne, forming a radical anion intermediate.• the paired electrons and the single electron adopt an anti-geometry.

Mechanism: Steps 2 and 3• NH3 as proton source.• One more electron from Na.

Mechanism – Step 4• NH3 as proton source.

11

Alkyne reduction strategy

P. 9.11: Identify reagents that you could use to achieve each of the following transformations.

12

• Hydrohalogenation affords Markovnikov addition of H and X to an alkyne, same as with an alkene.

• Excess HX affords a geminal dihalide.

9.6 Hydrohalogenation of Alkynes

addition to an alkene addition to an alkyne

geminal dihalide

• If the mechanism was analogous to HX addition to an alkene, it would require the formation of a vinyl carbocation.

• Vinyl carbocations are extremely unstable, so this mechanism is unlikely.• Kinetic studies suggest the rate law is 1st order with respect to the alkyne and 2nd

order with respect to HX.• Mechanism = a termolecular process.

(involving three molecules)

13

Radical mechanism (chapter 10):• HBr with peroxides promotes anti-Markovnikov addition, just like with alkenes• This only works with HBr (not with HCl or HI; chapter 10)

Interconversion of dihalides and alkynes:• Hydrohalogenation of alkynes,

and elimination of dihalides represent complimentary reactions:

P. 9.13: Predict the major product(s) expected for each of the following reactions.

14

Acid catalyzed Markovnikov hydration of Alkynes:9.7 Hydration of Alkynes

• The process is generally catalyzed with HgSO4 to compensate for the slow reaction rate that results from the formation of vinylic carbocation.

• The enol then tautomerizes to the ketone.• Process is called keto-enol tautomerization• The enol and the ketone are tautomers of one another• Equilibrium generally favors the ketone, since it is thermodynamically more stable.

15

P. 9.16: The following enols cannot be isolated. They rapidly tautomerize to produce ketones. In each case, draw the expected ketone and show a mechanism for its formation under acid-catalyzed conditions (H3O+).

SBP. 9.3: Under normal conditions, 1-cyclohexenol cannot be isolated or stored in a bottle, because it undergoes rapid tautomerization to yield cyclohexanone. Draw a mechanism for this tautomerization:

16

P. 9.19: Identify the alkyne you would use to prepare each of the following ketones via acid-catalyzed hydration.

P. 9.18: Draw the major product(s) expected when each of the following alkynes is treated with aqueous acid in the presence of mercuric sulfate (HgSO4).

17

Hydroboration-oxidation of alkynes: same as for alkenes

• Base-catalyzed tautomerization mechanism: Enol is deprotonated to form an enolate, which is protonated at the carbon to produce the aldehyde.

• Regioselective for anti-Markovnikov addition• produces an enol that tautomerizes to aldehyde • In this case, tautomerization is base-catalyzed

(OH-)

• Unlike an alkene, which only possesses one π bond, an alkyne possesses two π bonds. As a result, two molecules of BH3 can add across the alkyne.

• To prevent the second addition, a dialkyl borane (R2BH) is employed instead of BH3.

The bulky alkyl groups provide steric hindrance to prevent a second addition

18

P. 9.21: Identify the alkyne you would use to prepare each of the following ketones via acid-catalyzed hydration.

P. 9.20: Draw the major product for each of the following reactions

19

Controlling the Regioselectivity of Alkyne Hydration:• For a terminal alkyne:

– Markovnikov hydration yields a ketone– Anti Markovnikov hydration yields an aldehyde

• For an internal alkyne:• Markovnikov hydration yields a ketone• Anti Markovnikov hydration yields an ketone

P. 9.22: Identify reagents that you could use to achieve each of the following transformations

20

• Halogenation of alkynes yields a tetrahalide.• Two equivalents of halogen are added with

excess X2.

9.8 Halogenation of Alkynes

• When one equivalent of halogen is added to an alkyne, both anti and syn addition is observed.

• The mechanism for alkyne halogenation is not fully understood. If it was like halogenation of an alkene, only the anti product would be obtained.

• Ozonolysis of an internal alkyne produces two carboxylic acids.

9.9 Ozonolysis of Alkynes

• Ozonolysis of a terminal alkyne yields a carboxylic acid and carbon dioxide.

21

Mechanism of Alkyne Ozonolysis

OOO

OOO

Et

O O

O Et H2O OHO

OOH

Et+

Zn, H2O H2O

O O

Et O

O

Et

O

H2O

P. 9.22: Draw the major products that are expected when each of the following alkynes is treated with O3 followed by H2O.

22

• Recall that terminal alkynes are completely converted to an alkynide ion with NaNH2.

9.10 Alkylation of Terminal Alkynes

• Acetylene can undergo two successive alkylations.

• Alkylation of an alkynide ion is SN2 substitution, and so it works best with methyl and 1˚ halides. (E2 elimination dominates with 2˚/3˚ halides)

23

• Halogenation of an alkene followed by elimination yields an alkyne

• These reactions give us a handle on interconverting single, double and triple bonds.

• Recall the methods for converting triple bonds to double or single bonds

• But, what if you want to reverse the process or decrease saturation?

9.11 Synthesis Strategies

24

P. 9.29: Propose an efficient synthesis for each of the following transformations.

SBP. 9.6: Propose an efficient synthesis for the following transformation.

25