Ch 10- Alkynes.pdf

Chapter 10

10.1 Alkynes

10.1 Alkynes

• Given the presence of two pi bonds and their associated electron density, alkynes are similar to alkenes in their ability to act as a nucleophile

10.1 Alkyne Uses

• It is used in blow torches and as a precursor for the synthesis of more complex alkynes

• More than 1000 different alkyne natural products have been isolated

• One example is

histrionicotoxin, which can be isolated from South American frogs and is used on poison-tipped arrows by South

• An example of a synthetic alkyne is ethynylestradiol

• How do you think a CC triple bond affects the

molecules geometry? Its rigidity? Its intermolecular

10.1 Alkyne Uses

• Ethynylestradiol is the active

ingredient in many birth control pills

• The presence of the triple bond increases the potency of the drug compared to

10.2 Alkyne Nomenclature

• 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 CC triple bond

2. Identify and Name the substituents

3. Assign a locant (and prefix if necessary) to each substituent

giving the CC 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

• 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 CC triple bond

10.2 Alkyne Nomenclature

• Alkynes are named using the same procedure we used in Chapter 4 to name alkanes with minor modifications

3. Assign a locant (and prefix if necessary) to each substituent

giving the CC triple bond the lowest number possible

10.2 Alkyne Nomenclature

• Alkynes are named using the same procedure we used in Chapter 4 to name alkanes with minor modifications

4. List the numbered substituents before the parent name in alphabetical order. Ignore prefixes (except iso) when ordering alphabetically

10.2 Alkyne Nomenclature

• In addition to the IUPAC naming system, chemists often use common names that are derived from the common parent name acetylene

10.2 Alkyne Nomenclature

10.3 Alkyne Acidity

• Recall that terminal alkynes have a lower pK_a than other hydrocarbons

• Acetylene is 19 pK_a units more acidic than ethylene, which is 1019 times stronger

10.3 Alkyne Acidity

• Because acetylene (pK_a=25) is still much weaker than water (pK_a=15.7), a strong base is needed to make it react, and water cannot be used as the solvent

• Recall from chapter 3 we used the acronym, ARIO, to rationalize differences in acidity strengths

10.3 Alkyne Acidity

10.4 Preparation of Alkynes

10.4 Preparation of Alkynes

• Such eliminations usually occur via an E2 mechanism

• Geminal dihalides can be used

• Vicinal dihalides can also be used

10.4 Preparation of Alkynes

• Often, excess equivalents of NaNH₂ are used to shift the equilibrium toward the elimination products

• NH₂1- _{is quite strong, so if a terminal alkyne is produced,} it will be deprotonated

10.4 Preparation of Alkynes

10.5 Reduction of Alkynes

• Like alkenes, alkynes can readily undergo hydrogenation

• Two equivalents of H₂ are consumed for each alkynealkane

conversion

• The cis alkene is produced as an

10.5 Reduction w/ a Poisoned Catalyst

selectively react with the alkyne

10.5 Reduction w/ a Poisoned Catalyst

10.5 Dissolving Metal Reductions

• Dissolving metal conditions can give Anti addition producing the trans alkene

• Ammonia has a boiling point = -33°C, so the

temperature for these reactions must remain very low

• Mechanism: Step 1

10.5 Dissolving Metal Reductions

• Note the single-barbed and double-barbed (fishhook) arrows.

10.5 Dissolving Metal Reductions

• Mechanism: Step 1

• Why is the first intermediate called a radical anion?

• Mechanism: step 2 and 3

• Mechanism: step 4

• Do the pK_a values for NH₃ and the alkene favor the proton transfer?

• Predict the product(s) for the following reaction

• Familiarize yourself with the reagents necessary to manipulate alkynes

• Like alkenes, alkynes also undergo hydrohalogenation

• Draw the final product for the reaction above

• Do the reactions above exhibit Markovnikov

• You might expect alkynes to undergo

hydrohalogenation by a mechanism similar to alkenes

• Yet, the mechanism above does not explain all observed phenomena

– A slow reaction rate, 3rd _{order overall rate law, like 1°}

carbocations, vinylic carbocations are especially

10.6 Hydrohalogenation of Alkynes

• Kinetic studies on the hydrohalogenation of an alkyne suggest that the rate law is 1st order with respect to the alkyne and 2nd _{order with respect to HX}

• What type of collision would result in such a rate law? Unimolecular, bimolecular, or termolecular?

• Reaction rate is generally slow for termolecular collisions. WHY?

• Considering the polarizability of the alkyne, does the mechanism explain the regioselectivity?

• Peroxides can be used in the hydrohalogenation of

alkynes to promote anti-Markovnikov addition just like with alkenes

• Which product is E and which is Z?

• The process proceeds through a free radical mechanism that we will discuss in detail in Chapter 11

• Like alkenes, alkynes can also undergo acid catalyzed Markovnikov hydration

• The process is generally catalyzed with HgSO₄ to

compensate for the slow reaction rate that results from the formation of vinylic carbocation

• HgSO₄ catalyzed hydration involves the mecury (II) ion interacting with the alkyne

• Can you imagine what that interaction might look like and how it will increase the rate of reaction for the process?

• The enol/ketone tautomerization generally cannot be prevented and favors the ketone greatly

• Tautomers are constitutional isomers that rapidly interconvert. How is that different from resonance?

• Hydroboration-oxidation for alkynes proceeds through the same mechanism as for alkenes giving the anti

-Markovnikov product

• It also produces an enol that will quickly tautomerize

• In this case, the tautomerization is catalyzed by the

• In general, we can conclude that a C=O double bond is more stable than a C=C double bond. WHY?

• After the –BH₂ and –H groups have been added across the C=C double bond, in some cases, an undesired

second addition can take place

• To block out the second unit of BH₃ from reacting with the intermediate, bulky borane reagents are often used

• Some bulky borane reagents are shown below

• Predict products for the following reaction

• Draw the alkyne reactant and reagents that could be used to synthesize the following molecule

• Markovnikov hydration leads to a ketone

• Anti-Markovnikov hydration leads to an aldehyde

• Alkynes can also undergo halogenation

• Two equivalents of halogen can be added

• You might expect the mechanism to be similar to the halogenation of alkenes, yet stereochemical evidence suggests otherwise – see next slide

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

• The halogenation of an alkene undergoes anti addition ONLY

• The mechanism for alkyne halogenation is not fully elucidated

• When alkynes react under ozonolysis conditions, the pi system is completely broken

• The molecule is cleaved, and the alkyne carbons are

• Predict the product(s) for the following reaction

• As acids, terminal alkynes are quite weak

• Yet, with a strong enough base, a terminal alkyne can be deprotonated and converted into a good nucleophile

• What has a higher pK_a, NH₃ or R-CC-H? WHY?

• The alkynide ion can attack a methyl or 1° alkyl halide electrophile

• Such reactions can be used to develop molecular complexity

• Alkynide ions usually act as bases with 2° or 3° alkyl halides to cause elimination rather than

• Acetylene can be used to perform a double alkylation

• Why will the reaction be unsuccessful if the NaNH₂ and Et-Br are added together?

• Complex target molecules can be made by building a

• Recall the methods for increasing the saturation of alkenes and alkynes

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

• Halogenation of an alkene followed by two

dehydrohalogenation reactions can decrease saturation

• We will have to wait until chapter 11 to see how to

convert an alkane into an alkene, but here is a preview

• In the alkene to alkyne conversion above, why is water needed in part 3) of that reaction?

• Give necessary reaction conditions for the multi-step conversions below

Additional Practice Problems

Additional Practice Problems

Additional Practice Problems

• Give a set of reagents that could be used to synthesize

cis-2-pentene from an addition reaction.

• Give a set of reagents that could be used to synthesize

Additional Practice Problems

• Give a set of reagents that could be used to synthesize a ketone from an addition reaction.

Additional Practice Problems