Alkyne Notes

8/12/2019 Alkyne Notes

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Alkynes are molecules that incorporate a CC triple

bond.

10.1 Alkynes

Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e10 -1


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Chapter 92

Introduction

Alkynes contain a triple bond.

General formula is CnH2n-2.

Two elements of unsaturation for eachtriple bond.

Some reactions resemble the reactions ofalkenes, like addition and oxidation.

Some reactions are specific to alkynes.


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Given the presence of two pi bonds and their associatedelectron density, alkynes are similar to alkenes in their

ability to act as a nucleophile.

Converting pi bonds to sigma bonds generally makes a

molecule more stable. WHY?

10.1 Alkynes



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Chapter 94


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Acetylene is the simplest alkyne.

It is used in blow torches and as a precursor for the synthesis

of more complex alkynes.

More than 1000 different alkyne natural products havebeen isolated.

One example is histrionicotoxin,

which can be isolated from South

American frogs, and is used onpoison-tipped arrows by South

American tribes.

10.1 Alkyne Uses



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An example of a synthetic alkyne is ethynylestradiol.

Ethynylestradiol is the active

ingredient in many birth control

pills.

The presence of the triple bond increases the potency

of the drug compared to the natural analog.

How do you think a CC triple bond affects the

molecules geometry? Its rigidity? Its intermolecular

attractions?

10.1 Alkyne Uses



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Alkynes are named using the same procedure we used inChapter 4 to name alkanes with minor modifications:

1. Identify the parent chain, which should include the CC triple

bond.

2. Identify and name the substituents.

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

the CC triple bond the lowest number possible.

4. List the numbered substituents before the parent name in

alphabetical order. Ignore prefixes (except iso) when orderingalphabetically.

5. The CC triple bond locant is placed either just before the parent

name or just before the -yne suffix.

10.2 Alkyne Nomenclature



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Alkynes are named using the same procedure we usedin Chapter 4 to name alkanes with minor

modifications:

1. Identify the parent chain, which should include the CC

triple bond.

2. Identify and name the substituents.




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modifications:

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

giving the CC triple bond the lowest number possible.

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.




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modifications:

4. List the numbered substituents before the parent name in

alphabetical order. Ignore prefixes (except iso) when

ordering alphabetically.

5. The CC triple bond locant is placed either just before the

parent name or just before the -yne suffix.




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In addition to the IUCAP naming system, chemistsoften use common names that are derived from the

common parent name acetylene.

You should also be aware of the terminology below.

Practice with SKILLBUILDER 10.1.




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Name the molecule below.

Recall that when triple bonds are drawn, their angles

are 180.




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Chapter 913

Nomenclature: IUPAC

Find the longest chain containing the triple

bond.

Change -aneending to -yne.

Number the chain, starting at the end closest

to the triple bond.

Give branches or other substituents a number

to locate their position.


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Chapter 914


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Chapter 915

Examples of Nomenclature

All other functional groups, except ethers and

halides have a higher priority than alkynes.

If th i ti b t d bl b d d t i l b dN l t


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If there is a tie between a double bond and triple bond

the double bond gets the lower number but name ends with yneNomenclature


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Chapter 917

Physical Properties

Nonpolar, insoluble in water.

Soluble in most organic solvents.

Boiling points are similar to alkane of same

size.

Less dense than water.

Up to four carbons, gas at room temperature.


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Chapter 918

Acetylene

Acetylene is used in welding torches.

In pure oxygen, temperature of flame reaches2800C.

It would violently decompose to its elements,but the cylinder on the torch contains crushedfirebrick wet with acetone to moderate it.


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Chapter 919

Synthesis of Acetylene

Heat coke with lime in an electric furnace to

form calcium carbide.

Then drip water on the calcium carbide:

lime

This reaction was used to produce light for miners lamps and for the stage.

C CaO3 + +CaC2 CO

H C C H Ca(OH)2CaC2

+ 2 H2O +


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Chapter 920

Molecular Structure of Acetylene

Triple-bonded carbons have sphybrid orbitals.

A sigma bond is formed between the carbons by overlapof the sporbitals.

Sigma bonds to the hydrogens are formed by using thesecond sporbital.

Since the sporbitals are linear, acetylene will be a linearmolecule.


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Chapter 921

Bond Lengths

Triple bonds are shorter than double or singlebonds because of the two pi overlapping orbitals.


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Chapter 922

Acidity Table


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Chapter 923

Acidity of Alkynes

Terminal alkynes, are more acidic than other

hydrocarbons due to the higher scharacter of the sp

hybridized carbon.

Terminal alkynes can be deprotonated quantitativelywith strong bases such as sodium amide (-NH2).

Hydroxide and alkoxide bases are not strong enough

to deprotonate the alkyne quantitatively.


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Recall that terminal alkynes have a lower pKathanother hydrocarbons.

Acetylene is 19 pKaunits more acidic than ethylene,

which is 1019times stronger.

Does that mean that terminal alkynes are strong acids?

10.3 Alkyne Acidity



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Because acetylene (pKa=25) is still much weaker thanwater (pKa=15.7), a strong base is needed to make it

react.

Recall from Chapter 3 that we used the acronym ARIO

to rationalize differences in acidity strengths. Use ARIOto explain why acetylene is a stronger acid than

ethylene which is stronger than ethane.

10.3 Alkyne Acidity



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Use ARIO to rationalize the equilibria below.

A bases conjugate acid pKamust be greater than 25 for

it to be able to deprotonate a terminal

alkyne.

10.3 Alkyne Acidity



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Chapter 927


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Chapter 928

Formation of Acetylide Ions

H+can be removed from a terminal alkyne by

sodium amide, NaNH2.

The acetylide ion is a strong nucleophile that can

easily do addition and substitution reactions.


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Acidity of Terminal Alkynes

NaOH can not be used to deprotonate C-H of terminal alkyne

NaNH2is a stronger base than NaOH


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Other bases that can be used to deprotonate C-H of terminal

alkyne


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Like alkenes, alkynes can also be prepared byelimination.

10.4 Preparation of Alkynes



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Such eliminations usually occur via an E2 mechanism: GEMINAL dihalides can be used.

VICINAL dihalides can also be used.

E2 requires anti-periplanar geometry.




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Often, excess equivalents of NaNH2are used to shiftthe equilibrium toward the elimination products.

NH21-

is quite strong, so if a terminal alkyne isproduced, it will be deprotonated.

That equilibrium will greatly favor products.




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A proton source is needed to produce the alkyne.

Predict the products in the example below.

Practice with CONCEPTUAL CHECKPOINT

10.7.




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Like alkenes, alkynes can readily undergohydrogenation.

Two equivalents of H2are

consumed for eachalkynealkane conversion.

The cis alkene is produced as an intermediate. WHY

cis?

10.5 Reduction of Alkynes



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Chapter 936

Catalytic Hydrogenation of Alkynes

Two molecules of hydrogen can add across the triple bondto form the corresponding alkane.

A catalyst such as Pd, Pt, or Ni needs to be used for thereaction to occur.

Under these conditions the alkyne will be completelyreduced; the alkene intermediate cannot be isolated.

10 5 Reduction of Alkynes


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A deactivated or poisoned catalyst can be used toselectively react with the alkyne.

Lindlars catalyst and P-2 (Ni2B complex) are common

examples of a poisoned catalysts.


Poisoned Catalyst




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Is this a syn or antiaddition?

Practice with CONCEPTUAL CHECKPOINT 10.9.


Poisoned Catalyst



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Chapter 940

Mechanism

Both substrates, the hydrogen and the alkyne, have to beadsorbed on the catalyst for the reaction to occur.

Once adsorbed, the hydrogens add to the same side of thedouble bond (syn addition) giving the product a cisstereochemistry.



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Reduction with H2gives synaddition.

Dissolving metal conditions can give antiaddition

producing the trans alkene.

Ammonia has a boiling point of33C, so the

temperature for these reactions must remain very low.

Why cant water be used as the solvent?


Dissolving Metal Reductions



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Chapter 942

Reduction of Alkynes with Metal

Ammonia

To form a trans alkene, two hydrogens must be

added to the alkyne anti stereochemistry, so thisreduction is used to convert alkynes to trans

alkenes.


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Chapter 943

Reduction of Alkynes with Metal

Ammonia

Use dry ice to keep ammonia liquid.

As sodium metal dissolves in the ammonia,

it loses an electron.

The electron is solvated by the ammonia,

creating a deep blue solution.

NH3 + Na + Na+

NH3e-


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Chapter 944

Mechanism of Metal ReductionStep 1:An electron adds to the alkyne, forming a radical

anion.

Step 2:The radical anion is protonated to give a radical.

Step 3:An electron adds to the alkyne, forming an

anion.

Step 4:Protonation of the anion gives an alkene.



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Mechanismstep 1: Note the single-barbed and double-barbed (fishhook)

arrows.

Why does Na metal so readily give up an electron?






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Mechanismstep 1:

Why is the first intermediate called a RADICAL ANION? The radical anion adopts a trans configuration to

reduce repulsion.






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Mechanismstep 2 and 3:

Draw the product for step 3 of the mechanism.






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Mechanismstep 4:

Do the pKavalues for NH3and the alkene favor the

proton transfer?






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Predict the product(s) for the following reactions.






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Chapter 950



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Familiarize yourself with the reagents necessary tomanipulate alkynes.



Summary



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Like alkenes, alkynes also undergo hydrohalogenation.

Draw the final product for the reaction above.

Do the reactions above exhibit Markovnikov

regioselectivity?

10.6 Hydrohalogenation of Alkynes



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Modeled after the hydrohalogenation of alkenes, you mightexpect alkynes to react by the same mechanism.

Yet, the mechanism above does not explain all observedphenomena:

A slow reaction rate

3rdorder overall rate law

Vinylic carbocations are especially unstable




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Kinetic studies on the hydrohalogenation of an alkynesuggest that the rate law is 1storder with respect to

the alkyne, and 2ndorder with respect to HX.

What type of collision would result in such a rate law?

Unimolecular, bimolecular, or termolecular?




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Reaction rate is generally slow for termolecular

collisions. WHY? Considering the polarizability of the alkyne, does the

mechanism explain the regioselectivity?

May involve multiple competing mechanisms.




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Peroxides can be used in the hydrohalogenation ofalkynes 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.





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Chapter 957

Addition of HX

One mole of HCl, HBr, and HI add to alkynes to form vinyl

halides.

If two moles of HX is added, product is a geminal dihalide. The addition of HX is Markovnikov and will produce a

geminal dihalide.

h f d l d


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Chapter 958

Mechanism of Hydrogen Halide

Addition

The triple bonds abstract a proton from the hydrogenhalide forming a vinyl cation.

The proton adds to the least substituted carbon.

The second step of the mechanism is the attack by thehalide.

k k dd f


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Chapter 959

Anti-Markovnikov Addition of

Hydrogen Bromide to Alkynes

By using peroxides, hydrogen bromide can be added to a

terminal alkyne anti-Markovnikov.

The bromide will attach to the least substituted carbon

giving a mixture of cis and trans isomers.

d i f lk


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Like alkenes, alkynes can also undergo acid catalyzedMarkovnikov hydration.

The process is generally catalyzed with HgSO4to

compensate for the slow reaction rate that results

from the formation of vinylic carbocation.

10.7 Hydration of Alkynes


10 7 H d i f Alk


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HgSO4catalyzed hydration involves the mercury (II) ioninteracting with the alkyne.

Can you imagine what that interaction might look like

and how it will increase the rate of reaction for the

process?

Why is the intermediate called an enol?



10 7 H d i f Alk


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The enol/ketone TAUTOMERIZATION generally cannotbe prevented and favors the ketone greatly.

TAUTOMERS are constitutional isomers that rapidly

interconvert. How is that different from resonance?





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Chapter 963

Hydration of Alkynes

Mercuric sulfate in aqueous sulfuric acidadds HOH to one pi bond with aMarkovnikov orientation, forming a vinylalcohol (enol) that rearranges to a ketone.

Hydroborationoxidation adds HOH withan anti-Markovnikov orientation, and

rearranges to an aldehyde.

M i I C t l d H d ti f


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Chapter 964

Mercuric Ion Catalyzed Hydration of

Alkynes

Water can be added across the triple bond in a reactionanalogous to the oxymercurationdemercuration of alkenes.

The hydration is catalyzed by the mercuric ion. In a typical reaction, a mixture of mercuric acetate in

aqueous sulfuric acid is used. The addition produces an intermediate vinyl alcohol (enol)

that quickly tautomerizes to the more stable ketone oraldehyde.

M h i f M i I


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Chapter 965

Mechanism of Mercuric Ion

Catalyzed Hydration

The electrophilic addition of mercuric in (Hg+2) creates avinyl carbocation.

Water attacks the carbocation and after deprotonation,forms an organomercurial alcohol.

Hydrolysis of the alcohol removes the mercury, forming avinyl alcohol commonly referred to as enol.


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Chapter 966

KetoEnolTautomerism

Enols are not stable and they isomerize to the

corresponding aldehyde or ketone in a processknown as keto-enol tautomerism.


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Chapter 967

10 8 H d b ti O id ti


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Hydroboration-oxidation for alkynes proceeds throughthe 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

base (OH-) rather than by an acid.

10.8 Hydroboration-Oxidation

Copyright 2012 John Wiley & Sons, Inc.

Klein, Organic Chemistry 1e10 -68



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In general, we can concludethat a C=O double bond is

more stable than a C=C

double bond. WHY?






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After theBH2andH groups have been added acrossthe C=C double bond, in some cases, an undesired

second addition can take place.

To block out the second unit of BH3from reacting with

the intermediate, bulky borane reagents are

often used.






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Some bulky borane reagents are shown below.





10 8 Hydroboration Oxidation


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Predict products for the following reaction.

Draw the alkyne reactant and reagents that could be

used to synthesize the following molecule.




10 8 Hydration Regioselectivity


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Markovnikov hydration leads to a ketone. Anti-Markovnikov hydration leads to an aldehyde.


10.8 Hydration Regioselectivity



10 9 Alkyne Halogenation


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Alkynes can also undergo halogenation. Two equivalents of halogen can be added.

You might expect the mechanism to be similar to thehalogenation of alkenes, yet stereochemical evidence

suggests otherwise.

10.9 Alkyne Halogenation



10 9 Alkyne Halogenation


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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.

10.9 Alkyne Halogenation



10 10 Alkyne Ozonolysis


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When alkynes react under ozonolysis conditions, the pisystem is completely broken.

The molecule is cleaved, and the alkyne carbons arefully oxidized.

Practice with CONCEPTUAL CHECKPOINT

10.25.

10.10 Alkyne Ozonolysis



10 10 Alkyne Ozonolysis


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Predict the product(s) for the following reaction.

10.10 Alkyne Ozonolysis




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Permanganate Oxidation of Alkynes


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Chapter 979


to Diketones

Under neutral conditions, a dilute potassiumpermanganate solution can oxidize a triple bond into an

diketone. The reaction uses aqueous KMnO4to form a tetrahydroxyintermediate, which loses two water molecules to producethe diketone.



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Chapter 980


to Carboxylic Acids

If potassium permanganate is used under basic conditions

or if the solution is heated too much, an oxidativecleavage will take place and two molecules of carboxylic

acids will be produced.

O l i


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Chapter 981

Ozonolysis

Ozonolysis of alkynes produces carboxylic acids(alkenes gave aldehydes and ketones).

Used to find location of triple bond in anunknown compound.

HO CO

CH2 CH3CH3 CO

OHH2O(2)

O3(1)CH3 C C CH2 CH3 +


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10 11 Alkylation of Terminal Alkynes


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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.

Which has a higher pKa, NH3or R-CC-H? WHY?

10.11 Alkylation of Terminal Alkynes





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The alkynide ion can attack a methyl or 1 alkyl halideelectrophile.

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

substitution.






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Acetylene can be used to perform a double alkylation.

Why will the reaction be unsuccessful if the NaNH2and Et-Br

are added together?

Complex target molecules can be made by building acarbon skeleton and converting functional groups.






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Chapter 986

Acetylide Ions in SN2 Reactions

One of the best methods for synthesizing substitutedalkynes is a nucleophilic attack by the acetylide ion on anunhindered alkyl halide.

SN2 reaction with 1alkyl halides lengthens the alkyne

chain. Unhindered alkyl halides work better in an SN2 reaction:

CH3X > 1.


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Chapter 988

Acetylide Ions as Strong Bases

Acetylide ions are also strong bases. If the SN2

reactions is not possible, then an elimination (E2)

will occur.

Solved Problem 1


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Chapter 989

Show how to synthesize 3-decyne from acetylene and any necessary alkyl halides.

Another name for 3-decyne is ethyl n-hexylacetylene. It can be made by adding an ethyl group and a

hexyl group to acetylene. This can be done in either order; we begin by adding the hexyl group.

Solved Problem 1

Solution


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Chapter 990


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Chapter 991

10 12 Synthetic Strategies


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Recall the methods for increasing the saturation ofalkenes and alkynes.

But, what if you want to reverse the process

or decrease saturation?

10.12 Synthetic Strategies





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Halogenation of an alkene followed by twodehydrohalogenation 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.

What conditions would you use in step B?






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In the alkene to alkyne conversion above, why is water

needed in step 3) of that reaction?







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Give necessary reaction conditions for the multi-stepconversions below.





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Chapter 996


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Alkyne Notes

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