chapter 9: alkynes, from carey organic chemsitry

7/29/2019 Chapter 9: Alkynes, from Carey Organic Chemsitry

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Chapter 9: AlkynesChapter 9: Alkynes

9.1. Sources of Alkynes. Acetylene9.1. Sources of Alkynes. Acetylene

Industrial preparation of acetylene isIndustrial preparation of acetylene isby dehydrogenation of ethyleneby dehydrogenation of ethylene

CHCH CHCH 800800CC HH CC CHCH HH22++

11501150CC

++22 22

CostCost of of ener ener makesmakes acet leneacet lene aa moremore

Chem 211 B. R. Kaafarani 1

expensiveexpensive industrialindustrial chemicalchemical thanthan ethyleneethylene..


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9.2. Nomenclature

Acetylene and ethyne are both acceptable IUPAC

names for HCHC CHCH

Higher alkynes are named in much the same

-

of -ene.

HCHC CCHCCH33PropynePropyne

HCHC CCHCCH22CHCH3311--ButyneButyne

(CH(CH33))33CCCC CCHCCH33

Chem 211 B. R. Kaafarani 24,44,4--DimethylDimethyl--22--pentynepentyne


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..Physical Properties of AlkynesPhysical Properties of Alkynes

The physical properties of alkynes are similar toThe physical properties of alkynes are similar to

those of alkanes and alkenes.those of alkanes and alkenes.



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9.4. Structure and Bonding in9.4. Structure and Bonding in

Linear geometry for acetyleneLinear geometry for acetylene

CC CCHH HH

120 pm120 pm

106 pm106 pm 106 pm106 pm

CC CCCHCH33 HHpmpm



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Cyclic Alkynes

Cyclononyne is the

smallest cycloalkyne stableenough to be stored at

room temperature for a

reasonable length of time.

Cyclooctyne polymerizes

on standing.



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Bonding in acetylene is based on

-

x oge er y r ze e s or a

and one of the three 2p orbitals

22pp 22pp

22ss

22ss Each carbon has two half-


form s bonds.


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aandnd BondsBonds in Acetylenein Acetylene

Each carbon is connected to a hydrogen by a bond.

The two carbons are connected to each other by a

bond and two

bonds.

Two bonds in acetylene at


r g t ang es to eac ot er.


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TheThe regionregion ofof highesthighest

negativenegative chargecharge lieslies

TheThe regionregion ofof highesthighest

negativenegative chargecharge encirclesencircles

aboveabove andand belowbelow thethemolecularmolecular planeplane inin

thethe moleculemolecule aroundaround itsitscentercenter inin acetyleneacetylene..


..


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Table 9.1. Comparison of ethane, ethylene, and acetylene

CC distanceCC distance 153 pm153 pm 134 pm134 pm 120 pm120 pm

Ethane Ethylene Acetylene

CH distanceCH distance

111 pm111 pm

110 pm110 pm

106 pm106 pm

CC BDECC BDE

..

368 kJ/mol368 kJ/mol

..

611 kJ/mol611 kJ/mol 820 kJ/mol820 kJ/mol

CH BDECH BDE 410 kJ/mol410 kJ/mol

s 3s 3


s 2s 2


ssHybridization of CHybridization of C

% scharacter% scharacter 25%25% 33%33% 50%50%


pKapKa 6262 4545 2626


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9.5. Acidity of Acetylene and

Terminal Alkynes



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In general, hydrocarbons are

CompoundCompound ppKKaa

HFHF 3.23.2

22

NHNH 3636HCHC CHCH 2626

4545HH22CC CHCH22

CHCH44 6060

Acet lene is a weak acid but not nearl as weak asAcet lene is a weak acid but not nearl as weak as


alkanes or alkenes.alkanes or alkenes.


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Electronegativity of carbon

--6060

CC HH HH++

++ CC : sp3

HH++ ++

1010--4545

CC CC CC CC

:

sp2

1010--2626

++ : sp


closer to the nucleus and more strongly held.closer to the nucleus and more strongly held.


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Objective:Objective:

Prepare a solution containing sodium acetylide.Prepare a solution containing sodium acetylide.

NaCNaC CHCH

Will treatment of acetylene with NaOH be effective?Will treatment of acetylene with NaOH be effective?

HH22OONaOHNaOH ++ HCHC CHCH NaCNaC CHCH ++



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No H droxide is not a stron enou h base todeprotonate acetylene.

HH22OONaOHNaOH++

HCHC CHCH NaCNaC CHCH++

HOHO

....

:: HH CC CHCH HOHO HH

....

++ ++ CC CHCH::

....weaker acidweaker acid

KK = 26= 26

stronger acidstronger acid

KK = 16= 16

In acid-base reactions, the equilibrium lies to


e s e o e wea er ac .


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Solution: Use a stronger base. Sodium amideis a

stronger base than sodium hydroxide.

NHNH33NaNHNaNH22 ++ HCHC CHCH NaCNaC CHCH ++

.... ....++ ++

stronger acidstronger acid weaker acidweaker acid

pp aa == aa ==

Ammonia is a weaker acid than acet lene.


The position of equilibrium lies to the right.


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9.6. Preparation of Alkynes by Alkylationo ce y ene an erm na ynes

There are two main methods for thepreparation of alkynes:

Carbon-carbon bond formation:

a y at on o acety ene an term na a ynes

Functional-group transformations:

e m na on



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Alkylation of acetylene and

RRCC CCHH

RRCC CCRR



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Alkylation of acetylene and terminal

RR XXSS

NN22

XX::++CC

::HHCC CCRRHHCC ++

The alkylating agent is an alkyl halide, and thereaction is nucleophilic substitution.

The nucleophile is sodium acetylide or the sodium

salt of a terminal (monosubstituted) alkyne.



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Example: Alkylation of acetylene

NaNHNaNH22

NHNH33

CHCH33CHCH22CHCH22CHCH22BrBr

HCHC CC CHCH22CHCH22CHCH22CHCH33

(70(70--77%)77%)



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Example: Alkylation of a terminal alkyne

CCHH(CH(CH33))22CHCHCHCH22CC

NaNHNaNH22, NH, NH33

CCNaNa(CH(CH33))22CHCHCHCH22CC

CHCH33BrBr

CCCHCH33(CH(CH33))22CHCHCHCH22CC

Chem 211 B. R. Kaafarani 20(81%)(81%)


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Example: Dialkylation of acetylene

HHCC CCHH

1. NaNH1. NaNH22, NH, NH

33

2.2. CHCH33CHCH22BrBr

CCHHCHCH33CHCH22CC LIMITATION!1. NaNH1. NaNH22, NH, NH33

2.2. CHCH33BrBr

Effective only with

primary alkyl halides.

CCCHCH33CHCH33CHCH22CCtertiary alkyl halides

undergo elimination.


(81%)(81%)


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E2 predominates over SN2 when

HH CCCC::HHCC

CC XX

++CCHHCC HH CC CC XX::++



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9.7. Preparation of Alkynes by Elimination

Double Dehydrohalogenation

XXHH HHHH

CC CC CC CC

XXHH XX XX


of terminal alkynes.


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Geminal dihalide Alkyne

1. 3NaNH1. 3NaNH22, NH, NH33

2. H2. H22OO

CHCH CCCC

(56(56--60%)60%)



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Geminal dihalide Alkyne

(CH(CH33))33CCCCHH22CHCHClCl22aa 22,, 33

(CH(CH33))33CCCCHH CHCHClCl

s ows ow

NaNHNaNH22, NH, NH33

(CH(CH )) CCCC CHCH

(slow)(slow)

NaNHNaNH22, NH, NH33HH22OO (fast)(fast)

33 33



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Vicinal dihalide Alkyne

CHCH33(CH(CH22))77CCHHCCHH22BrBr

BrBr

1. 3NaNH1. 3NaNH22, NH, NH33

2. H2. H22OO

(54%)(54%)



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9.9. Hydrogenation of Alkynes

''catcat

'' ++22 22

==

alkene is an intermediatealkene is an intermediate



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Heats of Hydrogenation

CHCH33CHCH22CC CHCH CHCH33CC CCCHCH33

292 kJ/mol292 kJ/mol 275 kJ/mol275 kJ/mol

Alkyl groups stabilize triple bonds in the same

.

bonds are more stable than terminal ones.



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Partial Hydrogenation

RCHRCH22CHCH22R'R'catcatRCRC CR'CR' catcat RCHRCH CHR'CHR'

Alkynes could be used to prepare alkenes if a

catalyst were available that is active enough tocatalyze the hydrogenation of alkynes, but not active

enough for the hydrogenation of alkenes.



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Lindlar Palladium

HHHHRCHRCH22CHCH22R'R'

catcatRCRC CR'CR'

catcatRCHRCH CHR'CHR'

There is a catalyst that will catalyze the

hydrogenation of alkynes to alkenes, but not that ofalkenes to alkanes. It is called the Lindlar catalyst and

consists of palladium supported on CaCO3, which has

.syn-Hydrogenation occurs; cisalkenes are formed.



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++ HH22CHCH33(CH(CH22))33CC C(CHC(CH22))33CHCH33

Lindlar PdLindlar Pd Lindlar Pd: Pd

on CaCO3;

Pb(CH3CO2)2

33 22 33 22 33 33

CCCC

added to it.

HH HH

Quinoline



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9.10. Metal-Ammonia Reduction of Alkynes

AlkynesAlkynestranstrans--AlkenesAlkenes

'''' ''

Partial ReductionPartial Reduction

22 22

Another way to convert alkynes to alkenes is by

ammonia.

trans-Alkenes are formed.



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Example

CHCH33CHCH22CC CCHCCH22CHCH33

Na, NNa, NHH33

CHCH33CHCH22 HH

CCCC

CHCH22CHCH33HH

82%82%



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Mechanism

Metal (Li, Na, K) is reducing agent; H2 is not involved.

(1) Electron transfer

ro on rans er

(3) Electron transfer(4) Proton transfer



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Mechanism

MM++

alkyne to give an anion radical.

MM ..++R'R'RR CC CC RR R'R'CC.... ..

CC

Step (2) Transfer of a proton from the solvent (liquid

''.... .. ..

''

RR.

....

HH ....


HH NHNH22 NHNH22

::


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Step (3): Transfer of an electron from the metal to

..RR MM

++

RR....

.

MM..++R'R'CC CC

''

CCCC

Step (4) Transfer of a proton from the solvent (liquid

HH NHNH22

....

....

ammon a o e car an on.

RRCC CC::

CCCC

RR 22::

Chem 211 B. R. Kaafarani 36R'R'HH R'R'HH

S ff f (E) (Z)


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Suggest efficient syntheses of (E)- and (Z)-2- heptene

from propyne and any necessary organic or inorganic

reagents.

1. NaNH1. NaNH222. CH2. CH CHCH CHCH CHCH Br Br

a,a, 33HH22, Lindlar Pd, Lindlar Pd



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9.11. Addition of Hydrogen Halides to

''

HBrHBr

CHCH33(CH(CH22))33CC CHCH CHCH33(CH(CH22))33CC CHCH22

(60%)(60%)

Alkynes are slightlyAlkynes are slightly lesslessreactive than alkenes.reactive than alkenes.



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Termolecular Transition State

....rrHH ::

....

CHCHRCRC

....BrBrHH ::

....

== 22


R ti ith t l f h d


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Reaction with two moles of a hydrogen

2 H2 HFF

CC CCCHCH CHCH CHCH CHCH

FFHH


(76%)(76%)


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Free-radical addition of HBr occurs

w en perox es are presen

HBrHBrCHCH33(CH(CH22))33CC CHCH CHCH33(CH(CH22))33CCHH CHCHBrBr

(79%)(79%)

Regioselectivity opposite to Markovnikov's rule.


9 12 H d ti f Alk


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9.12. Hydration of Alkynes

Expected reaction:Expected reaction:++

RCRC CR'CR' HH22OO++ RCHRCH CR'CR'

enolenol

RCHRCH22CR'CR'HH++

RCRC CR'CR' HH22OO++

OO


e onee one

Tautomerism


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Tautomerism

RCHRCH CR'CR' RCHRCH22CR'CR'

enolenol

OHOH OO

Ketone (keto form)Ketone (keto form)

Enols are regioisomers of ketones, and exist in equilibrium

with them. Keto-enol equilibration is rapid in acidic media.

Ketones are more stable than enols and predominate at

e uilibrium.

Tautomers are constitutional isomers that equilibrate by


.

tautomerism.

Mechanism of conversion of enol to ketone


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....Mechanism of conversion of enol to ketone

OO HH

CC CC

::

HH

OO HH....

::HH++OO::

HH CC CCHH++

HH

HH

HH....OO::

Chem 211 B. R. Kaafarani 44HH

++


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Key carbocation intermediate is stabilized

by electron delocalization (resonance)

OO HH....

:: OO....

HH++

CC CCHH

++

CC CCHH



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Example

CHCH CHCH CC C CHC CH CHCH

HH22O, HO, H++

viavia

HgHg2+2+OHOH

OO

CHCH33(CH(CH22))22CHCH22C(CHC(CH22))22CHCH33


Markovnikov's rule followed in


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Markovnikov s rule followed in

OO

HH22O, HO, H22SOSO44

H SOH SOCHCH33(CH(CH22))55CCHCCH33CHCH33(CH(CH22))55CC CHCH

(91%)(91%)

viavia

CHCH33(CH(CH22))55CC CHCH22


9 13 Additi f H l t Alk


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9.13. Addition of Halogens to Alkynes

Example

ClCl

+ 2+ 2 ClCl22 CCClCl22CHCH CHCH33HCHC CCHCCH33

(63%)(63%)


Addition isAddition is antianti


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Addition isAddition is antianti

CHCH CHCH BrBr

BrBr22CHCH33CHCH22CC CCHCCH22CHCH33 CC CC

CHCH22CHCH33BrBr

(90%)(90%)

be isolated when 1:1molar ratio is used.


9 14 Ozonolysis of Alkynes


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9.14. Ozonolysis of Alkynes

Gives two carboxylic acids by cleavage of triple bondGives two carboxylic acids by cleavage of triple bond

CHCH33(CH(CH22))33CC CCHH

1. O1. O33

Example

.. 22OO OO

++CHCH33(CH(CH22))33CCOHOH HOHOCCOHOH


chapter 9: alkynes, from carey organic chemsitry

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