Chapter 9Chapter 9AlkynesAlkynes

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9.19.1Sources of AlkynesSources of Alkynes

Industrial preparation of acetylene isby dehydrogenation of ethylene.

CH3CH3

800°C

1150°C

Cost of energy makes acetylene a moreexpensive industrial chemical than ethylene.

H2C CH2

H2C CH2 HC CH

H2+

H2+

Acetylene

Naturally Occurring Alkynes

C(CH2)4COHCH3(CH2)10C

O

Some alkynes occur naturally. For example,

Tariric acid: occurs in seed of a

Guatemalan plant.

HNOH

Histrionicotoxin: defensive toxin in poison dart frogs of Central and South America

9.29.2NomenclatureNomenclature

Acetylene and ethyne are both acceptableIUPAC names for HC CH

Higher alkynes are named in much the sameway as alkenes except using an -yne suffixinstead of -ene.

HC CCH3

Propyne

HC CCH2CH3

1-Butyne or But-1-yne

(CH3)3CC CCH3

4,4-Dimethyl-2-pentyne or 4,4-Dimethyl-pent-2-yne

Nomenclature

The physical properties of alkynes are The physical properties of alkynes are similar to those of alkanes and alkenes.similar to those of alkanes and alkenes.

9.39.3Physical Properties of AlkynesPhysical Properties of Alkynes

9.49.4Structure and Bonding in Structure and Bonding in

Alkynes:Alkynes:spsp Hybridization Hybridization

Linear geometry for acetylene

C CH H

120 pm

106 pm 106 pm

C CCH3 H

121 pm

146 pm 106 pm

Structure

Cyclononyne is the smallest cycloalkyne stable enough to be stored at room temperaturefor a reasonable length of time.

Cyclooctyne polymerizeson standing.

Cycloalkynes

C C

2s

2p

2sp

Mix together (hybridize) the 2s orbital and one of the three 2p orbitals.

2p

sp Hybridization in Acetylene

Atomic orbitals Hybridized

Each carbon has two half-filled sp orbitalsavailable to form bonds.

Each carbon isconnected to ahydrogen by a bond. The twocarbons are connectedto each other by a bond and two bonds.

Figure 9.2 (a)

Bonds in Acetylene

One of the two bonds in acetylene isshown here.The second bond is at rightangles to the first.

Figure 9.2 (b)

Bonds in Acetylene

This is the secondof the two bonds in acetylene.

Figure 9.2 (c)

Bonds in Acetylene

The region of highest negative charge lies above and below the molecular plane in ethylene.

Figure 9.3 Electrostatic Potential in Acetylene

The region of highest negative charge encircles the molecule around itscenter in acetylene.

C—C distance

C—H distance

H—C—C angles

C—C BDE

C—H BDE

% s character

pKa

153 pm

111 pm

111.0°

368 kJ/mol

410 kJ/mol

sp3

25%

62

134 pm

110 pm

121.4°

611 kJ/mol

452 kJ/mol

sp2

33%

45

120 pm

106 pm

180°

820 kJ/mol

536 kJ/mol

sp

50%

26

hybridization of C

Ethane Ethylene Acetylene

Table 9.1 Structural Features of Ethane, Ethylene, and Acetylene

9.59.5Acidity of Acetylene and Acidity of Acetylene and

Terminal Alkynes:Terminal Alkynes:

HH CC CC

In general, hydrocarbons are exceedingly weak acids, but alkynes are not nearly as weak as alkanes or alkenes.

Compound pKa

26

45

CH4 60

H2C CH2

Acidity of Hydrocarbons

HCHC CHCH

Electrons in an orbital with more s character are closer to the

nucleus and more strongly held.

Carbon: Hybridization and Electronegativity

C H H+ +pKa = 62

sp3C :–

H+ + sp2

H

C C C C:

–

pKa = 45

H+ + spC C H C C :–

pKa = 26

Objective:Prepare a solution containing sodium acetylide

Will treatment of acetylene with NaOH be effective?

NaC CH

H2ONaOH + HC CH NaC CH +

Sodium Acetylide

NO

Hydroxide is not a strong enough base to deprotonate acetylene.

weaker acidpKa = 26

stronger acidpKa = 15.7

In acid-base reactions, the equilibrium lies tothe side of the weaker acid.

Sodium Acetylide

HO..

.. : ..HO H..

C CH–

H C CH+ + :–

Solution: Use a stronger base. Sodium amideis a stronger base than sodium hydroxide.

NH3NaNH2 + HC CH NaC CH +

Ammonia is a weaker acid than acetylene.The position of equilibrium lies to the right.

–

H2N..

: H C CH H..

+ + C CH:–

stronger acidpKa = 26

weaker acidpKa = 36

H2N

Sodium Acetylide

9.69.6Preparation of Alkynes byPreparation of Alkynes by

Alkylation of Acetylene and Alkylation of Acetylene and Terminal AlkynesTerminal Alkynes

Carbon-carbon bond formationalkylation of acetylene and terminal alkynes

Functional-group transformationselimination

There are two main methods for the preparationof alkynes:

Preparation of Alkynes

H—C C—H

R—C C—H

R—C C—R

Alkylation of Acetylene and Terminal Alkynes

Remove 1st Hwith NaNH2,

Remove 2nd H with NaNH2,

then alkylatewith RX.

then alkylatewith RX.

R XSN2

X–:+C–:H—C C—RH—C +

The alkylating agent is an alkyl halide, andthe reaction is nucleophilic substitution.

The nucleophile is sodium acetylide or the sodium salt of a terminal (monosubstituted) alkyne (these are strong bases).

Effective only with methyl and 1o alkyl halides, 2o and 3o alkyl halides undergo elimination.

Alkylation of Acetylene and Terminal Alkynes

NaNH2

NH3

HC CH HC CNa

CH3CH2CH2CH2Br

(70-77%)

HC C CH2CH2CH2CH3

Example: Alkylation of Acetylene

NaNH2, NH3

CH(CH3)2CHCH2C

CNa(CH3)2CHCH2C

CH3Br

(81%)

C—CH3(CH3)2CHCH2C

Example: Alkylation of a Terminal Alkyne

1. NaNH2, NH3

2. CH3CH2Br

H—C C—H

C—HCH3CH2—C

(81%)

1. NaNH2, NH3

2. CH3Br

C—CH3CH3CH2—C

Example: Dialkylation of Acetylene

Alkylation of an acetylide is effective only with methyl or primary alkyl halides

Secondary and tertiary alkyl halides undergo elimination.

Limitation on Alkylation of Acetylides

E2 predominates over SN2 when alkyl

halide is secondary or tertiary.

C–:H—C

E2

+CH—C —H C C X–:+

Acetylide Ion as a Base

H C

C X

9.79.7Preparation of Alkynes byPreparation of Alkynes by

Elimination ReactionsElimination Reactions

Elimination of vicinal and geminal Elimination of vicinal and geminal dihalides yield alkynes.dihalides yield alkynes.

Geminal dihalide Vicinal dihalide

X

C C

X

H

H

X X

C C

HH

The most frequent applications are in preparation of terminal alkynes.

Preparation of Alkynesby Double Dehydrohalogenation

(CH3)3CCH2—CHCl2

1. 3NaNH2, NH3

2. H2O

(56-60%)

(CH3)3CC CH

Geminal dihalide Alkyne

NaNH2, NH3

H2O

(CH3)3CCH2—CHCl2

(CH3)3CCH CHCl

(slow)

NaNH2, NH3

(CH3)3CC CH

(slow)

NaNH2, NH3

(CH3)3CC CNa(fast)

Geminal dihalide Alkyne

(Dehydrohalogenation)

(Dehydrohalogenation)

(Loss of a proton) (Regain

proton)

Br

CH3(CH2)7CH—CH2Br

1. 3NaNH2, NH3

2. H2O

(54%)

CH3(CH2)7C CH

Vicinal dihalide Alkyne

9.89.8Reactions of Alkynes Reactions of Alkynes

Acidity (Section 9.5)Hydrogenation (Section 9.9)Metal-Ammonia Reduction (Section 9.10)Addition of Hydrogen Halides (Section 9.11)Hydration (Section 9.12)Hydroboration-oxidation (not in text)Addition of Halogens (Section 9.13)Ozonolysis (Section 9.14)

Reactions of Alkynes

9.99.9Hydrogenation of Alkynes Hydrogenation of Alkynes

RCH2CH2R'cat

catalyst = Pt, Pd, Ni, or Rh

Twice the H2 is needed here compared to reaction

with an alkene since there are two pi bonds in the alkyne. An alkene is an intermediate.

RC CR' + 2H2

Hydrogenation of Alkynes

Heats of Hydrogenation

292 kJ/mol 275 kJ/mol

Alkyl groups stabilize triple bonds in the same way that they stabilize doublebonds. Internal triple bonds are more stable than terminal ones.

CH3CH2C CH CH3C CCH3

RCH2CH2R'

Alkynes can be used to prepare alkenes if acatalyst is used that is active enough to catalyze the hydrogenation of alkynes, but notactive enough for the hydrogenation of alkenes.

cat

H2RC CR' cat

H2RCH CHR'

Partial Hydrogenation

A catalyst that will catalyze the hydrogenationof alkynes to alkenes, but not of alkenes to alkanes is called the Lindlar catalyst and consists ofpalladium supported on CaCO3, which has been

poisoned with lead acetate and quinoline. (Poisoning reduces activity of the catalyst.)

syn-Hydrogenation occurs; cis alkenes are formed.

Lindlar Catalyst

RCH2CH2R'cat

H2RC CR' cat

H2RCH CHR'

+ H2

Lindlar Pd

CH3(CH2)3C C(CH2)3CH3

CH3(CH2)3 (CH2)3CH3

H H(87%)

CC

Example

9.109.10Metal-Ammonia Reduction Metal-Ammonia Reduction

of Alkynes of Alkynes

Alkynes Alkynes transtrans-Alkenes-Alkenes

Another way to convert alkynes to alkenes isby reduction with sodium (or lithium or potassium)in ammonia. This reaction goes by a multistep mechanism.

In this reaction, trans-alkenes are formed.

Partial Reduction

RCH2CH2R'RC CR' RCH CHR'

CH3CH2C CCH2CH3

CH3CH2

CH2CH3H

H

(82%)

CC

Na, NH3

Example

Four steps:

(1) electron transfer

(2) proton transfer

(3) electron transfer

(4) proton transfer

The metal (Li, Na, K) is the reducing agent; H2 is not involved in this reaction.

Mechanism

Step (2): Transfer of a proton from the solvent (liquid ammonia) to the anion radical.

H NH2

..

R R'C..

.–C

.R'

R

C C

HNH2

..

–:

Mechanism

M .+R R'C C R R'C.. .–

C

M+

Step (1): Transfer of an electron from a metal atomto the alkyne to give an anion radical.

Step (3): Transfer of an electron from a 2nd metalatom to the alkenyl radical to give a carbanion.

M.+.

R'

R

C C

H

M+

R'

R

C C

H

..–

Mechanism

Step (4): Transfer of a proton from the solvent(liquid ammonia) to the carbanion.

..H NH2

R'

R

C C

H

..–

R'H

H

CC

R NH2

..

–:

Mechanism

Strategy

Propose efficient syntheses of (E)- and (Z)-2-heptene from propyne and other necessary reagents.

1. NaNH2

2. CH3CH2CH2CH2Br

Na, NH3H2, Lindlar Pd

Synthesis

9.119.11Addition of Hydrogen Halides Addition of Hydrogen Halides

to Alkynes to Alkynes

HBr

Br

(60%)

Alkynes are slightly less reactive than alkenes.

Markovnikov’s rule is followed.

CH3(CH2)3C CH CH3(CH2)3C CH2

Follows Markovnikov's Rule

CH

..BrH :..

RC

..BrH :..

Observed rate law: rate = k[alkyne][HX]2

Termolecular Rate-determining Step

CH3CH2C CCH2CH3

2 HF

(76%)

F

F

C C

H

H

CH3CH2 CH2CH3

Two Molar Equivalents of Hydrogen Halide

HBr

Regioselectivity opposite to Markovnikov's rule. As with alkenes, the anti-Markovnikov product is formed in presence of peroxide.

CH3(CH2)3C CH

(79%)

CH3(CH2)3CH CHBrperoxides

Free-radical Addition of HBr

9.129.12Hydration of Alkynes Hydration of Alkynes

expected reaction:

enol

H+

RC CR' H2O+

OH

RCH CR'

observed reaction:

RCH2CR'

O

H+

RC CR' H2O+

ketone

Hydration of Alkynes

Enols are tautomers of ketones, and exist in equilibrium with them.

Keto-enol equilibration is rapid in acidic media.

Ketones are more stable than enols andpredominate at equilibrium.

enol

OH

RCH CR' RCH2CR'

O

ketone

Enols

: H

+O

O H

C CH

H

..:

Mechanism of Conversion of Enol to Ketone

O H

C CH+

O

H

H

:

..:

:

O H

C C

H

H

O: :

H+

..:

Mechanism of Conversion of Enol to Ketone

OH

C C

H

H

O:

H

+..

:

Carbocation is stabilized by electron delocalization (resonance).

H O

C CH

..H+

Key Carbocation Intermediate

O

C CH+

..:

H2O, H+

CH3(CH2)2C C(CH2)2CH3

Hg2+

(89%)

O

CH3(CH2)2CH2C(CH2)2CH3

via

OH

CH3(CH2)2CH C(CH2)2CH3

Example of Alkyne Hydration

H2O, H2SO4

HgSO4

CH3(CH2)5CCH3

(91%)

Markovnikov's rule followed in formation of enol

via

CH3(CH2)5C CH2

OH

CH3(CH2)5C CH

O

Regioselectivity

9.139.13Addition of Halogens to Alkynes Addition of Halogens to Alkynes

+ 2Cl2

Cl

Cl

(63%)

CCl2CH CH3HC CCH3

Example

Both pi bonds react with excess X2.

Br2

CH3CH2

CH2CH3Br

Br

(90%)

CH3CH2C CCH2CH3 C C

Addition is anti

One addition 1mol of X2.

Addition is anti as with an alkene.

9.149.14Ozonolysis of Alkynes Ozonolysis of Alkynes

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

1. O3

2. H2O

CH3(CH2)3C CH

+CH3(CH2)3COH

(51%)

O

HOCOH

O

Example

Reduction is not needed in step 2.

Hydroboration-Oxidation of Hydroboration-Oxidation of Terminal Alkynes Terminal Alkynes

Gives the anti-Markovnikov enolGives the anti-Markovnikov enol

This is similar to hydroboration-oxidation of alkenes. Disiaborane (a bulky borane) is used to prevent subsequent reaction with the alkene. Addition of the borane is syn as before.

RC CH RCH CH-BSia2

Disecondaryisoamyl borane, Sia2BH

Sia2BH

THF

H2O2

HOˉRCH CH-BSia2 RCH CH-OH

an enol which rearranges to an aldehyde.

End of Chapter 9End of Chapter 9AlkynesAlkynes