CCAlkynes. Synthesis of Acetylene Heat coke with lime in an electric to form calcium carbide. Then...

C C

Alkynes

Synthesis of Acetylene

Heat coke with lime in an electric to form calcium carbide.

Then drip water on the calcium carbide.

coke lime

*This reaction was used to produce light

for miners’ lamps and for the stage.

*

The Structure of Alkynes

A triple bond is composed of a s bond and two p bonds

Question

Arrange ethane, ethene, and ethyne in order of increasing C-C bond length.

A) ethane < ethene < ethyne

B) ethene < ethane < ethyne

C) ethyne < ethene < ethane

D) ethane < ethyne < ethene

H C C

Acidity of Acetylene

and Terminal Alkynes

In general, hydrocarbons are very weak acids

Compound pKa

HF 3.2

H2O 16

NH3 36

45

CH4 60

H2C CH2

Acidity of Hydrocarbons

Acetylene is a weak acid, but not nearlyas weak as alkanes or alkenes.

Compound pKa

HF 3.2

H2O 16

NH3 36

45

CH4 60

H2C CH2

HC CH 26

Acetylene

Question

Which one of the following is the strongest acid?

A) water

B) ammonia

C) 1-butene

D) 1-butyne

C H H+ +

H+ +

H+ +

10-60

10-45

10-26

sp3C :

sp2

sp

H

C C

C C H

C C

C C :

:

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

nucleus and more strongly held.

Carbon: Hybridization and Electronegativity

Question

Which one of the following statements best explains the greater acidity of terminal alkynes (RCºCH) compared with monosubstituted alkenes (RCH=CH2)?

A) The sp-hybridized carbons of the alkyne are less electronegative than the sp2 carbons of the

alkene.B) The two p bonds of the alkyne are better able to

stabilize the negative charge of the anion by resonance.

C) The sp-hybridized carbons of the alkyne are more electronegative than the sp2 carbons of the alkene.

D) The question is incorrect - alkenes are more acidic than alkynes.

The stronger the acid, the weaker its conjugate base

top 252

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

NH3NaNH2 + HC CH NaC CH +

–H2N

..: H C CH H

..+ + C CH:

–

stronger acidpKa = 26

weaker acidpKa = 36

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

H2N

Sodium Acetylide

Question

Which of the following bases is strong enough to completely deprotonate propyne?

A) NH3

B) CH3OH

C) NaNH2

D) NaOH

Preparation of Various Alkynes

by alkylation reactions with

Acetylide or Terminal Alkynes

Synthesis Using Acetylide Ions: Formation of C–C Bond

H—C C—H

R—C C—H

R—C C—R

Alkylation of Acetylene and Terminal Alkynes

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.

Alkylation of Acetylene and Terminal Alkynes

NaNH2

NH3

CH3CH2CH2CH2Br

(70-77%)

HC CH HC CNa

HC C CH2CH2CH2CH3

Example: Alkylation of Acetylene

Question

Which alkyl halide will react faster with the acetylide ion (HCºCNa) in an SN2 reaction?

A) bromopropane

B) 2-bromopropane

C) tert-butyl iodide

D) 1-bromo-2-methylbutane

NaNH2, NH3

CH3Br

CH(CH3)2CHCH2C

CNa(CH3)2CHCH2C

(81%)

C—CH3(CH3)2CHCH2C

Example: Alkylation of a Terminal Alkyne

1. NaNH2, NH3

2. CH3CH2Br

(81%)

H—C C—H

1. NaNH2, NH3

2. CH3Br

C—HCH3CH2—C

C—CH3CH3CH2—C

Example: Dialkylation of Acetylene

Effective only with primary alkyl halides

Secondary and tertiary alkyl halides undergo elimination

Limitation

Question

A.

B.

1) NaNH2

2) Pentyl chloride

C.

D.

NaCl + NH3 + ????

What is the product of the following reaction?

Answer

A.

B.

1) NaNH2

2) Pentyl chloride

C.

D.

NaCl + NH3 + ????

What is the product of the following reaction?

B) SEE: Skillbuilder 10.5.

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

Question

Consider the reaction of each of the following with cyclohexyl bromide. For which one is

the ratio of substitution to elimination highest?

A) NaOCH2CH3, ethanol, 60°C

B) NaSCH2CH3, ethanol-water, 25°C

C) NaNH2, NH3, -33°C

D) NaCºCH, NH3, -33°C

Preparation of Alkynes

by Elimination Reactions

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

Question

In addition to NaNH2, what other base can be

used to convert 1,1-dichlorobutane into

1-butyne?

A) NaOCH3

B) NaOH

C) NaOCH2CH3

D) KOC(CH3)3

CH3(CH2)7CH—CH2Br

Br

1. 3NaNH2, NH3

2. H2O

(54%)

CH3(CH2)7C CH

Vicinal dihalide Alkyne

Question

Which of the following compounds yield 1-heptyne on being treated with three moles of sodium amide (in liquid ammonia as the solvent) followed by adding water to the reaction mixture?

A) 1,1,2,2-tetrachloroheptaneB) 1-bromo-2-chloroheptaneC) 1,1,2-trichloropentaneD) all of the above

Reactions of Alkynes

Acidity

Hydrogenation

Metal-Ammonia Reduction

Addition of Hydrogen Halides

Hydration

Addition of Halogens

Ozonolysis

Reactions of Alkynes

Hydrogenation of Alkynes

Atomic Force Microscopy of Acetylene Lawrence Berkeley Laboratory (LBL)

C

C

H

H

C

C

H

H

Calculated image (Philippe Sautet)

p orbital

pz

TIP

H

O+

Imaging: acetylene on Pd(111) at 28 K

Molecular Image Tip cruising altitude ~700 pmΔz = 20 pm

Surface atomic profile

Tip cruising altitude ~500 pm

Δz = 2 pm

1 cm(± 1 μm)

The STM image is a map of the pi-orbital of distorted acetylene

Why don’t we see the Pd atoms?Because the tip needs to be very close to image the Pd atoms and would knock the molecule away

If the tip was made as big as an airplane, it would be flying at 1 cm from the surface and waving up an down by 1 micrometer

M. Salmeron (LBL)

Excitation of frustrated rotational modes in acetylene molecules on Pd(111) at T = 30 K

Tip

e-

((( ) ( )))

M. Salmeron (LBL)

RCH2CH2R'cat

catalyst = Pt, Pd, Ni, or Rh

alkene is an intermediate

RC CR' + 2H2

Hydrogenation of Alkynes

RCH2CH2R'

Alkenes could be used to prepare alkenes if acatalyst were available 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

There is a catalyst that will catalyze the hydrogenation

of alkynes to alkenes, but not that of alkenes to alkanes.

It is called the Lindlar catalyst and consists ofpalladium supported on CaCO3, which has been

poisoned with lead acetate and quinoline.

syn-Hydrogenation occurs; cis alkenes are formed.

RCH2CH2R'cat

H2RC CR' cat

H2RCH CHR'

Lindlar Palladium

+ H2

Lindlar Pd

CH3(CH2)3 (CH2)3CH3

H H(87%)

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

CC

Example

Alkynes trans-Alkenes

Metal-Ammonia Reduction

of Alkynes

RCH2CH2R'

Another way to convert alkynes to alkenes is

by reduction with sodium (or lithium or potassium)

in ammonia.

trans-Alkenes are formed.

RC CR' RCH CHR'

Partial Reduction

CH3CH2

CH2CH3H

H

(82%)

CH3CH2C CCH2CH3

CC

Na, NH3

Example

Question

How would you accomplish the following conversion?

A) NaNH2

B) H2, Lindlar Pd

C) Na, NH3

D) either B or C

four steps

(1) electron transfer

(2) proton transfer

(3) electron transfer

(4) proton transfer

Metal (Li, Na, K) is reducing agent; H2 is not involved; proton comes from NH3

Mechanism

Question

Select the most effective way to synthesize cis-2-pentene from 1-propyne.

A) 1) NaNH2 2) CH3CH2Br 3) H2, Pd

B) 1) NaNH2 2) CH3Br 3) H2, Lindlar Pd

C) 1) NaNH2 2) CH3CH2I 3) H2, Lindlar

Pd

D) 1) NaNH2 2) CH3CH2Br 3) Na,NH3

Question

Which reagent would accomplish the transformation of 3-hexyne into trans-3-hexene?

A) H2/Ni

B) H2, Lindlar Pd

C) Na, NH3

D) NaNH2, NH3

Suggest an efficient syntheses of (E)- and (Z)-2-heptene from propyne and any necessary organic or inorganic reagents.

Problem

Problem Strategy

Problem Strategy

1. NaNH2

2. CH3CH2CH2CH2Br

Na, NH3H2, Lindlar Pd

Problem Synthesis

Question

Which would be the best sequence of reactions to use in order to prepare cis-3-nonenefrom 1-butyne?

A) 1. NaNH2 in NH3; 2. 1-bromopentane; 3. H2, Lindlar Pd

B) 1. NaNH2 in NH3; 2. 1-bromopentane; 3. Na, NH3

C) 1. H2, Lindlar Pd; 2. NaNH2 in NH3; 3. 1-bromopentane

D) 1. Na, NH3; 2. NaNH2 in NH3; 3. 1-bromopentane

Addition of Hydrogen Halides

to Alkynes

HBr

Br

(60%)

Alkynes are slightly less reactive than alkenes

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

Follows Markovnikov's Rule

(76%)

CH3CH2C CCH2CH3

2 HF

F

F

C C

H

H

CH3CH2 CH2CH3

Two Molar Equivalents of Hydrogen Halide

HBr

(79%)

regioselectivity opposite to Markovnikov's rule

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

Free-radical Addition of HBr

expected reaction:

enolobserved reaction:

RCH2CR'

O

H+

RC CR' H2O+

H+

RC CR' H2O+

OH

RCH CR'

ketone

Hydration of Alkynes

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 andpredominate at equilibrium

enol

OH

RCH CR' RCH2CR'

O

ketone

Enols

O H

C C

H+O

H

H

:

..:

Mechanism of conversion of enol to ketone

O H

C C

H+O

H

H

:

..:

Mechanism of conversion of enol to ketone

O H

C CH+

O

H

H

:

..:

:

Mechanism of conversion of enol to ketone

O H

C C

H

H

O: :

H+

..:

Mechanism of conversion of enol to ketone

Carbocation is stabilized by electron delocalization (resonance).

H O

C CH

..H+

Key Carbocation Intermediate

O

C CH+

..:

O H

C C

H

H

O: :

H+

..:

Mechanism of conversion of enol to ketone

Useful for symmetrical starting alkynes

to produce a single product.

Unsymmetrical starting alkynes that are not terminal produce a mixture of ketones…non-regioselectively.

H2O, H2SO4

HgSO4

CH3(CH2)5CCH3

(91%)via

Markovnikov's rule followed in formation of enol, Useful with terminal alkynes.

CH3(CH2)5C CH2

OH

CH3(CH2)5C CH

O

Regioselectivity

Aldehyde vs. Ketone

Question

What is the product of the acid catalyzed hydration of 1-hexyne?

A) B)

C) D)

Question

A. I = C, D, E, F; II = A, B

B. I = B, D, E, F; II = A, C

C. I = B, C, D, F; II = A, E

D. I = A, D, E, F; II = B, C

E. I = A, C, D, F; II = B, E

I) Which reactions give ketones? II) Which reactions give aldehydes?

Addition of Halogens to Alkynes

+ 2 Cl2

Cl

Cl

(63%)

CCl2CH CH3HC CCH3

Example

Br2

CH3CH2

CH2CH3Br

Br

(90%)

CH3CH2C CCH2CH3 C C

Addition is anti

+ NBS Br-C CCH3

NBS Example

NBS = N-bromosuccinimide

HC CCH3

AgNO3

O

O

NBr

gives two carboxylic acids by cleavage of triple bond

Ozonolysis of Alkynes

1. O3

2. H2O

CH3(CH2)3C CH

+CH3(CH2)3COH

(51%)

O

HOCOH

O

Example

Question

What product is formed when 2-butyne is subjected to ozonolysis?

A) B)

C) D)

Answer

What product is formed when 2-butyne is subjected to ozonolysis?

A) B)

C) D)

Alkynes

Synthesis & Functions

Can you identify and name the function?

Example

Question

What is the structure of Compound Y in the following synthetic sequence?

A) pentane

B) cis-2-pentene

C) trans-2-pentene

D) 2-pentyne

Answer

What is the structure of Compound Y in the following synthetic sequence?

A) pentane

B) cis-2-pentene

C) trans-2-pentene

D) 2-pentyne

Natural Products

Polyynes

Compound 1 is isolated from the root and bark of mistletoe, Paramacrolobium caeruleum (Loranthaceae). The stems and leaves of members of this family have been used for the treatment of cancer in Indonesia.

Compound 2, thiarubrine B, has been isolated from Giant Ragweed, Ambrosia trifida. Native cultures in Canada and Africa use plants with similar natural products to treat skin infections and intestinal parasites.

Compound 3, oplopandiolacetate, is found in the bark and roots of Devil's club, Oplopanax horridus. It is used medicinally by native Americans to treat a variety of ailments. Other polyynes found in plants include oenanthotoxin, cicutoxin, and falcarinol .

Compound 4, dihydromatricaria acid, is found in the soldier beetle, Cantharidae, who are related to the Lampyridae or firefly family, but unable to produce light. They provide biological control of a number of insect pests including grasshoppers, aphids, caterpillars and other soft bodied insects.

Histrionicotoxin is isolated from the skin of frogs in the Dendrobatidae family. It comes from insects in their diet and is used by indigenous South American tribes as a poison on arrows.

+ NBS Br-C CCH3

PolyyneCu (I) Coupling Reactions

HC CCH3

AgNO3