Hougland CHE275 Chapter9 Slides

8/13/2019 Hougland CHE275 Chapter9 Slides

http://slidepdf.com/reader/full/hougland-che275-chapter9-slides 1/14

Organic Chemistry

CHE 275

Chapter 9

Alkynes

Industrial preparation of acetylene is

by dehydrogenation of ethylene

CH3CH3

800°C

1150°C

cost of energy makes acetylene a more

expensive industrial chemical than ethylene

H2C CH2

H2C CH2HC CH

H2+

H2+

Acetylene

Naturally Occurring

Alkynes

C(CH2)4COHCH3(CH2)10C

O

Tariric acid: occurs

in seed of a

Guatemalan plant

Some alkynes occur naturally. For example,

Histrionicotoxin: defensive toxin in the poisondart frogs of Central and South America

HNOH

HC CH

Acetylene and ethyne are both acceptable

IUPAC names for

Higher alkynes are named in much the same

way as alkenes except using an -yne suffix

instead of -ene.

HC CCH3

Propyne

HC CCH2CH3

1-Butyne or But-1-yne

(CH3)3CC CCH3

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

Nomenclature

The physical properties of alkynes are

similar to those of alkanes and alkenes.

Physical Properties

of Alkyneslinear 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 cycloalkynestable enough to be

stored at room temperature

for a reasonable length

of time.

Cyclooctyne polymerizes

on 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

2sp

Mix together (hybridize) the 2s orbital

and one of the three 2p orbitals

2p

Each carbon has two

half-filled sp orbitals

available to form bonds.

sp Hybridization in

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.

Bonds inAcetylene

One of the two

bonds in

acetylene is

shown here.The second

bond is at right

angles to the first.

Bonds in

Acetylene

This is the second

of the two bonds in

acetylene.

Bonds in

Acetylene



The region of highest

negative charge lies

above and below the

molecular plane in

ethylene.

Electrostatic Potential in

Acetylene

The region of highest

negative charge

encircles the

molecule around its

center in acetylene.

Comparison of Ethane,

Ethylene, and 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

H C C

Acidity of Acetyleneand Terminal Alkynes In general, hydrocarbons are

exceedingly weak acids

Compound pKa

HF 3.2

H2O 15.7

NH3 36

45

CH4 60

H2C CH2

Acidity of Hydrocarbons

Alkynes are not nearly as weak acids

as alkanes or alkenes

Compound pKa

HF 3.2

H2O 15.7

26

NH3 36

45

CH4 60

H2C CH2

Acidity of Alkynes

HC CH

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 = 60

sp3C : –

H+ + sp

2H

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 acid

pKa = 26

stronger acid

pKa = 15.7

In acid-base reactions, the equilibrium lies to

the side of the weaker acid.

Sodium Acetylide

HO..

..: HO H

..

.. C CH –

H C CH+ + : –

Solution: Use a stronger base. Sodium amide

is 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 acid

pKa = 26

weaker acid

pKa = 36

H2N

Sodium AcetylideInteractive Question

Which one of the following is the strongest

acid?

A) water

B) ammonia

C) 1-butene

D) 1-butyne

Carbon-carbon bond formationalkylation of acetylene and terminal alkynes

Functional-group transformations

elimination

There are two main methods for the preparation

of alkynes:

Preparation of Alkynes

H—C C—H

R—C C—H

R—C C—R

Alkylation of Acetylene

and Terminal Alkynes



R X

SN2

X – :

+C

–

:H—C

C—RH—C

+

The alkylating agent is an alkyl halide, and

the reaction is nucleophilic substitution.

The nucleophile is sodium acetylide or the

sodium salt of a terminal (monosubstituted)

alkyne.


and Terminal Alkynes

NaNH2

NH3

HC CH HC CNa

CH3CH2CH2CH2Br

(70-77%)

HC C CH2CH2CH2CH3


NaNH2, NH3

CH(CH3)2CHCH2C

CNa(CH3)2CHCH2C

CH3Br

(81%)

C—CH3

(CH3

)2

CHCH2

C

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

Interactive Question

Which alkyl halide will react faster with the

acetylide ion (HCCNa) in an SN2 reaction?

A) bromopropaneB) 2-bromopropane

C) tert-butyl iodide

D) 1-bromo-2-methylbutane

Effective only with primary alkyl halides

Secondary and tertiary alkyl halidesundergo elimination

Limitation



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


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) NaCCH, NH3, -33°C

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 Alkynes by

Double Dehydrohalogenation

(CH3)3CCH2 —CHCl2

1. 3 NaNH2, 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

CH3(CH2)7CH —CH2Br

Br

1. 3 NaNH2, NH3

2. H2O

(54%)

CH3(CH2)7C CH

Vicinal dihalide

Alkyne




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

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)

Addition of Halogens (Section 9.13)

Ozonolysis (Section 9.14)

RCH2CH2R'cat

catalyst = Pt, Pd, Ni, or Rh

alkene is an intermediate

RC CR' + 2H2

Hydrogenation of

AlkynesHeats of Hydrogenation

292 kJ/mol 275 kJ/mol

Alkyl groups stabilize triple bonds in the

same way that they stabilize double

bonds. Internal triple bonds are more

stable than terminal ones.

CH3CH2C CH CH3C CCH3

RCH2CH2R'

Alkynes could be used to prepare alkenes if a

catalyst were available that is active enough to

catalyze the hydrogenation of alkynes, but not

active enough for the hydrogenation of alkenes.

cat

H2RC CR'

cat

H2RCH CHR'

Partial Hydrogenation

There is a catalyst that will catalyze the hydrogenationof alkynes to alkenes, but not that of alkenes to alkanes.

It is called the Lindlar catalyst and consists of

palladium 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)3C C(CH2)3CH3

CH3(CH2)3 (CH2)3CH3

H H

(87%)

CC

ExampleUseful In Industry

• Large scale synthesis of vitamin A performed by

Hoffman-LaRoche

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

CH3CH2C CCH2CH3

CH3CH2

CH2CH3H

H

(82%)

CC

Na, NH3

Example

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

Mechanism

Step (1): Transfer of an electron from the metal

to the alkyne to give an anion radical.

M .+R R'C C R R'C.. . –

C

M+

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

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

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

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

heptene from propyne and any necessary organic

or inorganic reagents.

Strategy

1. NaNH2

2. CH3CH2CH2CH2Br

Na, NH3H2, Lindlar Pd

Synthesis




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

Retrosynthetic Analysis

• Write out the reactions in reverse to first

+ Br

Strategies for Synthesis

• Then write out reactions in the forward direction

• Be sure to look out for incompatabilities

Problem

• Devise a synthesis of 2-bromopentene from acetylene

Br

2-Bromopentane


Br

Br + Na

Solution



Problem

• Can we synthesize 1-hexanol from acetylene?

OH

1-Hexanol


OH

Br Na+

Solution

HBr

Br

(60%)

Follows Markovnikov's rule

Alkynes are slightly less reactive than alkenes

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

Addition of HX

CH

..Br H :..

RC

..Br H :..

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

CH3(CH2)3C CH

(79%)

CH3(CH2)3CH CHBr peroxides

Free-radical

Addition of HBr expected reaction:

enol

H+

RC CR' H2O+

OH

RCH CR'

observed reaction:

RCH2CR'

O

H+

RC CR' H2O+

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 and

predominate 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 CH+

O

H

H

:

..

:

:


of enol to ketone

O H

C C

H

H

O: :

H+

..

:


of enol to ketone



OH

C C

H

H

O:

H

+..

:


of enol to ketone

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

viaOH

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

Mechanism of Hg(II)-Catalyzed

Hydration of Alkynes

• Addition of Hg(II) to alkyne gives a vinylic cation

• Water adds and loses a proton

• A proton from aqueous acid replaces Hg(II)

Hydroboration/Oxidation of

Alkynes

• BH3 (borane) adds to alkynes to give a vinylic borane

• Oxidation with H2O2 produces an enol that converts to the

ketone or aldehyde

• Process converts alkyne to ketone or aldehyde with

orientation opposite to mercuric ion catalyzed hydration



+ 2 Cl2

Cl

Cl

(63%)

CCl2CH CH3HC CCH3

Addition of Halogens to

Alkynes

Br 2CH3CH2

CH2CH3Br

Br

(90%)

CH3CH2C CCH2CH3 C C

Addition is anti

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

Summary: Chapter 9

Summary: Chapter 9

9.1 Sources of Alkynes

9.2 Nomenclature

9.3 Physical Properties of Alkynes

9.4 sp hybridization

9.5 Acidity of Alkynes

9.6 Preparation of Alkynes by Alkylation

9.7 Preparation of Alkynes by Elimination

9.8 Reactions of Alkynes

9.9 Hydrogenation of Alkynes

Summary: Chapter 9Summary: Chapter 9

9.10 Dissolving Metal Reductions of Alkynes

9.11 Addition of HX to Alkynes

9.12 Hydration of Alkynes

9.13 Addition of X2

to Alkynes

9.14 Ozonolysis of Alkynes

Hougland CHE275 Chapter9 Slides

Documents

Transcript of Hougland CHE275 Chapter9 Slides