TOPIC 4: ALKYNES: STRUCTURE, REACTIVITY AND SYNTHESIS

Alkynes are hydrocarbons that contain a carbon-carbon triple

bond, which is the strongest and shortest type of carbon-carbon bond

that exists. Many of the reactions of alkynes are similar in nature to

those of alkenes, but there are important differences too. In this unit,

we will examine the structure and reactivity of alkynes as well as how

many of the reactions you have learned about thus far are used in the

important process of organic synthesis.

I. NAMING ALKYNES * Alkynes follow the general set of naming rules we saw for

alkanes and alkenes, with the name of the parent chain ending in –yne.

1. Find the parent hydrocarbon. Find the longest carbon chain that contains the triple bond. Name accordingly using –yne as the suffix.

2. Number the carbon atoms in the chain. Begin at the end nearest the triple bond. If the triple bond is equidistant from the two end points, begin at the end nearest the first branch point.

3. Write the full name. • Number the substituents according to their positions on the chain, and arrange alphabetically, just as for alkanes.• Indicate the position of the triple bond by giving the number of the first carbon where it begins.

• If more than one triple bond exists, use numbers to indicate the position of each and use the suffixes –diyne, -triyne, etc.

• Compounds containing both double and triple bonds are called enynes. Numbering of an enyne chain begins nearer the first multiple bond, whether double or triple. A number is used to indicate the positions of both the double and triple bond. Where a tie exists, the double bond receives the lower number.

1

2

3

4

56

78

5-ethyl-4-methyl-1-octen-6-yne

II. PREPARATION OF ALKYNES: ELIMINATION REACTIONS

ELIMINATION OF X2 FROM 1,2-DIHALIDES

* Alkynes can be prepared by elimination of X2 from a 1,2-dihalide by treatment with excess strong base or by elimination of HX from a vinylic halide by treatment with strong base.

C

C

H Br

Br H

2 KOHCC

+ 2 H2O + 2 KBr

ELIMINATION OF HX FROM VINYLIC HALIDES

VINYLIC:

- Refers to a substituent directly attached to a double bond

C

C

H

Br

NaNH2 CC

+ NH3 + NaCl

III. REACTIONS OF ALKYNES: ADDITION OF HX AND X2 Based on electronic similarity between alkynes and alkenes,

you may expect the chemical reactivity of the two functional groups to be similar. While there are indeed many similarities, significant differences also exist.

CH3C ≡ CHCH3C = CH

H

Br

CH3C ─ C─H

Br

Br

H

H HBrHBr

add’n of 1 eq. ofHBr

add’n of excessHBr yields dihalide

ELECTROPHILIC ADDITION OF HX

- Reaction can be stopped after addition of 1 equivalent of HX- Follows Markovnikov’s Rule (X adds to more sub. carbon)- trans stereochemistry of H and X typically occurs- Addition of excess HX forms the dihalide

ELECTROPHILIC ADDITION OF X2

- Reaction can be stopped after addition of 1 equivalent of X2

to give the di-substituted product

- Excess addition of X2 yields the tetra-substituted product

- Trans stereochemistry results

CH3CH2C ≡ CH Br2

CH3CH2

Br

Br

C = C

H

Br2 CH3CH2CBr2CHBr2

H BrRC CH

Br

C C

H

HR

C C

H

HR

Br

Reaction Mechanism:

The reaction mechanism of electrophilic addition of HX to an alkyne is similar to that of an alkene. It occurs in two steps with a vinylic carbocation intermediate forming.

Vinylic carbocationintermediate

* Although the mechanism for the addition of HX to an alkyne mirrors that of the mechanism for the addition of HX to an alkene, most other alkyne additions occur through more complex mechanistic pathways.

IV. HYDRATION OF ALKYNES: ADDITION OF H2O

* Like alkenes, alkynes can be hydration via two different

methods. Addition of H2O in the presence of acid yields the

Markovnikov product while indirect addition of H2O via

hydroboration yields the anti-Markovnikov product. Unlike in the case of alkenes however, the product of hydration is not exactly an alcohol. This is due to the existence of what is known as tautomerisation.

CC

OH

CC

O

H

Rapid

Enol tautomer

(less favored)

Keto tautomer

(more favored)

ENOL:

- a vinylic alcohol (ene + ol) (-OH is directly attached to a double bonded carbon)

- less stable than its keto counterpartC

C

OH

TAUTOMERS:- describes constitutional isomers that interconvert rapidly- in keto-enol tautomerism, the keto tautomer is favored (*see example above)

*Note: tautomerisation is not the same as resonance. In resonance, atoms do not move, only electrons do. In tautomerisation, atoms switch places. Also, resonance is a concept of how molecules exist: they do not actually switch back and forth between resonance structures. In tautomerisation, molecules actually do shift back and forth between isomeric structures.

A. Acid- Catalyzed Hydration of Alkynes: Markovnikov Product

- Reaction of an alkyne w/ H2O in the presence of acid to form an enol at more substituted carbon (Markonikov product)

- Enol undergoes tautomerisation to give the favored ketone

product

* See board for mechanism

CH3C CHH

C C

H

HCH3

H2O

more substituted vinyllic carbocation forms

C C

H

HCH3

O+

HH

H2O

C C

H

HCH3

OH

tautomerisation

CCH3CH3

O

*

Reaction Mechanism:

*Note: This reaction is most useful when applied to a terminal alkyne because only one product forms. When the reaction occurs with an internal alkyne, a mixture of ketone products form.

C CR R'H2O

H2SO4

CR

O

CH2R'C

O

R'RCH2

an internal alkyne mixture

+

C CR HH2O

H2SO4

CR

O

an external alkyne single product

CH3

B. Hydroboration of Alkynes: anti- Markovnikov Product

- Reaction of an alkyne w/ BH3 in the presence of H2O2/ OH─ to form an enol at less substituted carbon (Markonikov product)

- Enol undergoes tautomerisation to give the favored keto

product

* See board for mechanism*Again, this reaction is most useful when applied to a

terminal alkyne because only one product, in this case an aldehyde, forms. When the reaction occurs with an internal alkyne,

a mixture of products form.

C CR H1. BH3

2. OH- /

H2O2

CR

O

H

an external alkyne single aldehyde product

Reaction Mechanism:

CH3C CH1. BH3

C C

HCH3

Transition State: BH3 adds to less substituted carbon

H BH2

C C

BH2

HCH3

H

C C

OH

HCH3

H

*

2. H2O2

OH-

OH replaces BH2

tautomerisation

CH2

CCH3

O

H*

V. REDUCTION OF ALKYNES

* Alkynes are easily reduced, which means that they form an

increase in bonds to hydrogen, by addition of H2 over a metal

catalyst. As a result, alkynes can be reduced to an alkene or

further to an alkane. Depending on what reagents are used in

the reduction, either product can be selected for.

CH CH CH2 CH2 CH3 CH3

Ethyne

alkyne

Ethene

alkene

Ethane

alkane

reduction reduction

A. Complete Reduction: Alkyne to Alkane

- Reduction to the alkane occurs when the alkyne is reacted w/

H2 in the presence of palladium on carbon (Pd/C) as a catalyst

H2

Pd/C

alkyne alkane

B. Incomplete Reduction: Alkyne to Alkene

- Incomplete reduction to the alkene occurs when the alkyne is

reacted with H2 in the presence the less active Lindlar catalyst

- The use of H2/ Lindlar catalyst produces CIS alkenes.

PATH 1: Formation of Cis Alkenes

H2

Lindlar

catalyst

H

H

alkyne cis alkene

- Incomplete reduction to the alkene also occurs when the alkyne

is reacted w/ Na or Li metal in liquid ammonia (NH3).

- The use of Na or Li/ NH3 produces TRANS alkenes.

PATH 2: Formation of Trans Alkenes

Li

NH3

H

H

alkyne trans alkene

VI. OXIDATIVE CLEAVAGE OF ALKYNES

* Alkynes, like alkenes, can by cleaved by reaction with a

powerful oxidizing agent like ozone. A triple bond is generally

less reactive that a double bond however, and yields of

cleavage products are sometimes low. Like alkenes, the

products of alkyne cleavage produce carbonyls, but as part of

a different functional group.

Oxidative Cleavage of Alkynes:

- The products of cleavage of an internal alkyne (R─C≡C─R’) are

two carboxylic acids

- The products of cleavage of an external alkyne (R─C≡C─H) are a

carboxylic acid and CO2

O3

Zn/H3O+

internal alkyne

O

OH OH

O

1 21 2

+

HO3

Zn/H3O+

external alkyne

O

OH1 2

1 2

+ CO2

VII. ALKYNE ACIDITY: FORMATION OF ACETYLIDE ANIONS* According to the Brønsted- Lowry definition, an acid is any

substance that donates H+. Although we usually think of oxyacids (H2SO4, H3PO4) or halogen acids (HCl, HBr) in this context, any compound containing a hydrogen atom can be an acid under the right circumstances.

The most striking difference between alkynes and alkenes and alkanes is that terminal alkynes are weakly acidic.

Acidity of Simple Hydrocarbons

Type Example Ka pKa

Alkyne HC ≡ CH 10─25 25

Alkene H2C= CH2 10─44 44

Alkane H3C− CH3 10─60 60 Weaker acid

Stronger acid

When a terminal alkyne is treated with a strong base, such as sodium amide (Na+NH2

─), the terminal hydrogen is removed and an

ACETYLIDE ANION is formed:

CR C H

external alkyne

NH2 Na CR C Na NH3

Acetylide anion

The presence of a negative charge and an unshared electron pair

on carbon makes an acetylide anion extremely ________________.nucleophilic

ALKYLATION REACTION:

- Substitution reaction in which a new alkyl group becomes

attached to the starting alkyne

- The nucleophilic acetylide anion attacks a positively polarized carbon atom in a haloalkane

- This a BIG deal a new carbon – carbon bond is formed!

CR C Na C

H

H

H

Br CR C C

H

H

H NaBr

The reaction conditions for acetylide anion formation and alkylation are often shown together as a two step reaction

process.

1. NaNH2

2. CH3CH2Br

acetylide anion formation

alkylation

CH3CH2CH2C CH CH3CH2CH2C CCH2CH3

-Acetylide anion alkylation is limited to primary alkyl bromides

and iodides.

-In addition to their reactivity as nucleophiles, acetylide anions are

sufficiently strong bases that they can cause dehydrohalogenation

(loss of H + halogen) instead of substitution when they react with

secondary and tertiary alkyl halides.

H

HH

Br

CR C Na

H

HH

R

+ NaBr

H

H

+ HBr2 o alkyl bromide

elimination product (acetylide anion acts as a base)

substitution product (acetylide anion undergoes alkylation)