Alkynes - Organic Chemistry

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ALKYNES

Alkynes (Cn H2n-2) are hydrocarbons that contain a carbon-carbon triple bond. STRUCTURE AND NOMENCLA NOMENCL AT URE In the IUPAC system of nomenclature, the rules for alkynes are similar to those for alkenes, except that the ending –yne replaces –ene. Ethyne or acetylene (its common name) is the simplest member of the alkyne family family . Propyne is the second. se cond.

H

C

C

H

CH3

ethyne (acetylene)

C

C

H

propyne

Alkynes of four or more carbons can have different positions for the carbon-carbon triple bond and can be classified as either internal or terminal depending dependi ng on t he location of t he triple bond. Note t hat terminal alkynes have a hydrogen directly attached to the sp-hybridized carbon. Internal

CH3

C

Terminal C

CH3

CH3CH2

C

C

CH2CH3

CH3CH2CH2CH2

C

C

H

C

C

1-hexyne

3-hexyne CH3

C

1-butyne

2-butyne CH3CH2

C

CH CH3

4-methyl-2-pentyne

CH3

H3C

CHCH2

C

CH3 4-methyl-1-pentyne

C

H

H


2

Cycloalkynes Cycloalkynes are are the cyclic analogs of alkynes. alkynes. The eightmembered ring is the smallest ring that can form a stable cycloalkyne.

cyclooctyne

PREPARATION: In general, alkynes may be prepared from the dehydrohalog dehydrohalogenation enation of vicinal or 1,2 – dihalides with strong base, ba se, e.g. conc. KOH or NaNH2. Br

H 2 NaNH2

H ex.

C

or conc KOH

C

Br

Br H C H

2 NaNH2

C

C

or conc KOH

Br

C

PHYSICAL PHYSICAL PROPERTIES Alkynes are very weakly polar and have essentially the same physical properties as alkanes and alkenes. T he examples b elow illustrate the effect of molecular size and chain-branching on boiling point. H

C

C

CH2CH2CH3

H

C

BP 40 °C

C

CH2CH2CH2CH3

BP 72 °C CH3

H

C

C

CH CH3

BP 29 °C

CH3

H

C

C

C CH3

BP 38 °C

CH3


3

REACTIONS T he π electrons electrons in alkynes are held loosely as in the alkenes. Thus, T hus, alkynes also undergo addition reactions, many of which are electrophilic addition reacti reactions. ons. As a general rule, r eagents add to alkynes in the sam sa me way that they add to alkenes. However, for alkynes to be transfor transform med to a saturated product, two molar equivalents of the adding reagent are necessary.

C

I.

X

C

Y

C

C

X

Y

X

Y

X

Y

C

C

X

Y

Hydrogenation

2H 2 Ni, Pt or Pd

C

C

Na or Li liq NH3

H

H

C

C

H H alkane H C

C

-alkene trans -alkene H2 Lindlar's catalyst

H

H H

C

C

-alkene cis -alkene


4

Examples: II. II. Electrophilic Addition Like alkenes, the electrophilic addition reactions of alkynes follow Markovnikov Markovnikov orientation.

2H2 Ni, Pt or Pd

CH3

C

C

Na or Li liq NH3

CH3

H2 Lindlar's catalyst

CH3

H

H

C

C

CH3

H H butane H

CH3 C

C

CH3

H -2-butene trans -2-butene

H

H C

C

CH3

CH3 -2-butene cis -2-butene

1. Addition of Halogens

C

C

X2

X2 = Cl2, Br2

C

C

X

X

X2

X

X

C

C

X

X

Example:

C

C

HX

HX = HCl, HBr, HI

C

C

H

X

HX

H

X

C

C

H

X


5

C

C

H

Cl2, CCl4

2 Br2, CCl4

Cl

Cl

C

C

H

Br

Br

C

C

Br

Br

H

2. Addition of Hydrogen Halides

C

HX

C

HX = HCl, HBr, HI

C

C

H

X

HX

H

X

C

C

H

X

Example: Cl CH3

C

3.

C

H

HCl

CH3

C

C

Cl

H

H

HCl

CH3

C

CH3

Cl

Addition of Water: Hydration Hy dration

Alkynes are not hydrated as easily as alkenes because of the lower reactivity of the triple bond towards electrophilic addition. Aqueous H2 SO4 by itself has has little effect effect on the t riple riple bond. However, in the presence of HgSO 4 catalyst, hydration occurs readily. H

H C

C

+ H2O

H2SO4 HgSO4

C

C

C

C

OH

H

O


6

Consider the me chanism of the th e hydration of propyne. propyne. (Note: (N ote: T his should apply to all alkynes). 2+

Hg (1) CH3

C

C

2+

Hg CH3

H

C

CH3 H

C

C

H

O H

SN2 attack (T.S. has carbocation character)

CH3

C

C

H

C

C

Hg H

CH3

O

C

C

H

OH H

Hg CH3

+

Hg

+

(3)

H

+

2+

Hg (2)

C

H H

OH

H2SO4 (destroys C-Hg bond)

CH3

C

C

H

OH enol

H (4)

CH3

C

C

O

H

H

keto-enol tautomerism

CH3

C

CH3

O acetone (a ketone)

Although the immediate product is an enol, it is not isolated because it rearranges to a more stable isomer through a process known as keto-enol tautomerism . Tautomers are structural isomers that are readily interconvertible through a rapid equilibrium process. III. III.

Acidity ci dity of Acetylene, Propyne and T erminal Alkynes

The sp-hybridiz sp-hybridized ed carbons of the C=C act a ct as though they are ar e electronegative. T he Csp-H bond is polar p olarized ized towards carbon to a higher degree than th an in a lkanes and alkenes. As a result, acetylene, propyne and the termi terminal nal alkynes are weak weak Bronsted Lowry acids. acids. The hydrogen R

C

C

H

+

+ H R C C acetylide anion bonded to the sp-hybridized carbon can be released as a proton, i.e.:


7

Acetylene is a weaker acid than water but it is a stronger acid than ammonia. Propyne and the t erminal alkynes alkynes also show c omparable acidities. Relative acidities :

H2O > H

C

C

H

> NH3

> RH

The corresponding acetylide anions are strong bases. Relative Basicities :

HO

-

< H

C

<

C

-

-

NH2

< R

Reactions of Terminal Alkynes A.

Formation of Metal Acetylide ce tylide Salts AgNO3 alcoholic R

C

C

-

+

R C C Ag silver acetylide

H Na metal or NaNH2

-

+

R C C Na sodium acetylide

Examples: CH3CH2

C

CH3

B.

C

C C

H CH3

AgNO3 alcoholic Na

CH3CH2

C

C

-

N. R.

Formation of Higher Alkynes lk ynes

Acetylide salts react with primary alkyl halides to form bigger alkynes from smaller ones. Example: R

C

C

-

M+

+ R'CH2X 1° alkyl halide

R

C

C

R'

+

MX

+

Ag


8

CH2C

-

C

+

Na + CH3CH2Br

CH2C

C

CH2CH3

The reaction is a nucleophilic substitution reaction and is successful only with primary alkyl halides. halides. Being a very ver y strong base, the acetylide anion will promote elimination in secondary and tertiary alkyl halides. ANALYSIS OF ALKYNES •

They have the sam sa me response to simple si mple chemical chemical tests as alkenes + Br2 /CCl 4 – decolorization of the red-or re d-orange ange bromine color c olor + cold, dilute dilute neutral neutral KMnO KMnO 4 – formation formation of brown b rown precipitate precipitate and disappearance of the purple color of permanganate

•

Acidic alkynes form insoluble acetylides with alcoholic AgNO 3.

R •

C

C

H

+

-

+

Ag

+

R C C Ag insoluble gray precipitate

Proof of structure is best carried out using ozonolysis.

R

C

C

R'

1. O3 2. Zn, HOAc

internal alkyne

O R

C

O OH + R'

C

carboxylic acids R

C

C

terminal alkyne

H

1. O3 2. Zn, HOAc

O R

C

OH +

CO2

OH

Alkynes - Organic Chemistry

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