By:Dr. Shatha Alaqeel

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The alkynes comprise a series of carbon and hydrogen based compounds that contain one triple bond. This group of compounds is a homologous series with the general molecular formula of CnH2n—2

The alkyne triple bond is composed of one σ and two π covalent bonds.

The simplest alkyne, ethyne (also known as acetylene), has two carbon atoms and the molecular formula of C2H2. The structural formula for ethyne is:

CH CH2

This involves the mixing of one s- and one p-orbital forming two sp-hybrid orbitals. The two sp-hybrid orbitals are oriented in a linear arrangement and bond angle is 180°.

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Derived from acetylene

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CH CH

Acetylene

CH C-CH3CH3-C C-CH3

Methylacetylene Dimethylacetylene

CH3-C C-CH2CH3

Ethylmethylacetylene

1. Identify the longest continuous chain of carbon atoms that contains the carbon-carbon triple bond. the - ane ending is changed to – yne

2. Number the carbon atoms of the longest continuous chain, starting at the end closest to the triple bond.

3. The position of the triple bond is indicated by placing the lower of the pair of numbers assigned to the triple-bonded carbon atoms in front of the name of the alkyne.

4. The location and name of any substituent atom or group is indicated.

5.5.DoubleDouble and and tripletriple bonds are considered to bonds are considered to have equal priorityhave equal priority: thus in a molecule : thus in a molecule with both a double and triple bond, whichever is close to the end of the chain with both a double and triple bond, whichever is close to the end of the chain determines the direction of numbering.determines the direction of numbering.

6.6.In case where double and triple bonds would have the In case where double and triple bonds would have the same position numbersame position number, the , the double bond takes the lower numberdouble bond takes the lower number..

7.7.In writing the final name In writing the final name ‘’ene’’ comes before ‘’yne’’‘’ene’’ comes before ‘’yne’’ regardless which takes the regardless which takes the lower number (i.e. alphabetical order).lower number (i.e. alphabetical order).

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CH3 C CH CH3 C C CH2 CH2 Br

IUPAC : Propyne Common : Methyl acetylene

5-bromo-2-pentyne

CH3 CH

CH3

CH2 C C CH

CH3

CH3

IUPAC :2,6-Dimethyl-3-heptyne Common :Isobutylisopropylacetylene

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6-Ethyl-4-nonyne

Pentyne

C CHHC

CH3

CH

F

F

4,4-Difluoro-3-methyl-1-butyne

C CH-CH2-CH3

H

CHC

Hex-3-en-1-yne(triple bond closer to the end of chain)Note: An''e'' is dropped from ''ene''

due to it is followed by a vowel

HC C CH2 CH CH2

Pent-1-en-4-ynedouble and triple bonds have have the same position number

thus ene take lower number

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2345

6-Ethyl-4-nonyne

C C

Br

H3C

5-Bromo-2-pentyneNot 1-Bromo-3-pentyne

1 2 34

6

1 2

3 4 5

7

8

9

1

5

1 2 3 4 5 6

234

Nonpolar, insoluble in water.

Soluble in most organic solvents.

Boiling points similar to alkane of same size.

Less dense than water.

Up to 4 carbons, gas at room temperature.

Terminal alkynes, R-CC-H, are more acidic than

other hydrocarbons.

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1. Dehydrohalogenation of alkyl dihalides

C C

X

H

X

H

Strong BaseC C + 2 HX

CH3 Br

BrH

H

KOH/alcohol

Heat

CH3 Br

H

NaNH2

HeatCH3 CH

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2. Reaction of sodium Acetylide with Primary Alkyl Halides

C CHR + Na liq NH3 C C

- Na

+R + H21/2

C C- Na

+R + R' X C CR R' + NaX

Sodium acetylide

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11. Addition of hydrogen ( Hydrogenation. Addition of hydrogen ( Hydrogenation))

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Alkynes can be reduced toAlkynes can be reduced to transtrans-alkenes-alkenes using using Na or Li in liquid NHNa or Li in liquid NH33

Alkynes can be partially reduced to Alkynes can be partially reduced to ciscis-alkenes-alkenes with with HH22 in the presence of in the presence of poisoned poisoned

catalystscatalysts..

HH

+ H2C C

Pd(BASO4) or (NiB)

Cis- alkene

H

H

+ H2C C

Na or Li / liq. NH3

Trans -alkene

C C C C

X X

+ X2 + X2 C C

X X

XX

Tetrahalide(X= Br or Cl)

C C C C

H X

+ HX + C C

H X

XH

A gem dihalide(X= Br , Cl, I)

HX

A haloalkene

3 .Addition of hydrogen halide

C C C C

OH

H

C C

H

OH

+ OH HH2SO4, HgSO4

an enol unstable carbonyl more stable

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