ALKYNES

 

UNSATURATED HYDROCARBON (ALKYNES)

Alkynes are a homologous series of unsaturated hydrocarbons containing triple bond. It has a functional group of (≡) and general molecular formular of C_nH_2n-2 where n= 1,2,3, ... n for successive members of the group. 

The first member of the alkyne family is ethyne (acetylene).

 Alkynes are named by replacing ending –ane  of the corresponding alkane with –yne.

 

NOTE: Since alkynes contain triple bonds between C≡C therefore n=1 is not visible.

When n=	General Molecular Formulae  CnH2n-2	Name
2.	C2H2x2-2 = C2H2	Ethyne
3.	C3H2x3-2 = C3H4	Propyne
4.	C4H2x4-2 = C4H6	Butyne
5.	C5H2x5-2 = C5H8	Pentyne
6.	C6H2x6-2 = C6H10	Hexyne
7.	C7H2x7-2 = C7H12	Heptyne
8.	C8H2x8-2 = C8H14	Octyne
9.	C9H2x9-2 = C9H16	Nonyne
10.	C10H2x10-2 = C10H18	Decyne
11.	C11H2x11-2 = C11H20	Undacyne
12.	C12H2x12-2 = C12H22	Dodecyne
13.	C13H2x13-2 = C13H24	Tridecyne
14.	C14H2x14-2 = C14H26	Tetradecyne
15.	C15H2x15-2 = C15H28	Pentadecyne
16.	C16H2x16-2 = C16H30	Hexadecyne
17.	C17H2x17-2 = C17H32	Heptadecyne
18.	C18H2x18-2 = C18H34	Octadecyne
19.	C19H2x19-2 = C19H36	Nonadecyne
20.	C20H2x20-2 = C20H38	Icosyne/Eiocosyne
21.	C21H2x21-2 = C21H40	Heneicosyne
22.	C22H2x22-2 = C22H42	Docosyne
23.	C23H2x23-2 = C23H44	Tricosyne
24.	C24H2x24-2 = C24H46	Tetracosyne
25.	C25H2x25-2 = C25H48	Pentacosyne
26.	C26H2x26-2 = C26H50	Hexacosyne
27.	C27H2x27-2 = C27H52	Heptacosyne
28.	C28H2x28-2 = C28H54	Octacosyne
29.	C29H2x29-2 = C29H56	Nonacosyne
30.	C30H2x30-2 = C30H58	Triacontyne

 

 MOLECULAR STRUCTURES OF ALKYNES

N	ALKYNES	STRUCTURAL FORMULAR	MOLECULAR FORMULAR
2.	C2H2 Ethyne	H-C≡C-H	HC≡CH
3.	C3H4 Propyne	H  H-C-C≡C-H       H	CH3C≡CH
4.	C4H6 Butyne	H H  H-C-C-C≡C-H      H H	CH3CH2C≡CH
5.	C5H8 Pentyne	H H H  H-C-C-C-C≡C-H      H H H	CH3(CH2)2C≡CH
6.	C6H10 Hexyne	H H H H  H-C-C-C-C-C≡C-H      H H H H	CH3(CH2)3C≡CH
7.	C7H12 Heptyne	H H H H H  H-C-C-C-C-C-C≡C-H      H H H H H	CH3(CH2)4C≡CH
8.	C8H14 Octyne	H H H H H H  H-C-C-C-C-C-C-C≡C-H       H H H H H H	CH3(CH2)5C≡CH
9.	C9H16 Nonyne	H H H H H H H  H-C-C-C-C-C-C-C-C≡C-H      H H H H H H H	CH3(CH2)6C≡CH
10.	C10H18 Decyne	H H H H H H H H  H-C-C-C-C-C-C-C-C-C≡C-H       H H H H H H H H	CH3(CH2)7C≡CH
11.	C11H20 Undecyne	H H H H H H H  H H  H-C-C-C-C-C-C-C-C-C-C≡C-H       H H H H H H H H  H	CH3(CH2)8C≡CH
12.	C12H22 Dodecyne	H H H H H H H H HH  H-C-C-C-C-C-C-C-C-C-C-C≡C-H       H H H H H H H H HH	CH3(CH2)9C≡CH
13.	C13H24 Tridecyne	H H H H H H H H H H H  H-C-C-C-C-C-C-C-C-C-C-C-C≡C-H       H H H H H H H H H H H	CH3(CH2)10C≡CH
14.	C14H26 Tetradecyne	H H H H H H H H H H H H  H-C-C-C-C-C-C-C-C-C-C-C-C-C≡C-H       H H H H H H H H H H H H	CH3(CH2)11C≡CH
15.	C15H28 Pentadecyne	H H H H H H H H H H H H H  H-C-C-C-C-C-C-C-C-C-C-C-C-C-C≡C-H       H H HH H H H H H H H H H	CH3(CH2)12C≡CH
16.	C16H30 Hexadecyne	H H H H H H H H H H H H H H  H-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C≡C-H       H H H H H H H H H H H H H H	CH3(CH2)13C≡CH
17.	C17H32 Heptadecyne	H H H H H H H H H H H H H H H  H-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C≡C-H       H H H H H H H H H H H H H H H	CH3(CH2)14C≡CH
18.	C18H34 Octadecyne	H H H H H H H H H H H H H H H H  H-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C≡C-H      H H H H H H H H H H H H H H H H	CH3(CH2)15C≡CH
19.	C19H36 Nonadecyne	H H H H H H H H H H H H H H H H H  H-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C≡C-H       H H H H H H H H H H H H H H H H H	CH3(CH2)16C≡CH
20.	C20H28 Eiocosyne	H H H H H H H H H H H H H H H H H H  H-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C≡C-H       H H H H H H H H H H H H H H H H H H	CH3(CH2)17C≡CH

NOMENCLATURE OF ALKYNES

The nomenclature of alkynes is similar to that of alkenes in many ways as shown in the structures below. The only difference lies on the type of bonds, in alkenes (double bond) and alkynes (triple bond).

 

(i)         CH₃-CH₂-C≡CCH₃                     (ii)        CH₃CH₂CH₂C≡CCH₃    

            Pent-2-yne                                           hex-2-yne

            

             CH₃                                                        CH₃
              |                                                              | 
(iii)       CHC≡CCH₃                               (iv)       CH₂-C≡C-CH₂
              |                                                                                 |
             CH₃                                                                           CH₃
             4-methylpent-2-yne                                       hex-3-yne

                   

1.                         CH₃                                                                     CH₃       CH₃
2. (v)                    |                                                                              |            | 
3.                  CH₃CHC≡CCHCH₃                                (vi)       CH₃C-C≡C-C-CH₃
4.                                     |                                                                   |            |
5.                                    CH₃                                                             CH₃      CH₃
6.             2,5-dimethylhex-3-yne                                    2,2,5,5-tetramethylhex-3-yne

                                 CH₃

(vii)      CH₃CH-C ≡CC-CH₂CH₃                                 (viii)     CH₃C≡CCH₂

                   CH₃         CH₃                                                           CH₃

             2,5,5-trimethylhept-3-yne                              pent-2-yne

                      CH₃                                                                              CH₃ 

(ix)       CH≡CC-C=C-------CH—CH₂CH₂C≡CH               (x)        CH₃C-CHC≡CC≡CC≡CH 

                      CH₃              CH₂CH₃                                                    CH₂CH₃  

            6-ethyl,3,3-dimethyldec-1,6-diyne                 8-ethyl, 8-methynon-1,3,5-triyne

                                                                                              Cl

(xi)       CH₃C≡CCHCH₃                                    (xii)      CH₃-C-C≡CH 

                          Cl                                                                  Cl

            4-chloropent-2-yne                                         3,3-dichlorobut-1-yne

(xiii)     CH₃CHC≡CC≡CCHCH₃                          (xiv)     CH₃CHC≡CCHC≡CH  

                   Cl                Br                                                      Cl         Cl

            2-bromo, 7-chlorooct-3,5-diyne                     3,6-dichlorohept-1,3-diyne

                        H    

                    H-C-H      

             H H H           

(xv)  H-C-C-C-CC≡CH

             H H H        

                   H-C-H

                       H

            3,3-dimethylhex-1-yne

LABORATORY PREPARATION OF ETHYNES (ALKYNES)

Ethyne is prepared in the laboratory by adding cold water into calcium dicarbide (CaC₂). Much heat is evolved and sand is placed beneath the flask to protect the flask from breakage. Ethyne is collected over water. The chief impurity, phosphine, PH₃ is absorbed by the acidified CuSO₄ solution.

EQUATION FOR THE REACTION

CaC₂  +  2H₂O → Ca(OH)₂  +  C₂H₂.

                                                 Ethyne

 

 

PHYSICAL PROPERTIES OF ETHYNE

1. It is colourless gas

2. It has sweet smell when pure

3. Almost insoluble in water

4. It is neutral to litmus

5. It is strongly exothermic

CHEMICAL PROPERTIES OF ETHYNE

 Alkynes such as ethyne also undergoes addition reaction – a reaction in which one molecule of a compound is simply added on to the alkynes at the position of the carbon – carbon triple bond (C≡C) and this is converted to carbon – carbon single bond (C-C) that is, the alkanes. Examples of addition reaction are:

1.     Reaction of ethyne with hydrogen in the presence of nickel as a catalyst

                                       Ni
            CH≡CH + 2H₂   →   CH₃CH₃   
            ethyne                           ethane

2. Reaction of ethyne with bromine to produce 1,1,2,2-tetrabromoethane. The reddish brown colour of bromine is destroyed.

            CH≡CH + 2Br₂ → CHBr₂-CHBr₂

Ķ. Reaction of ethyne with chlorine to produce hydrogen chloride

            CH≡CH + Cl₂ → 2C+ 2HCl

4. Reaction of ethyne with oxygen or combustion reaction of ethyne (alkynes) to produce carbon(iv)oxide and water

            2CH≡CH + 5O₂  →4CO₂ + 2H₂O

5. Polymerization reaction of ethyne to produce benzene.

            3C₂H_{2 →} C₆H₆ 

6. Reaction of ethyne with water in the presence of dilute H₂SO₄ and mercury as a catalyst to produce ethanal

            CH≡CH + H₂O →CH₃CHO

7. Reaction of ethyne with KMnO₄ to produce 1,2-ethan-diol (glycol)

            CH≡CH  +KMnO₄ →CH₂-CH₂
                                                 |        | 
                                                 OH   OH
                                               1,2-ethan-diol

USE OF ETHYNE

1. In oxy-acetylene flame for welding and cutting of metals

2. In oxy-acetylene torch

3. In preparation of acetic acid

4. as a starting material for making polyvinylchloride (PVC) which is used in electrical insulation and water proofing.

TESTS TO DISTINGUISHED BETWEEN ALKANES, ALKENES AND ALKYNES.

To distinguishe between the three aliphatic hydrocarbons, the following test can be performed to clearly the differentiate the classes of hydrocarbons, that is, the alkanes, alkenes and alkynes.

NOTE: All alkanes are saturated compounds while both alkenes and alkynes are unsaturated.

TEST 1: 

To the suspected hydrocarbons, add an acidified solution of KMnO₄ or K₂Cr­₂O₇ solution. Alkanes have no effect in any of these solutions while both alkenes and alkynes decolorized. Acidified KMnO₄ solution changes from purple to colourless, while K₂Cr₂O₇ changes from orange to green.

TEST 2: 

To the suspected hydrocarbons, add the solution of Ammonical copper (I) chloride. Alkanes and alkenes have no effect, but alkynes form a yellowish or reddish –brown precipitate.

2NH₄OH_(aq)+ 2CuCl  + C₂H₂ → CuC₂  + 2NH₄Cl  + 2H₂O.

TEST: To the suspected hydrocarbon, add solution of Ammonical silver tronitrate (V). Alkanes and alkenes have no effect, but alkynes form a yellowish precipitate.  

2NH₄OH + 2AgNO₃ + C₂H₂ → 2AgC + 2NH₄NO₃ + 2H₂O.