ALKYNES at a glance

 UNSATURATED HYDROCARBON (ALKYNES)

Alkynes are a homologous series of unsaturated hydrocarbons containing at least one 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

 

 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

NOMENCLATURE OF ALKYNES

The nomenclature of alkynes is similar to that of alkenes in many respects 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              

                    Pent-2-yne  

  (ii)        CH₃CH₂CH₂C≡CCH₃    

                       hex-2-yne

            C1H₃                                         C1H₃
                 |                                                            |   
(iii)       C2HC3≡C4C5H₃              (iv)    C2H₂-C3≡C4-C5H₂
                                      |                                                                  |
                                     CH₃                                                            C6H₃
             4-methylpent-2-yne                                 hex-3-yne

               CH₃                                                        CH₃          CH_{3
                          |}                                                                           |                 |
(v)        CH₃CHC≡CCHCH₃                (vi)       C1H₃C2-C3≡C4-C5-C6H₃
                                         |                                                  |                 |
                                  CH₃                                           CH₃          CH₃
            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=CCH—CH₂CH₂C≡CH               (x)        CH₃C-CHC≡CC≡CC≡CH 
                        |              |                                                                 |
                      CH₃      CH₂CH₃                                                      CH₂CH₃  

    6-ethyl,3,3-dimethyldec-1,6-diyne                 8-ethyl, 8-methylnon-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-C6-C5-C4-C3C2≡C1H
              |    |     |     |
             H H H       |
                               |
                          H-C-H
                               |
                              H
            3,3-dimethylhex-1

LABORATORY PREPARATION OF ETHYNES (ALKYNES)

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

 

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

3. 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.

The following test can be performed to distinguished clearly the different classes of hydrocarbons, that is, the alkanes, alkenes and alkynes.

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:  Add the solution of Ammonical copper (I) chloride to the suspected hydrocarbons. it will form a yellowish or reddish –brown precipitate with terminal alkynes (alkynes with the triple bond in front of or behind the first C- atom). Alkanes and alkenes show no reaction.

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

OBJECTIVE QUESTION

1.When alkynes are hydrogenated completely, they produce compounds with the general molecular formula 

a. CnHn

b. CnH2n+2

c. CnH2n

d.CnH2n-2

2. 

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