Easykemistry: 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 C_nH_2n-2	Name
2.	C₂H_2x2-2= C₂H₂	Ethyne
3.	C₃H_2x3-2= C₃H₄	Propyne
4.	C₄H_2x4-2= C₄H₆	Butyne
5.	C₅H_2x5-2= C₅H₈	Pentyne
6.	C₆H_2x6-2= C₆H₁₀	Hexyne
7.	C₇H_2x7-2= C₇H₁₂	Heptyne
8.	C₈H_2x8-2= C₈H₁₄	Octyne
9.	C₉H_2x9-2= C₉H₁₆	Nonyne
10.	C₁₀H_2x10-2= C₁₀H₁₈	Decyne
11.	C₁₁H_2x11-2= C₁₁H₂₀	Undacyne
12.	C₁₂H_2x12-2= C₁₂H₂₂	Dodecyne
13.	C₁₃H_2x13-2= C₁₃H₂₄	Tridecyne
14.	C₁₄H_2x14-2= C₁₄H₂₆	Tetradecyne
15.	C₁₅H_2x15-2= C₁₅H₂₈	Pentadecyne
16.	C₁₆H_2x16-2= C₁₆H₃₀	Hexadecyne
17.	C₁₇H_2x17-2= C₁₇H₃₂	Heptadecyne
18.	C₁₈H_2x18-2= C₁₈H₃₄	Octadecyne
19.	C₁₉H_2x19-2= C₁₉H₃₆	Nonadecyne
20.	C₂₀H_2x20-2= C₂₀H₃₈	Icosyne/Eiocosyne
21.	C₂₁H_2x21-2= C₂₁H₄₀	Heneicosyne
22.	C₂₂H_2x22-2= C₂₂H₄₂	Docosyne
23.	C₂₃H_2x23-2= C₂₃H₄₄	Tricosyne
24.	C₂₄H_2x24-2= C₂₄H₄₆	Tetracosyne
25.	C₂₅H_2x25-2= C₂₅H₄₈	Pentacosyne
26.	C₂₆H_2x26-2= C₂₆H₅₀	Hexacosyne
27.	C₂₇H_2x27-2= C₂₇H₅₂	Heptacosyne
28.	C₂₈H_2x28-2= C₂₈H₅₄	Octacosyne
29.	C₂₉H_2x29-2= C₂₉H₅₆	Nonacosyne
30.	C₃₀H_2x30-2= 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 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 (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

CH₃ CH₃ CH₃

(v) CH₃CHC≡CCHCH₃ (vi) CH₃C-C≡C-C-CH₃

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=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

(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-C-H

H H H

(xv) H-C-C-C-CC≡CH

H H H

H-C-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

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: 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.

Easykemistry

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Thursday, 4 July 2024

ALKYNES

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