This chapter discusses alkynes, also known as acetylenes, which are unsaturated hydrocarbons containing carbon-carbon triple bonds. The key points covered include: - Alkynes follow the general formula CnH2n-2 and have sp hybridized carbons in the triple bond. Nomenclature involves changing the -ane ending to -yne. - Alkynes can undergo addition reactions like hydrogenation, halogenation, and hydration. Hydration produces ketones or aldehydes depending on the alkyne. - Acetylene is the simplest and most common alkyne. It is a colorless, flammable gas that is produced commercially

1. Chapter 4 Alkynes
Introduction
1. Alkynyl Groups
2. Nomenclature of Alkynes
3. Isomerism in Alkynes
4. Physical Properties of Alkynes
5. Chemical Properties of Alkynes
6. Preparation of Alkynes
7. Acetylene
Table of Contents

2. Chapter 4
Warm up
• Why is acetylene gas
preferred to mold or to cut
metals rather than
electricity?
• Talk about the picture
given beside.
Alkynes

3. Chapter 4 Introduction
• Unsaturated hydrocarbons containing C≡C bond are called
alkynes.
• Carbon atoms having triple bond make sp hybridization.
• They are also known as acetylenes.
• General formula is Cn H2n-2.
• Simplest member is ethyne, or acetylene, C2H2.
• Change –ane ending with –yne to name alkynes.

4. Chapter 4 Introduction

5. Chapter 4 1.Alkynyl Groups
• When one hydrogen is removed from an alkyne an alkynyl
group is formed.
• General formula of alkenyl group is CnH2n-3.
• CnH2n-2 → CnH2n-3
Alkyne Alkynyl
Example 1
C
H C
ethyne
H C
H C
ethynyl
(acetynyl)

6. Chapter 4 1.Alkenyl Groups
10% of the molecular weight of an alkyne is hydrogen. What is
the molecular formula of the alkyne? (C:12, H:1)
Example 2
A 2.7 g of an alkyne is burned and produces 8.8 g of carbon
dioxde. What is the molecular formula of the alkyne? (C:12,
H:1)
Example 3
When an alkyne is burned, the mass of water produced is the
same as the mass of alkyne used up. What is the molecular
formula of the alkyne? (C:12, H:1)
Example 4

7. Chapter 4 2. Nomenclature of Alkynes
• Select longest carbon atom chain containing a triple bond.
• Number the carbon atoms in the chain so that the triple bond
would be between the carbons with lowest designated number.
Name the following alkynes.
Example 5
a.
H3C
H2
C
C
C
CH3
b.
H3C
CH
C
C
CH3
CH3
2-pentyne
4-methyl-2-pentyne

8. Chapter 4 2. Nomenclature of Alkynes
c.
HC
C
CH
C
H2
CH
CH3
CH2
H3C
Cl
d.
H3C
CH
C
C
CH
CH3
C2H5
C2H5
5-chloro-3-ethyl-1-hexyne 3,6-dimethyl-4-octyne
e. H3C
C
C
C
CH
1,3-pentadiyne
f.
H3C
CH
C
C
C
CH
C2H5
5-methyl-1,3-heptadiyne

9. Chapter 4 2. Nomenclature of Alkynes
Example 6
Draw the structures of the following alkynes.
a. 2-butyne
b. 3-methyl-1-butyne
c. 4-chloro-2-pentyne
d. 2,5-dimethyl-3-hexyne
e. 3-methyl-2-hexene-5-yne

10. Chapter 4 3. Isomerism in Alkynes
• In alkynes only structural isomerism is possible, according to
position of triple bond. For the first and second members
there is only one possible structure.
• Structural isomers of alkynes have different physical
properties.
H3C
C
C
CH3
2-butyne
H3C
C
H2
C
CH
1-butyne

11. Chapter 4 4. Physical Properties of
Alkynes
• The physical properties of alkynes are similar to those of
alkenes and alkanes.
• Acetylene, propyne and butyne are gases at room
temperature, others are liquids.
• Boiling points increase as the number of carbon atom
increases.
• They are quite soluble in organic solvents, but insoluble in
water.
• Branching as it is other hydrocarbons decreases the boiling
point.

12. Chapter 4 5. Chemical Properties of
Alkynes
• Alkynes are unsaturated compounds therefore their
chemical properties are similar to those of alkenes.
1. Combustion Reactions
CnH2n -2 + (3n-1)/2 O2 → n CO2 + (n-1) H2O
A 2.7 g of sample of an alkyne, when burned, produces 8.8 g of
CO2. What is the molecular formula of the alkyne? (C:12, H:1)
Example 7

13. Chapter 4 5. Chemical Properties of
Alkynes
1. Combustion Reactions

14. Chapter 4 5. Chemical Properties of
Alkynes
2. Addition Reactions
+ H2
alkyne
Ni, Pt
CnH2n-2
alkene
CnH2n
+ H2
Ni, Pt
alkane
CnH2n+2 Hydrogenation
C CH
H3C + 2H2
H2
C CH3
H3C
propyne propane
Ni, Pt

15. Chapter 4 5. Chemical Properties of
Alkynes
2. Addition Reactions
Halogenation
C CH
H3C + 2Cl2 C CH
H3C
propyne
1,1,2,2-tetrachloropropane
Cl Cl
Cl Cl
C CH
R + 2X2 C CH
R
X X
alkyne
tetrahaloalkane
X X

16. Chapter 4 5. Chemical Properties of
Alkynes
2. Addition Reactions
Hydrohalogenation
C CH
R + C CH
R
X H
dihaloalkane
alkyne
2HX
H
X
• Hydrohalogenation follows Markovnikov’s rule.
C CH
H3C + 2HBr C CH
H3C
Br H
propyne
2,2-dibromopropane
Br H

17. Chapter 4 5. Chemical Properties of
Alkynes
2. Addition Reactions
Hydration
C CH
R + H2O C C
R
HO H
alkyne
enol(unstable)
H
C
R
H3C
ketone
O
H2SO4
HgSO4
C C
R + H2O C C
R
HO R'
alkyne
enol(unstable)
H
C
R
CH2
ketone
O
H2SO4
HgSO4
R'
R'
C C
H + H2O
C C
H
HO H
acetylene
enol(unstable)
H
C H
H3C
acetaldehyde
O
H2SO4
HgSO4
H

18. Chapter 4 5. Chemical Properties of
Alkynes
C CH
H3C + H2O C C
H3C
HO H
propyne
1-propene-2-ol
H
C
H3C
H3C
acetone
O
H2SO4
HgSO4
C C
H3C + H2O C C
H3C
H
C2H5
2-pentyne 2-propene-3-ol
OH
C
H2
C C2H5
3-pentanone
O
H2SO4
HgSO4
C2H5 H3C
• There is an equilibrium between the keto group and enol
group that is known as keto-enol tautomerization.

19. Chapter 4 5. Chemical Properties of
Alkynes
3. Substitution Reactions with Metals
+ Ag(NH3)2
+
CH
C
R CAg
C
R
Silver acetylide
white ppt
+ NH4
+
+ NH3
CH
C
R CCu
C
R + NH3
+ Cu(NH3)2
+
Copper acetylide
red ppt
+ NH4
+
C
C
R + Ag(NH3)2
+
No reaction
R
non-terminal
alkyne

20. Chapter 4 5. Chemical Properties of
Alkynes
Example 8
Write equations for the following reactions.
a. 3-hexyne + Cl2(excess) →
b. 2-pentyne + H2(excess) →
c. 2-butyne + H2O (H+, HgSO4) →
d. 2-pentyne + HCl (excess) →

21. Chapter 4 5. Chemical Properties of
Alkynes
Example 9
How many grams of silver acetylide is precipitated by the
reaction of 2.6 grams of acetylene with ammonical silver
nitrate solution? (Ag: 108, C:12, H:1)
Solution
+ 2Ag(NH3)2
+
CH
HC CAg
AgC + 2NH4
+
+ 2NH3
2.6 g ? g
26 g of C2H2 240 g of Ag2C2
2.6 g X g
2.6 x 240
26
X = = 24 g produced

22. Chapter 4 6. Preparation of Alkynes
1. From Acetylene Salts
+ Na(1 mol)
CH
HC
NH3 Liq.
CNa
HC + 1/2H2
CNa
HC
Propyl bromide
H2
C
H2
C CH3
Br
+ C
HC
1-pentyne
H2
C
H2
C CH3
Acetylene
2. Dehalogenation of Dihaloalkanes, Elimination Reaction
R-CH-CH2 + 2KOH + 2KBr + 2H2O
Alkyne
Alcohol
Br
dihaloalkane
Br
CH
C
R
ether

23. Chapter 4 7. Acetylene
• Colorless gas has a very light odor.
• Soluble in water and highly soluble in acetone.
• It boils at –83°C and can be liquefied at 1°C.
• When it is liquefied, becomes extremely explosive, so it is
dissolved in acetone.
Physical Properties
Chemical Properties
• It burns with a bright flame.
• It undergoes addition of hydrogen, hydrogen halide and
water reactions.
• And some polymers like polyvinyl chloride and vinyl cyanide
can be produced from acetylene..

24. Chapter 4 7. Acetylene
3. Preparation
3C + CaO CaC2 + CO
2000o
C
Coke Lime Calcium Carbide
CaC2 + 2H-OH
Acetylene
HC CH + Ca(OH)2

25. Chapter 4 7. Acetylene
Example 10
When a 10 g sample of the mixture of methane and acetylene
reacts with aqueous ammonical cuprous chloride, a 30.2 g
precipitate is formed. What is the percentage by mass of
methane in the mixture? (Cu: 63.5, Cl:35.5, C:12, H:1)
Example 11
How many liters of oxygen gas at STP are needed to burn a
sample of acetylene which is obtained from 6.4 g of calcium
carbide? (Ca: 40, C:12, H:1)

26. Chapter 4 7. Acetylene

27. End of the chapter 4