Experiment 3B: Properties of Alkynes (LAB)

Question 1

What alkyne was prepared for the experiment?

Answer 1

Acetylene, or C2H2

Question 2

It was set up for the reaction of the prepared alkyne

Answer 2

Gas generator

Question 3

For the preparation of the alkyne, it was placed in a distilling flask wherein water was dropped on it via medicine dropper

Answer 3

Calcium carbide or CaC2

Question 4

The alkyne gas was collected underwater. This process of collection is called:

Answer 4

Water displacement

Question 5

Equation for Alkyne Preparation:

Answer 5

CaC2 + 2 H2O –> C2H2 + Ca(OH)2

Question 6

Why should C2H2 be kept away from all flames?

Answer 6

It is very combustible

Question 7

I. Baeyer’s Test for Unsaturation

Answer 7

1 mL of KMnO4 is added to the first vial with C2H2

Question 8

Results for Baeyer’s Test:

Answer 8

• POSITIVE RESULT;

Unsaturation was detected

• Mixture turns from purple to brown; brown precipitates form

KMnO4 oxidizes C2H2, leading to the disappearance of its purple color. The yielded result is the formation of a brown precipitate

Question 9

What are the formed brown precipitates called in Baeyer’s test?

Answer 9

Mangnese dioxide or MnO2

Question 10

Other byproducts in Baeyer’s Test with Acetylene:

Answer 10

• Carbon dioxide or CO2
• Potassium Hydroxide or KOH

Question 11

Equation for Baeyer’s Test:

Answer 11

3 C2H2 + 10 KMnO4 + 2 H2O –> 10 MnO2 + 6 CO2 + 10 KOH

Question 12

II. Reaction with Bromine Water

Answer 12

1 mL of Br2 is added to vial with C2H2. It is then stoppered and shaken.

Question 13

Results of Reaction w Br2

Answer 13

Mixture shows slight yellow tint

Question 14

When C2H2 reacts with the 2 moles of Br2 (2 Br2), what is happened and formed?

Answer 14

Acetylene loses its triple bond, and a tetrahaloalkane is formed

Question 15

What specific tetrahaloalkane was formed?

Equation:

Answer 15

1,1,2,2-tetrabromoethane

C2H2 + 2 Br2 –> HCBr2-CBr2H

Question 16

III. Reaction with Alcoholic Iodine

Answer 16

1 mL of alc. I2 was added to the third vial with C2H2.

More C2H2 should be added if no reaction is found.

Question 17

Result with Alc. I2

Answer 17

• Mixture turned brownish red

NO REACTION

Question 18

Why did Alc. I2 not give a reaction in C2H2?

Answer 18

• I2 is weakly reactive and has low electronegativity levels
• C2H2 is highly soluble in alcohol, therefore no reaction is to be expected

Question 19

Alc. I2 + C2H2 –>

Answer 19

CHI ~~~ CHI (no reaction)

Question 20

IV. Formation of Cuprous acetylide

Answer 20

5 mL of of ammoniacal Cu2Cl2 (cuprous chloride) is added to the fourth vial with C2H2

Question 21

What indicates presence of Cuprous acetylide or CuC = CCu

Answer 21

Formation of chocolate brown precipitate

Question 22

Acetylene is an acetylenic hydrogen atom, meaning that it has a carbon attached to a triple bond and its hydrogens can be replaced by _____ due to its somewhat _____.

Answer 22

Metals; acidic nature

Question 23

The formation of ______ are to be expected in this substitution reaction, copper being one of them

Answer 23

Metal salts

Question 24

Acetylene reacted with the ______ and _____ present in the ammoniacal cuprous chloride solution

Answer 24

Molten sodium, Copper Chloride / Cu2Cl2

Question 25

Formula of Cuprous Acetylide:

Answer 25

Cu2Cl2 + C2H2 + 2 NH4OH –> CuC ≈ CCu + 2NH4Cl + 2 H2O