Chemical Properties Of Alkynes

Question 1

In combustion of acetylene/ethyne, the aforementioned alkyne burns with air to give this intensely hot flame used for cutting metals and welding

Answer 1

Oxyacetylene

Question 2

In Baeyer’s test, alkynes are oxidized in _____ by permanganate.

What shows a positive result?

Answer 2

Aqueous solution

Formation of a brown precipitate, MnO2

Question 3

In addition reactions, alkynes as well as alkenes can be saturated to alkanes. What are the differences in their reactions?

Answer 3

Alkene ver: H2, X2, HX

Alkyne ver: two compounds are used; therefore

HYDROGENATION - 2H2 (two H2 compounds instead of one)

R-C ≈ C-R + 2H2 ->[Pt, Pd, Ni] R-CH2-CH2-R

HALOGENATION - 2X2 (two X2 atoms instead of one)

R-C ≈ C-R + 2X2 -> R-CX2-CX2-R

ADDITION OF HYDROGEN HALIDE - 2XH (two hydrogen-halide compounds instead of one)

R-C ≈ CH + 2HX -> R-CX2-CH3

Question 4

In the Hydrogenation of alkynes to alkanes, what cannot be done?

Answer 4

Hydrogenation cannot be stopped at the halfway stage, which is the alkene stage.

Question 5

In Hydrogenation of alkyne to alkene, what must be done?

Answer 5

A slightly deactivated or poisoned catalyst must be prepared.

Question 6

How is a poisoned catalyst made? And what does it do during the hydrogenation of alkynes to alkenes?

Answer 6

• Lead acetate is added to palladium to form a poisoned catalyst
• The doubled 2H2 atom becomes one again (only H2)

R-C ≈ C-R + H2 ->[poisoned Pd] R-CH-CH

Question 7

In Addition of Hydrogen Halide, HCl, HBr, and HI add to alkynes in two steps.

What happens to the reaction after it is stopped after the first step (adding only 1 HX)?

What happens after the two steps are completed?

Answer 7

• If the reaction is stopped, a haloalkene is obtained
• If two HX molecules are added, a gem-dihalide is formed

Question 8

What does the “gem-“ prefix mean?

Answer 8

Coming from *Latin** geminus (twin), it means that both halogens are on the same Carbon atom

Question 9

In the substitution of alkynes, acetylenic hydrogen atoms are somewhat acidic. What are these?

Answer 9

Acetylenic hydrogen atoms are hydrogen atoms attached to a Carbon bearing triple bonds

Question 10

Becaus Acetylenic hydrogen atoms are somewhat acidic, they can be replaced by _____, thus forming _____

Answer 10

Metals; metal salts

Question 11

What metal salts can be readily prepared?

Answer 11

• Sodium
• Silver
• Copper (I)

Question 12

Acetylene reacts with _____ and with _____ or _____ to form insoluble precipitates

Answer 12

• Molten sodium / 2NH4OH
• Copper (I) chloride / Cu2Cl2
• Ammoniacal silver nitrate / 2AgNo3

Question 13

Why are silver or copper acetylides used?

Answer 13

They are unstable when dry and can be detonated by heat or shock