Organic Chemistry> alkynes...

please explain me about ''acidity of alkynes'' with some examples...

chinthalapudi prashanth , 14 Years ago

Grade 11

1 Answers

SAGAR SINGH - IIT DELHI

Last Activity: 14 Years ago

Dear student,

ACIDITY OF ALKYNES

Acetylene and other terminal alkynes (1- alkynes) are weakly acidic in character

–

Ex. CH ≡ CH + NaNH₂ ————→ H – C ≡ C Na⁺ + 1/2 H₂

^{(Monosodium acetylide)}

The acetylenic hydrogen of alkynes can be replaced by copper (I) and silver (I) ions. They react with ammonical solutions of cuprous chloride and silver nitrate to form the corresponding copper and silver alkynides.

CH ≡ CH + 2[Cu(NH₃)₂]Cl ———→ Cu – C ≡ C – Cu + 2NH₄Cl + 2NH₃ Dicopper acetylide (Red ppt)

CH ≡ CH + 2[Ag(NH₃)₂]NO₃ ———→ AgC ≡ C – Ag + 2NH₄NO₃ + 2NH₃ Disilver acetylide (white ppt)

This reaction can be used to distinguish between 2-alkynes and 1-alkynes. 1-alkynes will give this test while 2-alkynes, will not give this test.

CH₃ – C ≡ CH + 2[Ag(NH₃)₂] NO₃ ———→ CH₃ – C ≡ C – Ag

^1-propyne

CH₃ – C ≡ C – CH₃ + 2[Ag(NH₃)₂]NO₃ ————→ No reaction

EXPLANATION FOR THE ACIDIC CHARACTER

It explained by sp hybridization. We know that an electron in s–orbital is more tightly held than in a p-orbital. In sp hybridization s-character is more (50%) as compared to sp² (33%) or sp³ (25%), due to large s-character the carbon atom is quite electronegative.