Physical Properties:
The first three members are gases, next eight members are liquids and the higher alkynes are solids. They are all colorless and odourless except acetylene which has a gasoline odour. Alkynes are slightly soluble in water but dissolve readily in organic solvents such as benzene, acetone, ethyl alcohol, etc. The melting and boiling points show a regular increase with increase in molecular mass.
Chemical Reactions:
Alkynes give some kind of addition reaction as do alkenes. However, with alkynes the addition may take place in one or two steps, depending upon reaction conditions.
1) Addition of hydrogen: When alkyne is heated with two molecules of hydrogen in presence of Pd, Pt or Ni, give alkane.
2) Addition of halogen: Chlorine and bromine reacts with alkyne at ordinary temperature to form tetrahalides.
CH ≡ CH + 2Br2 ——> Br2—CH — CH—Br2
Ethyne Tetra bromoethane
3) Addition of Water: Alkynes reacts with water in presence of HgSO4 and H2SO4 to produce acetaldehyde.
4) Addition of hydrogen halide: Alkynes reacts with halogen acids (HCl, HBr or HI) to form alkyl halides.
Acidic Character of Alkynes:
Acetylene and 1-alkynes are acidic in nature be cause they readily donate protons to strong bases.
R—C≡C: H + Base ——-> R—C:– + H—Base
Moreover, acetylene is a stronger acid than ethylene, which in turn is a stronger acid than ethane. The acidity of acetylene or 1-alkynes can be explained by the atomic orbital description of the C—H bonds. We know that s-electrons are closer to the nucleus than are p-electrons. Therefore, the more s-character there is in a hybrid atomic orbital, the closer the electrons of the bond are to the nucleus. The amount of s-character in various hybrid orbitals is as follows:
The s-character of the C—H bond of acetylene or 1 -alkynes (sp—s σ bond) is greater than that of an alkene C—H bond (sp2—s σ bond), or an alkane C—H bond (sp3—s σ bond). Thus the electrons of the C—H bond in 1 -alkynes are more strongly held by the carbon nucleus. This makes it easier to remove the hydrogen (≡C—H) as a proton.