Properties of Alkynes

The third type of hydrocarbon is Alkynes which contain at least one triple bond between a pair of carbon atoms. Since it is also an unsaturated hydrocarbon some of its properties will be similar to alkenes. Let us take a look at some physical and chemical properties of alkynes.

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Physical Properties of Alkynes

  • The properties of alkynes pretty much follow the same pattern of those of alkanes and alkenes.
  • Alkynes are unsaturated carbon that shares a triple bond at the carbon site
  • All alkynes are odourless and colourless with the exception of ethylene which has a slight distinctive odour.
  • The first three alkynes are gases, and the next eight are liquids. All alkynes higher than these eleven are solids
  • Alkynes are slightly polar in nature
  • The boiling point and melting point of alkynes increases as their molecular structure grows bigger. The boiling point increases with increase in their molecular mass
  • Also, the boiling points of alkynes are slightly higher than those of their corresponding alkenes, due to the one extra bond at the carbon site.

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Chemical Properties of Alkynes

Acidic nature

Coming to the chemical properties of alkynes, we begin with their slightly acidic nature. Now Alkynes are slightly electronegative in nature. The triply bonded carbon atoms in alkynes are sp hybridized, Whereas like in alkanes the single bond atoms are sp3 hybridized, causing the difference in the electronegativity. This makes it easier for them to attract the shared electron pair of the C-H bond.

So when we react a strong base like NaNH2 with ethyne, we will get sodium acetylide and liberated hydrogen (H2) gas. But such reactions will not happen in alkanes and alkenes. The conclusion being that the hydrogen atoms attached to the carbon-carbon triple bond in alkynes are slightly acidic in nature. It is to be noted the other hydrogen atoms baring these ones are not acidic.

HC ≡ CH + Na → HC ≡ C Na+ + 1/2H2

Addition Reactions

Under suitable conditions (temperature and pressure) alkynes will undergo hydration reactions quite easily. Alkynes will react with halogens, hydrogen and other such elements to give a saturated compound as a product. Since they have a triple bond, two atoms of H2 or halides or halogens can be added to its structure.

1] Addition of Dihydrogen

The reaction occurs in presence of a catalyst such as Nickel or Platinum or Palladium. Here the addition of hydrogen to the alkyne gives us an alkene.

C3H4(g) + 2H2(g)  C3H8(g)

2] Addition of Halogens

When alkynes and halogens like Bromine react, halogen will add itself to the structure of the alkynes and result in halogen substituted alkenes. The resulting product will be tetrabromopropane.

Properties of Alkynes

(Source: chemistryassignment)

3] Addition of Water

Just like other hydrocarbons (alkanes and alkenes) alkynes also do not react with a water molecule. This is called immiscibility. But if alkynes is bubbled through dilute sulphuric acid (about 40%) in presence of the catalyst mercuric Sulphate, then a reaction occurs. The products will be carbonyl compounds, and such a reaction can be called a hydration reaction.

Properties of Alkynes

(Source: chemistryassignment)

4] Polymerization

Alkynes can undergo linear and cyclic polymerization under suitable conditions. They polymerize to give compounds that have a higher molecular weight than the original alkyne. Like for example, ethyne will polymerize to give polyacetylene or polyethene (of higher molecular weight), This is an example of linear polymerization.

For cyclic polymerization, high temperatures and the presence of a catalyst is required. Like passing ethyne through a red-hot iron tube at a minimum of 877K which gives benzene.

Solved Example for You

Question: Which of the following reacts with metal by displacing H atom :

  1. C2H2
  2. C2H4
  3. C2H6
  4. None of the above

Solution: The correct answer is A. Only end alkynes have acidic hydrogen molecules which react and can be displaced by a metal.

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