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Chemistry · Ch 10 — Hydrocarbons

Properties

10.4.4

Properties

Physical Properties

The physical properties of alkynes follow the same general trends observed in alkenes and alkanes. The first three members (ethyne, propyne, and but-1-yne) are gases at room temperature. The next eight members are liquids, and the higher members are solids. All alkynes are colourless. Ethyne has a characteristic odour, while the other members are odourless.

Alkynes are weakly polar in nature. They are lighter than water and immiscible with it, but they dissolve readily in organic solvents such as ethers, carbon tetrachloride, and benzene. Their melting points, boiling points, and densities increase with increasing molar mass.

Note

The weak polarity of alkynes arises from the slight polarisation of the C–H bond due to the high electronegativity of the sp-hybridised carbon. This is much weaker than the polarity of, say, an O–H bond, so alkynes are still essentially non-polar and behave like typical hydrocarbons with respect to solubility.

Chemical Properties

Alkynes undergo three major types of reactions: acidic character (involving the terminal hydrogen), addition reactions (across the triple bond), and polymerisation reactions.

A. Acidic Character of Alkynes

Sodium metal and sodamide (NaNH2\mathrm{NaNH_2}) are strong bases. They react with ethyne to form sodium acetylide with the liberation of dihydrogen gas. These reactions are not observed with ethene or ethane, which shows that ethyne is acidic in comparison to them.

Why is ethyne acidic?

The hydrogen atoms in ethyne are attached to sp-hybridised carbon atoms. In ethene, they are attached to sp2\mathrm{sp^2}-hybridised carbons, and in ethane, to sp3\mathrm{sp^3}-hybridised carbons. The sp hybrid orbital has 50% s-character — the highest among the three hybridisation states. Because s-orbitals are closer to the nucleus than p-orbitals, a higher s-character means the carbon atom is more electronegative. This greater electronegativity attracts the shared electron pair of the C–H bond more strongly towards the carbon, making the hydrogen more easily released as a proton (H+\mathrm{H^+}). Hence, the hydrogen atoms attached to the triply bonded carbon are acidic.

Watch out

Only the hydrogen atoms directly attached to the triply bonded carbon are acidic. In an alkyne like but-2-yne (CH3C≡CCH3\mathrm{CH_3C \equiv CCH_3}), there is no hydrogen on the triple bond, so it shows no acidic character.

Reactions demonstrating acidic character:

  1. Ethyne with sodium metal:

HC≡CH+Na⟶HC≡C−Na++12H2\mathrm{HC \equiv CH + Na \longrightarrow HC \equiv C^- Na^+ + \frac{1}{2} H_2}

(Monosodium ethynide)

HC≡C−Na++Na⟶Na+C−≡C−Na++12H2\mathrm{HC \equiv C^- Na^+ + Na \longrightarrow Na^+ C^- \equiv C^- Na^+ + \frac{1}{2} H_2}

(Disodium ethynide)

2. Propyne with sodamide:

CH3−C≡C−H+NaNH2⟶CH3−C≡C−Na++NH3\mathrm{CH_3-C \equiv C-H + NaNH_2 \longrightarrow CH_3-C \equiv C^- Na^+ + NH_3}

(Sodium propynide)

These reactions are not shown by alkenes and alkanes, so they can be used to distinguish alkynes from alkenes and alkanes.

Trend in acidic behaviour:

The relative acidity of hydrocarbons follows this order:

HC≡CH>H2C=CH2>CH3−CH3\mathrm{HC \equiv CH > H_2C=CH_2 > CH_3-CH_3}

And among alkynes themselves:

HC≡CH>CH3−C≡CH≫CH3−C≡C−CH3\mathrm{HC \equiv CH > CH_3-C \equiv CH \gg CH_3-C \equiv C-CH_3}

The terminal alkyne (ethyne) is the most acidic. But-2-yne, with no terminal hydrogen, shows no acidic character at all.

Tip

The acidity trend directly follows the s-character of the carbon hybrid orbital: sp (50%) > sp² (33%) > sp³ (25%). More s-character means a more electronegative carbon, which stabilises the negative charge on the conjugate base (the acetylide ion) better.

B. Addition Reactions

Alkynes contain a triple bond, which consists of one sigma bond and two pi bonds. They can add two molecules of dihydrogen, halogen, hydrogen halides, etc. The addition occurs in two steps, with the formation of a vinylic carbocation intermediate. The product formed depends on the stability of this vinylic cation. Addition to unsymmetrical alkynes follows Markovnikov's rule. Most addition reactions of alkynes are electrophilic addition reactions.

Important

The triple bond is a region of high electron density, making it susceptible to attack by electrophiles. The first addition gives an alkene; the second addition gives an alkane or a saturated derivative.

(i) Addition of Dihydrogen

Alkynes add hydrogen in the presence of a catalyst (Pt, Pd, or Ni) to first form an alkene, and then an alkane.

HC≡CH+H2→Pt/Pd/Ni[H2C=CH2]→H2CH3−CH3\mathrm{HC \equiv CH + H_2 \xrightarrow{Pt/Pd/Ni} [H_2C=CH_2] \xrightarrow{H_2} CH_3-CH_3}

CH3−C≡CH+H2→Pt/Pd/Ni[CH3−CH=CH2]→H2CH3−CH2−CH3\mathrm{CH_3-C \equiv CH + H_2 \xrightarrow{Pt/Pd/Ni} [CH_3-CH=CH_2] \xrightarrow{H_2} CH_3-CH_2-CH_3}

(Propyne → Propene → Propane)

Note

The reaction can be stopped at the alkene stage by using a poisoned catalyst like Lindlar's catalyst (Pd/CaCO₃ with lead acetate), which gives the cis-alkene. Using sodium in liquid ammonia gives the trans-alkene.

(ii) Addition of Halogens

Alkynes add two molecules of chlorine or bromine. The reddish-orange colour of bromine in carbon tetrachloride is decolourised, which is used as a test for unsaturation.

HC≡CH+Br2⟶CHBr=CHBr→Br2CHBr2−CHBr2\mathrm{HC \equiv CH + Br_2 \longrightarrow CHBr=CHBr \xrightarrow{Br_2} CHBr_2-CHBr_2}

(1,2-Dibromoethene → 1,1,2,2-Tetrabromoethane)

(iii) Addition of Hydrogen Halides

Two molecules of hydrogen halides (HCl, HBr, HI) add to alkynes to form geminal dihalides (dihalides in which both halogen atoms are attached to the same carbon atom).

HC≡CH+HBr⟶[CH2=CHBr]→HBrCH3−CHBr2\mathrm{HC \equiv CH + HBr \longrightarrow [CH_2=CHBr] \xrightarrow{HBr} CH_3-CHBr_2}

(Bromoethene → 1,1-Dibromoethane)

The addition follows Markovnikov's rule. In the first step, the hydrogen adds to the carbon with more hydrogens, giving the more stable vinylic carbocation. In the second step, the same rule applies again.

Watch out

The product is a geminal dihalide (both halogens on the same carbon), not a vicinal dihalide (halogens on adjacent carbons). This is a key difference from the addition to alkenes.

(iv) Addition of Water (Hydration)

Like alkenes, alkynes do not react with water under normal conditions. However, one molecule of water adds to alkynes on warming with mercuric sulphate (HgSO4\mathrm{HgSO_4}) and dilute sulphuric acid at 333 K. This reaction forms carbonyl compounds (aldehydes or ketones) via an enol intermediate that tautomerises.

HC≡CH+H2O→HgSO4,H2SO4,333K[CH2=CH−OH]⟶CH3−CHO\mathrm{HC \equiv CH + H_2O \xrightarrow{HgSO_4, H_2SO_4, 333K} [CH_2=CH-OH] \longrightarrow CH_3-CHO}

(Ethyne → Vinyl alcohol (enol) → Acetaldehyde)

CH3−C≡CH+H2O→HgSO4,H2SO4,333K[CH3−C(OH)=CH2]⟶CH3−CO−CH3\mathrm{CH_3-C \equiv CH + H_2O \xrightarrow{HgSO_4, H_2SO_4, 333K} [CH_3-C(OH)=CH_2] \longrightarrow CH_3-CO-CH_3}

(Propyne → Propen-2-ol (enol) → Acetone)

Important

The addition follows Markovnikov's rule. For ethyne, the product is an aldehyde. For all other terminal alkynes, the product is a ketone. This is a key method for preparing carbonyl compounds from alkynes. …

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