Hydrocarbons — Part 2: Alkynes and aromatic hydrocarbons: visual lesson, CBSE Class 11 Chemistry

Lesson

Alkynes and Aromatic Hydrocarbons

Part 2 — the carbon–carbon triple bond and the special stability of benzene.

In this lesson you will

Explain the acidity of terminal alkynes and write their preparation and addition reactions
Describe the structure of benzene by resonance and apply the Hückel (4n + 2) rule
Write the mechanism of electrophilic substitution and the five key reactions of benzene
Predict the directive influence of substituents; state the toxicity of benzene and polynuclear hydrocarbons

Table of an alkyne and aromatic hydrocarbons and their uses
Compound	Role
Ethyne	Oxyacetylene flame for welding
Benzene	Starting material for plastics and dyes; a toxic solvent
Toluene	Solvent in paints
Naphthalene	Moth balls (fused rings)

Concept

Alkynes: structure

In ethyne the carbons are sp hybridised: linear, 180°, one σ and two π bonds. The C≡C bond is short and strong.

Table comparing bond length, enthalpy, hybridisation and s-character of ethane, ethene and ethyne carbon bonds
Bond	Length / pm	Enthalpy / kJ mol⁻¹	Hybridisation	Percent s-character
C–C in ethane	154	347	sp³	25
C=C in ethene	134	611	sp²	33
C≡C in ethyne	120	837	sp	50

Concept

Acidity of terminal alkynes

An sp carbon holds its electrons closer to the nucleus (50% s-character), so it is more electronegative and the ≡C–H hydrogen is weakly acidic. Order of acidity: ethyne > ethene > ethane. Only terminal alkynes react with sodium or sodium amide.

\ce{2CH#CH + 2Na -> 2CH#CNa + H2}

\ce{CH#CH + NaNH2 -> CH#CNa + NH3}

Table of acidity of ethyne, ethene and ethane and which react with sodium or sodium amide
Hydrocarbon	Carbon bearing H	Approximate pKa	Reacts with Na or NaNH₂?
Ethyne	sp	25	Yes
Ethene	sp²	44	No
Ethane	sp³	50	No
But-2-yne	No ≡C–H	—	No

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