CBSE Class 11 Chemistry Hydrocarbon Notes Set 06

These Class 11 CBSE Chemistry notes for Chapter 9 Hydrocarbons, part two, explain alkynes (structure, acidic character and addition reactions) and aromatic hydrocarbons (benzene structure, resonance, Hückel's rule, electrophilic substitution, directive influence and carcinogenicity). Four solved examples show typical conversions and the use of directing groups.

Important Notes and Formulas

Alkynes: Structure, Nomenclature and Isomerism

1. Alkynes have the general formula C_nH_2n−2. Each triple-bond carbon is sp hybridised and linear (bond angle 180°). The C≡C is made of one σ bond and two π bonds. Its length is 120 pm and its bond enthalpy is 823 kJ/mol.
2. Nomenclature: use the suffix -yne with the longest chain containing the triple bond, numbered to give the triple bond the lowest number: HC≡CH is ethyne, CH₃C≡CH is propyne, CH₃CH₂C≡CH is but-1-yne and CH₃C≡CCH₃ is but-2-yne.
3. Isomerism: chain and position isomerism (but-1-yne and but-2-yne). Alkynes do not show geometrical isomerism because each triple-bond carbon has only one other group, and the molecule is linear about the bond.

Preparation of Alkynes

Calcium carbide itself is made by heating quick lime with coke: CaO + 3C → CaC₂ + CO (at about 2000 °C).

Properties of Alkynes

The first three members (ethyne, propyne, butyne) are gases and the next members up to C₁₂ are liquids. They are colourless, and are insoluble in water but soluble in organic solvents. Ethyne is lighter than air. Their boiling points are slightly higher than those of the corresponding alkanes and alkenes.

1. Acidic character: the carbon of a terminal alkyne is sp hybridised (50% s-character), so it is more electronegative and the ≡C–H hydrogen is weakly acidic. The order of acidity is HC≡CH > CH₂=CH₂ > CH₃CH₃. Terminal alkynes react with sodium (2HC≡CH + 2Na → 2HC≡CNa + H₂) and give a white precipitate with ammoniacal silver nitrate (Tollens' reagent) and a red precipitate with ammoniacal cuprous chloride. Internal alkynes do not, so the test distinguishes terminal alkynes.
2. Addition of H₂: HC≡CH + 2H₂ → CH₃CH₃ (Pt, Pd or Ni). With Lindlar's catalyst the reaction stops at the cis-alkene, and with Na in liquid NH₃ at the trans-alkene.
3. Addition of halogens: HC≡CH + Br₂ → BrCH=CHBr and then CHBr₂CHBr₂ (1,1,2,2-tetrabromoethane). The reddish-orange colour of bromine in CCl₄ is discharged.
4. Addition of HX (Markovnikov): HC≡CH + HBr → CH₂=CHBr; with excess HBr, CH₃CHBr₂ (a gem-dihalide). Propyne + 2HBr → CH₃CBr₂CH₃.
5. Addition of water: with dilute H₂SO₄ and HgSO₄ at 333 K, ethyne gives ethanal (through the unstable enol CH₂=CHOH), and higher alkynes give ketones: CH₃C≡CH + H₂O → CH₃COCH₃.
6. Polymerisation: ethyne, passed through a red hot iron tube at 873 K, undergoes cyclic trimerisation to give benzene (3C₂H₂ → C₆H₆). Linear polymerisation gives polyacetylene, a conducting polymer.
7. Combustion: 2HC≡CH + 5O₂ → 4CO₂ + 2H₂O (oxyacetylene flame, used for welding).

Aromatic Hydrocarbons: Structure of Benzene

1. Benzene, C₆H₆, is a planar, cyclic molecule. Kekulé proposed a ring of six carbons with alternating single and double bonds, in two forms that rapidly interconvert; the actual structure is a resonance hybrid of these.
2. Each carbon is sp² hybridised and forms σ bonds with two carbons and one hydrogen (all angles 120°). The remaining unhybridised p orbital on each carbon overlaps sideways with both neighbours, so six π electrons are delocalised above and below the ring.
3. All six C–C bonds have the same length, 139 pm, between those of a C–C (154 pm) and a C=C (134 pm). Benzene has a high resonance (stabilisation) energy, so it undergoes substitution rather than addition.
4. Aromaticity (Hückel's rule): a planar, cyclic, completely conjugated system with (4n + 2) π electrons (n = 0, 1, 2, ...) is aromatic. Benzene has 6 π electrons (n = 1). Cyclobutadiene (4 π electrons) and cyclooctatetraene (8) are not aromatic.
5. Nomenclature: toluene (methylbenzene), ethylbenzene, xylene (dimethylbenzene: o-, m- and p-xylene, i.e. 1,2-, 1,3- and 1,4-), nitrobenzene, chlorobenzene. Positions are numbered 1 to 6; ortho = 1,2; meta = 1,3; para = 1,4.

Preparation of Benzene

Properties of Benzene

Aromatic hydrocarbons are non-polar, colourless liquids or solids with a characteristic aroma. They are immiscible with water but soluble in organic solvents, and they burn with a sooty flame because of their high carbon content. Benzene is a liquid with a boiling point of 353 K.

Electrophilic Substitution Reactions

A typical aromatic reaction is electrophilic substitution. The mechanism has three steps: (1) generation of the electrophile E⁺, (2) attack of E⁺ on the π cloud to form a resonance-stabilised carbocation (the arenium ion, or sigma complex) and (3) loss of H⁺ from the arenium ion to restore the aromatic ring.

In the nitrating mixture, nitric acid acts as a base and accepts a proton from sulphuric acid: HNO₃ + 2H₂SO₄ → NO₂⁺ + H₃O⁺ + 2HSO₄⁻.

Addition and Other Reactions of Benzene

1. Hydrogenation: C₆H₆ + 3H₂ → C₆H₁₂ (cyclohexane) with Ni at 453–473 K.
2. Chlorination in sunlight: C₆H₆ + 3Cl₂ → C₆H₆Cl₆ (benzene hexachloride, BHC, gammexane), by a free radical addition.
3. Combustion: C₆H₆ + 15/2 O₂ → 6CO₂ + 3H₂O, with a sooty, luminous flame.

Directive Influence of Substituents

1. Groups that donate electrons (+R, +I or hyperconjugation) increase the electron density at the ortho and para positions and stabilise the arenium ion formed by attack there.
2. Groups that withdraw electrons (–R or –I) reduce the electron density, mainly at the ortho and para positions, so the meta position is attacked.
3. Halogens withdraw electrons by the inductive effect (deactivating) but donate by resonance through their lone pairs, which directs the incoming group to the o- and p-positions.
4. Examples: toluene + nitrating mixture → o-nitrotoluene and p-nitrotoluene; nitrobenzene + nitrating mixture (fuming, higher temperature) → 1,3-dinitrobenzene (m-dinitrobenzene); phenol + Br₂ → o- and p-bromophenol.

Carcinogenicity and Toxicity

Benzene is toxic and is also carcinogenic: prolonged exposure harms the bone marrow and may cause leukaemia. Polynuclear hydrocarbons, which have more than two benzene rings fused together (for example 1,2-benzanthracene and benzo[a]pyrene), are formed by the incomplete combustion of organic materials such as tobacco, coal and petroleum. They enter the body and cause cancer by damaging DNA.

Solved Examples

Example 1. How will you convert (a) ethyne to benzene, (b) benzene to toluene and (c) benzene to m-bromonitrobenzene?
Answer:
(a) 3HC≡CH → C₆H₆ (red hot iron tube, 873 K).
(b) Friedel–Crafts alkylation: C₆H₆ + CH₃Cl → C₆H₅CH₃ + HCl (anhydrous AlCl₃).
(c) First nitrate: C₆H₆ + HNO₃ → C₆H₅NO₂ (conc. H₂SO₄, 323–333 K). Then brominate: C₆H₅NO₂ + Br₂ → m-BrC₆H₄NO₂ (FeBr₃). The –NO₂ group directs the bromine to the meta position, so the order of steps matters.

Example 2. Which of the following are aromatic according to Hückel's rule? (i) benzene (ii) cyclobutadiene (iii) cyclooctatetraene (iv) naphthalene
Answer:
Count the π electrons in a planar, cyclic, fully conjugated ring: aromatic if the count is 4n + 2 (2, 6, 10, ...).
(i) Benzene: 6 π electrons, aromatic. (ii) Cyclobutadiene: 4, not aromatic. (iii) Cyclooctatetraene: 8, not aromatic (and not planar). (iv) Naphthalene: two fused rings with 10 π electrons (n = 2), planar and conjugated, so aromatic.
Aromatic: (i) and (iv).

Example 3. Write the structures and names of the main products when (a) toluene and (b) nitrobenzene are nitrated. Explain the difference.
Answer:
(a) Toluene gives o-nitrotoluene and p-nitrotoluene (CH₃-C₆H₄-NO₂ with NO₂ at positions 2 or 4). The –CH₃ group is electron-releasing, so it activates the ring and directs to the o and p positions.
(b) Nitrobenzene gives 1,3-dinitrobenzene. The –NO₂ group is strongly electron-withdrawing, so it deactivates the ring and directs the incoming group to the meta position. More drastic conditions are required.

Example 4. Arrange benzene, toluene, nitrobenzene and chlorobenzene in the order of decreasing reactivity towards electrophilic substitution, and give a reason.
Answer:
Toluene > benzene > chlorobenzene > nitrobenzene.
The –CH₃ group activates the ring (+I and hyperconjugation). Benzene is the reference. The –Cl group deactivates weakly (its –I effect outweighs its +R effect). The –NO₂ group deactivates strongly (–I and –R).