What went wrong when the king rode under the arch?

It was too low and knocked off his crown

How does the chain of blame unfold in the poem?

Each accused person blames another, ending with the king's own changes

Give the general formulae of alkanes, alkenes and alkynes.

\(C_nH_{2n+2}\), \(C_nH_{2n}\) and \(C_nH_{2n-2}\) respectively.

What is the Wurtz reaction used for?

Preparing higher symmetrical alkanes by treating alkyl halides with sodium in dry ether.

Why is the staggered conformation of ethane the more stable?

Its hydrogen atoms are farthest apart, giving minimum torsional strain and lowest energy.

Name a reagent that tests for unsaturation.

Bromine in carbon tetrachloride, or cold dilute alkaline potassium permanganate (Baeyer's reagent), both of which are decolourised.

What is the peroxide (anti-Markovnikov) effect?

In the presence of peroxide, HBr adds to an unsymmetrical alkene contrary to Markovnikov's rule, by a free-radical mechanism.

Why are terminal-alkyne hydrogens acidic?

They are bonded to sp-hybridised carbon with high s-character, so the hydrogen is released as a proton relatively easily.

Why is benzene unusually stable?

Its six pi electrons are delocalised over all six carbons, so resonance lowers its energy and all carbon-carbon bonds are equal.

Name the electrophilic substitution reactions of benzene.

Nitration, halogenation, sulphonation and the Friedel-Crafts alkylation and acylation.

Why are polynuclear aromatic hydrocarbons hazardous?

Many of them are carcinogenic and toxic.

Hydrocarbons

Q: State Markovnikov's rule.

When an unsymmetrical reagent adds to an unsymmetrical alkene, the negative part adds to the carbon with fewer hydrogens.

Class 11 Chemistry · Chemistry Part II · Chapter 3

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CBSE Class 11 · Chemistry

Organic basics: 11 marks, the biggest unit

Marks booster + cheatsheet: key concepts in 2 lines each, common traps
10 marks: Hydrocarbons
Every reaction with reagent and condition
Common traps that cost marks, listed
IUPAC naming rules
Oxidation-number rules and balancing

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CBSE Class 11 · Chemistry

Chemistry Part I: 45 of 70 theory marks

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9 marks each: Atom and Thermodynamics
Constants and formulas, with page numbers
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pH, buffers and solubility product

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Summary

Hydrocarbons, compounds of carbon and hydrogen only, are the principal sources of energy and the feedstock for polymers, dyes and drugs. They are classified as saturated alkanes (general formula \(C_nH_{2n+2}\), with single bonds), unsaturated alkenes (\(C_nH_{2n}\)) and alkynes (\(C_nH_{2n-2}\), with multiple bonds), and aromatic hydrocarbons. Alkanes are prepared by hydrogenation of unsaturated hydrocarbons, reduction of alkyl halides, the Wurtz reaction, decarboxylation and Kolbe electrolysis; they are fairly unreactive but undergo free-radical halogenation, combustion, controlled oxidation, isomerisation and pyrolysis, and their staggered conformation is more stable than the eclipsed because of lower torsional strain. Alkenes contain a sigma plus pi double bond, show chain, position and geometrical (cis-trans) isomerism, and undergo electrophilic addition of hydrogen, halogens, hydrogen halides (Markovnikov rule, with the anti-Markovnikov peroxide effect for HBr), water and sulphuric acid, as well as oxidation, ozonolysis and polymerisation. Alkynes have a triple bond between sp-hybridised carbons, terminal alkynes showing acidic hydrogens. Aromatic chemistry centres on benzene, whose unusual stability is explained by resonance delocalisation of its six pi electrons and the Hückel \((4n+2)\) rule; benzene undergoes electrophilic substitution — nitration, halogenation, sulphonation and Friedel-Crafts reactions — with substituents directing further substitution to ortho/para or meta positions. Polynuclear aromatic hydrocarbons are noted as carcinogenic and toxic.

Classification of hydrocarbonsPreparation and reactions of alkanesConformations of ethaneAlkenes and the double bondMarkovnikov and peroxide effectsAlkynes and acidity of terminal hydrogenStructure and aromaticity of benzeneElectrophilic substitution of benzeneCarcinogenicity and toxicity

Key terms

Conformation

One of the spatial arrangements of a molecule interconvertible by rotation about a single bond, such as the staggered and eclipsed forms of ethane.

Markovnikov rule

In the addition of an unsymmetrical reagent to an unsymmetrical alkene, the negative part adds to the carbon bearing fewer hydrogen atoms.

Peroxide effect

The anti-Markovnikov addition of HBr to alkenes in the presence of peroxide, via a free-radical mechanism.

Aromaticity

The special stability of a planar, cyclic, fully conjugated ring containing \((4n+2)\) pi electrons (Hückel rule).

Ozonolysis

The cleavage of an alkene by ozone, followed by reductive work-up, to give carbonyl compounds and locate the double bond.

Friedel-Crafts reaction

The electrophilic substitution of benzene by an alkyl or acyl group using an anhydrous aluminium chloride catalyst.

Extra questions & answers

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The general formula of an alkane is:

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