Hydrocarbons
Class 11 · Chemistry
4 ideas in Class 11 Chemistry, explained in plain words, with games and quizzes to practise.
Alkanes: Structure, Conformations and Reactions
Part of: Saturated and Unsaturated Chains
Tetrahedral carbons spin almost freely, resting staggered between eclipsed peaks 12.5 apart. Alkanes burn clean, substitute by radical chains in light, couple by Wurtz, and shed carboxyl by heating with soda lime.
Why it matters. Fuels, plastics and solvents start here. Conformer gaps explain why single bonds spin freely.
Formulas
- Alkane family: CnH2n+2
- Wurtz: 2RX + 2Na gives R-R + 2NaX in dry ether
- Decarboxylation: RCOONa + soda lime gives RH + Na2CO3 on heating
- Methane burns: CH4 + 2O2 gives CO2 + 2H2O
Key words
- Alkane
- : A hydrocarbon with only single bonds between carbons, holding the maximum possible hydrogen.
- Conformation
- : A shape a molecule takes by spinning around a single bond without breaking anything.
- Staggered form
- : The resting shape with neighbouring groups spaced apart and the lowest energy.
- Eclipsed form
- : The crowded shape with neighbouring groups lined up and the highest energy.
- Free radical
- : A short-lived particle with one unpaired electron that starts chain substitution.
- Wurtz reaction
- : Joining two alkyl halide molecules with sodium metal to build a longer alkane.
- Decarboxylation
- : Heating the sodium salt of an acid with soda lime to remove carbon dioxide.
Try it in two minutes
- Twist a model from staggered to eclipsed.
- Burn a candle and name products.
- Couple two methyls with sodium.
Try at home. Compare wax and oil flames.
Alkenes: Structure and Electrophilic Addition
Part of: Saturated and Unsaturated Chains
Pi electrons invite electrophiles: proton first, then anion to the stabler carbocation. Peroxides flip HBr to anti-Markovnikov by radicals. Ozone slices double bonds into carbonyls, revealing skeletons.
Why it matters. Polymers, alcohols and acids all enter through addition. One rule predicts the major product.
Formulas
- Alkene family: CnH2n
- Markovnikov: H of HX adds to the carbon with more H atoms
- Peroxide effect: with peroxides, Br adds to the carbon with more H (HBr only)
- Ozonolysis cuts each C=C into two carbonyl pieces
Key words
- Alkene
- : A hydrocarbon with one double bond between two carbons.
- Pi bond
- : The sideways overlap above and below the bond axis that makes double bonds reactive.
- Markovnikov rule
- : The H of HX joins the double-bond carbon that already carries more hydrogen.
- Carbocation
- : A positively charged carbon that grows stabler with more alkyl groups around it.
- Peroxide effect
- : HBr adds opposite to Markovnikov when organic peroxides are present.
- Ozonolysis
- : Ozone cuts a double bond into carbonyl pieces, revealing the carbon skeleton.
Try it in two minutes
- Add HBr to propene on paper.
- Flip with peroxides.
- Slice an alkene by ozonolysis.
Try at home. Compare oil and fat saturation tests.
Alkynes: Acidity and Addition
Part of: Triple Bonds and Rings
sp carbons hold electrons tight, making terminal alkynes mildly acidic and testable by silver and copper precipitates. Additions follow Markovnikov twice over; water with mercury gives ethanal. Polymers and rings grow from acetylene.
Why it matters. Acidity tests sort terminal from internal. Double addition builds ketones and plastics.
Formulas
- Alkyne family: CnH2n-2
- Ethyne hydrates with Hg2+ and acid to give ethanal
- Terminal alkyne + ammoniacal silver nitrate gives a white precipitate
Key words
- Alkyne
- : A hydrocarbon with one triple bond between two carbons.
- Terminal alkyne
- : An alkyne with the triple bond at the chain end, carrying one acidic hydrogen.
- Acetylide
- : The salt left after a strong base removes the acidic hydrogen of a terminal alkyne.
- Keto-enol pair
- : Two forms differing only in H position that interconvert during alkyne hydration.
- Polymerisation
- : Small alkyne units joining into long chains or rings.
Try it in two minutes
- Rank three hydrocarbons by acidity.
- Test a terminal alkyne with silver.
- Add water across acetylene.
Try at home. Compare vinegar with alkyne acidity ideas.
Aromatic Hydrocarbons
Part of: Triple Bonds and Rings
Flat rings with 4n+2 pi electrons share a stable sextet that substitutes instead of adding. Activators enrich ortho-para; deactivators leave meta. Friedel-Crafts, nitration and sulphonation build derivatives; some fused rings cause cancer.
Why it matters. Medicines, dyes and plastics grow on benzene rings. Directing rules place each newcomer.
Formulas
- Benzene: C6H6
- Huckel rule: flat ring with (4n + 2) pi electrons is aromatic
- Activating groups direct ortho and para; deactivating groups direct meta
Key words
- Aromatic compound
- : A flat ring with a shared pi cloud that undergoes substitution, not addition.
- Huckel rule
- : A flat ring is aromatic when it holds (4n + 2) pi electrons.
- Activating group
- : A group that pushes electrons into the ring and directs new groups ortho and para.
- Deactivating group
- : A group that pulls electrons from the ring and directs new groups to meta.
- Electrophilic substitution
- : A reaction where an electron-seeking particle replaces a ring hydrogen.
- Friedel-Crafts reaction
- : Adding an alkyl or acyl group to benzene with anhydrous AlCl3.
Try it in two minutes
- Count pi electrons in three rings.
- Nitrate toluene and note spots.
- Compare addition versus substitution.
Try at home. Smell mothballs with care and note the ring.
What your child will practise
- Alkanes: Structure, Conformations and Reactions: Ocean Puzzle (Clue Puzzle)
- Alkanes: Structure, Conformations and Reactions: Ocean Quick Fire (Quick Fire)
- Alkenes: Structure and Electrophilic Addition: Jungle Puzzle (Clue Puzzle)
- Alkynes: Acidity and Addition: Space Quest (Flip and Match)
- Aromatic Hydrocarbons: Desert Quick Fire (Quick Fire)
- Quiz: Alkanes: Structure, Conformations and Reactions (8 questions)
- Quiz: Alkenes: Structure and Electrophilic Addition (8 questions)
- Quiz: Alkynes: Acidity and Addition (8 questions)
- Quiz: Aromatic Hydrocarbons (8 questions)