09Class 11 Chemistry
Hydrocarbons short notes
Short notes on alkanes, alkenes, alkynes and aromatic compounds, with their key reactions. Markovnikov's rule and aromaticity come up often.
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Alkanes
- Made by hydrogenating unsaturated compounds, the Wurtz reaction, or decarboxylation.
- Show free radical substitution, as in halogenation under light.
- Conformations of ethane: staggered is more stable than eclipsed.
Alkenes
- Made by dehydrohalogenation, dehydration or partial reduction of alkynes.
- Markovnikov's rule: in HX addition, the H goes to the carbon with more hydrogens.
- With peroxide, HBr adds in the anti-Markovnikov way (Kharasch effect), but HCl and HI don't.
- Cold dilute KMnO₄ gives a diol, and the purple colour disappears (Baeyer's test).
- Ozonolysis splits the double bond into carbonyl compounds.
Alkynes
- Calcium carbide with water gives ethyne.
- Terminal alkynes are acidic, and react with sodium and with ammoniacal AgNO₃.
- Lindlar's catalyst gives cis alkenes, and Na in liquid ammonia gives trans alkenes.
- Ethyne polymerises to benzene through a red-hot iron tube.
Aromatic compounds
- Hückel's rule: planar, cyclic, fully conjugated, with (4n + 2) π electrons.
- Benzene mostly undergoes electrophilic substitution: nitration, halogenation, sulphonation and Friedel-Crafts reactions.
- Activating groups (−OH, −NH₂, −CH₃) direct ortho and para.
- Deactivating groups (−NO₂, −CN, −COOH) direct meta.
- Halogens deactivate, but direct ortho and para.
Where students slip up
- Applying the peroxide effect to HCl or HI. It only works with HBr.
- Calling cyclooctatetraene aromatic. It has 8 π electrons and isn't planar.
- Treating all deactivating groups as meta-directing. Halogens are the exception.
Try it on your weakest chapter.
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