Lesson Notes By Weeks and Term v4 - SHS 2
CHARACTERIS ATION OF ORGANIC COMPOUNDS
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CHARACTERIS ATION OF ORGANIC COMPOUNDS
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Subject: Chemistry
Class: SHS 2
Term: 2nd Term
Week: 18
Grade code: 2.3.2.LI.3
Strand code: 3
Sub-strand code: 1
Content standard code: 2.3.2.CS.1
Indicator code: 2.3.2.LI.3
Theme: CHEMISTRY OF CARBON COMPOUNDS
Subtheme: CHARACTERIS ATION OF ORGANIC COMPOUNDS
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Welcome, learners! Today, we are exploring a very special and important organic compound called benzene. Benzene is the parent compound of a whole family of substances known as aromatic compounds, so-called because many of them have distinct, often pleasant, smells. You encounter benzene derivatives every day, from the paracetamol you take for a headache to the plastics used in making takeaway food packs and even in the petrol we use for our cars and "tro-tros". Understanding benzene is crucial because its unique structure gives it unusual stability and chemical properties that are different from other unsaturated hydrocarbons like alkenes.
2.1 The Puzzle of Benzene's Structure (C₆H₆)
Benzene was discovered by Michael Faraday in 1825. Its molecular formula is C₆H₆. Looking at this formula, we can see a high carbon-to-hydrogen ratio. Compared to a saturated alkane with 6 carbons (Hexane, C₆H₁₄), benzene is highly unsaturated. This suggests it should have many double or triple bonds and be very reactive, like alkenes. However, experiments showed that benzene is surprisingly unreactive. This was a major puzzle for chemists for many years.
Evidence for a Ring Structure: Benzene reacts with chlorine to form only *one* monosubstituted product (C₆H₅Cl). This indicates that all 6 hydrogen atoms in benzene are chemically equivalent. A straight-chain structure would have different types of hydrogen atoms. Benzene reacts with hydrogen under high pressure and temperature to form cyclohexane (C₆H₁₂). This confirms it has a six-carbon ring structure. 2.2 Models for the Structure of Benzene
A. Kekulé's Structure (1865) The German chemist August Kekulé proposed that benzene consists of a six-membered ring of carbon atoms with alternating single and double bonds. Each carbon atom is bonded to one hydrogen atom. Structure: A planar hexagon with alternating C=C and C-C bonds. Problems with Kekulé's Structure: Reactivity: If it had three C=C double bonds like an alkene, it should readily undergo addition reactions (e.g., decolourise bromine water). Benzene does *not* decolourise bromine water, indicating it is much more stable. Isomers: Kekulé's model predicts two different versions of 1,2-dichlorobenzene, depending on whether the chlorine atoms are across a double bond or a single bond. However, only *one* form of 1,2-dichlorobenzene has ever been found. Bond Lengths: X-ray diffraction shows that all carbon-carbon bonds in benzene are of the same length (0.139 nm). This is intermediate between a typical C-C single bond (0.154 nm) and a C=C double bond (0.134 nm). Kekulé's model, with its alternating bonds, would predict two different bond lengths.