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: 15
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, students! So far in our study of organic chemistry, we have looked at straight-chain and branched compounds like alkanes and alkenes. Today, we delve into a special class of cyclic compounds called aromatic compounds, with benzene (C₆H₆) as our main example. Benzene is a fascinating molecule whose unique structure gives it incredible stability and special chemical properties. Understanding benzene is crucial because it is the parent compound for thousands of useful substances, from medicines like paracetamol that we buy at the pharmacy, to plastics used in making "take-away" packs, and even the colourful dyes used in our traditional Kente cloths.
A. The Puzzle of Benzene's Structure (C₆H₆)
The molecular formula of benzene is C₆H₆. If we compare this to hexane (C₆H₁₄), we can see that benzene is highly unsaturated (has fewer hydrogen atoms than the corresponding alkane). This suggests it should have multiple double or triple bonds and be very reactive, much like alkenes. However, experimental evidence shows benzene is surprisingly stable and unreactive. This created a puzzle for 19th-century chemists. i. The Kekulé Structure (1865) The German chemist August Kekulé proposed a structure for benzene as a cyclic ring of six carbon atoms with alternating single and double bonds. Each carbon atom is bonded to one hydrogen atom.
This structure could exist in two forms that are in rapid equilibrium. ii. Evidence Against the Kekulé Structure While clever, Kekulé's model could not explain three key experimental facts: Bond Lengths: X-ray crystallography shows that all six carbon-carbon bonds in benzene are of the same length (139 pm). This length is intermediate between a typical C-C single bond (154 pm) and a C=C double bond (134 pm). If Kekulé's model were correct, there should be three shorter double bonds and three longer single bonds. Chemical Reactivity: Alkenes (like cyclohexene) readily decolourise bromine water (Br₂(aq)) in an addition reaction. Benzene does not react with bromine water. It only reacts with pure bromine in the presence of a catalyst, and it undergoes a substitution reaction, not addition. This points to a much more stable structure. Enthalpy of Hydrogenation: The hydrogenation of one C=C double bond in cyclohexene releases about 120 kJ/mol of energy. C₆H₁₀ (cyclohexene) + H₂ → C₆H₁₂ (cyclohexane) ΔH = -120 kJ/mol Based on Kekulé's structure with three double bonds ("cyclohexatriene"), we would predict the enthalpy of hydrogenation of benzene to be 3 x (-120 kJ/mol) = -360 kJ/mol. However, the experimentally measured value is only -208 kJ/mol. This means benzene is 152 kJ/mol more stable than predicted by the Kekulé model. This extra stability is called the resonance energy or delocalisation energy. B. The Modern View: Resonance and Delocalisation
The modern understanding of benzene's structure solves the problems of the Kekulé model. Hybridisation: Each carbon atom in the benzene ring is sp² hybridised. This means it forms three sigma (σ) bonds: one with a hydrogen atom and two with its neighbouring carbon atoms. All these bonds lie in a single plane, forming a flat hexagonal ring. The Pi (π) System: Each carbon atom has one unhybridised p-orbital, containing one electron, sticking out at a right angle (above and below) to the plane of the ring. Delocalisation: These six p-orbitals overlap sideways with their neighbours all around the ring. Instead of forming three separate π bonds, they combine to form a continuous, doughnut-shaped cloud of electron density above and below the plane of the ring. The six π-electrons are said to be delocalised – they are not confined to specific pairs of carbon atoms but are shared equally among all six.