CHM 211 — Organic Chemistry I: Lecture Note

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LA-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS Course code: CHM 211 Course title: Organic Chemistry I Course unit: 2 Units Series 1: Aromatic Compounds and Their Chemistry Part 1 Fundamentals of Aromatic Compounds - 1.1.1 Definition and general features of aromatic compounds - 1.1.2 Structure and resonance in benzene - 1.1.3 Criteria for aromaticity (Hückel’s rule) Part 2 Reactions of Aromatic Compounds - 1.2.1 Electrophilic substitution reactions: nitration, halogenation, sulphonation, Friedel–Crafts reactions - 1.2.2 Orientation and reactivity in substituted aromatics - 1.2.3 Other reactions of aromatic compounds: oxidation and reduction Part 3 Applications and Problem-Solving in Aromatic Chemistry - 1.3.1 Industrial importance of aromatic compounds - 1.3.2 Worked examples and problem-solving in aromatic chemistry Introduction Aromatic compounds occupy a central place in organic chemistry because of their unique stability and widespread occurrence in everyday life. From the fragrance of perfumes to the structure of many pharmaceuticals, these compounds demonstrate the importance of understanding how ring systems behave and react. This Series will set the foundation for appreciating the chemistry of aromatic compounds, preparing you to tackle more complex reactions and applications later in the course. The aim of this Series is to equip you with the knowledge and skills to define, explain, and analyse the structures, properties, and reactions of aromatic compounds, as well as to apply these concepts in solving practical problems. We shall commence this Series by exploring the fundamentals of aromatic compounds, including their definition, general features, and the resonance structure of benzene, followed by the criteria for aromaticity. Next, we will move on to the reactions of aromatic compounds, focusing on electrophilic substitution, orientation effects, and other important transformations such as oxidation and reduction. Finally, we will wrap up by considering the industrial importance of aromatic compounds and engaging with worked examples that will strengthen your problem-solving skills in aromatic chemistry.

LA-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS Series Learning Outcomes Upon completing this Series, you should be able to: define the general features of aromatic compounds, explain the resonance structure of benzene, analyse aromaticity using Hückel’s rule, describe the major electrophilic substitution reactions of aromatic compounds, evaluate orientation and reactivity patterns in substituted aromatic compounds, explain other important reactions of aromatic compounds such as oxidation and reduction, assess the industrial importance of aromatic compounds, and apply problem-solving strategies to worked examples in aromatic chemistry. 1.1 Fundamentals Of Aromatic Compounds 1.1.1 Definition and general features of aromatic compounds An aromatic compound is an organic molecule that contains one or more planar ring systems with delocalised π-electrons, which confer unusual stability compared to non-aromatic compounds. The most common example is benzene, a six-membered ring with alternating double bonds that are not localised but shared across the ring. This delocalisation leads to a property known as aromatic stability, making these compounds resistant to addition reactions that are typical of alkenes. Aromatic compounds are characterised by their cyclic structure, planarity, and electron delocalisation. They often exhibit unique chemical behaviour, such as undergoing substitution rather than addition reactions.

LA-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS Fill in the blank: Aromatic compounds are unusually stable because of __________ of π-electrons across the ring. Figure 1.1 Benzene structure with delocalised electrons 1.1.2 Structure and resonance in benzene The resonance structure of benzene refers to the representation of its bonding as alternating single and double bonds. However, in reality, the electrons are delocalised, and all carbon–carbon bonds are of equal length. This concept is explained using resonance theory, where benzene is depicted as a hybrid of two canonical structures. This resonance stabilisation explains why benzene does not behave like a typical alkene. Instead of undergoing addition reactions, it prefers substitution reactions that preserve the aromatic ring. Fill in the blank: In benzene, all carbon–carbon bonds are of equal __________ due to resonance stabilisation.

LA-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS Figure 1.2 Resonance structures of benzene 1.1.3 Criteria for aromaticity (Hückel’s rule) To determine whether a compound is aromatic, chemists use Hückel’s rule, which states that a planar, cyclic, conjugated molecule is aromatic if it contains (4n + 2) π-electrons, where (n) is a non-negative integer. For benzene, (n = 1), giving 6 π-electrons, which satisfies the rule. Other compounds, such as naphthalene and anthracene, also follow Hückel’s rule, making them aromatic. Compounds that fail to meet these criteria are either non-aromatic or anti-aromatic, depending on their electron count and structure. Fill in the blank: According to Hückel’s rule, a molecule is aromatic if it contains __________ π-electrons, where n is a non-negative integer.

LA-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS Figure 1.3 – Hückel’s Rule and Aromaticity Pilot questions 1.1 Which property of aromatic compounds explains their resistance to addition reactions? What feature of benzene’s structure makes all its carbon–carbon bonds equal in length? State Hückel’s rule for aromaticity. How many π-electrons are present in benzene? Why do aromatic compounds prefer substitution reactions over addition reactions? 1.2 Reactions Of Aromatic Compounds 1.2.1 Electrophilic substitution reactions: nitration, halogenation, sulphonation, Friedel–Crafts reactions Aromatic compounds typically undergo electrophilic substitution reactions, which are reactions where an electrophile (an electron-deficient species that seeks electrons) replaces a

LA-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS hydrogen atom on the aromatic ring. This type of reaction preserves the aromaticity of the ring, which is why it is preferred over addition reactions. Common examples include: Nitration, where a nitro group (-NO₂) is introduced using concentrated nitric acid and sulphuric acid. Halogenation, where halogens such as chlorine or bromine are introduced in the presence of a Lewis acid catalyst like FeCl₃. Sulphonation, where a sulphonyl group (-SO₃H) is introduced using concentrated sulphuric acid. Friedel–Crafts reactions, which involve alkylation or acylation of the aromatic ring using alkyl or acyl halides with a Lewis acid catalyst. Fill in the blank: In electrophilic substitution reactions, the aromatic ring retains its __________ because the π-electrons remain delocalised. 1.2.2 Orientation and reactivity in substituted aromatics When an aromatic ring already contains a substituent, the position at which a new substituent enters is influenced by the nature of the existing group. This is known as orientation. Substituents can be classified as activating groups (which increase reactivity and direct substitution to the ortho and para positions) or deactivating groups (which decrease reactivity and often direct substitution to the meta position). For example, a methyl group activates the ring and directs substitution to ortho and para positions, while a nitro group deactivates the ring and directs substitution to the meta position. Fill in the blank: Activating groups generally direct new substituents to the __________ and __________ positions of the aromatic ring. Here’s a clear table that organises common substituents on benzene, showing whether they are activating or deactivating, and their typical orientation effects: Substituent Classification Orientation effect –OH (hydroxyl) Activating Ortho/para-directing –NH₂ (amino) Activating Ortho/para-directing –CH₃ (methyl) Activating Ortho/para-directing –OCH₃ (methoxy) Activating Ortho/para-directing –Cl, –Br (halogens) Deactivating Ortho/para-directing –NO₂ (nitro) Deactivating Meta-directing –CN (cyano) Deactivating Meta-directing –COOH (carboxyl) Deactivating Meta-directing

LA-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS Substituent Classification Orientation effect –CHO (formyl) Deactivating Meta-directing –SO₃H (sulphonyl) Deactivating Meta-directing Table 1.1 Orientation effects of common substituents 1.2.3 Other reactions of aromatic compounds: oxidation and reduction Apart from substitution, aromatic compounds can also undergo oxidation and reduction reactions. Oxidation often occurs at side chains rather than the ring itself. For example, toluene can be oxidised to benzoic acid using strong oxidising agents. Reduction reactions, on the other hand, may involve hydrogenation of the ring under high pressure and in the presence of a catalyst, converting benzene into cyclohexane. These reactions are important in industrial processes, such as the production of polymers and intermediates for pharmaceuticals. Fill in the blank: Oxidation of toluene typically produces __________ acid as the major product. Pilot questions 1.2 What type of reaction allows aromatic compounds to retain their aromaticity while introducing new substituents? Name two common electrophilic substitution reactions of benzene. Which type of substituents direct new groups to the ortho and para positions? What is the major product when toluene undergoes oxidation? Why do aromatic compounds prefer substitution reactions over addition reactions? 1.3 Applications And Problem-Solving In Aromatic Chemistry 1.3.1 Industrial importance of aromatic compounds Aromatic compounds play a vital role in industry due to their stability and versatility. They are used as raw materials in the manufacture of dyes, plastics, pharmaceuticals, and explosives. For example, benzene is a precursor for styrene, which is polymerised to form polystyrene, a widely used plastic. Similarly, toluene is employed in the production of solvents and explosives such as trinitrotoluene (TNT). The importance of aromatic compounds extends to the pharmaceutical industry, where many drugs contain aromatic rings that contribute to their biological activity. Aspirin, paracetamol, and many antibiotics are examples of medicines derived from aromatic chemistry. Fill in the blank: Benzene

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