Topic 6: Organic Chemistry I — Edexcel A-Level Chemistry

Topic 6: Organic Chemistry I introduces the fundamental principles of organic chemistry, focusing on the chemistry of alkanes, alkenes, and haloalkanes. You will explore the concept of homologous series, functional groups, and the nomenclature of organic compounds. This topic also covers the mechanisms of key reactions, including free-radical substitution, electrophilic addition, and nucleophilic substitution, which are essential for understanding how organic molecules transform.

Understanding organic chemistry is crucial because it forms the basis for more advanced topics such as organic synthesis, polymers, and biochemistry. The skills you develop here—such as drawing mechanisms, predicting products, and explaining reaction conditions—are directly applicable to later topics and to real-world applications like pharmaceuticals and materials science. Mastery of this topic will also strengthen your ability to analyse and interpret organic reactions in exams.

This topic builds on your GCSE knowledge of hydrocarbons and introduces the systematic approach required at A-Level. You will learn to name compounds using IUPAC rules, identify isomers, and understand how the structure of a molecule influences its reactivity. By the end of this topic, you should be able to write balanced equations, draw reaction mechanisms, and explain the factors that affect reaction rates and yields.

• →Nomenclature: Understand how to name alkanes, alkenes, and haloalkanes using IUPAC rules, including the use of prefixes, suffixes, and locants.
• →Isomerism: Recognise structural isomers (chain, position, functional group) and stereoisomers (E/Z isomerism in alkenes).
• →Reaction mechanisms: Be able to draw and explain the mechanisms for free-radical substitution (alkanes), electrophilic addition (alkenes), and nucleophilic substitution (haloalkanes).
• →Reactivity trends: Understand how bond polarity and bond enthalpy affect the reactivity of alkanes, alkenes, and haloalkanes.
• →Practical techniques: Know how to carry out tests for unsaturation (bromine water) and distinguish between primary, secondary, and tertiary haloalkanes using silver nitrate.

• Correct calculation of oxidation numbers in compounds and ions, including peroxides and metal hydrides.
• Correct identification of oxidation and reduction based on electron transfer and oxidation number changes.
• Correct identification of oxidising and reducing agents.
• Correct identification of disproportionation reactions.
• Correct use of Roman numerals to indicate oxidation numbers.
• Correct construction of full ionic equations from ionic half-equations.

• 💡Always check that the sum of oxidation numbers in a neutral compound equals zero and in an ion equals the charge of the ion.
• 💡Remember that oxidising agents are reduced (gain electrons) and reducing agents are oxidised (lose electrons).
• 💡When balancing half-equations, ensure the total charge on both sides is equal.
• 💡Practice identifying oxidation numbers in various contexts, especially for s- and p-block elements.
• 💡Always show the movement of electrons using curly arrows in mechanisms. Ensure arrows start from a bond or lone pair and point to the atom where the electrons are going. Missing or incorrect arrows lose marks.
• 💡When naming compounds, remember to number the carbon chain to give the lowest possible locants to functional groups and substituents. Check for alphabetical order when listing substituents.
• 💡For practical questions on haloalkane reactivity, remember that silver nitrate test results depend on the halogen (AgCl white, AgBr cream, AgI yellow) and the rate of precipitation (tertiary > secondary > primary).

• Confusing the direction of electron transfer in oxidation and reduction.
• Incorrectly assigning oxidation numbers in complex ions or species.
• Failing to balance both atoms and charges when constructing ionic half-equations.
• Misidentifying the species being oxidised or reduced in a disproportionation reaction.
• Misconception: Alkanes are unreactive because they are saturated. Correction: Alkanes do undergo reactions, such as combustion and free-radical substitution, but they require specific conditions (e.g., UV light for substitution).
• Misconception: In electrophilic addition, the electrophile is always H⁺. Correction: The electrophile can be a positive ion (e.g., H⁺ from HBr) or a molecule with a partial positive charge (e.g., Br₂ in the presence of a catalyst).
• Misconception: All haloalkanes undergo nucleophilic substitution at the same rate. Correction: The rate depends on the halogen (I > Br > Cl > F) and the class of haloalkane (tertiary > secondary > primary for SN1, but primary > secondary > tertiary for SN2).