[H2 Chemistry] 2022 Topic 15 Halogen derivatives

The lecture picks up from where the organic chemistry series left off, focusing on the topics covered in the previous year, including stereochemistry, nomenclature, and reactions of alkanes. The instructor reminds students of the introductory concepts such as cis-trans isomerism and steroid isomerism, and emphasizes the importance of understanding nomenclature as they progress through functional groups. The lecture then delves into the reactions of alkanes, particularly free radical substitution, using propane as an example to illustrate the formation of chloroalkanes under UV light conditions.

Building upon the foundation laid in the previous lecture, the instructor explores alkenes, specifically the electrophilic addition mechanism. The stability of carbocations formed during this process is discussed, with a focus on how the nature of the carbocation affects the reaction's favorability. The lecture then transitions to the topic of halogen derivatives, highlighting their significance in organic chemistry and their potential health risks, particularly as carcinogens. The naming and classification of halogenated alkanes are also covered, with examples provided to illustrate the concepts.

The lecture continues with a detailed examination of halogen derivatives, including their physical properties and the concept of constitutional isomerism. The degree of unsaturation is introduced as a tool for determining the saturation of compounds, and the instructor demonstrates how to calculate it. Various isomers of halogenated compounds are discussed, with a step-by-step guide on how to draw and name them according to IUPAC rules. The importance of understanding the structure and properties of these compounds is emphasized, especially in the context of their potential impact on human health.

This section delves into the physical properties of halogenated alkanes, including their color, solubility, density, and boiling points. The influence of halogen size on the strength of dispersion forces and, consequently, boiling points is explained. The instructor clarifies the misconception that polarity alone determines boiling points, instead highlighting the role of polarizability of the electron cloud. A detailed analysis of the boiling points of propane, chloro, bromo, and iodopropane is provided, with an exercise to apply this knowledge.

The lecture discusses the methods of preparing halogenated alkanes and arines, including free radical substitution of alkanes and electrophilic addition to alkenes. The limitations of these methods are highlighted, such as the difficulty in controlling the substitution process. The instructor also covers the preparation of halogenoarines through electrophilic substitution, using bromobenzene as an example. The importance of understanding these preparation methods for the synthesis of various organic compounds is emphasized.

The instructor introduces nucleophilic substitution reactions in halogenoalkanes, explaining the mechanisms of SN1 and SN2, and the conditions under which they occur. The concept of elimination reactions as a competing process to nucleophilic substitution is also presented. The lecture provides a detailed explanation of how the structure of the halogenoalkane affects the likelihood of these reactions, with a focus on the electron deficiency of the carbon atom and its susceptibility to nucleophilic attack.

This section provides an in-depth look at the mechanisms of nucleophilic substitution reactions, specifically SN1 and SN2. The instructor describes the stepwise process of SN1, emphasizing the formation of a carbocation intermediate and the subsequent attack by the nucleophile. In contrast, the SN2 mechanism is highlighted as a single-step reaction with a direct displacement of the halogen by the nucleophile. The importance of understanding these mechanisms for predicting reaction outcomes and controlling synthetic pathways is discussed.

The lecture explores the factors that influence the rate of nucleophilic substitution reactions, such as the structure of the substrate and the nature of the nucleophile. The stability of carbocation intermediates in SN1 reactions is discussed, with a focus on how the substitution pattern affects the reaction rate. The instructor also explains the steric and electronic effects that can lead to exceptions in the typical reactivity patterns of primary, secondary, and tertiary substrates.

This section highlights the versatility of halogenoalkanes as starting materials in organic synthesis. The instructor discusses various reactions, including hydrolysis to form alcohols, substitution with cyanide to form nitriles, and reduction to form primary amines. The lecture emphasizes the importance of understanding these reactions for constructing more complex organic molecules and the practical considerations for carrying out these syntheses in the laboratory.

The lecture delves into the nucleophilic substitution reactions involving ammonia, leading to the formation of primary, secondary, tertiary amines, and quaternary ammonium salts. The instructor explains the mechanism of these reactions, emphasizing the competition between ammonia and the formed amine for the halogenated substrate. The lecture also discusses the practical aspects of these reactions, such as the conditions required for the formation of quaternary ammonium salts.

The instructor introduces elimination reactions in halogenoalkanes, discussing the conditions and reagents required for their occurrence. The lecture explains the thermodynamic driving force behind elimination reactions and how they can be favored over substitution reactions. The section also covers the prediction of elimination products based on the number of unique beta hydrogens and the stability of the resulting alkenes.

This section focuses on laboratory tests to distinguish between different halogenoalkanes based on the color of silver halide precipitates formed upon reaction with silver nitrate. The instructor outlines the steps for these tests, including nucleophilic substitution to release free halides, neutralization with nitric acid, and the addition of silver nitrate. The lecture also discusses the solubility of silver halides in ammonia as a confirmation test.

The lecture concludes with a discussion on the environmental impact of halogenoalkanes, particularly the use of chlorofluorocarbons (CFCs) and their role in ozone depletion. The instructor highlights the measures taken to protect the ozone layer, such as reducing the use of CFCs and exploring alternatives like pure alkanes. The section encourages students to consider the broader implications of chemical usage and the search for environmentally friendly alternatives.