Bài giảng Hoá hữu cơ: Chương 7 - TS. Lê Ngọc Hà Thu

Alkyl halides, also known as haloalkanes, represent a pivotal class of organic compounds characterized by the presence of a carbon-halogen (C-X) bond. These organohalides are of immense significance in organic chemistry, serving as versatile intermediates in numerous synthetic pathways. The inherent polarity of the C-X bond renders the carbon atom susceptible to nucleophilic attack, thus facilitating a wide array of chemical transformations. This chapter provides a comprehensive overview of alkyl halides, commencing with their definition, diverse classification based on carbon hybridization and halogen position, and systematic nomenclature. Crucially, it explores their fundamental reactivity, highlighting the prevalence of nucleophilic substitution and elimination reactions, which underpin their utility in constructing complex molecular architectures.

Sinh viên hóa học hữu cơ, đặc biệt là những người đang nghiên cứu về cấu trúc, danh pháp, tính chất vật lý và các cơ chế phản ứng của hợp chất halogen hữu cơ.

This chapter provides a detailed examination of organohalides, with a particular focus on alkyl halides (or haloalkanes). It begins by defining organohalides as organic compounds featuring a C-X bond, where X is a halogen. The text meticulously categorizes these compounds based on carbon hybridization (sp³, sp², sp), distinguishing between alkyl, allylic, benzylic, vinylic, and aryl halides, with alkyl halides being the primary subject. A significant portion is dedicated to their nomenclature, covering both common and IUPAC systems, including special prefixes like "gem-" and "vic-" for dihalides, and "per-" for fully substituted halogenoalkanes. Furthermore, alkyl halides are classified by the degree of the carbon atom bonded to the halogen (primary, secondary, tertiary), which is crucial for understanding their reactivity. The physical properties are discussed, noting that the C-X bond imparts a dipole moment leading to higher boiling points compared to alkanes. The core of the chapter delves into the chemical reactivity of alkyl halides, emphasizing that the polar C-X bond makes the carbon electrophilic, thus prone to reactions with nucleophiles. This leads to two major reaction pathways: nucleophilic substitution and elimination reactions. The text then elaborates on the SN2 mechanism (bimolecular nucleophilic substitution) as one of the key reaction mechanisms. It explains SN2 as a concerted, one-step process involving a transition state where both the alkyl halide and the nucleophile contribute to the rate-determining step. Key characteristics, such as the backside attack by the nucleophile and the resulting inversion of configuration, are detailed. The limitations of the SN2 mechanism are also addressed, specifically its unsuitability for organohalides with sp² or sp hybridized carbons (vinylic and aryl halides) due to steric hindrance preventing the required linear alignment in the transition state. This comprehensive understanding of alkyl halide properties and reaction mechanisms is vital for their application in organic synthesis.