Cycloalkane Handwritten Notes (Organic Chemistry 2, Unit 5)
Download comprehensive handwritten notes on Cycloalkanes, covering Unit 5 of Organic Chemistry 2 for B.Pharm (3rd Semester). These notes define cycloalkanes, discuss their hybridization and IUPAC nomenclature, detail various preparation methods (from dihalides, Dieckmann, Simmons-Smith, aromatic hydrocarbons, dicarboxylic acids), and explain chemical reactions including substitution and ring-opening addition reactions. View online or download PDF/notes for free.
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Cycloalkanes: Structure, Synthesis, and Reactivity (Unit 5, Organic Chemistry 2)
This detailed set of handwritten notes focuses on Cycloalkanes, a fundamental topic within Unit 5 of Organic Chemistry 2, specifically designed for B.Pharm students in their 3rd semester. Cycloalkanes are saturated hydrocarbons that contain a closed ring of carbon atoms. They share many properties with open-chain alkanes but exhibit unique characteristics due to their cyclic structure, particularly ring strain in smaller rings.
Fundamentals of Cycloalkanes: Definition, Hybridization, and Nomenclature
The notes begin by answering the fundamental question: "What is cycloalkane?" They are defined as cyclic alkanes, meaning all carbon-carbon bonds within the ring are single bonds. Each carbon atom in a cycloalkane is sp³ hybridized, forming a tetrahedral geometry around it, similar to open-chain alkanes. The notes thoroughly cover the "IUPAC nomenclature" for cycloalkanes, explaining how to name substituted cycloalkanes, prioritize functional groups, and correctly identify ring sizes. Understanding these basic concepts is crucial before delving into their synthesis and reactions.
Methods of Preparation of Cycloalkanes
A significant portion of the notes is dedicated to the various "methods of preparation of cycloalkanes." Several synthetic routes are explored, allowing for the formation of different ring sizes:
- From Dihalides: This method involves the intramolecular Wurtz reaction or similar coupling reactions where a 1,ω-dihaloalkane reacts with a metal (like Na or Zn) to form a cyclic alkane, particularly effective for smaller rings.
- Dieckmann Reaction: Primarily used for the synthesis of five- and six-membered cyclic β-keto esters, which can then be decarboxylated and reduced to yield cycloalkanones or cycloalkanes. This reaction involves an intramolecular Claisen condensation.
- Simmons-Smith Reaction: A highly useful method for the synthesis of cyclopropanes. This reaction involves the reaction of an alkene with diiodomethane and a zinc-copper couple (or zinc dust) to add a methylene group across the double bond, creating a cyclopropane ring in a stereospecific manner.
- From Aromatic Hydrocarbons: Larger cycloalkanes or polycyclic systems can be prepared by hydrogenation of aromatic hydrocarbons under specific conditions, leading to the saturation of the aromatic rings.
- From Calcium Salts of Dicarboxylic Acids: Heating the calcium salts of dicarboxylic acids can lead to the formation of cyclic ketones, which can then be reduced to the corresponding cycloalkanes. This method is particularly useful for forming five-, six-, or seven-membered rings.
Chemical Reactions of Cycloalkanes
The reactivity of cycloalkanes varies significantly with ring size, primarily due to ring strain. The notes categorize their "chemical reactions" into two main types:
- Substitution Reaction: Like open-chain alkanes, larger cycloalkanes (five-membered and above, due to their relative lack of ring strain) primarily undergo free radical substitution reactions, such as halogenation (e.g., chlorination, bromination) in the presence of light or heat, where a hydrogen atom is replaced by a halogen.
- Addition Reaction (Ring Opening Reaction): Smaller cycloalkanes, particularly cyclopropane and cyclobutane, possess significant ring strain due to the deviation of bond angles from the ideal 109.5° (e.g., 60° in cyclopropane). This strain makes them more reactive and capable of undergoing ring-opening addition reactions. Cyclopropane, for instance, can react with hydrogen (hydrogenation), halogens, or hydrogen halides to open the ring and form an open-chain product, behaving more like an alkene in some respects. Cyclobutane can also undergo ring opening, though it requires more vigorous conditions than cyclopropane. Larger rings are generally stable and do not undergo ring-opening reactions readily.
This comprehensive set of handwritten notes provides a clear and detailed understanding of cycloalkanes, their unique properties stemming from their cyclic nature, and their diverse synthetic pathways and reactions, making it an invaluable resource for B.Pharm students.
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