Organic Chemistry Lecture #2

Organic Chemistry Lecture #2

• The lecture covers organic chemistry concepts, including reaction mechanisms, carbocations, and radicals, along with the nomenclature of alkanes.

Past Paper Review Question

• The question asks which statement about sp³ orbital hybridization in methane is false.
• Option (a) states hybridization results from the combination of one s orbital and three p orbitals, which is correct.
• Option (b) states that it forms three new identical orbitals, which is also correct.
• Option (c) states that each new orbital possesses an unpaired electron which is false.
• Option (d) states the angle between the orbitals is 109.5°, which is correct.
• Option (e) states that there is the same probability of finding an electron in any of the new orbitals, which is correct.

Lecture Outline

• Introduces reaction mechanisms and their steps.
• Explains homolytic and heterolytic cleavage of bonds, along with bond dissociation energy.
• Covers reaction intermediates, carbocations, and radicals.
• Discusses nucleophiles and electrophiles, their properties and reactions.
• Delves into alkanes, focusing on homologous series, nomenclature, structural isomerism (skeletal, positional, functional group), and examples of cyclic alkanes.
• Introduces alkenes, which are unsaturated hydrocarbons, and explains how their nomenclature is different from alkanes.
• Also introduces alkynes, their structure and nomenclature.

Introduction to Reaction Mechanisms

• Reaction mechanisms are sequential descriptions of the steps involved in chemical reactions, including bond breaking and bond formation to produce products.
• The steps have intermediate steps which lead to the final products.

Homolytic Cleavage

• Bonds consist of two electrons.
• The movement of one electron is denoted by a fishhook arrow which indicates the direction of electron flow in a chemical reaction.
• Homolytic cleavage results in radicals.

Heterolytic Cleavage

• Movement of an electron pair is denoted by a curved arrow showing the flow of the electrons in a reaction.

Relative Stability of Carbocations

• Tertiary carbocations are more stable than secondary carbocations, and secondary carbocations are more stable than primary carbocations.
• Methyl cations are the least stable.
• Stability is enhanced by elements that disperse the positive charge, such as through the inductive effect.

Relative Stability of Radicals

• Tertiary radicals are more stable than secondary radicals, and secondary radicals are more stable than primary radicals.
• Methyl radicals are the least stable type.

Nucleophiles and Electrophiles

• Electrophiles are electron-poor species that seek electrons.
• Nucleophiles are electron-rich species that donate electrons to electrophiles.
• Arrows in reactions show electron density moving from areas of high to low density.

Bond Dissociation Energy

• The energy required to break a bond homolytically is comparable to the energy released when the atoms form the molecule.
• This measure is helpful for determining energies needed for bond dissociation.
• Bond dissociation energies are used to calculate enthalpy changes for reactions.
• They can be used to estimate relative stabilities of radicals.
• Multiple bonds require more energy for dissociation than single bonds.
• Selected homolytic bond dissociation energies are tabulated.

Alkanes

• Natural sources of alkanes include petroleum and natural gas (methane, ethane).
• Data on US energy consumption by sources is presented.
• Alkanes form a homologous series, characterized by a consistent structure (CnH2n+2).
• Members in the series differ by methylene (CH2) groups.
• Nomenclature of alkanes follows a standard system.

Nomenclature

• International Union of Pure and Applied Chemistry (IUPAC) nomenclature system for alkanes is described.
• The -ane suffix is used for saturated hydrocarbons.
• For unbranched alkanes, naming is based on the number of carbon atoms.
• For branched alkanes, the longest continuous carbon chain defines the parent alkane.

Nomenclature - Substituents

• Substituents like alkyl groups attached to the main chain are named and their positions identified using numbers.
• Common substituents (propyl, butyl, isobutyl, sec-butyl, tert-butyl, halogens) are presented.
• The IUPAC naming method of hydrocarbons uses one word.

Cyclic Alkanes

• Nomenclature of cyclic alkanes involves placing the prefix "cyclo" before the alkane name corresponding to the number of carbon atoms.
• Alkyl or halogen substituents are named similarly to alkanes and their positions are identified.
• Listing and examples with diagrams are provided for better understanding.

Representation of Alkanes, Alkenes, and Alkynes

• Different ways to represent the structure of alkanes, alkenes, and alkynes (dash, condensed, and bond-line).
• Specific examples of these structures are presented in their respective sections.

Nomenclature Priorities of Selected Functional Groups

• Priority order of functional groups in naming organic molecules is outlined.

Nomenclature in Alkenes

• Nomenclature of alkenes includes specifying the presence of a carbon-carbon double bond using the ending "ene".
• Naming follows the longest continuous carbon chain that contains the double bond, which becomes the parent structure.
• The position of the double bond is noted as a number in the name.
• For multiple double bonds, prefixes for their number are used in the name.

Nomenclature in Alkenes (cont'd)

• The carbon chain is numbered from the end closer to the double bond to assign the lowest possible number to the double bond.
• The position of the first carbon forming the double bond is used in naming the parent structure and is also used for the prefixes.
• Substituent positions are numbered and indicated in the name.
• Examples and rules for numbering when multiple double bonds exist are given.

Naming Cyclic Alkenes

• Rules for naming cyclic alkenes and substituents are presented.

Structural Isomerism

• Structural isomers possess the same molecular formula, but have different structural formulas.
• They are further separated into skeletal, positional and functional group isomers.
• Examples of each type are given with diagrams.

Physical Properties of Alkanes

• Alkanes are generally insoluble in water.
• They exhibit low boiling points due to weak intermolecular forces.
• Branched alkanes have lower boiling points compared to their linear counterparts.

Preparation of Alkanes

• Alkanes are formed through various methods, including fractional distillation of crude oil, from alkenes, from carbonyl compounds with Wolff-Kishner or Clemmensen reduction, and from alkyl halides.

Fractional Distillation

• A detailed discussion and image of the crude oil distillation tower is presented.
• This explains how hydrocarbons separate by boiling points in a fractional distillation unit, and are then grouped in various categories and fractions depending on the boiling point.
• The example given is for petroleum fractions.

Catalytic Cracking

• This method breaks down large alkanes into smaller alkanes and alkenes with the addition of hydrogen.
• The process uses high temperature and catalysts.
• An example is given of how the process converts a larger alkane to smaller hydrocarbons and hydrogen gas.

Preparation from Alkenes (Catalytic Hydrogenation)

• Methods for converting alkenes and alkynes back to alkanes using catalytic hydrogenation.
• This process uses catalysts such as platinum (Pt), nickel (Ni), or palladium (Pd) under pressure to add hydrogen across the double or triple bond.

Clemmensen Reduction

• Used for compounds that are stable in acids, and converts C=O bonds into CH₂ bonds.
• Employs zinc amalgam and concentrated hydrochloric acid (HCl).

Reaction with Grignard Reagents

• Grignard reagents are organomagnesium compounds with a carbon-metal bond.
• They react with specific functional groups like compounds with acidic hydrogens, alcohols, and alkynes.
• Reaction mechanisms are shown in diagrams.

Past Paper Question (IUPAC Naming)

• A sample question about naming a molecule using the IUPAC system is presented.
• The correct IUPAC name among the options provided is highlighted.

Additional Notes

• A further summary is provided of the main differences.
• A key concept is the similar but different aspect of the topic in an engaging way, with an image, to attract the students’ attention.

This lecture explores essential concepts in organic chemistry including reaction mechanisms, the nature of carbocations and radicals, and the nomenclature of alkanes. Additionally, it reviews sp³ orbital hybridization in methane through critical questions and explanations of correct and false statements related to the topic.