Washu Chem 2561

These notes provide an overview of fundamental concepts in organic chemistry, including bonding, nomenclature, structure, properties, and reactions of alkanes, alkenes, alkynes, alcohols and ethers. Key topics covered are stereochemistry, reaction mechanisms (free radical halogenation and electrophilic addition), and functional group transformations.

Note 1: Introduction to Organic Chemistry and Bonding Organic chemistry is the study of carbon-containing compounds. Carbon's unique ability to form stable catenated (chain-like) structures and multiple bonds is the foundation of organic chemistry. The electronic structure of carbon, specifically its four valence electrons, allows it to form four covalent bonds. This tetravalency is crucial. Hybridization theory (sp3, sp2, and sp) explains the observed geometries around carbon atoms. sp3 hybridization leads to tetrahedral geometry (e.g., methane), sp2 to trigonal planar geometry (e.g., ethene), and sp to linear geometry (e.g., ethyne). The strength and length of carbon-carbon bonds vary depending on the hybridization: single bonds (sp3-sp3) are the longest and weakest, triple bonds (sp-sp) are the shortest and strongest. Bond polarity arises from differences in electronegativity between atoms in a bond (e.g., C-O bond is polar). This polarity influences intermolecular forces and reactivity. A common misconception is that organic compounds are only found in living organisms; many are synthesized in labs.

Note 2: Alkanes: Structure, Nomenclature, and Properties Alkanes are saturated hydrocarbons with the general formula CnH2n+2. They contain only single carbon-carbon and carbon-hydrogen bonds. Nomenclature follows IUPAC rules: identify the longest continuous carbon chain as the parent alkane, number the chain to give substituents the lowest possible numbers, and name substituents alphabetically. Branched alkanes are named as alkyl substituents attached to the parent alkane. Cycloalkanes have the general formula CnH2n and are named with the prefix "cyclo-". Physical properties, such as boiling point and melting point, increase with increasing molecular weight due to stronger London dispersion forces. Branching decreases boiling points. Alkanes are relatively unreactive due to the strong, nonpolar C-C and C-H bonds. Combustion is a highly exothermic reaction. A key application is as fuels (e.g., methane, propane, butane). A common misconception is that all cyclic compounds are aromatic.

Note 3: Stereochemistry: Chirality, Enantiomers, and Diastereomers Stereochemistry deals with the three-dimensional arrangement of atoms in molecules. A chiral center is a carbon atom bonded to four different groups. A molecule containing a chiral center is chiral, meaning it is non-superimposable on its mirror image. Enantiomers are stereoisomers that are mirror images of each other. They have identical physical properties except for the direction they rotate plane-polarized light. Optical activity is the ability of a chiral molecule to rotate plane-polarized light. A racemic mixture is an equimolar mixture of enantiomers and shows no net optical rotation. Diastereomers are stereoisomers that are not mirror images. They have different physical properties. Meso compounds contain chiral centers but are achiral due to an internal plane of symmetry. The R/S nomenclature system is used to assign absolute configurations to chiral centers. Common misconceptions include thinking that all molecules with chiral centers are chiral (consider meso compounds) and confusing enantiomers with diastereomers.

Note 4: Reactions of Alkanes: Halogenation Alkanes undergo substitution reactions with halogens (e.g., Cl2, Br2) in the presence of light or heat, known as free radical halogenation. The mechanism involves three steps: initiation (formation of radicals), propagation (chain reaction), and termination (radical combination). Chlorination is less selective than bromination, meaning that chlorination results in a mixture of products due to the higher reactivity of chlorine radicals. Bromination is more selective because the transition state for bromination is more sensitive to steric hindrance. The relative reactivity of different hydrogen atoms (primary, secondary, tertiary) towards halogenation follows the order: tertiary > secondary > primary. Regioselectivity refers to the preference for halogenation at a particular position. The Hammond postulate states that the transition state resembles the species (reactants or products) that it is closer in energy to. The reaction has limited synthetic utility due to the formation of mixtures of products. The common misconception is that halogenation only occurs at the most substituted carbon.

Note 5: Alkenes: Structure, Nomenclature, and Reactions (Addition Reactions) Alkenes are hydrocarbons containing at least one carbon-carbon double bond. The general formula is CnH2n. The carbon atoms involved in the double bond are sp2 hybridized and have trigonal planar geometry. Nomenclature follows similar IUPAC rules as alkanes, with the double bond taking priority in numbering. Alkenes undergo addition reactions to the double bond. Electrophilic addition is a characteristic reaction. Markovnikov's rule states that in the addition of HX to an alkene, the hydrogen atom adds to the carbon with more hydrogen atoms already attached (the more substituted carbon). This is due to the formation of the more stable carbocation intermediate. Carbocation stability follows the order: tertiary > secondary > primary. Rearrangements (hydride or alkyl shifts) can occur to form more stable carbocations. Hydration (addition of water) requires an acid catalyst. Hydroboration-oxidation is an anti-Markovnikov addition of water. A common misconception is that addition reactions always follow Markovnikov's rule.

Note 6: Alkynes: Structure, Nomenclature, and Reactions Alkynes are hydrocarbons containing at least one carbon-carbon triple bond. The general formula is CnH2n-2. The carbon atoms involved in the triple bond are sp hybridized and have linear geometry. Terminal alkynes (alkynes with a triple bond at the end of the chain) are acidic due to the sp hybridized carbon. They can be deprotonated with strong bases like NaNH2 to form acetylide ions. These acetylide ions are strong nucleophiles and can undergo SN2 reactions with alkyl halides to form new carbon-carbon bonds. Internal alkynes (alkynes with the triple bond in the middle of the chain) are not acidic. Alkynes undergo addition reactions similar to alkenes, but with the possibility of adding two equivalents of reagent. Catalytic hydrogenation can reduce alkynes to alkanes. Lindlar's catalyst can be used to reduce alkynes to cis-alkenes. A common misconception is that alkynes are less reactive than alkenes, which isn't always true (depends on the reaction).

Note 7: Alcohols: Structure, Nomenclature, Properties, and Reactions Alcohols contain a hydroxyl (-OH) group bonded to a saturated carbon atom. Nomenclature involves identifying the longest carbon chain containing the hydroxyl group and adding the suffix "-ol". Alcohols are polar due to the electronegativity difference between oxygen and hydrogen. They can form hydrogen bonds, which significantly increase their boiling points compared to alkanes of similar molecular weight. Alcohols can act as acids (proton donors) or bases (proton acceptors). Primary alcohols can be oxidized to aldehydes and then to carboxylic acids. Secondary alcohols can be oxidized to ketones. Tertiary alcohols cannot be oxidized under normal conditions. Alcohols can undergo dehydration to form alkenes in the presence of acid. The mechanism involves the formation of a carbocation intermediate. A common misconception is that all alcohols are readily oxidized.

Note 8: Ethers: Structure, Nomenclature, Properties, and Reactions Ethers contain an oxygen atom bonded to two alkyl or aryl groups (R-O-R'). Nomenclature involves naming the two alkyl/aryl groups attached to the oxygen atom followed by "ether". Ethers are relatively inert and are often used as solvents. They are slightly polar due to the C-O bonds, but cannot form hydrogen bonds with themselves. Ethers can form hydrogen bonds with water, making them more soluble in water than alkanes of similar molecular weight. Ethers can undergo cleavage under strongly acidic conditions. Epoxides are cyclic ethers and are much more reactive than acyclic ethers due to ring strain. Epoxides can be opened by nucleophiles under acidic or basic conditions. Williamson ether synthesis involves the reaction of an alkoxide with a primary alkyl halide via an SN2 mechanism. A common misconception is that ethers are completely unreactive.

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