Organic Chemistry Unit 3 ReviewAlkanes: Structure and Stereochemistry

Key Concepts and Definitions

• Alkanes are saturated hydrocarbons consisting of only carbon and hydrogen atoms connected by single bonds
• Homologous series describes a group of compounds with similar structures and properties that differ by a constant unit (CH2 for alkanes)
• Constitutional isomers have the same molecular formula but different connectivity of atoms
• Stereoisomers have the same connectivity but different spatial arrangements of atoms
  • Enantiomers are non-superimposable mirror images of each other
  • Diastereomers are stereoisomers that are not mirror images
• Conformations are different spatial arrangements of atoms that can interconvert by rotation around single bonds (conformers)
• Newman projections represent the view along a carbon-carbon bond with the front carbon represented as a dot and the back carbon as a circle
• Sawhorse projections depict molecules as if they were bent 90° in the middle, providing a side-view perspective

Alkane Structure and Bonding

• Alkanes have a tetrahedral geometry around each carbon atom with bond angles of approximately 109.5°
• Carbon atoms in alkanes are sp3 hybridized, meaning they have four equivalent sp3 hybrid orbitals that form single bonds with other atoms
• The C-C and C-H bonds in alkanes are nonpolar covalent bonds due to the small difference in electronegativity between carbon and hydrogen
• Alkanes can form straight-chain, branched, or cyclic structures
  • Straight-chain alkanes have all carbon atoms connected in a single continuous chain (n-pentane)
  • Branched alkanes have one or more alkyl groups attached to the main chain (isopentane)
  • Cycloalkanes have carbon atoms connected in a ring structure (cyclohexane)
• Alkyl groups are derived from alkanes by removing one hydrogen atom and are named by replacing the -ane suffix with -yl (methyl, ethyl)

Nomenclature of Alkanes

• IUPAC nomenclature is a systematic method for naming organic compounds based on their structure
• The longest continuous chain of carbon atoms determines the base name of the alkane (meth-, eth-, prop-, but-, pent-, hex-, hept-, oct-, non-, dec-)
• Substituents are named and numbered based on their position along the main chain, with the lowest possible numbers assigned
  • Alkyl groups are named by replacing the -ane suffix of the parent alkane with -yl (methyl, ethyl, propyl)
  • Multiple identical substituents are indicated using prefixes (di-, tri-, tetra-) and separated by commas (2,3-dimethylpentane)
• The prefix "n-" is used to denote a straight-chain alkane when necessary to distinguish it from branched isomers (n-butane vs. isobutane)
• Cycloalkanes are named by adding the prefix "cyclo-" to the corresponding alkane name (cyclopentane, cyclohexane)
• Bicyclic and polycyclic alkanes are named using a combination of prefixes and numbers to indicate the number of rings and bridges (bicyclo[2.2.1]heptane)

Conformations of Alkanes

• Conformations are different spatial arrangements of atoms that can interconvert by rotation around single bonds
• The energy barrier for rotation around C-C single bonds is relatively low, allowing for rapid interconversion between conformations at room temperature
• Staggered conformations have the maximum separation between substituents on adjacent carbon atoms and are more stable than eclipsed conformations
  • Anti conformations have substituents 180° apart and are the most stable (anti-butane)
  • Gauche conformations have substituents 60° apart and are slightly less stable than anti conformations (gauche-butane)
• Eclipsed conformations have substituents on adjacent carbon atoms aligned with each other and are less stable due to steric hindrance and electron repulsion
• The energy difference between staggered and eclipsed conformations is called the torsional barrier or rotational barrier
• Conformational analysis is the study of the relative stabilities and interconversion of different conformations

Stereochemistry and Isomerism

• Stereochemistry is the study of the three-dimensional arrangement of atoms in molecules and its effect on properties and reactivity
• Chirality is the property of a molecule being non-superimposable on its mirror image
  • Chiral molecules have at least one stereocenter (carbon atom bonded to four different groups)
  • Enantiomers are non-superimposable mirror images of each other and have opposite configurations at all stereocenters
• Optical activity is the ability of a chiral substance to rotate plane-polarized light
  • Enantiomers rotate plane-polarized light in opposite directions but have equal magnitudes of rotation
• Diastereomers are stereoisomers that are not mirror images and have different physical properties
  • Diastereomers can have different configurations at one or more stereocenters but not all of them
• Meso compounds have multiple stereocenters but are achiral due to an internal plane of symmetry (meso-2,3-dibromobutane)
• Cis-trans isomerism occurs in cycloalkanes and alkenes when substituents can be on the same side (cis) or opposite sides (trans) of the ring or double bond

Physical Properties of Alkanes

• Alkanes are nonpolar and hydrophobic due to the absence of polar bonds and the symmetric distribution of charge
• The melting points and boiling points of alkanes increase with increasing molecular weight and chain length
  • Straight-chain alkanes have higher melting and boiling points than branched isomers due to more efficient packing and stronger intermolecular forces
• Alkanes are less dense than water and are insoluble in water but soluble in organic solvents
• The vapor pressure of alkanes decreases with increasing molecular weight and chain length
• Alkanes have low reactivity due to the strength and stability of C-C and C-H single bonds
  • Higher alkanes (> C16) are essentially unreactive at room temperature
• The heat of combustion of alkanes increases linearly with the number of carbon atoms, making them excellent fuels

Chemical Reactions of Alkanes

• Combustion is the most common reaction of alkanes, producing carbon dioxide and water when burned in excess oxygen
  • Complete combustion: CnH2n+2 + (3n+1)/2 O2 -> n CO2 + (n+1) H2O
• Halogenation is the substitution of hydrogen atoms with halogen atoms (F, Cl, Br, I) via a radical mechanism
  • Chlorination and bromination occur more readily than fluorination and iodination
  • Selectivity decreases in the order tertiary > secondary > primary due to the stability of the corresponding alkyl radicals
• Pyrolysis is the thermal decomposition of alkanes at high temperatures (> 500°C) in the absence of oxygen, producing a mixture of smaller alkanes and alkenes
• Catalytic cracking is the breaking of C-C bonds in larger alkanes to produce smaller alkanes and alkenes using a catalyst (zeolites, aluminosilicates)
  • Catalytic cracking is used in the petroleum industry to convert heavy fractions into more valuable lighter products (gasoline, diesel)
• Isomerization is the rearrangement of atoms within a molecule to form a structural or stereoisomer
  • Branched alkanes can be converted to straight-chain alkanes and vice versa using a catalyst (platinum on alumina)

Applications and Real-World Examples

• Fossil fuels (natural gas, petroleum, coal) are primarily composed of alkanes and are the main source of energy for transportation and electricity generation
• Gasoline is a mixture of C5-C12 alkanes and is used as a fuel for internal combustion engines
• Diesel fuel is a mixture of C12-C18 alkanes and is used in diesel engines, which have higher efficiency than gasoline engines
• Kerosene is a mixture of C10-C16 alkanes and is used as a fuel for jet engines and as a starting material for the production of other chemicals
• Liquefied petroleum gas (LPG) is a mixture of propane and butane and is used as a fuel for cooking and heating
• Paraffin wax is a mixture of high molecular weight alkanes (C20-C40) and is used in candles, polishes, and coatings
• Polyethylene is a polymer of ethylene (C2H4) and is the most widely used plastic in the world, with applications in packaging, containers, and insulation
• Cycloalkanes are found in petroleum and are used as solvents and starting materials for the production of other chemicals (cyclohexane -> nylon, adipic acid)