Chem 40 Ch2

• Alkanes (only C-C and C-H s bonds).

  1) Acyclic alkanes (saturated hydrocarbons): CnH2n+2

  linear and branched chains.

  2) Cyclic, Cycloalkanes: one or more rings. CnH2n

  
• Substituents: Alkyl Groups (-R)

  • Carbon substituents bonded to the main chain.

  • Naming an alkyl group: –ane to –yl.

  methane (CH4) to methyl (CH3-)

  ethane (CH3CH3) to ethyl (CH3CH2-).
• Physical properties of alkanes include low density, solubility in non-polar solvents, and phase changes based on carbon atom numbers
• The relationship between the strength of intermolecular forces and the boiling point and melting point of alkanes:

  • Alkanes are non-polar molecules with weak van der Waals forces
  • Boiling point and melting point increase as the number of carbon atoms in the molecule increases

  
• Radical Electron Movement

  • classified as either initiation, termination, or propagation

  1. Initiation occurs when radicals are created.

  2. Propagation occurs when radicals are moved from

  one location to another.

  3. Termination occurs when radicals are destroyed.
• Chlorination of methane involves a chain reaction with self-sustaining propagation steps
• Classification of Carbon and Hydrogens

  • Primary (1o) C & H

  • Secondary (2o) C & H

  • Tertiary (3o) C & H

  • Quaternary (4o) C & H

  
• Naming Alkanes

  • The suffix -ane : alkanes

  
• Naming Cycloalkanes

  • The prefix cyclo: cycloalkane

  
• Nomenclature

  1. Common name: traditionally given long ago
  2. Systematic name: IUPAC system
  3. Others
  4. Generic name: officially approved name for drug
  5. Trade name: assigned by company or manufacturer
• IUPAC Nomenclature

  1. The parent name: is the longest carbon chain (main chain).
  2. The suffix: functional group.
  3. The prefix: the identity, location, and number of substituents.

  
• IUPAC Nomenclature

  1.Find the parent chain

  -the longest consecutive chain of carbons

  -If there is more than one possible parent chain, choose the one with the most substituents attached

  -If the parent chain is cyclic, add the prefix, “cyclo” to the parent name.
• IUPAC Nomenclature

  2.Identify substituents

  -count the number of carbons in each side group.

  -substituent names end in -yl instead of -ane.

  -name the parent and substituents above

  -a ring can be either a parent chain or a substituent depending on the number of carbons

  STEPS:

  1. Number the longest carbon chain WITHIN the substituent. Start with the carbon directly attached to the main chain.
  2. Name the substituent
  3. Name and number the substituent's side group
• In IUPAC nomenclature, the number or locant is used to locate where each substituent is attached to the parent chain
• Number the parent chain so that a substituent has the lowest locant possible
• A prefix (di, tri, tetra, penta, etc) is used if multiple substituents are identical
• Tie breaker in IUPAC nomenclature: assign the lowest number alphabetically
• Locants are assigned to each substituent and listed before the parent chain name in alphabetical order
• Prefixes are NOT used for alphabetical purposes in IUPAC nomenclature
• Only prefixes “iso, neo, and cyclo” are used for alphabetical purposes
• Alkanes: only van der Walls force

  1. Phase: C1 ~ C4 : gas C5 ~ C18 : liquid C19 ~ : solid
  2. Boiling point: cycloalkane > n-alkane > branched(packing effect & surface area)
  3. Melting point: C2n > C2n+1 (symmetry à packing effect)
  4. Density & solubility:• has the lowest density in all organic compounds• soluble in non-polar solvents

  
• Conformations of Acyclic Alkanes

  1. Conformations: different spatial arrangement(sigma bond rotation), interconvertible.
  2. Configuration: different bond connectivity.

  
• Eclipsed vs Staggered

  1. Eclipsed conformation: the C-H bonds are aligned.
  2. Staggered conformation: the C-H bonds staggered

  
• Conformations and Dihedral Angle

  • Rotation of a bond by 60° : eclipsed conformation same with staggered conformation
  • For ethane: staggered conformation: dihedral angle of 60° eclipsed conformation: dihedral angle of 0°

  
• Newman projection: End-on representations for conformations.

  
• Conformations of Butane

  
• Anti and Gauche Conformations

  1. Anti: A staggered conformation with two larger groups 180°.
  2. Gauche: A staggered conformation with two larger groups 60° Higher in energy due to steric strain

  
• Conformational Strains in Alkane

  1. Torsional strain: instability due to the eclipsed conformation
  2. Steric strain: Repulsive van der Waals force(too closeness of groups)

  
• Conformational Strains in Alkane

  
• Barrier to Rotation - The energy difference between the lowest and highest energy conformations is called barrier to rotation.

  
• Ring Strains in Cycloalkanes (C3-C10)

  1. Torsional strain
  2. Steric strain
  3. Angle strain : an increase in energy when tetrahedral bond angles deviate from the optimum angle of 109.5°

  Cycloalkanes with > three C atoms in the ring are not flat molecules. They are puckered to reduce strain

  
• Free radicals form when bonds break homolytically.

  Free radicals do not have a formal charge but are unstable because of an incomplete octet

  Free radicals are stabilized by hyperconjugation

  
• Radical Electron Movement

  • Chain reaction: self-sustaining propagation steps

  • Chain reaction: the products from one step are reactants for a different step in the mechanism

  
• Halogenation Thermodynamics

  
• Halogenation Regioselectivity

  Bromination at the 3° position happens 1600 times more often than at the 1° position

  
• Relative Proportions of Products

  Based on radical stability, isomeric products will result in halogenation.

  
• Percent Yield of each product.

  
• Cycloalkane Reactivity