Organic Chemistry
Chapter 2
Alkanes and Cycloalkanes
Nanoplasmonic Research Group
Fully-saturated hydrocarbons:
Alkanes & Cycloalkanes
How to READ their structures: NOMENCLATURE
Physical Properties:
Interaction & Conformation
Chemical Properties: Reactions
Nanoplasmonic Research Group
How to READ them ?
• Systematic methods for naming compounds (IUPAC)
• RULES (see page 44 middle ~ page 45)
1.
Find out the longest continuous chain
If 2 different chains of equal length are present, choose the one with
the greater number of branch points
2. Number the substituents
If the first branch points occurs at the same carbon number on both
ends, begins at the end that has the second nearest branch pints
3. Write the name as one word
Put the substituents in alpabetical order: di-, tri-, tetra-, sec-, tert-,
ignored when alphabetizing, iso-, neo- are included when
alphabetizing
How to read substituents ?
• Methyl, ethyl, propyl, etc
• Iso, sec-, tert-, etc
• Fluoro, chloro, bromo, iodo
Physical Properties: Intermolecular
Interaction
• Nonpolar due to the fact that C-C & C-H bonds
are nearly purely covalent (no dipole moment)
• Interaction between alkanes
– Induced dipole-induced dipole moment
– Van der Waals attraction
– M.W. dependence
• Van der Waals interaction
– Permanent dipole-permanent dipole forces (Keesom)
– Permanent dipole-induced dipole forces (Debye)
– Induced dipole-induced dipole forces (London)
Conformations of alkanes
• A consequence of rotating one carbon
atom with respect to the other carbon
atom
Conformational Energy of Ethane
Conformational Analysis Summary
• Torsional energy
– Higher energy associated with eclipsed conformation
• Torsional strain
– Resistantce to rotating to an eclipsed conformation
• Steric strain
– Repulsive interactions that occurs when atoms are forced
closed together than their atomic radii allow
• Gauche: spatial relationship with a 60 torsion angle
• Interactions
–
–
–
–
H-H eclipsing (torsional strain): 1.0 kcal/mol
H-Me eclipsing (mostly torsional strain) 1.4 kcal/mol
Me-Me eclipsing (steric and torsional strain) 2.6 kcal/mol
Me-Me gauche interaction (steric strain) 0.9 kcal/mol
Why is staggered form lower in
energy
• Hyperconjugation
– Stabilizing overlap between sigma bond and
antibonding orbitals that does not occur in
the eclipsed conformer
• Electron-electron repulsion
Conformational Energy of Butane
Naming Cyclohexane
• Find parent (ring or chain, depending on
which is larger)
• Label point of attachment of alkyl, halo, etc
• Continue numbering so that the second
substituent is the lowest possible number
• If 2 or more groups could potentially get
the same number, use alphabetical order as
a tie-breaker
Cyclohexane
Disubstituted Cyclohexane (I)
Disubstituted Cyclohexane (II)
• If 2 substituents are on cyclohexane the
lowest energy conformation
– Has both substituents equatorial (if possible)
– t-Bu is NEVER axial
• Cis-Trans Isomerism
– Do not interconvert each other (see page 59
bottom)
The relationships of the various
types of isomers