Alkanes and Cycloalkanes

Nanoplasmonic Research Group

Organic Chem-istry

Chapter 2

Fully-saturated hydrocarbons: Alkanes & Cycloalkanes

Nanoplasmonic Research Group

How to READ their structures: NOMENCLATURE

Physical Properties: Interaction & Conformation

Chemical Properties: Reactions

How to READ them ?• Systematic methods for naming compounds (IUPAC)• RULES (see page 44 middle ~ page 45)

1. Find out the longest continuous chain

2. Number the substituents

3. Write the name as one word

If 2 different chains of equal length are present, choose the one with the greater number of branch points

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

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 (Kee-

som)– Permanent dipole-induced dipole forces (Debye)– Induced dipole-induced dipole forces (London)

Conformations of alkanes• A consequence of rotating one car-

bon 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 en-ergy

• 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 pos-

sible)– 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