Chapter 3 Flashcards
Boiling Point of Alkanes
As the number of carbons in an alkane increases, the boiling point will increase due to the larger surface area and the increased Van der Waal attractions
Melting points of Alkanes
- Melting points increase as the carbon chain increases
- Alkanes with an even number of carbons have higher melting points than those with an odd number of carbons
- Branched alkanes have higher melting points than unbranched alkanes
Methane Representation
- Tetrahedral
- sp3 hybrid carbon with angles of 109.5 degrees
Ethane Representations
- Two sp3 hybrid carbons
- Rotation about the C-C sigma bond
Conformations definition
Conformations are different arrangements of atoms caused by rotation about a single bond
Conformations of Ethane
Pure confomers cannot be isolated in most cases, because the molecules are constantly rotating through all the possible conformations
ethane eclipsed conformation
ethane staggered conformation
Ethane Conformations Energy Diagram
The torsional energy of ethane is lowest in the staggered conformation
Propane Conformations
propane eclipsed conformation
propane staggered conformation
Propane conformation energy diagram
The staggered conformations of propane are lower in energy than the eclipsed conformations. Since the methyl group occupies more space than a hydrogen, there is a greater torsional strain for propane than for ethane.
butane totally eclipsed conformation
butane gauche (staggered) conformation
butane eclipsed conformation
butane anti (staggered) conformation
Steric Strain in Butane
- The totally eclipsed conformation is higher in energy because it forces the two end methyl groups so close together that their electron clouds experience a strong repulsion
- This kind of interference between two bulky groups is called steric strain
Butane conformation energy diagram
Geometric Isomers
Torsional Strain
- Torsional strain reflects barrier to rotating about the C-C single bond.
- For butane, high torsional energy for the totally eclipsed conformer is due to steric strain (two groups trying to occupy the same space).
Angle Strain in Cycloalkanes
- When a cycloalkane has an angle other than 109.5º, there will not be optimum overlap and the compound will have angle strain.
- This is because all the C atoms are sp3 hybridized
Torsional Strain in Cycloalkanes
- Torsional strain arises when all the bonds are eclipsed
Ring Strain in Cycloalkanes
- Ring strain reflects contributions of angle strain and torsional strain.
Cyclopropane
- The bond angles are compressed to 60° from the usual 109.5° bond angle of sp3 hybridized carbon atoms
- This severe angle strain leads to nonlinear overlap of the sp3 orbitals and “bent bonds” (sometimes called banana bonds)
- Leads to interesting reactivity.
Torsional strain in cyclopropane
- All the C—C bonds are eclipsed, generating torsional strain that contributes to the total ring strain.
Planar Cyclobutane
In a planar conformation:
- Angle strain from the compressing of the bond angles to 90°
- Torsional strain from eclipsing of the bonds
Non-planar Cyclobutane
- Cyclic compound with four carbons or more adopt non-planar conformations to relieve torsional strain.
- Cyclobutane adopts the folded conformation (“envelope”) to decrease the torsional strain caused by eclipsing hydrogens.
Cyclopentane
- The conformation of cyclopentane is slightly folded, like the shape of an envelope. This puckered conformation reduces the eclipsing of adjacent methylene (CH2) groups.
Cyclohexane Chair Conformation
Cyclohexane Boat Conformation
Conformational Energy Diagram of Cyclohexane
Axial and Equitorial Positions
Chair-Chair Interconversion
- Chair conversion results in switching all the axial substituents into the equatorial positions, and vice versa.
Equitorial Methyl Group
Axial Methyl in Methylcyclohexane
Tert-butylcyclohexane
- Very large substituents (like t-butyl) will ONLY occupy equatorial positions
- Chair will be locked in this conformation
Cis-1,3-dimethylcyclohexane
- Cis-1,3-dimethylcyclohexane can have both methyl groups in axial positions or both in equatorial positions.
- The conformation with both methyl groups equatorial is more stable.
Tert-butylcyclohexane
- Very large substituents (like t-butyl) will ONLY occupy equatorial positions
- Chair will be locked in this conformation
Cis-1,4-ditertbutylcyclohexane
The most stable conformation of cis-1,4-di-tertbutylcyclohexane is the twist boat. Both chair conformations require one of the bulky t-butyl groups to occupy an axial position.
cis - 1,2-dimethylhexane positions
trans - 1,2-dimethylhexane positions
cis - 1,3-dimethylhexane positions
trans - 1,3-dimethylhexane positions
cis - 1,4-dimethylhexane positions
trans - 1,4-dimethylhexane positions
