Chapter 3

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 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

• Tetrahedral • sp3 hybrid carbon with angles of 109.5 degrees

• Two sp3 hybrid carbons • Rotation about the C-C sigma bond

Conformations are different arrangements of atoms caused by rotation about a single bond

Pure confomers cannot be isolated in most cases, because the molecules are constantly rotating through all the possible conformations

The torsional energy of ethane is lowest in the staggered conformation

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.

• 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

• 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).

• 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 arises when all the bonds are eclipsed

• Ring strain reflects contributions of angle strain and torsional strain.