Video Transcript
This is a three-part question. Part (a) is the boiling point of
2-methylpentane lower or higher than the boiling point of hexane? Part (b) is the boiling point of
hexane lower or higher than the boiling point of 2,3-dimethylbutane? Part (c) which of the following
correctly explains this difference in boiling points? (A) Straight-chain alkanes can pack
more tightly than branched-chain alkanes, and this increases the boiling point due
to greater intermolecular forces. Or (B) branched-chain alkanes can
pack more tightly than straight-chain alkanes, and this increases the boiling point
due to greater inermolecular forces
We are asked to compare the boiling
point of two substances, 2-methylpentane and hexane. We do not know the boiling point
values of these two compounds. But since we are given the two
compound names, we can start by drawing their structures. 2-Methylpentane has five carbons in
the base chain, and all the bonds between these carbons are fully saturated or
single bonds. And there is a methyl group on
carbon number two. So if we number our carbons one,
two, three, four, and five, we can place our methyl group on the second carbon and
then fill in the rest of the hydrogens. This is the structural formula of
2-methylpentane. And its molecular formula is C6,
because there are six carbons, H14, for the 14 hydrogen atoms. It is a branched-chain alkane, with
the methyl group forming the branch.
Now let’s investigate hexane in a
bit more depth. Hexane is a six-carbon chain, and
the bonds between the carbons are fully saturated or single bonds. Here are the six carbons. There are no other side groups or
functional groups, so we can fill in all the hydrogens. We know that carbon forms four
bonds, and because all the carbon-carbon bonds are saturated, we can fill in the
hydrogens until each carbon has a total of four bonds. And this gives us a molecular
formula for hexane of C6H14. Hexane is a straight-chain alkane
because it has no side branches.
Notice that 2-methylpentane and
hexane have the same molecular formula. They both have six carbon atoms and
14 hydrogen atoms. However, their structures are
different. We say they have different
structural formulas. Compounds with the same molecular
formula but different structural formulas are called isomers. These two compounds are structural
isomers. They are molecules or compounds
with the same molecular formula but a different bonding arrangement of atoms.
Let’s simplify their
structures. Drawing organic structures in a
simplified form like this will help us compare or understand their relative boiling
points. Since we are asked about the
boiling points, we can assume that these two substances are in the liquid phase. Liquid molecules have a fair amount
of motion and movement, and single bonds can rotate. Long chains can also bend. However, hexane molecules are often
likely to be packed together like this. Although this is a simplistic
diagram, we can see that because hexane is a straight-chain alkane molecule, the
molecules can pack relatively closely, even though they have movement, because
they’re in the liquid phase.
The Van der Waals forces of
attraction between these nonpolar molecules, in this case London dispersion forces,
is relatively high because the molecules are close together. However, in liquid 2-methylpentane,
the molecules cannot pack as closely together because of their branched
structure. And as a result, the Van der Waals
forces of attraction between these nonpolar molecules is relatively weaker.
The close packing in hexane,
because of its structure and the associated stronger Van der Waals forces of
attraction, should therefore result in a relatively high boiling point. Because more energy is required to
overcome these forces of attraction and separate the molecules during boiling. But because of the looser packing
in 2-methylpentane due to its branch structure and its relatively weaker Van der
Waals forces of attraction between molecules, we can deduce that there is a
relatively low boiling point. Because less energy would be
required to separate the molecules during boiling and overcome the Van der Waals
forces of attraction.
So we can conclude that the boiling
point of 2-methylpentane is probably lower than the boiling point of hexane. And in reality, this is the
case. Hexane’s boiling point is 69
degrees Celsius and its structural isomer 2-methylpentane has a boiling point of 60
degrees Celsius.
Part (b) is the boiling point of
hexane lower or higher than the boiling point of 2,3-dimethylbutane?
Again, we are asked to compare
boiling point, this time between hexane and 2,3-dimethylbutane. The structural formulas of these
compounds are drawn here. Hexane is a six-carbon chain. It is a straight-chain alkane. 2,3-Dimethylbutane has four carbons
in its base chain, with 2 methyl groups, one on carbon number two and one on carbon
number three. It is a branched-chain alkane with
two branches. Again, these two compounds are
structural isomers of each other. They both have six carbon atoms and
14 hydrogen atoms. But the atoms are arranged in a
different way. The bonding is different. We say they are chain isomers of
each other because although they have the same molecular formula, their chain
arrangement is different.
Let’s simplify their structures and
have a look at how they’d pack in a liquid phase. Remember, there is constant
movement. But hexane’s molecules, because of
their straight-chain nature, will arrange themselves closer to each other. And as a result, the intermolecular
Van der Waals forces are stronger. Molecules of 2,3-dimethylbutane
pack or arrange themselves less closely. And therefore, their intermolecular
forces of attraction are weaker. More energy is required to overcome
the attractive forces between hexane molecules. And so, hexane will have a higher
boiling point. And their boiling points are for
hexane 69 degrees Celsius and its isomer 2,3-dimethylbutane, 58 degrees Celsius.
Part (c) which of the following
correctly explains this difference in boiling points? (A) Straight-chain alkanes can pack
more tightly than branched-chain alkanes, and this increases the boiling point due
to greater intermolecular forces. Or (B) branched-chain alkanes can
pack more tightly than straight-chain alkanes, and this increases the boiling point
due to greater inermolecular forces.
We have already seen in two
examples that straight-chain alkanes can get closer together than branched-chain
alkanes, even when they have the same number of carbon and hydrogen atoms, in other
words when they are isomers of each other. The closer the molecules can get,
the greater the intermolecular forces of attraction and the higher the boiling
point.
