Introduction:
The purpose of this
project is to explore the effect of steric hindrance on the boiling points
of aldehydes and ketones. The carbonyl group of aldehydes and ketones possesses
a sizable dipole moment that is responsible for the enhanced boiling points
of these types of compounds. This project will attempt to determine
the effect of steric hindrance on the dipole-dipole interactions of the
carbonyl group. Since increased steric hindrance is associated with
branching, and since branching of a carbon skeleton is known to lower boiling
points, care will be needed to separate that effect from steric effects
on the carbonyl group.
Data Collection
and Analysis:
The boiling points
of common alkanes and their branched isomers were collected. The
following table summarizes the collected results. For simplicity and ease
of comparison, only the 2-methyl branched alkanes were compared.
|
Number of Carbons
|
N-alkane
|
Boiling point
|
Branched Alkane
|
Boiling point
|
|
1
|
methane
|
-164
|
|
|
|
2
|
ethane
|
-89
|
|
|
|
3
|
propane
|
-42
|
|
|
|
4
|
butane
|
0
|
isobutane
|
-11
|
|
5
|
pentane
|
36
|
2-methylbutane
|
27
|
|
6
|
hexane
|
69
|
2-methylpentane
|
60
|
|
7
|
heptane
|
98
|
2-methylhexane
|
90
|
|
8
|
octane
|
126
|
2-methylheptane
|
118
|
From the analysis,
branched alkanes shown in pink, have lower boiling points than the respective
straight chain alkanes. This is attributed to the increased surface
area of straight chain alkanes.When
temperatures begin to lower, straight chain alkanes can compact together
easier causing stronger London Dispersion forces between the molecules.This
leads to increased boiling points. Theoretically, the same should
happen when comparing straight chain and branched aldehydes and ketones.
The question will be, does the dipole of the functional group affect the
boiling points.Branching should
lower the boiling points but will the strong dipole-dipole attraction in
the carbonyl group inhibit this?
The only structural
difference between an aldehyde and ketone is the position of the C=O functional
group. The C=O has a similar dipole moment whether at the end of
a chain as in the aldehyde or in the middle of the chain as in the ketone
as shown in figure 1.
Figure 1
Therefore it is not
surprising when comparing the position of the C=O, the difference in boiling
points is relatively small. There is a slight lowering of boiling point
the more internal the functional group is. This is probably due to
increased stability and less polarization of the entire molecule when a
molecule becomes more symmetrical.
Boiling points of
C7H14O
|
Position of C=O
|
Name
|
Boiling Point
|
|
Carbon 1 (aldehyde)
|
heptanal
|
153
|
|
Carbon 2
|
2-heptanone
|
150
|
|
Carbon 3
|
3-heptanone
|
147
|
We would also suspect
that the same trend in boiling points that are seen with n-alkanes and
branched alkanes can be seen with aldehydes and ketones. For simplicity
purposes only a branching of a methyl group on the second carbon will be
compared.
Aldehydes
|
Number of Carbons
|
n-Aldehyde
|
Boiling Point
|
Branched Aldehyde(2-methyl)
|
Boiling Point
|
|
1
|
methanal
|
-21
|
|
|
|
2
|
ethanal
|
21
|
|
|
|
3
|
propanal
|
49
|
|
|
|
4
|
butanal
|
75
|
2-methylpropanal
|
61
|
|
5
|
pentanal
|
103
|
2-methylbutanal
|
92
|
|
6
|
hexanal
|
129
|
2-methylpentanal
|
119
|
|
7
|
heptanal
|
155
|
2-methylhexanal
|
?
|
|
8
|
octanal
|
171
|
2-methylheptanal
|
?
|
Ketones
|
Number of carbons
|
n-2-ketone
|
Boiling Point
|
branched 2-ketones
|
Boiling point
|
|
3
|
2-propanone
|
56
|
|
|
|
4
|
2-butanone
|
80
|
|
|
|
5
|
2-pentanone
|
102
|
3-methyl-2-butanone
|
95
|
|
6
|
2-hexanone
|
127
|
4-methy-2-pentanone
|
117
|
|
7
|
2-heptanone
|
151
|
5-methyl-2-hexanone
|
144
|
|
8
|
2-octanone
|
173
|
6-methyl-2-heptanone
|
173
|
As seen by the graph,
both branched aldehydes and ketones are lower in boiling points than their
straight-chained isomers.It can
also be noted again that there is minimal difference in the boiling points
of a straight-chained aldehyde and its respective straight chain ketone.This
gives further evidence to the argument position of the carboxyl affects
the boiling point minimally.
In order to look at
the effect of steric hindrance of the carboxyl group, we need to cancel
out the effect of branching.To do
this, we will compare branched alkanes, aldehydes and ketones with the
same number of carbons.
As seen, similarly
branched aldehydes and ketones have higher boiling points than their respective
branched alkanes.It can be noted
that the difference in boiling points gets less as the mass becomes larger.
This must mean the effect of the dipole on boiling point is more pronounced
in smaller molecules.
The position of the
branching also has an effect on steric hindrance.The
closer the methyl group is to the carboxyl, the more steric hindrance should
be.This should be reflected in a
lower boiling point. (For comparison, ketones will be used.Boiling
points of branched C7H14O aldehydes were difficult
to locate. Based on the data above, the results should be similar.)
C7H14O
Ketones
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
With one branched methyl
group, the closer the methyl is to the carboxyl the lower the boiling point
becomes.Something different seems
to be happening with dimethyl branching.It
seems symmetrical branching lowers the boiling point.However,
the lowest boiling points occur when the branching is farther from the
carboxyl group.
Conclusion:
Aldehydes and ketones
have a much higher boiling point than the alkanes. This is attributed to
the dipole moment of the carbonyl group. The carbonyl group not only adds
a dipole moment to the molecule, it also adds mass/surface area - which
will increases London forces. As the molecules get larger, the difference
between an aldehyde/ketone and its corresponding alkane gets smaller. The
reason for this is that the non-polar region of the carbon chain is getting
larger as the polar region (C=O) is staying the same. As a result, the
dipole effect becomes less significant as the hydrocarbon chain gets larger.It
seems from the data that increased steric hindrance by placing branching
closer to the carboxyl group will decrease boiling point as long as one
methyl branch is encountered.If
multiple branching occurs, symmetrical branching results in a lower boiling
point than unsymmetrical branching.However,
if the multiple branching occurs farther from the carboxyl, the boiling
point is lowered..
Bibliography:
Online sources:
Beilstein’s
Crossfire Data Base
Chem Finder
at www.chemfinder.com
Print sources:
Organic Chemistry:
A Short Course, 10th edition, by Hart, Craine, and Hart
Organic Chemistry,
4th edition by L. G. Wade, Jr.
Chemistry
of Organic Compounds, 3rd edition,
by C.R.Noller.