What is the effect of steric hindrance on the boiling points of aldehydes and ketones?

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
 
Branches
name
B.P
0
3-heptanone
147
1
3-methyl-2-hexanone
137
1
4-methyl-2-hexanone
139
1
5-methyl-2-hexanone
144
2
3,3-dimethyl-2-pentanone
131
2
3,4-dimethyl-2-pentanone
135
2
4,4-dimethyl-2-pentanone
126

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:
          Beilsteinís 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.

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