How is the boiling point of a molecule related to its structure?
(Adapted from: Thomas Greenbowe, Iowa State University)
To determine the boiling points of organic liquids using microscale distillation.
To discover relationships between boiling point and the structure of organic compounds.
Microscale distillation Boiling Points
See MSDS for hydrocarbons used.
The boiling point (bp) is an important physical property of a substance and can be used to help identify it or, if known, offer information about its purity. Pure substances have a narrow boiling point range while mixtures may show multiple or broad ranged boiling temperatures. A number of definitions for boiling point appear in texts but offer little information about the underlying causes for the differences in bp between one substance and another. One goal of this experiment is to uncover some of the factors that determine the temperature at which an organic compound boils.
In this experiment the boiling points of various organic compounds are to be determined and then compared with their structures to discover trends. The boiling points of similar organic functional groups are to be graphically represented and trends evaluated and explanations proposed. The goal is to answer the question:
Each member of the class will be assigned a simple liquid organic compound. The boiling point of the assigned compound will be determined by the process of simple distillation, a frequently used and important laboratory technique for isolation and purification of organic liquids.
Simple distillation using Williamson microscale glassware will allow distillation
Williamson Microscale Simple Distillation of small quantities of compounds thus reducing waste and minimizing potentially odorous and flammable vapors. Heating mantels or sand baths will eliminate the need for flames and so reduce the potential for ignition of flammable organic compounds.
Each group of students will be assigned a collection of organic compounds of a particular organic functional group to investigate.
1.
For your assigned group of compounds: a.
Provide molecular formulas, structures, and molar masses (molecular weights MW). b.
Propose a hypothesis (or hypotheses) which provides the basis for your prediction to answer the question: How is the boiling point of a molecule related to its structure?
2.
Determine the boiling point of your sample according to the procedures for microscale distillation using Williamson apparatus in your lab techniques text. Record your results. a.
Distill about 2-3 mLs of your assigned liquid. Remember boiling stones. b.
Use a heating mantle/ sand bath as your heat source. c.
Use an ice bath to cool the distillate.
1
CH241 Lab 1 Boiling Points of Liquids(F14)

d.
As you distill record the temperature often at regular intervals. e.
Determine the boiling point plateau of your compound (graph if necessary). The temperature at which a graphical representation of boiling temperatures plateaus is the boiling point (bp) f.
Share your data with the rest of the class. g.
Place your freshly distilled sample in a clean glass vial to be handed in.
3.
Arrange the bp data from your group in the order of increasing bp. How well do the boiling points map to the hypothesis your group developed for the relationship between molecular structure and bp? Propose an explanation to answer the question: How is the boiling point of a molecule related to its structure?
How do you explain boiling trends within a particular functional group?
4.
Pool data from the entire class. Create a plot of bp versus MW from data collected for each functional group investigated by the class.
5.
Examine and compare the class data to answer the question: How is the boiling point of a molecule related to its structure? comparing one functional group to another.
How do you explain boiling point trends for various functional groups of a particular molecular weight?
6.
Summarize your findings and propose conclusions supported by data. Evaluate any applicable factors such as techniques used, experimental errors, safety, waste considerations etc………
7.
Attach or rewrite and then answer any post lab questions at the end of your notebook experimental write up.
(Complete the missing information)
Group 1
Compound n-Pentane
Formula Structure
C
5
H
12
M.W.
72
B.P.
Hexane
Heptane
Octane
Decane C
10
H
22
H
H
C
H
H H H H H H H
C C C C C C C
H H H H H H H
H
114
142
Group 2
Compound
1-Propanol
Formula Structure
C
3
H
8
O
M.W. B.P.
1-Butanol
1-Pentanol
1-Hexanol
1-Octanol
H
H
C
H
H H
C C
H H
H O H
C C H
H H
74
102
130
2
CH241 Lab 1 Boiling Points of Liquids(F14)

Group 3
Compound
1-Bromopropane
Formula Structure
1-Bromobutane
1-Bromopentane
1-Bromohexane
C
4
H
9
Br
H
H
C
H
H H
C C
H H
H Br
C C H
H H
Group 4
Compound
2-Propanone
(Acetone)
2-Butanone
Formula Structure
C
3
H
6
O H O H
H C C C
H H
H
2-Pentanone
2-Heptanone C
7
H
14
O
Group 5
Compound
2-propanol
2-butanol
2-pentanol
2-hexanol
Formula Structure
C
3
H
8
O
H
H
C
H
O
H
H H
C C C
H H H
H
C
6
H
14
O
CH241 Lab 1 Boiling Points of Liquids(F14)
M.W.
151
165
B.P.
M.W. B.P.
M.W.
60
102
B.P.
3

100
80
60
40
20
0
180
160
140
120
0 20 40 60 80 100 120 140 160 180
1.
Using the concepts developed by this experiment answer the following:
A.
1-butanol and diethyl ether are structural isomers of the formula C
4
H
10
O but differ markedly in bp.
Formulate an explanation that clarifies why their boiling points are so different.
H H H H H H H H
Alkanes
1 o
Alcohols
2 o
Alcohols
1 o
Halides
2 o
Halides
Ketones
H C C O C C H H C C C C O H
H H H H H H H H
Diethyl ether b.p. 35 o
C
1-butanol b.p. 118 o
C
B.
The four alcohols shown in the table below have the same MW and molecular formula but differ in bp. Draw the structures of each and propose a reasonable explanation that justifies this bp trend.
1-butanol
2-butanol
2-methyl-1-propanol
2-methy-2-propanol
74
74
74
74
118
100
108
83
CH241 Lab 1 Boiling Points of Liquids(F14)
4

1.
List any health and safety concerns to note when working with hydrocarbons.
2.
A. Draw structures for the following compounds showing each bond:
Propane 1-Bromopropane 1-Propanol Propanal
Rank ____ Rank ____ Rank ____ Rank ____
B. On the above structures use arrows to predict the direction of polarity and

+ and

- to indicate polar atoms in any polar covalent bonds.
C. Using your best intuition; Rank the above compounds according to decreasing overall polarity (#1 most polar

#4 least polar). Explain why you chose the order you did.
3.
Draw the Williamson Apparatus for doing a microscale distillation. Show proper connections and placement of clamps.
4.
A student, Boy Ling, carried out a simple distillation on a compound known to boil at 124 o
C and reported an observed boiling point of 116-117 o
C. Analysis showed that the compound was pure and the thermometer was accurate. a.
What procedural error might the student have made in setting up the distillation apparatus? b.
What should he do to correct the situation?
5
CH241 Lab 1 Boiling Points of Liquids(F14)