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Experiment 2- Functional Groups
Introduction
The properties and reactivity of organic compounds are determined largely by the
functional groups which they contain. A functional group is an arrangement of atoms,
often including atoms other than carbon within a molecule, which lend certain predictable
properties to the molecule. The functional group(s) present determine the class of
compound. For instance the hydroxy functional group (-OH) will be present in the class
of compounds known as alcohols. One can usually generalize properties- for example,
molecules with a carboxylic acid functional group have high boiling points, are relatively
water soluble, have an unpleasant smell, and react with bases. An ester functional group
affords different properties- esters are usually insoluble in water, have lower boiling
points than carboxylic acids, and generally smell sweet or fruity.
One of the most readily predictable physical properties is boiling point. Three
major types of intermolecular forces contribute to higher boiling points. All compounds
possess London Forces. While relatively weak, the overall attraction due to London
Forces depends on the surface area of the molecule. Thus, all else being equal, large
molecules have higher boiling points than small molecules. Some compounds also have
an interaction termed hydrogen bonding which is present when an OH or NH group is
incorporated in the molecule. Hydrogen bonding is a fairly strong interaction and can
result in a high boiling point for even a small molecule, the classic example being water.
A weaker interaction is a dipolar attraction which predominates in polar molecules.
Water solubility is also fairly predictable by an analysis of a molecule's structure
and how well it can interact with the solvent molecules. Water is a polar, hydrogen
bonding solvent. Small polar compounds are soluble in water. Larger organic
compounds can also be soluble if they are capable of hydrogen bonding or exist as a salt.
Increasing the number of hydrogen bonding groups will increase the solubility. Ionic
compounds also tend to have high water solubility.
Laboratory Activities
A. Test the water solubility of several organic compounds and make generalizations
on the effect of structure on solubility
B. Identify the presence and effect of hydrogen bonding and size of molecule on
boiling point.
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Experiment 2
Laboratory Record Name:_______________________
Part A- - Test the water solubility of each of the following organic compounds.
Insoluble compounds may either sink, float or be suspended. Soluble compounds will
form a clear, homogenous solution (only one layer or phase).
Procedure:
Prepare 7 test tubes, each with 1 mL of water, and add each compound, one per test tube.
For liquid compounds add 3 drops from a pipette. For solid compounds add a small
amount using the tip of a scoopit or spatula (with the sample extending about 2 mm from
the tip). Swirl or stopper and shake to encourage dissolution, but avoid skin contact.
Record whether each substance was soluble (s) or insoluble (I). Don’t contaminate the
samples- use the proper pipette or spatula for each. Wash the test tubes when done.
physical state (solid,
compound structure liquid or gas) class of compound water solubil
O
sodium benzoate
C O Na
O
methyl benzoate C O CH3
H
benzyl alcohol C O H
H
CH3
2-propanol CH
H3 C OH
O
benzoic acid C OH
O
acetic acid C
H3 C OH
toluene CH3
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1. Observe the samples of acetic acid and benzoic acid. Which has the higher melting
point? Use your knowledge of intermolecular forces and account for this observation.
2. Account for the observation that even though methyl benzoate has a higher molecular
weight than benzoic acid, it has a lower melting point.
Use your solubility data to organize the compounds into groups based on water solubility.
Write each structure in the appropriate column based on its water solubility.
soluble in water insoluble in water
Answer each question below based on your observations.
1. Consider the soluble compounds. Are they able to hydrogen bond to water? Any
exceptions?
2. Compare soluble and insoluble compounds with identical functional groups (list all
compounds from above to which this applies). What are the differences between them
which might explain the difference in solubility?
3. Account for the difference in solubility between benzoic acid and sodium benzoate
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Part B- Each compound listed has a formula weight of approximately 60 g/mol.
Consider the functional group and indicate the class of compound for each entry. Indicate
using a checkmark whether that functional group is capable of hydrogen bonding to (a)
other molecules with the same functional group (b) water
Hydrogen Bonds to:
compound structure boiling class of cmpds with same water
point compound functional group
butane CH3CH2CH2CH3 0°C
2-butene CH3CH=CHCH3 1°C
1-propanol CH3CH2CH2OH 97°C
O
propanal 46°C
H3 C-H2 C C H
O
methyl 34°C
H C O-CH 3
formate
O
acetic acid 116°C
H3 C C OH
propylamine CH3CH2CH2NH2 48°C
O
acetamide 221°C
H3 C C NH2
methoxyethan CH3CH2-O-CH 3 11°C
e
Rank the boiling points of the following compounds by writing highest, middle, and
lowest under the structure. Like the compounds above, they have approximately equal
formula weights but different functional groups. The relative boiling points for each
functional group will behave the same as those in the table above.
O
CH2 CH2 OH H
C CH3
1. OH C
H
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O
H2 CH3
H3 C C H3 C O CH3
2. H3 C C C CH C
CH NH2 CH H2 H2
CH3 CH3
CH3
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Part C- Determine the relationship in properties for compounds with the same
functional group but different molecular weights.
Use the data in table 1 to graph the boiling point vs. the molecular weight for a series of
primary alcohols.
250
225
200
175
150
BOILING POINT (°C)
125
100
75
50
25
0
0 20 40 60 80 100 120 140 160
MOLECULAR WEIGHT
Answer the following questions:
1. According to your graph, what happens to the boiling point as the molecular weight
increases?
2. Based on your graph, predict the boiling point of 1-decanol, an alcohol with a
molecular weight of 158 g/mol.
3. Account for the observation that heptane (MW 100 g/mol) has a greater molecular
weight than 1-pentanol (MW 88 g/mol) yet a boiling point that is significantly less (98°C
for heptane and 136°C for 1-pentanol).
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Table 1
compound molecular weight boiling point
methanol 32 65 °C
ethanol 46 78 °C
propanol 60 97 °C
butanol 74 117 °C
pentanol 88 136 °C
hexanol 102 156 °C
heptanol 116 176 °C
octanol 130 196 °C
nonanol 144 215 °C