A
homologous series is a group of organic compounds with similar properties
due to their molecule structure that differs in the number of CH2-groups:
The hydrocarbons methane methane
(H-(CH2)-H) , ethane
(H-(CH2)2-H),
propane
(H-(CH2)3-H)
and butane
(H-(CH2)4-H)
and many other hydrocarbons are members of the
homologous series of alkanes. The
similarity of their molecule structures is related to similar properties of
alkanes. Increase in the length of the molecule chains
causes increase of densities, boiling and melting points.
For
chemists "alcohol" is ethanol
(H-(CH2)2-OH).
Together with methanol
(H-(CH2)-OH,
propanol
(H-(CH2)3-OH)
and butanol
(H-(CH2)4-OH)
they are four members of the homologous series of alkanols.
In the following experiment you
will compare viscosity (fluidity) of four alkanols.
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org15
Homologous
series of alkanols: Run out speed
Material
Tray, 4 insulin syringes
with integrated needles, 4 high ampoules in a stand (Vacutainer package),
scissors, 4 yellow pipette tips, 4 tooth sticks, 4 dropper bottles with methanol,
ethanol, 1-propanol and 1-butanol.
Experiment
1. Cut off the tip of
the needles.
2. Use the toothsticks to fix the syringe cylinders inside the top of the
ampoules (Fig.
1).
3. Fix the pipette tips on the narrow openings of the dropper bottles with
the 4 alkanols ((Fig.
2 ).
4. Start at the cylinder for 1-butanol to add 1.0 mL samples of the alkanols.
5. Wait until the methanol has completely dropped into the ampoule
((Fig.
2 ).
Observation
(Fig.
2 ).
a) 1 mL (40 units) of methanol
have left the syringe cylinder after about 30 minutes (Fig.
2.1 ).
b) The runout speed of ethanol, 1-propanol and 1-butanol decreases in this
order:
. . Ethanol 40 - 13
units (0.67 mL), 1-propanol 40
- 18
units (0.54 mL), 1-butanol
40 - 23
units (0.42 mL).
Explanation
Methanol has a hydrocarbon rest
CH3-,
ethanol has a hydrocarbon rest CH3-CH2-,
propanol has a hydrocarbon rest CH3-CH2-CH2-,
butanol has a hydrocarbon rest CH3-CH2-,CH2-CH2-
The longer the hydrocarbon rest
is the stronger are the forces acting between the alcohol molecules (cohesion)
and between the molecules of the syringes and those of the alcohols (adhesion).