Org Chem II Laboratory (Majors)
The Properties of Alcohols
I.
Kelly
INTRODUCTION
Alcohols are an interesting and important group of organic compounds. They
are used in beveages, in medicines, as solvents and as synthetic intermediates. The
characteristic functional group is the hydroxyl group (-OH) attached to a saturated
carbon.
Alcohols are classed into three categories: promary (1˚), secondary (2˚) and
tertiary (3˚); the classification is based on the type of carbon to which the hydroxyl
group is attached. If the carbon is attached to only one other carbon, it is a primary
carbon. If the carbon is attached to two other carbons, it is a secondary carbon and the
alcohol is a secondary alcohol and so on.
H
R C OH
H
H
R C OH
R'
1˚
R''
R C OH
R'
2˚
3˚
Alcohols have a much higher boiling point than ethers or alkanes of similar
molecular weight. This is due to the association of alcohols in the liquid phase through
hydrogen bonding. For example, the boiling point of butyl alcohol is118˚C whereas the
boiling point of the isomeric diethyl ether is 36˚C.
Hydrogen Bonds
R
-∂
O
+∂ H
II.
H +∂
O
-∂
R
Procedure
Before the advent of modern spectroscopic methods,the determination of
chemical properties was ofparamount im portance for the identification, characterization and determination of structure of pure substances. Many reagents or conditions
were found to be specific with substances that contained sertain functional groups. Thus
these reactions can be used to confirm or deny the presence of a specific functional
group. The following experiment is designed to acquaint you with the chemical
properties of alcohols. You will examine the solubility of various alcohols in water as
well as several of their chemical properties. From this data you will arrive at
conclusions as to the Structure-Chemical Activity relationship of alcohols. You will
then draw on these conclusions to deduce the structure of an unknown alcohol sample.
I.
Solubility of Alcohols
Because of the hydrogen bonding possible between the polar hydroxyl group of
an alcohol and water, many alcohols are soluble in water. However as the carbon
content of an alcohol goes up, the solubility decreases. This is often described as the 5Carbon Rule, since the point at which an alcohol is no longer soluble in water is often
(but not always) reached when it contains 5 or more carbons. However, alcohol
structure (either 1˚, 2˚ or 3˚) also has an effect on solubility.
PROCEDURE
1. Add each of the following alcohols dropwise to 1 mL of water. Count the
number of drops needed to cause a phase separation, but do not use more than 10
drops. If you have added 10 drops of alcohol, and no phase separation has been
observed, the alcohol should be classified as completely miscible in water.
Record your observations as very soluble(6-9 drops), soluble (2-5 drops), or
insoluble (1-drop)
1. absolute ethyl alcohol
2. n-butyl alcohol
3. sec-butyl alcohol
4. tert-butyl alcohol
5. cyclohexanol
6. Unknown
II.
Alcohols as a source of protons
This is a test for reactive hydrogens, that is hydrogens which are
sufficiently acidic that they will react with sodium metal. Note that many compounds
will give a reaction with sodium metal even if that compound does not have a reactive
hydrogen such as alcohols do. This is often do to small amounts of water in the sample.
CAUTION SODIUM IS VERY REACTIVE AND MUST BE HANDELED WITH
CARE. USE ONLY SMALL PIECES AND HANDLE IT WITH FORCEPTS.
SODIUM REACTS VIOLENTLY WITH WATER! CARRY OUT ALL REACTIONS
IN A HOOD. IT IS NORMALLY STORED UNDER .
PROCEDURE (in the Hood)
1. Cut a small piece of sodium metal about half the size of a pea and add to
a beaker containing about 50 mL of water. Do this in a hood! Record you
observations on this reaction.After the reaction is complete, add 1-2 drops of
phenolphthalein indicator to the aqueous solution. Write an equation
for the
reaction of sodium with water.
2. Cut small pieces of sodium metal about half the size of a pea and add, in the
hood, to 3 mL of each of the alcohols, below, in a 6-inch test tube. Devise a
method to describe the rate of the reaction (such as the rate of gas bubbles
evolved over time). Test the resulting solution with phenolphthalein. Write an
equation for the reaction of the alcohol with
sodium. If all of the sodium
doesn’t react in any one of the alcohols, transferthis remaining sodium to the
alcohol that reacts fastest. DO NOT POUR THE ALCOHOL DOWN THE
DRAIN UNTIL YOU ARE SURE NO SODIUM REMAINS.
1. absolute ethyl alcohol
2. n-butyl alcohol
3. sec-butyl alcohol
4. tert-butyl alcohol
5. cyclohexanol
6. Unknown
III.
Alcohols in Substitution Reactions(Lucas Test)
The hydroxyl (OH) group of an alcohol is a poor leaving group and is not
redily displaced by nucleophiles. However, in strongly acidic solutions, protonation of
the -OH can occur to give -OH2+ and water may be displaced. The alcohol may,
depending on its structure, give substitution by either Sn1 or Sn2 mechanisms.
PROCEDURE
1. Add 5-7 drops of each of the following alcohols to each of 6-inch test tubes
and add to each test tube 2-3 mL of Lucas Reagent prepared by dissolving 16 g of
test to be valid, the alcohol must be initially soluble in the Lucas Reagent. The
formation of a milky or opaque solution is evidence for formation of the
insoluble alkyl chloride. Note the time required for the mixture to become
cloudy or to separate into two layers. Place test tubes that have not reacted after
10 minutes in a beaker of boiling water for 15 minutes. Observe any changes.
Write an equation to describe the reaction, if any.
1.
2.
3.
4.
IV.
n-butyl alcohol
sec-butyl alcohol
tert-butyl alcohol
Unknown
Oxidation of Alcohols
Primary and secondary alcohols are oxidized rapidly to acids and ketones,
respectively, by Cr and Mn oxidizing agents. Tertiary alcohols are not easliy oxidized,
however, over time in an acidic environment, they are dehydrated to alkenes.
Oxidation of Primary, Secondary and Tertiary Alcohols with the Jones Reagent :
CrO 3/H2SO4
PROCEDURE
1. Prepare the Jones Reagent by adding 2 grams of sodium dichromate to 10 mL
of water. Then add, slowly, with stirring, 2 mL of Conc. sulfuric acid. Pour 2
mL each of this solution into four 6-inch test tubes and add 2 mL of the following
alcohols to different test tubes.
1. n-butyl alcohol
2. sec-butyl alcohol
3. tert-butyl alcohol
4. Unknown
2. Stir each tube and note any rise in temperature(use you hand as a guide) or
change in color. Report your observations. In an acidic solution, the dichromate
ion, which is usually an orange-red color, is oxidized to the Cr+3 ion. This ion is
usually a dark green color but may form a dark precipitate with the carboxylate
group.
V.
Iodoform Test for Methyl Carbinols
A methyl carbinol is a 2˚ alcohol in which at least one of the R groups attached to
the 2˚ carbon is a methyl group. A methyl carbinol can be easily oxidized to a methyl
ketone which is then able to undergo a process known as the iodoform reaction.
OH
R C CH3
H
O
R C CH3
O
R C CH2I
O
R C CHI2
O
R C CI3
I2 as
oxidising
agent
-
OH / I2
-
OH / I2
-
OH / I2
-
OH
O
R C CH3 + 2 HI Oxidation to methyl ketone
O
R C CH2I
O
R C CHI2
O
R C CI3
Successive iodination leading
to triiodomethyl substituent
O
R C O + CHI3 Formation of iodoform
PROCEDURE
1. Add 4 drops of isopropyl alcohol to 5 mL of dioxane in a test tube and shake
until all the sample has gone into solution. You may need to stopper the test tube to get
adequate shaking. Add 1 mL of 10% sodium hydroxide solution, and then iodinepotassium iodide solution, with shaking, until a slight excess yields a definite dark color
of iodine. If less than 2 mL of the iodine solution is decolorized, place the test tube in a
water bath maintained at a temperature of 60°. If the slight excess of iodine already
present is decolorized, continue the addition of iodine solution with shaking until a
slight excess of iodine solution again yields a definite dark color. The addition of
iodine is continued until the dark color is not discharged by 2 minutes’ heating at 60°.
This excess of iodine is removed by the addition of a few drops of 10% sodium
hydroxide solution with shaking. Now fill the test tube with water and allow to stand
for 15 minutes. Observe the results. Iodoform is a light yellow solid.
The iodine-potassium iodide solution has been prepared by adding 200 g of
potassium iodide with 100 g of iodine to 800 mL of distilled water and stirring until
solution is complete
Reaction with Carboxylic Acids (Esterification)
Acid catalyzed esterification is an equilibrium in which water is a biproduct (see
reaction a). If we use a concentrated acid to tie up the water produced (through heat
of solvation) and add an excess of carboxylic acid, we can drive the equilibrium toward
the right. After a few minutes of reaction we can convert all the excess acid to its nonvolatile salt (reaction b) and the only volitle material left will be the ester. We will
use this technique to form some common flavor components.
RCH2OH + R'CO2H
R'CO2H + NaOH
H+
RCH2OCOR' + H2O
R'CO2-Na+ + H2O
(a)
(b)
Procedure (IN THE HOOD)
1 Place 1 mL (≈5 drops) of acetic acid in one test tube, 1 mL of butyric acid
in a second, and about 0.5 g of salicylic acid in a third. Note the different odors.
2. Add 5 drops of amyl alcohol to the acetic acid, 5 drops of ethyl alcohol to
the butyric acid and 20 drops of methyl alcohol to the salicylic acid.
3. Add 2 drops of concentrated sulfuric acid to the first and second tubes and
5 drops to the third tube.
4. Place all three tubes in a beaker of water warmed to 60° and let stand for
10 minutes.
5. At the end of this time remove the tubes from the water and add 5 mL of
10% NaOH to each tube, stopper and shake.
6. The ester should float on the water solution as an insoluble oil. Note the
odor in each tube. After you have recorded your results, pour the contents of
each tube down the drain IN THE HOOD. Rinse all glassware well in the hood
before taking anything back to your desk.
III. Report
1.
Cover sheet with experiment title and name.
2.
Introduction and experimental description.
3.
Observations and data sheets.
4.
Conclusion( what is the structure of your unknown alcohol and how did
you arrive at it?).
5.
Answer the following questions.
1. Why does the size of the alkyl group in an alcohol affect the vigor of its
reaction with sodium metal?
2. Why does the length of the carbon chain in an alcohol effect its
solubility in water?
3. Why do the alcohols exhibit differences in rates during the Lucas Test?
DATA SHEETS
Include these Data Sheets in with your report
I.
SOLUBILITY TESTS
Alcohol
ethyl alcohol
n-butyl alcohol
sec-butyl alcohol
tert-butyl alcohol
Cyclohexanol
Unknown
A.
Drops of H 2 O
Account for the diferences in solubility of the different alcohols.
II.
ALCOHOLS AS A SOURCE OF PROTONS
Alcohol
Rate of reaction with Sodium
ethyl alcohol
n-butyl alcohol
sec-butyl alcohol
tert-butyl alcohol
cyclohexanol
Unknown
A.
Show below the general reactions of the sodium with water and the various
alcohols (except the unknown).
B.
Explain why the order of reactivity is as you observed.
III.
ALCOHOLS IN SUBSTITUTION REACTIONS
Alcohol
Rate of Reaction with HCl
n-butyl alcohol
sec-butyl alcohol
tert-butyl alcohol
Unknown
A.
Show, below, the complete mechanism for the reaction of Lucas Reagent with the
various alcohols(except the unknown).
B.
Offer an explanation of the relative rate of reaction for each of the alcohols.
IV.
OXIDATION OF ALCOHOLS
Alcohol
Rate of Reaction with Jones Reagent
n-butyl alcohol
sec-butyl alcohol
tert-butyl alcohol
Unknown
A.
Show a balanced equation for the reaction of the Jones Reagent with each of the
alcohols.
V.
IODOFORM TEST
A.
Show a balanced equation for the reaction of isopropyl alcohol with iodine in a
base solution.
VI.
FORMATION OF ESTERS
ACID
Acetic acid
Butyric acid
Salicylic acid
A.
ALCOHOL
amyl alcohol
ethyl alcohol
methyl alcohol
SMELL
Show the reactions for the formation of ester for each of the following.
a. Acetic acid + amyl alcohol
b. Butyric acid + ethyl alcohol
c. Salicylid acid and methyl alcohol