Synthesis Of An Alkyl Halide
A. B. Dioquino, K.P. Isleta, And M.V. Monzon1Institute Of
Chemistry, College Of ScienceUniversity Of The Philippines,
Diliman, Quezon City, PhilippinesDATE PERFORMED: December 8,
2009
ABSTRACT
Alkyl halides are generally formed from alcohols through
treatment with hydrogen halides. The purpose of this study is to
illustrate the mechanisms behind the SN1 reaction. Tert-butyl
chloride is synthesized from tert-butyl alcohol using cold,
concentrated hydrochloric acid. The organic layer was obtained
using a separatory funnel and reagents such sodium bicarbonate and
calcium chloride were added. Simple distillation was conducted in
order to obtain pure tert-butyl chloride. The computed theoretical
yield is 11.45mL and the obtained tert- butyl chloride is 2.7mL,
which gives a 23.58% percentage yield. This relatively low yield
may be due an inefficient methodology, and the presence of a
simultaneous elimination reaction.
INTRODUCTION Alkyl halides, or haloalkanes, are compounds that
have a halogen atom bonded to a saturated sp3-hybridized carbon
atom. They are are nonflammable and nonpolar compounds which give
them various uses. They are comercially used as flame retardants,
fire extinguishers, refrigerants, as well as solvents for dry
cleaning and degreasing applications.
This type of organic compound is generally formed through the
synthesis of alcohols with hydrogen halides (HCl, HBr, or HI). The
order of reactivity increases as one goes down the halogen group.
These compounds may be also classified according to the number of
alkyl groups attached to the carbon. Primary alkyl halides have a
single alkyl group attached, secondary alkyl halides have two
groups and tertiary alcohols have three. A tertiary alcohol with
the form R3COH, should be applied for a more effective reaction.
Primary and secondary alcohols react much more slowly and require
higher temperatures. In some cases, Lewis acids such as zinc
chloride are required for a more effective reaction. Thionyl
chloride (SOCl2) or phosphorous tribromide (PBr3) may also be used
for treatment with primary and secondary alcohols.In this
experiment, tert-butyl chloride is formed from the addition of
tert-butyl alcohol with concentrated hydrochloric acid. The alcohol
and hydrogen halide are placed in a separatory funnel which will
later on facilitate the isolation of components. Purification of
the tert-butyl chloride is performed through simple distillation to
remove excess water from the solution. Since tert-butyl is a
tertiary alcohol, it will readily produce an alkyl halide and water
at room temperature.
The purpose of this experiment is to acquaint students with the
mechanisms behind alkyl halide synthesis through the synthesis of
tert-butyl chloride from tert-butyl alcohol. It also aims to teach
laboratory techniques such as the usage of the separatory funnel
and simple distillation.
METHODOLOGY Ten milliliters of tert-butyl alcohol and 20
milliliters of cold, concentrated hydrochloric acid (HCl) were
measured with a graduated cylinder and placed in a separatory
funnel. The mixture was gently swirled under the fume hood. To
relieve internal pressure, the stop cock was opened after some time
to release gas. Afterwards, the separatory funnel was left to stand
in an iron ring for 20 minutes. While waiting, the simple
distillation apparatus was prepared. A cork that had been
previously drilled into was inserted with a laboratory thermometer.
Subsequently, the apparatus was assembled accordingly.
After 20 minutes had passed, about two milliliters of six molar
sodium chloride (NaCl) solution were measured in a graduated
cylinder and placed in the separatory funnel. The two layers were
then separated by slowly opening the stopcock. A drop of water was
placed into both layers to determine the organic layer. The aqueous
layer was discarded.
The organic layer was transferred to a flask containing sodium
bicarbonate (NaHCO3) and swirled gently. Afterwards, the liquid was
decanted into another flask. Anhydrous calcium chloride (CaCl2) was
added until there was undissolved excess solid. The liquid was then
decanted into a dry, 25-milliliter round bottom flask. Two boiling
chips were added to the distillation flask and the crude tert-butyl
chloride was distilled. The heat and rate of distillation were
controlled and the boiling point was recorded. The first one
milliliter of the distillate was discarded, and the remaining
solution was placed in a ten-milliliter graduated cylinder which
was previously cooled in an ice bath. The volume of the product was
measured and afterwards, it was placed in a vial, labeled, and
submitted to the instructor.
Synthesis of tert-butyl chloride from tert-butyl alcohol
involved the reaction of the alcohol with a hydrogen halide. In
this case, cold, concentrated hydrochloric acid was used as the
hydrogen halide. Cold HCl was used to hasten the reaction, and
avoid side products. Excess HCl was added to further stimulate
product formation and to ensure that all the alcohol will be used
in the reaction. The reactants underwent a nucleophilic
substitution reaction involving unimolecular rate-determining step,
also known as the SN1 reaction. Tertiary substrates are the most
favored substrates in this type of reaction. In treating an alcohol
with hydrogen halides, tertiary alcohols react rapidly as compared
to primary and secondary alcohols. By this principle, tert-butyl
alcohol, a tertiary alcohol, reacted rapidly via SN1 and E1
pathways with HCl, its acidic nucleophile. During the SN1 reaction,
the tert-butyl alcohol dissociates to form a carbocation, which
then reacts with the nucleophile. The mechanism is shown below.
Figure I. Mechanism for the reaction of tert- butyl alcohol with
HCl via SN1At first, the OH group is protonated by HCl which is a
fast reaction. Then the protonated alcohol dissociates
spontaneously, which occurs in a slow, rate-determining step. This
dissociation gave rise to a carbocation intermediate plus water.
Afterwards, the chloride ion attacks the stable carbocation, and
the water (H2O) leaves the resulting product. Water, in this part
of the reaction, is called the leaving group. In tert-butyl
alcohol, OH is considered as the leaving group. However, this
leaving group is not at its most stable form. In order for it to be
at its most stable form, the OH group is protonated. The hydrogen
atom from HCl is ionized and transformed into water. The chloride
ion then attaches itself to the carbocation to yield the neutral
substitution product, tert-butyl chloride.Unlike in an SN2
reaction, where displacement of leaving group and the attack of the
incoming nucleophile happen at the same time, an SN1 reaction
occurs first with the spontaneous loss of the leaving group then
the incoming nucleophile. In between, a stable carbocation
intermediate is formed. This two-step mechanism is the reason why
tertiary alcohols react with HCl must faster than the primary and
secondary alcohols. More stable carbocation intermediates involve a
faster SN1 reaction.
It can be seen in the reaction that the substrate is the only
species involved in the rate- determining step. This is because the
rate of an SN1 reaction depends only on the substrate
concentration, and not with the nucleophile concentration (first-
order process). In the synthesis via SN1 reaction, by-products can
be formed as consequences of the synthesis. This is because an
opposite reaction occurs with SN1 reaction. This reaction is called
the unimolecular elimination reaction, or E1. E1 reactions have the
same characteristics as that of SN1, but it involves loss of H+
from the intermediate carbocation following the dissociation of
process. A neutral alkene is formed from this step. SN1 and E1
reactions normally occur in competition when alkyl halides are
treated with a nonbasic nucleophile. In the synthesis of tert-
butyl chloride, 2- methylpropene is formed as a by-product. The
reaction mechanism of 2- methylpropene is shown below.
Figure II. Mechanism for the formation of 2- methylpropene as a
by- product via E1
The reaction mechanism above implies that the formation of
2-methylpropene is a first- order process. This reaction proceeds
because not all tert-butyl alcohol may have reacted via SN1. After
the protonation, the hydroxide ion leaves the substrate and forms
water. The water can act as a base and it can remove an H+ from the
carbocation. A bond will result from the C-H bond. 2-methylpropene
is then formed.
In the elimination reaction, there is heat required for the
reaction to proceed. As the temperature increases, the rate of E1
reaction increases as well. Therefore, by adding cold HCl, the
formation of a stable tertiary carbocation, through the SN1
mechanism, is maximized and favored. In addition, based on Le
Chateliers Principle to help favor the forward reaction, more
reactants (HCl) must be added.
As the cold HCl is mixed with tert-butyl alcohol, the gas
2-methylpropene (BP:-6.9oC) formed from the E1 reaction is released
by opening the stopcock. This is done to prevent the build up of
internal pressure. However, this release of gas may result in a
loss that could lead to a lower yield.
The mixture was allowed to stand for 20 minutes and then
separated. Sodium chloride (NaCl) solution was added to make the
layers more visible since it can draw out the water from the
organic layer to the aqueous layer. Crude tert- butyl chloride is
obtained. Since it still has impurities, solid sodium bicarbonate
(NaHCO3) is added in small amounts to neutralize any excess HCl.
This addition will cause effervesence of carbon dioxide (CO2).
Solid NaHCO3 is used rather than the aqueous NaHCO3 to ease the
separation of NaHCO3.
Drying the collected filtrate with a small amount of anhydrous
calcium chloride (CaCl2) followed. This was done before
distillation to remove traces of water left and unreacted alcohol
which may interfere in the process.The solution was decanted and
boiling chips were added before distillation. Boiling chips are
small, insoluble, porous objects made of calcium carbonate or
silicon carbide that can trap air and provide spaces where bubbles
of solvent vapor can form. When a boiling chip is heated in a
solvent, it releases tiny bubbles that ensure even boiling and
prevent bumping and boiling over. It does not make the boiling
faster and avoids product loss by preventing the bumping of the
solution during boiling.Subsequently, simple distillation was
conducted. Distillation of the solution was done to purify the
tert-butyl chloride by separating the impurities through their
different boiling points. Continuous flow of water on the condenser
during distillation is important to properly cool and condense the
hot product vapors. It affects the cooling capacity of the
condenser which would affect the product recovery.Table I.
Experimental Results
Weight of tert-butyl alcohol, g : 7.80Volume of tert-butyl
alcohol, g : 2.7Theoretical yield, mL : 11.45% yield : 23.58%
The synthesis via SN1 and E1 resulted in a tert-butyl chloride
with a volume of 2.7ml. Pure tert-butyl chloride has a colorless
appearance and our result yielded a similar substance. However,
impurities may still be present, such as minute traces of HCl. It
would be difficult to detect these errors because they come from
different sources. A possible source of these errors is the
addition of the NaHCO3 and CaCl2. It is possible that not enough
NaHCO3 was added to neutralize the excess HCl, or the addition of
anhydrous CaCl2 was not enough to react with all the unreacted
alcohol and water. These errors would lead to an impure
product.
Tert-butyl chloride is a volatile substance which can cause the
loss of product. The way the process was conducted may have
resulted in reagent loss. During separation of layers, the
separatory funnel is not covered and some tert- butyl chloride may
have vaporized. The separation of the layers through the stopcock
is another source of error. Some aqueous layer might have been
retained with the organic layer due to improper separation. Also,
the decantation and distillation of the solution may contribute to
product loss. Inadequate drying of the filtrate may have lead to
backward reaction and heating the distillation system too abruptly
and erratically will cause a sudden increase in temperature. These
possible errors may have accounted for the low yield.CONCLUSION AND
RECOMMENDATION
With a percent yield of 23.58%, it can be concluded that not all
of the tert-butyl alcohol reacted to form the desired product,
tert-butyl chloride. As expected, some of the product formed
2-methylpropene, which was released in a gaseous form and resulted
in a lower yield. However, the results show a relatively low
percent yield despite precautions. After adding certain reactants
to favor the forward reaction, the SN1 was not effectively
maximized and the E1 reaction was favored instead.
For further improvement, the usage of a very cold concentrated
hydrochloric acid is recommended. More anhydrous calcium chloride
should also be added until there is undissolved CaCl2. Careful
observation should also be observed in the steps that could result
in side reactions.REFERENCES[1] McMurry, J. Organic Chemistry 6th
edition. Brooks/Cole-Thomson Learning., USA. 2004
[2] Mackenzie, C.A. Experimental Organic Chemistry. Prentice
hall, Inc., New Jersey. 1967
[3] March, Jerry. Advanced Organic Chemistry. 4th edition. John
Wiley & Sons. Inc.
[4] Organic Chemistry Group. Organic Chemistry Laboratory
Manual. Institute of Chemistry, College of Science, University of
the Philippines, Diliman, Quezon City. 2008.APPENDIX (see
attached)
RESULTS AND DISCUSSION
: 7.80: 2.70: 11.45: 23.58%