CHARACTERISTIC REACTIONS OF ORGANIC HALIDES

Bennet Fiorello P. Pinoy, Angela Grace P. Regalado, Dana Izabel L. Rivera, Kobe Kahlil H. Rodriguez,
Marielle Lynn E. Ruiz, and Maria Leah Joyce C. Santos

Group 6 1F Medical Technology Inorganic ang Organic Chemistry Laboratory

ABSTRACT
Alkyl halides are compounds that are made up of alkyl groups with halogens as substituents (R-X). This type of
compound usually undergoes Nucleophilic Substitution (S N) reactions which could be SN1 or SN2. However, Beilstein
test, which uses flame to determine alkyl halides was not used in this experiment. This experiment aimed to differentiate
the SN1 and SN2 reactions and be able to explain their equation and mechanism on how it affected the speed of reaction
depending on the type of alkyl halide. The test compounds: n-butyl chloride, sec-butyl chloride, tert-butyl chloride, and
chlorobenzene, with two reagents: 2% ethanoic AgNO3 and 15% NaI in anhydrous acetone were used to see on which
mechanism, between SN1 and SN2 the test compounds would react or react the fastest. With that, reactions and reaction
speed were determined through the time it took for the sample to be cloudy or form white particles. In SN1 reaction,
where 2% ethanoic AgNO3 was used, tertiary alkyl halides reacted the fastest and primary alkyl halides reacted the
slowest given that tertiary alkyl halides were able to give the most stable carbocation. While in SN2 reaction, where
15% NaI in anhydrous acetone was used, primary alkyl halides reacted the fastest and tertiary alkyl halides reacted the
slowest as primary alkyl halides were less hindered by other Carbons or bulky groups, making it easier to detach.
However, Chlorobenzene did not react to any of the two reactions due to its resonance and stability.

INTRODUCTION different rates of reaction between the SN1 and

SN2 reactions. Some of the alkyl halides may react
Alkyl halides are classified as chemical to SN2 quickly, and some may react to SN1 faster,
compounds wherein one or more of the hydrogen or maybe not at all. Their difference in the rate of
atoms in the carbon chain is substituted with a reaction may be due to their difference in
halogen atom. Halogen atoms can be fluorine, structure, location of the halide, number of
chlorine, bromine or iodine. Alkyl halides has had Carbons attached, and stability.
several uses through chlorofluorocarbons which
are carbons that are attached with fluorine and The condensed structural formulas and
chlorine halides. Alkyl halide substitutions and skeletal structures for each sample used can be
addition have also been applied for researching on found in Table 1.
potential replacements for CFC’s as they make a
Table 1. Condensed Structural Formulas
negative impact on the environment.[1] Alkyl
Condensed
halides though not all, have been known to Skeletal
Sample Structural
produce a sweet smell, they are typically gaseous Structure
Formula
or liquid, and are always soluble with organic n-butyl CH3(CH2)3Cl
solvents while being slightly soluble in water. chloride
Numerous tests are present in determining the sec-butyl
characteristics of alkyl halides with the Beilstein chloride
Test, SN1 and SN2 reactivity tests. The Beilstein
test developed by a Russian chemist named tert-butyl
Friedrich Konrad Beilstein. The Beilstein test chloride
detects the presence of an organic halide aside
from fluorine halides, through the formation of
copper halide that produces a green flame.[2]
However, the test is no longer used today regularly
in organic chemical labs because of the formation chlorobenzene
of chloro-dioxins which are highly toxic and was
not used in this experiment.

In this experiment, you will compare the

reactivity of a series of alkyl halides under SN2 and
SN1 conditions. In order to determine how rapidly
the reaction is proceeding, we will run the
reactions under conditions which provide a visible
precipitate which we can observe. [3] The different
alkyl halides used in this experiment will undergo
 The objectives of this experiment were to
understand the difference between SN1 and SN2
reactions, their mechanisms, and the reason why
different alkyl halides (primary, secondary, and
tertiary) reacted differently on SN1 and SN2.

EXPERIMENTAL
A. Test Compounds and Samples used

The test compounds used were n-butyl

chloride, sec-butyl chloride, tert-butyl chloride,
and chlorobenzene. The reagents used were 2% Figure 2. Addition of 15% NaI in Anhydrous
ethanoic AgNO3, and 15% NaI in anhydrous Acetone
acetone.
RESULTS AND DISCUSSION
B. Procedure
1. Reaction with Alcoholic AgNO3 1. SN1 Reactivity: Reaction with Ethanoic
AgNO3
A few (5) drops each of n-butyl chloride,
sec-butyl chloride, tert-butyl chloride and
chlorobenzene were placed in separate test tubes.
After making sure that the mixture was not turbid,
20 drops of the 2% ethanoic AgNO3 were added to
each test tube (Figure 1). The mixtures were
shaken and the time for the precipitate to form
was recorded.

Figure 3. SN1 Reactivity Reactions (left to right):

n-butyl, sec-butyl, tert-butyl, and chlorobenzene

Table 2. SN1 Reactivity Reactions

Results observed/Reaction
Sample
Time
n-butyl Cloudy with white particle after
Figure 1. Addition of 2% Ethanoic AgNO3 chloride 220 seconds.
sec-butyl Cloudy with small dusty particles
2. Reaction with NaI in Acetone chloride after 193 seconds.
tert-butyl Immediate reaction or formation
A few (5) drops each of four samples were chloride of white particles (less than 5
placed in separate test tubes. After making sure seconds).
that the mixture was not turbid, 2 drops of the chlorobenzene No reaction; no cloudiness or
15% NaI in anhydrous acetone were added to each white particle formation.
test tube (Figure 2). The mixtures were shaken
and the time for the precipitate to form was
recorded. As seen on Figure 3, tert-butyl chloride was
the most cloudy and reactive compared to the sec-
butyl and n-butyl as the slowest in which the three
showed a reaction. However, Chlorobenzene
remained clear, and did not show any cloudiness
or reaction.

With the SN1 reactivity results reflected on

Table 2, the tertiary substrate is favored by the
SN1 reactions, followed by the secondary, then the intermediates are formed rapidly, the attack of the
primary substrates. The Chlorobenzene had no nucleophile to the carbocation will also occur at a
reaction to the Ethanoic AgNO3. faster rate. This explains why the reaction
between Ethanolic Silver Nitrate (AgNO3) and
The SN1 reactivity is indicative of a Tert-Butyl Chloride occurs at a rapid rate, evident
unimolecular nucleophilic substitution. It is the formation of precipitate immediately after the
considered unimolecular due to its Rate reactant was dropped into the substance.
Determining Step involving only one component.[4]
It is also to be expected to arrive with two different
products with a SN1 pathway, one with retention
and one with the inversion of stereochemistry. It
is said to be a racemic mixture, which means that
there are equal amounts of the two enantiomers.[5]

The SN1 reaction involves a leaving group Figure 5. Structural formula of t-butyl chloride
and a nucleophile to replace said group, not unlike In contrast, primary carbocations are
the SN2 reactions. Though, in SN1, the reactions highly unstable which entails a much slower
are unimolecular, thus, the rate of its reactions are reaction rate and are thus not commonly observed
dependent on the concentration of only one as reaction intermediates (Figure 6). As seen in
reactant. The reaction proceeds in two steps, with Figure 6, the structure of n-butyl chloride shows a
the substrate, first, slowly and spontaneously halogen bonded to a primary carbon, which entails
losing the leaving group, consequently generating that the compound would have less resonance,
a carbocation intermediate (Figure 4). and is thus less stable. Therefore, the carbocation
intermediates are generated slower, and
consequently, the attack of the nucleophile to the
carbocation also occurs at a slower rate. This
explains why the reaction occurs at a much slower
rate, and there was no precipitate formed.
Figure 4. Chemical Equation of an SN1 reaction

The second step involves the nucleophile

‘attacking’ the electrophilic carbocation to form a
new S bond. This step often occurs rapidly and
produces the product of the substitution. Figure 6. Structural formula of n-butyl chloride
The rate of the reaction is also heavily 2. SN2 Reactivity: Reaction with NaI in
dependent on the stability of a carbocation. The
Acetone
transition state resembles the structure of the
nearest stable species.[6] The stability of the
carbon group is also attributed to resonance, or
the number of its contributing structures. In SN1
reactivity, the nearest stable species are the
carbocation, therefore the more stable the
carbocation intermediate would be, the faster the
first bond-breaking step would occur. This is
because positively charged species are often very
electron poor and therefore, anything which
donates electron density will be able to stabilize it.
Conversely, a carbocation is said to easily be
destabilized by an electron-withdrawing group.[7]

Alkyl groups in tertiary alkyl halides are

weak electron donating group, they would stabilize
nearby carbocations. The structure of t-butyl
chloride, as shown in Figure 5, shows a halogen Figure 7. SN1 Reactivity Reactions (left to right):
bonded to a tertiary carbon. This arrangement of chlorobenzene, tert-butyl, sec-butyl, and n-butyl
carbons entails more canonical structures, which
contributes to the stability of the compound. Thus,
t-butyl chloride will easily form a carbocation
intermediate. Consequently, since carbocation
 Table 3. SN2 Reactivity Reactions Figure 9. Inversion of structure in SN2 Reaction
Results observed/Reaction
Sample A tertiary haloalkane, reacts the slowest, or
Time
n-butyl White translucent particles in does not undergo SN2 reactions at all. The addition
chloride yellowish solution formed after of a third R group to the molecule of a tertiary
2.53 seconds. haloalkane creates a carbon that is entirely
sec-butyl White translucent particles in blocked.[9] Meaning, the more R groups that the
chloride yellowish solution formed after electrophilic Carbon is attached to, the slower the
3.69 seconds.
SN2 reaction will be because electrons are held
tert-butyl White translucent particles in more tightly by the R groups which makes it
chloride yellowish solution formed after
harder to form a new bond (Figure 10).
4.13 seconds.
Nucleophilic substitution reactions at the ‘‘bulky’’
chlorobenzene No reaction; no cloudiness or
center is generally avoided since it is overly
white particle formation.
unreactive.[10]

As seen on Figure 7, n-butyl, sec-butyl, and

tert-butyl formed evident white particles which
proved a SN2 reaction, with the n-butyl as the
fastest and the tert-butyl the slowest. However,
Chlorobenzene still showed a limited or no reaction
to the NaI in anhydrous acetone.

With the SN2 reactivity results reflected on

Table 3, primary alkyl halides result in faster Figure 10. Descending order of the speed of SN2
nucleophilic substitution reactions, in comparison reaction
to secondary and tertiary haloalkanes, which
However, Benzene is less reactive in
result in nucleophilic substitution reactions that
occur at slower or much slower rates, respectively. nucleophilic substitution primarily due to its
structure consisting of conjugating pi bonds that is
Still, the Chlorobenzene did not react to the NaI in
stable enough to resist most reactions.
Anhydrous Acetone.
Additionally, because benzene is rich in electron,
In SN2 Reactivity, the reaction takes place it undergoes electrophilic substitution and not
in a single step, and bond-forming and bond- nucleophilic substitution because it repels
breaking occur simultaneously (Figure 8). The nucleophiles, which are also electron rich. The
number 2 in SN2 refers to the fact that this is a same principle applies to chlorobenzene. The
bimolecular reaction: the overall rate depends on electron pairs of chlorine are delocalized along
a step in which two separate molecules (the with the conjugated pi bonds of benzene to attain
nucleophile and the electrophile) collide.[8] stability, avoiding releasing electrons.[11]

CONCLUSION
Figure 8. Chemical Equation of an SN2 reaction To summarize, alkyl halides undergo
nucleophilic substitution reactions, namely: SN1
For the SN2 reaction to occur, the
and SN2 reactions. To differentiate the two
nucleophile must be able to overlap orbitals with
reactions, SN1 is unimolecular wherein its reaction
the electrophilic carbon center, resulting in the
is determined by the R-X group only and produces
expulsion of the leaving group.[9]
a racemic product. It also involves a two-step
The nucleophile, being an electron-rich reaction wherein it needs to undergo the
species, must attack the electrophilic carbon from carbocation stage first. With that, tertiary alkyl
the back side relative to the location of the leaving halides are the most reactive in SN1 since its
group which blocks the way.[8] This process results structure is the most substituted and would be
to an inversion of the stricture (Figure 9). able to yield the most stable carbocation for
nucleophilic substitution. On the other hand, S N2
is bimolecular wherein its reaction is determined
by the R-X group and the nucleophile which causes
an inversion of the product. Unlike SN1, SN2 is a
one-step reaction where the substitution and
expulsion occur simultaneously. With that,
primary alkyl halides react the fastest among the [9] Libretexts. (2019). Characteristics of the Sₙ2
test compounds due to its less substituted Carbon Reaction. Retrieved November 17, 2019,
which makes the electrons held loosely, making from
bond formation easier. https://chem.libretexts.org/Bookshelves/
Organic_Chemistry/Map%3A_Organic_Che
mistry_(Wade)/07%3A_Alkyl_Halides%3A
REFERENCES: _Nucleophilic_Substitution_and_Eliminatio
n/7.06%3A_Generality_of_the_SN2_React
[1] Libretexts. (2019). Uses of Alkyl Halides. ion.
Retrieved November 17, 2019, from
[10] Orgue, S. et al. (2015). Stereospecific
https://chem.libretexts.org/Bookshelves/
SN2@P reactions: Novel Access to Bulky P-
Organic_Chemistry/Supplemental_Module
Stereogenic Ligands. Chemical
s_(Organic_Chemistry)/Alkyl_Halides/Use
Communications, 51, 17548-17551. DOI:
s_of_Alkyl_Halides.
10.1039/c5cc07504a.
[2] Ahmed, N. (1970). Physical properties of alkyl
[11] Reusch, W. (2019). Libretexts: Nucleophilic
halides. Retrieved November 17, 2019,
Reactions of Benzene Derivatives.
from
https://chemeasylearn.blogspot.com/2018 Retrieved November 12, 2019, from
https://chem.libretexts.org/Bookshelves/
/07/physical-properties-of-alkyl-
Organic_Chemistry/Supplemental_Module
halides.html.
s_(Organic_Chemistry)/Arenes/Reactivity_
[3] Morgan, S. (n.d.). Nucleophilic Substitution. of_Arenes/Benzene/Nucleophilic_Reaction
Retrieved November 17, 2019, from s_of_Benzene_Derivatives.
http://cactus.dixie.edu/smblack/chemlabs
/Nucleophilic_Substitution.pdf

[4] Hunt, I. (n.d.). Nucleophilic Substitution: SN1

mechanism. University of Calgary:
Department of Chemistry. Retrieved
November 13, 2019, from
http://www.chem.ucalgary.ca/courses/35
0/Carey5th/Ch08/ch8-2.html

[5] Ionic substitution— SN1. (n.d.). Retrieved

November 13, 2019, from
http://www.chem.ucla.edu/~harding/note
s/notes_14D_SN1.pdf.

[6] Ashenhurst, J. (2019). Hammond’s postulate.

Retrieved November 13, 2019, from
https://www.masterorganicchemistry.com
/2011/09/28/hammonds-postulate/.

[7] Carbocation structure and stability. (2019).

Retrieved November 17, 2019, from
https://chem.libretexts.org/Bookshelves/
Organic_Chemistry/Map%3A_Organic_Che
mistry_(McMurry)/Chapter_07%3A_Alken
es%3A_Structure_and_Reactivity/7.09_Ca
rbocation_Structure_and_Stability.

[8] Lumen. (n.d.). Physical chemistry for SN2 and

SN1 reactions: The SN2 Reaction.
Retrieved November 17, 2019, from
https://courses.lumenlearning.com/suny-
potsdam-organicchemistry/chapter/8-2-
physical-chemistry-for-SN2-and-SN1-
reactions/.