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8/19/2019 Hydrohalogenation of alkenes and dehydrohalogenation of haloalkanes
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1.0 PROJECT PLANNING
1.1 OBJECTIVES:
1) To explain alkene and haloalkane together with the brief view on the substitution and
elimination reaction.
2) To study the hydrohalogenation of alkene and the effect of the rearrangement of
carbocation at the end products of the reaction.
3) To identify markovnikov and anti-markovnikov’s rule.
4) To explain the elimination reaction in dehydrohalogenation of haloalkanes.
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1.2 WORK DISTRIBUTION
NAMES
TASKS SITI NAJIHAH SYAZA IZNI TUAN NURUL WAN AINUN
INARAH HANAN SYAMILA
INTRODUCTION
DISCUSSION OF
ISSUES
ANALYSIS OF
ISSUES
WORK
DISRIBUTION
CONCLUSION
ISLAMISATION
BIBLIOGRAPHY
APPENDIX
ABSTRACT
Table 1 : Work distribution
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1.3 TIMELINE
MEETING DATE (DAY) TIME ACTIVITY
1 28/1/2016
(THURSDAY)
10.00AM-10.15AM -Consultation with sir
2 30/1/2016
(SATURDAY)
9.00PM-11.00 PM - Discuss the objectives
- Divide task among the
group members
3 2/2/2016
(TUESDAY
9.00PM-11.00 PM -Discuss the sub point
4 11/2/2016
(THURSDAY)
10.00AM-10.15AM -Consult with sir regarding
the objectives
5 13/2/2016
(SATURDAY)
10.00AM-12.00PM -Discuss the issue of
dehydrohalogenations of
haloalkane
-Discuss the issue of
hydrohalogenation reaction
of alkenes
6 20/2/2016
(SATURDAY)
10.00AM-12.00PM - All members complete their
given task
7 21/2/2016
(SUNDAY)
3.00PM-5.00PM -All members complete their
given task
8 26/2/2016
(FRIDAY)
11.00AM-11.15AM -Final consultation with sir
9 7/3/2016
(MONDAY)
3.15PM-4.15PM -Compilation of content
10 8/3/2016
(TUESDAY)
3.00PM-4.00PM - Final touch up
Table 2: Timeline
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2.0 ABSTRACT
In this project, we briefly explain on the substitution and elimination reaction.
Basically, substitution reactions occur during hydrohalogenation reaction of alkene while
elimination reactions occur in dehydrohalogenation reaction of haloalkanes. Next, we
identify the effect of rearrangement of carbocation at the end product of hydrohalogenation
reaction of alkene. The carbocation is shifted to the different carbon to achieve a more stable
state. It undergo three types of rearrangement which are hydride shift, alkyl shift and ring
expansion Other than that, we also discuss on the markovnikov and anti-markovnikov rules
on the process of hydrohalogenation reaction of alkene. . Markovnikov’s rule state that in an
unsymmetrical alkene, the hydrogen atom is attached to the carbon atom that had the most
hydrogen atoms. Anti-Markovnikov rule is when rather than the more substituted carbon, the
substituent is bonded to a less substituted carbon. Lastly, we describe the elimination reaction
that occur in dehydrohalogenation of haloalkanes. In this case, the removable of halogen in
the presence of base form an alkene.
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3.0 INTRODUCTION
In our study of the effect of intermediate stability on the final products of
hydrohalogenation of alkenes and dehydrohalogenation of haloalkane, we emphasize on the
carbocation stability. There are four objectives of our study. First, we want to explain alkene
and haloalkane together with the brief view on the substitution and elimination reaction.
Haloalkanes are easily converted into other type of functional groups. This is because they
can leave with their bonding pair to form stable halide ions. Haloalkane can undergo two type
of reaction; substitution reaction and elimination reaction.
Next, we would like to study the hydrohalogenation of alkene and the effect of the
rearrangement of carbocation at the end products of the reaction. The carbocation is shifted to
the different carbon to achieve a more stable state. The three types in rearranging the
carbocation are hydride shift, alkyl shift and ring expansion. Furthermore, we would like to
identify markovnikov and anti-markovnikov’s rule. Markovnikov’s rule state that in an
unsymmetrical alkene, the hydrogen atom is attached to the carbon atom that had the most
hydrogen atoms. Anti-Markovnikov rule is when rather than the more substituted carbon, the
substituent is bonded to a less substituted carbon. The last objective is to explain the
elimination reaction in dehydrohalogenation of haloalkanes. Halogen is removed from one
carbon of a haloalkane and from an adjacent carbon to form an alkene in the presence of base.
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4.0 CONTENT
4.1 DISCUSSION OF ISSUES
4.1.1 HALOALKANES
Haloalkanes can be classified into three classes which are primary (1°), secondary (2°)
and tertiary (3°). They are divided depending on the number of alkyl groups which attached
to the carbon atom which link to the halogen atom. Frequently, haloalkane can undergo two
type of reaction; substitution reaction and elimination reaction. Substitution reaction which in
this case it is nucleophilic substitution is when an atom replaces another atom specifically
halide ion. On the other hand, elimination reaction occurs when a small molecule, H-X, is
removed from larger molecule, alkyl halide, to produce a double bond which is an alkene.
4.1.2 REARRANGEMENT OF CARBOCATION
Rearrangement of carbocation also known as the movement of carbocation from less
stable state to more stable state through structural organization “shifts” within the molecule.
The carbocation is shifted to the different carbon to achieve a more stable state. The bonding
pair of electrons migrates to a carbocation from one of its neighbours. The bonding pair may
attach to the hydrogen or the alkyl group or in the ring. The three types in rearranging the
carbocation are hydride shift (shifting the hydrogen), alkyl shift (shifting the methyl group)
and ring expansion.
4.1.2.1 HYDRIDE SHIFT
For this type of reaction, the first step is the attack of alkene upon the electrophile.
The π bond attacks the hydrogen. Then, the π bond will break. According to Markovnikov’s
rule, hydrogen is bonded to the terminal carbon producing the secondary carbocation that is
located next to a tertiary carbon which is more stable.
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Figure 1: Attack of alkene on H-CL
The second step is the lone pair in the C-H bond will migrate from the tertiary carbon
to the secondary carbocation in order to form a new carbocation which is tertiary carbocation
that is more stable.
Step 2 – rearrangement (arrow c)
Figure 2: Rearrangement of carbocation
The last step is the nucleophile (chloride ion) attacks the carbocation in order to form the
alkyl halide.
Figure 3: Attach of nucleophile
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4.1.2.2 ALKYL SHIFT
Therefore, there is a carbocation that does not have suitable hydrogen atoms, either
secondary or tertiary, which are on the neighboring carbon atoms that are available for the
rearrangement. So, they will undergo another process of rearrangement known as alkyl shift
or alkyl group migration. There are time where a hydride shift would not lead to a more
stable carbocation. For instance, in this example if a hydride shift occurred, it will lead to a
less stable (primary) carbocation.
Figure 4: Hydride shift
Thus, the alkyl shift will take place. The process of hydride and alkyl shift is quite
similar in shifting the element, hydrogen atom, H or methyl group, R to get a more stable
state. The shifting group,which are the alkyl group carries its electron pair with it to furnish a
bond to the adjacent or neighboring carbocation. The shifted alkyl group and the positive
charge of the carbocation will switch their positions on the molecule to be more stable.
Reactions of tertiary carbocations is much faster than secondary carbocations. For instance,
3-dimethylbutene and hydrogen bromide.
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Figure 5: Alkyl shift process
4.1.2.3 RING EXPANSION
For this types of reaction, it usually occurs when an unstable cycloalkene is near a
carbocation. The migration of the CH2 from the ring not only produces tertiary carbon but
increase the size of the ring from 4-membered to 5-membered, which relieves considerable
ring strain present in the cyclobutane ring.
Figure 6: Ring expansion process
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4.1.3 ELEMINATION REACTION
All nucleophiles are based, it is because all of them have an electron pair either as a lone
pair or sometimes as a π-bond which can accept a proton. Dehydrohalogenation is one of type
of elimination. Halogen is removed from one carbon of a haloalkane and from an adjacent
carbon to form an alkene in the presence of base. Strong bases like OH -, OR -, NH2-, and
acetylide anions promotes elimination of haloalkanes effectively. The conjugate acid of the
base is commonly used as solvent. The more substituted and more stable alkene is the major
product of elimination reactions. The formation of the major product is common and it is said
to follow Saytzeff’s rule, or to undergo Saytzeff elimination.
There are two mechanisms of elimination reaction, which are E1 mechanism and E2
mechanism. E1 is an unimolecular elimination reaction. This reaction involve the removal of
haloalkane (HX) substituent and form double bond, just like unimolecular nucleophilic
substitution reaction, SN1. In an E1 reaction, the rate determining step is the loss of the
leaving group to form a carbocation. Hence, the more stable the carbocation is, the easier it is
to form, and the faster the E1 reaction will be. In addition, deprotonation of a hydrogen
occurs in an E1 reaction (usually one carbon away, or the beta position). The carbocation
results in the forming of alkene.For example, Bromine leaves haloalkane to form a
carbocation. Then, a proton is removed by base to form alkene.
Figure 7: E1 Mechanism
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Different from E1, E2 is a bimolecular reaction. This reactions remove two subsituents
and add a strong base, forming an alkene. In this reaction, the base removes the proton from
the alkyl halide that is oriented anti to the leaving group, and the leaving group leaves – all in
one concerted step. Based on the figure, hydrogen, which oriented 180° from Bromine, the
leaving group is removed. Then, double bond form.
Figure 8: E2 Mechanism
One of the similarities between E1 and E2 is in both cases, we form a new C-C π bond,
and break a C-H bond and a C–(leaving group) bond. In both reactions also, new π bond form
as the base removes the proton. On the other hand, one of the difference between these
mechanism is the rate of the E1 reaction depends only on the substrate, since the rate limiting
step is the formation of a carbocation while the rate of the E2 reaction depends on both
substrate and base, since the rate-determining step is bimolecular (concerted).
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4.2 ANALYSIS OF ISSUES
4.2.1 MARKOVNIKOV AND ANTI-MARKOVNIKOV RULES
4.2.1.1 MARKOVNIKOV’S RULES
Figure 10: Reaction between HBr and alkene
Markovnikov’s rule state that when an unsymmetrical molecule of the form HX adds
to an unsymmetrical alkene, the hydrogen atom from the HX becomes attached to the
unsaturated carbon atom that already had the most hydrogen atoms.
The chemical basis for Markovnikov's Rule is to form the most stable carbocation
during the addition process. When hydrogen ion is added to one carbon atom in the alkene, it
creates a positive charge on the other carbon and form a carbocation intermediate. Due to
induction and hyperconjugation, the more substituted the carbocation (the more bonds it has
to carbon or to electron-donating substituents), the more stable it will become. The major
product of the addition reaction will be formed from the more stable intermediate. Thus, the
major product of the addition of HX to an alkene has the hydrogen atom in the less
substituted position and X in the more substituted position as X is more electronegative than
H. But the other product will be the minor product with the opposite, conjugate attachment of
X as it has less substituted and less stable carbocation formed at some concentration.
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MECHANISM OF MARKOVNIKOV’S RULES
Example of mechanism is for reaction of alkenes with HBr.
Figure 11: The Mechanism of Markovnikov’s rule
Step 1 :
It is an acid/base reaction. To generate the more stable carbocation, protonation of the alkene
occur. The p electrons act as a Lewis base.
Step 2:
Nucleophilic bromide ion attack the electrophilic carbocation to creates the alkyl bromide
4.2.1.2 ANTI-MARKOVNIKOV’S RULES
Anti-Markovnikov rule is when rather than the more substituted carbon, the
substituent is bonded to a less substituted carbon. This process is quite rare, as carbocations
which are usually formed during alkene, or alkyne reactions prefer to bias the more
substituted carbon. This is because, in order to make the carbocation more stable, substituted
carbocation allow more hyperconjugation and induction to happen.
Anti-Markovnikov Radical Addition of Haloalkanes can only occur to HBr and
presence of Hydrogen Peroxide (H2O2) is crucial. Hydrogen Peroxide is vital for this process
because in the initiation step, it is the chemical which starts off the chain reaction. HI and
HCl cannot be used in radical reactions. This is because one of the radical reaction steps:
Initiation is Endothermic in HI and HCl radical reaction, this means the reaction is
unfavorable.
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MECHANISM OF ANTI-MARKOVNIKOV’S RULES
Figure 12: Initiation step
Radical reactions need an initiation step. A bromine radical is formed during initiation step.
Figure 13: Propagation step
In propagation step, addition of electrophilic bromine radical to the alkene generates the 3 o
radical. Then, radical attacks a H atom from another molecule of HBr to create the alkyl
bromide and another bromine radical.
4.2.2 DEHYDROHALOGENATION OF HALOALKANES
Dehydrohalogenation is carried out by heating a haloalkane with an alcoholic solution of
bases, such as OH- or OR - . During this process, a hydrogen atom and halogen atom are
removed from adjacent carbon atoms of haloalkanes. When bromoethane is heated with
concentrated ethanolic solution of sodium hydroxide, the elimintaion of HBr occurs and
ethene is formed.
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Figure 14: Example of dehydrohalogenation of haloalkane
If the reaction used more than three carbon atoms, it can be more than one elimination
product of a haloalkane. For example, 1-butene and 2-butene are produced when 2-
iodobutane is refluxed with an ethanolic solution of potassium hydroxide.
Figure 15: Saytzeff’s rule
According to the Saytzeff’s rule, dehydrohalogenation will yield an alkene in which the
C = C bond has the larger number of alkyl groups as the main product. Thus, based on the
example, 2-butene is the main product and 1-butene is the minor product.
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5.0 CONCLUSION
Based on our study, we can conclude that both hydrohalogenation of alkenes and
dehydrohalogenation of haloalkane are reverseable reactions. Allah swt stated in Surah Al-
Haj verse 104, ‘The Day that we roll up the heavens like a scroll rolled up for
books(completed) even as We produced the first creation, so shall We produced a new one: a
promise We have undertaken; truly shall We fulfill it’. From this verse, it shows that Allah
will created something at the beginning exactly same at the end.
Besides that, we can establish that markovnikov and anti-markovnikov’s rule are used to
determine which carbon atom that will be attached with the hydrogen atom during
substitution process. Meanwhile, eliminations follow Saytzeff’s rule to identify major and
minor products of dehydrohalogenation of haloalkane. Therefore, it proves that every Allah’s
creation has its own significance and roles in this world. Just like has been stated in Surah
Fatir verse 27, “See you not that Allâh sends down water (rain) from the sky, and We
produce therewith fruits of varying colours, and among the mountains are streaks white and
red, of varying colours and (others) very black”.
All in all, we should gain knowledge as the Prophet Muhammmad once said, “The
seeking of knowledge is obligatory for every Muslim”. This hadith shows that knowledge is
really important to Muslim in order to be a great caliph in this world.
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6.0 APPENDICES
Order of stability of carbocations
primary < secondary < tertiary
Figure 16: Order of carbocations’ stability
Table 3: Difference between Markovnikov’s and anti-Markovnikov’s rule
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7.0 BIBLIOGRAPHY
(n.d.). Reaction of Alkene With Hydrogen Halides. Retrieved from
http://www.mhhe.com/physsci/chemistry/carey/student/olc/graphics/carey04oc/ref/ch
06hydrohalogenation.html
Balasubramanian S., (n.d.). E1 reactions. Retrieved from
http://chemwiki.ucdavis.edu/Core/Organic_Chemistry/Reactions/Elimination_Reactio
ns/E1_Reactions
Brown W. H., Foote C. S., Iverson B. L., Anslyn E. V., (2009). Neuclophilic substitution and
β-elimination. Organic Chemistry: Fifth Edition. Canada, U.S.A: Brooks/Cole
Cengage Learning.
Clark J., (2000). Carbocations (or carbonium ions). Retrieved from
http://www.chemguide.co.uk/mechanisms/eladd/carbonium.html
James., (n.d.). Comparing the E1 and E2 reactions. Retrieved from
http://www.masterorganicchemistry.com/2012/10/10/comparing-the-e1-and-e2-
reactions/
Kan K., (n.d.). Radical Additions: Anti-Markovnikov Product Formation. Retrieved from
http://chemwiki.ucdavis.edu/Core/Organic_Chemistry/Hydrocarbons/Alkenes/Reactiv
ity_of_Alkenes/Radical_Additions--Anti-Markovnikov_Product_Formation
Tan Y. T., Shamuganathan S., (2014). Haloalkanes (Alkyl Halides). Chemistry for
Matriculation Semester 2. Selangor, Malaysia: Oxford Fajar Sdn. Bhd.
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