Hydrohalogenation of alkenes and dehydrohalogenation of haloalkanes

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

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http://www.mhhe.com/physsci/chemistry/carey/student/olc/graphics/carey04oc/ref/ch

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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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Hydrohalogenation of alkenes and dehydrohalogenation of haloalkanes

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