NTA Level 5 • Semester 1 • PST05106

Alkenes of Pharmaceutical Importance

Pharmaceutical Organic Chemistry • Source Session/Topic 9

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Session 9: Alkenes of Pharmaceutical Importance.

Total Session Time: 120 minutes

Prerequisites

None

Learning Tasks

By the end of this session students are expected to be able to:

• Define alkenes

• List alkenes and their isomers

• Explain nomenclature of alkenes

• Draw chemical structure of alkenes

• List chemical properties of alkenes

• Explain chemical reactions of alkenes

Resources Needed:

• Flip charts, marker pens, and masking tape.

• Black/white board and chalk/whiteboard markers.

SESSION OVERVIEW

SESSION CONTENTS.

STEP 1: Presentation of Session Title and Learning Tasks (5 minutes).

READ or ASK students to read the learning tasks and clarify.

ASK students if they have any questions before continuing.

STEP 2: Definition of Alkenes (5 minutes).

|Activity: Brainstorming (5 minutes) |

| |

|Ask students to brainstorm on the following question: |

| |

|What are Alkenes? |

| |

|ALLOW few students to respond. |

| |

|WRITE their responses on the flip chart/ board. |

| |

|CLARIFY and SUMMARISE by using the content below |

• These are unsaturated hydrocarbons, they contain two less hydrogen atoms

as compared to corresponding alkanes (sp2 hybrid), also known as OLEFINS

or ALKYLENES, general formula: (CnH2n).

• They contain carbon-carbon double bond, this is the distinguishing

feature of the alkenes.

• The simplest member of the alkene family is ethylene C2H2.

STEP 3: Alkenes and their Isomers (15 minutes).

|Activity: Buzzing (10minutes) |

| |

|ASK students to pair up and buzz on the following question for 5 |

|minutes. |

| |

|What is the isomer of Alkenes? |

| |

|ALLOW pairs to respond on the question. |

| |

|WRITE their response on the flip chart/board. |

| |

|CLARIFY and SUMMARIZE by using the content in the table 1 below |

Geometric isomerism

• Arises due to restricted rotation at the C-C double bonds

• Occurs only when two atoms/groups attached to each carbon of the double

bond are different from one another

• The cis-isomer has like groups on the same side of the double bond,

whereas the trans-isomer has like group on opposite sides of the double

bond

Example,

[pic]

Cis Butene (the methyl groups are on the same side)

[pic]

Trans Butene (the methyl groups are on the opposite side)

Structural isomerism

• Isomers which have the atoms of their molecules linked in a different

order

• This can come about in one of three ways:

o Chain Isomerism

[pic] [pic][pic]

▪ Chain isomers of the same compound are very similar.

▪ There may be small difference in physical properties such as

melting or boiling point due to different strengths of

intermolecular bonding.

▪ Their chemistry is likely to be identical.

o Positional Isomers

[pic] [pic]

▪ Positional isomers are also usually similar.

▪ There are slight physical differences, but the chemical

properties are usually very similar.

▪ However, occasionally, positional isomers can have quite

different properties

Example of isomerism is given by propanol

• It has the formula C3H8O (or C3H7OH) and two isomers propan-1-ol

(n-propyl alcohol; I) and propan-2-ol (isopropyl alcohol; II)

• Note that the position of the oxygen atom differs between the

two: it is attached to an end carbon in the first isomer, and to

the center carbon in the second.

• The number of possible isomers increases rapidly as the number

of atoms increases;

For example; the next largest alcohol, named butanol

(C4H10O), has four different structural isomers.

[pic] [pic]

STEP 4: Nomenclature of Alkenes (15 minutes).

• The IUPAC Rules are similar to those of alkanes, but few new rules must

be added to name and locate the double bond.

o Rule 1: Select as the parent structure the longest continuous chain

that contains the C-C double bond:

▪ C-C double bonds are designated by the ending -ene, if more than

one double bond is present, the ending is diene, triene, tetraene,

etc.

o Rule 2: Indicate by a number the position of the double bond in the

chain. Number it so that the C-atoms in the double bond have the

lowest possible numbers.

o Rule 3: The position of the double bond(s) is indicated by the

number(s) of the lower numbered carbon atom of each double bond. These

numbers are placed in front of the name of the compound.

Example,

[pic]

o Rule 4: In cyclic hydrocarbons, start numbering around the ring with

the carbons of the double bond indicates by numbers the positions of

alkyl groups attached to the parent chain.

Example,

[pic]

3-Methylcyclopenten

Table 1. Nomenclature of simple alkenes

|COMPOUND |COMMON NAME |IUPAC NAME |

|CH2=CH2 |Ethylene |Ethene |

|CH3CH=CH2 |Propylene |1-Propene |

|CH3CH2CH=CH2 |α-Butylene |1-Butene |

|CH3C(CH3)=CH2 |Isobutylene |2-Methylpropene |

|CH2=C(C2H5)CH2CH3 |- |2-Ethyl-1-butene |

|CH2=CHCl |Vinyl chloride |Chloroethene |

|CH2=CHCH2Cl |Allyl chloride |3-Chloropropene |

|CH3=CHCH=CH2 | |1,3-Butadiene |

STEP 5: Chemical Structure of Alkenes (15 minutes).

Definition

• The arrangement of chemical bonds between atoms in a molecule (or in an

iron or radical with multiple atoms) especially which atoms are

chemically bonded to what other atoms with what kind of chemical bonds,

together with any information on the geometric shape of the molecule

needed to uniquely identify the type of molecule.

OR

• It is the spatial arrangement of atoms in a molecule and the chemical

bonds that holds the atoms together. Example diatomic oxygen or nitrogen

molecules or DNA molecules.

Table 2: Chemical stuctures of alkenes CnH2n

| IUPAC Name | Molecular Formula |Condensed Structural |

| | |Formula |

|Ethane |C2H4 |CH2=CH2 |

|Propene |C3H6 |CH2=CHCH3 |

|1-butene |C4H8 |CH2=CHCH2CH3 |

|1-pentene |C5H10 |CH2=CH(CH2)2CH3 |

|1-hexene |C6H12 |CH2=CH(CH2)3CH3 |

|1-heptene |C7H14 |CH2=CH(CH2)4CH3 |

|1-octene |C8H16 |CH2=CH(CH2)5CH3 |

STEP 6: Chemical Properties of Alkenes (10 minutes).

|Activity: Brainstorming (5 minutes) |

| |

|Ask students to brainstorm on the following question: |

| |

|What are chemical properties of Alkenes? |

| |

|ALLOW few students to respond. |

| |

|WRITE their responses on the flip chart/ board. |

| |

|CLARIFY and SUMMARISE by using the content below |

• Alkenes are chemically more reactive than alkanes, this because alkenes

are unsaturated hydrocarbons that have a double bond, C=C, between two

carbon atoms, almost all of the chemical reactions of alkene occur at the

double bond.

Isomerization

• Alkenes when heated alone at high temperatures (500-700°C) or at lower

temperatures (200-300°C) isomerizes in the presence of catalyst, such as

Al2(SO4)3.

• Alkenes isomerism due to;

o The shifting of the double bond which tends to move towards the center

of chain, e.g., pentene-1 isomerizes to pentene-2.

CH3-CH2-CH2-CH=CH2[pic]CH3-CH2-CH=CH-CH3

pentene-1 pentene-2

o The migration of a methyl group, e.g., butene-1 isomerizes to 2-

methylpropene (iso-butene).

[pic]

• Alkenes undergoes electrophilic addition with halogens, water, hydrogen

and halogen acids to produce di halo alkanes, alcohol, alkane and halo

alkanes, also with oxygen to form epoxides.

• At elevated temperatures (500°C), higher alkenes give substitution

products with chlorine. For example, CH3-CH=CH2 + Cl2 [pic] ClCH2-CH=CH2

+HCl

• Alkenes has an ability to dimarize and polymerizes.

• Alkenes undergoes combustion to produce carbondioxide and water

CH2=CH2 + 3O2 → 2CO2 + 2H2O

STEP 7: Chemical Reactions and Uses of Alkenes (40 minutes).

|Activity: Small Group Discussion (20 minutes) |

| |

|DIVIDE students into small groups. |

| |

|ASK students to discuss in groups on the following questions |

|What are the chemical properties of Alkanes? |

| |

|[pic]REFER Students to Book |

| |

|ALLOW students to discuss for 15 minutes. |

| |

|ALLOW each group to present for 5 minutes. |

| |

|CLARIFY and SUMMARIZE by using the contents below |

• All alkenes have a common feature: a carbon-carbon double bond.

• The reactions of alkenes arise from the reactivity of the carbon-carbon

double bond.

• Because single bonds (sigma bonds) are more stable than pi bonds, the

most common reactions of double bonds transform the pi bond into a sigma

bond.

• For example, catalytic hydrogenation converts the C = C pi bond and the H-

H sigma bond into two C – H sigma bonds

• The double bond serves as an electron source

o These are because atoms are added & the double bond becomes a

single bond

[pic]

• Alkenes can undergo the following reactions:

o Electrophilic addition

o Dimerazation

o Polymerisation

o Combustion

• Electrophilic addition to alkenes

o Addition is the most common reaction of alkenes

o Most addition reactions involve a second step in which a nucleophile

attacks the carbocation (as in the second step of the SN1 reaction),

forming a stable addition product.

o In the product, both the electrophile and the nucleophile are bonded

to the carbon atoms that were connected by the double bond.

o Example 1;

[pic]

Step 1: Attack of the pi bond on the electrophile forms a carbocation.

[pic]

Step 2: Attack by a nucleophile gives the addition product.

[pic]

o Example 2: Ionic addition of HBr to 2-butene

o When gaseous HBr adds to 2-butene the proton in HBr is electrophilic;

it reacts with the alkene to form a carbocation.

o Bromide ion reacts rapidly with the carbocation to give a stable

product in which the elements of HBr have added to the ends of the

double bond.

o Step 1: Protonation of the double bond forms a carbocation.

[pic]

Step 2: Bromide ion attacks the carbocation.

[pic]

The electrophilic addition reactions to alkenes include the following

reactions

1. Catalytic hydrogenation =Addition of Hydrogen

• A solution of alkenes is shaken under slight pressure with H2(g) in the

presence of a small amount of Ni / pt catalyst.

• The hydrogen reduces the double bond to a single bond

• One molecule of H2 is absorbed for each double bond in the unsaturated

compound (heterogeneous)

[pic]

2. Addition of Halogens to Alkenes.

• Halogens add to alkenes to form vicinal dihalides.

• Alkenes are readily converted by Cl2 & Br2 into saturated compounds.

• The halogens will be attached to consecutive positions on carbon chain/

adjacent carbon atoms.

[pic]

EXAMPLE: Addition of Br2 to propene.

Step 1: Electrophilic attack forms a bromonium ion.

[pic]

Step 2: Bromide ion opens the bromonium ion

[pic]

• Chlorine and bromine commonly add to alkenes by the halonium ion

mechanism.

• Iodination is used less frequently because diiodide products decompose

easily.

• Any solvents used must be inert to the halogens; methylene chloride

(CH2CI2), chlorofonn (CHCI3 ), and carbon tetrachloride (CCI4) are the

most frequent choices.

3. Addition of Hydrogen Halides to Alkenes

• Alkenes are converted by HCl, HBr & HI into corresponding alkyl halide

[pic]

• Addition occurs at the double bond; following ‘Markovnikov’s rule’

• ‘Markovnikov’s rule’ The addition of a proton acid to the double bond of

an alkene results in a product with the acid proton bonded to the carbon

atom that already holds the greater number of hydrogen atoms’.

• Reactions that follow this rule are said to follow Markovnikov

orientation and give the Markovnikov product.

• We are often interested in adding electrophiles other than proton acids

to the double bonds of alkenes.

• Markovnikov's rule can be extended to include a wide variety of other

additions, based on the addition of the electrophile in such a way as to

produce the most stable carbocation

‘In an electrophilic addition to an alkene, the electrophile adds in such

a way as to generate the most stable intermediate’.

▪ Mechanism summarized

Step 1: Protonation of the pi bond forms a carbocation.

[pic]

Step 2: Attack by the halide ion gives the addition product .

[pic]

Example.

[pic]

• Where the structure permits, electrophilic addition is accompanied by

rearrangement

[pic]

• The more stable carbocation is preferred because the positive charge is

stabilized by the alkyl groups which release electrons

[pic]

Order of stability of carbocations: 3o>2o>1o

4. Hydration of Alkenes: Addition of Water

• An alkene may react with water in the presence of a strongly acidic

catalyst to form an alcohol.

• Formally, this reaction is a hydration (the addition of water), with a

hydrogen atom adding to one carbon and a hydroxyl group adding to the

other.

• Hydration of an alkene is the reverse of the dehydration of alcohols

[pic]

• The reaction follows ‘Markorvonikov’s rule’

• Hydration of an alkene is accomplished by adding excess water to drive

the equilibrium toward the alcohol.

EXAMPLE: Acid-catalyzed hydration of propene.

Step 1: Protonation of the double bond forms a secondary carbocation.

[pic]

Step 2: Nucleophilic attack by water gives a protonated alcohol

[pic]

Step 3: Deprotonation gives the alcohol

[pic]

• Dimerization of Alkenes

o Under proper conditions isobutene is converted by sulfuric acid/

phosphoric acid into a mixture of two (2) alkenes – C8H16

o Hydrogenation of these alkenes produce the same alkane 2,2,4-

trimethylpentane.

[pic]

o The alkenes produced contain exactly twice the number of carbon and

hydrogen atoms as the original isobutylene, they are known as

dimers of isobutylene

[pic]

• Polymerization of Alkenes

o A polymer is a large molecule composed of many smaller repeating

units (the monomers) bonded together.

o Alkenes serve as monomers for some of the most common polymers,

such as polyethylene, polypropylene, polystyrene, poly (vinyl

chloride), and many others.

o Alkenes generally undergo addition polymerization, the rapid

addition of one molecule at a time to a growing polymer chain.

o There is generally a reactive intermediate (cation, anion, or

radical) at the growing end of the chain; for that reason, addition

polymers are also called chain-growth polymers

o Polymerization is an important industrial process to produce

synthetic polymers such as poly(ethene)=polyethene, Teflon,

plastic, nylon & Bakelite

o In nature there are some natural polymers as well such as; starch,

cellulose, proteins & rubber

• Combustion

o Alkenes, like alkanes, are highly combustible.

o Alkenes burn with a luminous flame to give carbon dioxide and

water; the flame becomes luminous because of the higher carbon

content of alkenes than alkanes.

o Their combustion reactions are exothermic.

[pic]

o Due to the luminosity of the flame, the lower alkenes may be used

as illuminants.

Uses of Alkenes

• Ethene is used in the manufacture of polyethylene or polythene-a

plastic material.

• Propene is used in the manufacture of polypropene.

o These polymers are used in making plastic bags, pipes electrical

insulation.

• Ethene is used in the preparation of solvents like ethylene glycol,

dioxane.

o Ethylene glycol is also used as an antifreeze in automobile

radiators.

• Ethene is a plant hormone which controls growth, seed germination and

fruit development.

o Therefore, ethene is used for artificial ripening of fruits,

flower maturation.

• Alkenes are used as a starting material for the manufacture of many

compounds such as alkyl halides, ethylene oxide, ethanol and other

alkanols.

STEP 8: Key Points (05 minutes).

• Alkenes are organic compounds made up of carbon and hydrogen atoms with

one or more carbon- carbon double bonds.

• The IUPAC Rules is used for naming of alkenes.

• Alkenes undergo Electrophilic addition reactions, Dimerization,

Polymerization and Combustion reactions.

• The most important alkenes for the chemical industry are ethene, propene

and 1,3-butadiene which are used as starting materials in the syntheses

of alcohols, plastics, detergents, and fuels

STEP 9: Evaluation (05 minutes)

• What are alkenes?

• What are rules for naming alkenes?

• What are chemical reactions involving alkenes?

• What are the uses of alkenes?

References

Ternay, A.L (1976). Contemporary Organic Chemistry. Philadelphia, United

States: W.B. Saunders Co.

Morrison R.T and Boyd R N (1997). Organic Chemistry (6th Ed.). New Delhi,

India: Prentice Hall of India

Graham Solomon et al (2014). Organic Chemistry (11th Ed.). New Jeysey,

United States: John Willey and Sons.

Nadendla R. R. (2005). Principles of Pharmaceutical Organic Chemistry. New

Delhi, India: MacMillan Publishers

Bruice Y (2013). Organic Chemistry (7th ed.). New York, United States:

Prentice Hall Pearson.

Delgado J. N. Et al (1998). Wilson and Gisvold's Textbook of Organic

Medicinal and Pharmaceutical Chemistry (10th Ed.). Calfornia, United

States: Lippincott Williams

Bhassin S.K, Gupta R.(2013). Pharmaceutical organic chemistry (E-book

Kindle edition). New Delhi, India: Elsevier Publishing Services

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