NTA Level 5 • Semester 1 • PST05106

Alkynes of Pharmaceutical Importance

Pharmaceutical Organic Chemistry • Source Session/Topic 10

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Session 10: Alkynes 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 alkynes

• List alkynes and their isomers

• Explain nomenclature of alkynes

• Draw chemical structure of alkynes

• List physical properties of alkynes

• Explain chemical reactions of alkynes

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 Alkynes (10 minutes).

|Activity: Brainstorming (5 minutes) |

| |

|Ask students to brainstorm on the following question: |

| |

|What are Alkynes? |

| |

|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 four less hydrogen atoms

as compared to corresponding alkanes, also known as Acetylenes, general

formula: (CnH2n-2).

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

feature of the alkynes.

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

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

|Activity: Buzzing (10minutes) |

| |

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

|minutes. |

| |

|What are the isomers of Alkynes? |

| |

|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 |

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.

▪ 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 Isomer

▪ 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

STEP 4: Nomenclature of Alkynes (15 minutes).

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

must be added to name and locate the triple bond.

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

that contains the C-C triple bond:

▪ C-C triple bonds are designated by the ending -yne, if more than one

triple bond is present, the ending is diyne, triyne, tetrayne, etc.

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

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

lowest possible numbers.

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

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

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

Examples

o [pic] [pic]

o [pic][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.

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

• The sigma bond is sp-sp overlap

[pic]

• The two pi bonds are unhybridized p overlaps at 90(, which blend into

cylindrical shape.

[pic]

• Bond Lengths

• More s character, so shorter length than alkenes or alkanes.Three bonding

overlaps, so shorter

[pic]

• Acidity Table

[pic]

Table 2: Chemical stuctures of alkynes CnH2n-2

| IUPAC Name | Molecular Formula |Condensed Structural |

| | |Formula |

|Ethyne |C2H2 |CHCH |

|Propyne |C3H4 |CHCCH3 |

|1-butyne |C4H6 |CHCCH2CH3 |

|1-pentyne |C5H8 |CHC(CH2)2CH3 |

|1-hexyne |C6H10 |CHC(CH2)3CH3 |

|1-heptyne |C7H12 |CHC(CH2)4CH3 |

|1-octyne |C8H14 |CHC(CH2)5CH3 |

STEP 6: Physical Properties of Alkynes (10 minutes).

|Activity: Brainstorming (5 minutes) |

| |

|Ask students to brainstorm on the following question: |

| |

|What are physical properties of Alkynes? |

| |

|ALLOW few students to respond. |

| |

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

| |

|CLARIFY and SUMMARISE by using the content below |

• Nonpolar, insoluble in water

• Soluble in most organic solvent

• Boiling points similar to alkane of same size

• Less dense than water

• Up to 4 carbons, gas at room temperature

STEP 7: Chemical Reactions involving Alkynes (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 |

| |

• Reduction Reactions

o Addition of Hdrogen; Alkynes can be reduced to form alkenes and/or

alkanes depending on upon the reaction condition.

Examples;

o Reduction with Metal – Amonia: Alkynes on treatment with sodium or

lithium in liquid ammonia produce trans- alkenes as almost exclusive

product.

[pic]

o Catalytic reduction: Catalytic reduction of alkynes to alkanes

[pic]

• Electrophilic Addition

o Addition of halogens:Addition of halogens to alkyenes takes place in

two stages, first forming a trans-1, 2-dihaloalkene and then

1,1,2,2,-tetrahaloalkane

[pic]

o Addition of halogen acids: On treatment with halogens acids, alkynes

first form vinly halides followed by alkylidene halides. The second

molecule of halogens acid adds according to Markowkwnikoff’s rule

[pic]

o Hydration (addition of water): React with water in presence of

sulphuric acid and mercuric sulphate to form a vinyl alcohol which

readily tautomerizes to the corresponding carbonyl derivative.

[pic]

o Ozonization: Ozon adds to carbon-carbon triple bond forming ozonide.

The ozonides are hydrolyzed by water and the products – the 1,2-

diketones undergo oxidative cleavage to acids by hydrogen peroxide

formed in the reaction.

[pic]

o Hydroboration: Addition of borane to carbon –carbon triple bond

produces vinlyboranes which is oxidized by hydrogen peroxide to

given enol. Tautomerazation then gives either ketone or aldehyde

depending on the structure of alkyne reactant.

[pic]

STEP 8: Key Points (05 minutes)

• Alkynes or acetylenes are unsaturated hydrocarbons containing carbon

–carbon triple bond and having general formula (CnH2n-2).

• The IUPAC Rules is used for naming of alkynes

• Alkynes undergo reduction and electrophilic addition reactions.

• Alkynes occur in some pharmaceuticals including the contraceptives

noretynodrel, antiretroviral Efavirenz and antifungal terbinafine.

STEP 9: Evaluation (05 minutes)

• What are alkynes?

• What are rules for naming alkynes?

• What are chemical reactions involving alkynes?

• What are the uses of alkynes?

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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