NTA Level 5 • Semester 1 • PST05106

Aldehydes of Pharmaceutical Importance

Pharmaceutical Organic Chemistry • Source Session/Topic 16

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Session 16: Aldehydes 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 aldehydes

• List aldehydes and their isomers

• Explain nomenclature of aldehydes

• Draw chemical structure of aldehydes

• List chemical properties of aldehydes

• Explain chemical reactions of aldehydes

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 Aldehydes (10 minutes).

|Activity: Brainstorming (5 minutes) |

| |

|Ask students to brainstorm on the following question: |

| |

|What are Aldehydes? |

| |

|ALLOW few students to respond |

| |

|WRITE their responses on the flip chart/ board |

| |

|CLARIFY and SUMMARISE by using the content below |

Aldehydes are compounds of the general formula RCHO (R may be aliphatic or

aromatic group)

Examples of aldehydes

In aldehydes, the carbonyl group has a hydrogen atom attached to it

together with either

• a second hydrogen atom Or

• More commonly, a hydrocarbon group which might be an alkyl group or one

containing a benzene ring.

[pic]

ALDEHYDE

STEP 3: Aldehydes and their Isomers (10 minutes).

Aldehydes exhibit the following type of isomerism:

• Chain (nuclear) Isomerism.

o Aldehydes with 4 or more carbon atoms show chain isomerism. For

example:

[pic]

• Position isomerism.

o Aromatic aldehydes and higher ketones give position isomers. For

example:

[pic]

• Functional Isomerism.

o The general formula of aldehydes, ketones, unsaturated alcohols

oxiranes and oxolanes is CnH2nO

o C3H6O has isomers as:

[pic]

STEP 4: Nomenclature of Aldehydes (15 minutes).

I. The longest chain carrying the –CHO group is considered as the parent

structure. Aldehydes are named by replacing the ‘-e’ of the

corresponding alkane by the ending ‘-al’ [pic]

II. When R is aromatic just the word aldehyde is added to the aromatic name

III.The common name of simple aldehydes end with ‘aldehyde’

Examples:

Formaldehyde (=methanal)

Acetaldehyde (=ethanal)

Propionoaldehyde (=propanal)

Butyraldehyde (=butanal)

IV. If the aldehyde group is attached to a large unit (ring), the suffix

carbaldehyde is used.[pic]

[pic]

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

In an aldehyde, at least one of the attached groups must be a hydrogen

atom. The following compounds are aldehydes:

[pic]

• In condensed formulas, we use CHO to identify an aldehyde rather than

COH, which might be confused with an alcohol.

• This follows the general rule that in condensed structural formulas H

comes after the atom it is attached to (usually C, N, or O).

[pic]

Some structures of simple aldehydes

[pic]

STEP 6: Chemical Properties of Aldehydes (15 minutes).

|Activity: Buzzing (5minutes) |

| |

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

|minutes. |

| |

|What are the chemical properties of Aldehydes? |

| |

|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 |

• Aldehydes can be reduced to a variety of compounds under different

conditions with different reducing agents.

• Aldehydes are reduced to the corresponding alcohols by;

o Addition of hydrogen in the presence of catalysts like finely divided

platinum, palladium, nickel and ruthenium.

o Treatment with chemical reagents such as sodium borohydride (NaBH4) or

Lithium aluminium hydride (LiAlH4).

• Aldehydes are easily oxdised to carboxylic acids on treatment with common

oxidising agents like nitric acid, potassium permanganate, potassium

dichromate etc.

• Aldehydes respond to the Fehlings' test.

o Fehlings' solution is an alkaline solution of copper sulphate

containing sodium potassium tartrate (Rochelle Salt) as a complexing

agent.

o Aldehydes on warming with solution, give a red precipitate of cuprous

oxide as a result of the redox reaction.

o Aromatic aldehydes give very poor results in this test.

STEP 7: Chemical Reactions involving Aldehydes (35 Minutes).

|Activity: Small Group Discussion (15 minutes) |

| |

|DIVIDE students into small groups. |

| |

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

|What are the chemical reactions involving Aldehydes? |

| |

|[pic]REFER Students to Book |

| |

|ALLOW students to discuss for 10 minutes. |

| |

|ALLOW each group to present for 5 minutes. |

| |

|CLARIFY and SUMMARIZE by using the contents below |

• Aldehydes undergo many reactions to give a wide variety of useful

derivatives.

• Their most common reaction is nucleophilic addition, addition of a

nucleophile and a proton across the C = O double bond.

• The reactivity of the carbonyl group arises from the electronegativity of

the oxygen atom and the resulting polarization of the carbon-oxygen

double bond.

• The electrophilic carbonyl carbon atom is sp2 hybridized and flat,

leaving it relatively unhindered and open to attack from either face of

the double bond.

• As a nucleophile attacks the carbonyl group, the carbon atom changes

hybridization from sp2 to sp3

• The electrons of the pi bond are forced out to the oxygen atom to form an

alkoxide anion, which protonates to give the product of nucleophilic

addition.

[pic]

• Aldehydes are oxidized to carboxylic acids.

[pic]

Nucleophilic addition of Grignard Reagents

• A Grignard reagent (a strong nucleophile resembling a carbanion, R : -)

attacks the electrophilic carbonyl carbon atom to give an alkoxide

intermediate.

• Subsequent protonation gives an alcohol.

o [pic]

Hydride Reduction of Aldehydes

• Hydride reduction of an aldehyde is another example of nucleophilic

addition, with hydride ion (H : -) serving as the nucleophile.

• Attack by hydride gives an alkoxide that proto nates to form an alcohol.

[pic]

STEP 8: Key Points (05 minutes).

• An aldehyde is an organic compound containing a functional group with

the structure −CHO, consisting of a carbonyl center (a carbon double-

bonded to oxygen) with the carbon atom also bonded to hydrogen and to

an R group, which is any generic alkyl or side chain.

• In condensed structural formulas, the carbonyl group of an aldehyde is

commonly represented as −CHO.

• Aldehydes shows chain, position and functional isomerism

• Aldehydes can be easily oxidised to carboxylic acids due to the

presence of a hydrogen atom on carbonyl group which can be easily

converted to OH group.

• Reactions of aldehydes with alcohols produce either hemiacetals (a

functional group consisting of one —OH group and one —OR group bonded

to the same carbon) or acetals (a functional group consisting of two

—OR groups bonded to the same carbon), depending upon conditions

STEP 9: Evaluation (05 minutes).

• What are aldehydes?

• List two aldehydes and their isomers

• How are aldehydes named?

• Draw chemical structure of aldehydes

• What are chemical reactions of aldehydes?

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.). California, 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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