NTA Level 5 • Semester 1 • PST05106

General Properties of Organic Compounds

Pharmaceutical Organic Chemistry • Source Session/Topic 5

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Session 5: General Properties of Organic Compounds.

Total Session Time: 120 minutes + 10 minutes home assignment.

Prerequisites

None

Learning Tasks

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

• List physical properties of organic compounds

• Explain the general properties of organic compounds

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: Physical Properties of Organic Compounds (25 minutes).

|Activity: Buzzing (10minutes) |

| |

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

|minutes. |

| |

|What are the physical properties of organic compounds? |

| |

|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. |

The following are the physical properties of organic compounds;

Melting Point.

• It usually indicates the temperature in which a state of a compound

changes from solid to liquid state.

• There are few factors that affect the melting point such as:

o Size of a molecule:

▪ Melting Point identifies the characteristics of an organic

compound.

▪ Two different compounds consisting of a variant structural

arrangement of atoms or possess different configurations will have

difference of melting point.

▪ Two samples possessing same melting point will have same

configurations.

o Force of attraction between the molecules:

▪ Melting point of a compound is usually affected by the force of

attraction between the molecules.

▪ The existence of hydrogen bonds in organic compounds will result

to a higher melting point.

Boiling Point:

• Boiling Point varies depending on the surrounding environment.

• A boiling point of a liquid is high at high pressure and has a lower

boiling point when atmospheric pressure is low.

• Factors that affect boiling point and they are stated below.

o Polarity: Greater the polarity the higher the boiling point, that is,

polarity determines the force of attraction between the molecules.

Molecules are attracted by opposite charges in a polar compound.

o Carbon-carbon chain: Boiling point decreases with the increase in the

length of a carbon-carbon chain.

o Strength of Intermolecular forces: Various effects such as Vander

Waals dispersion hydrogen – bonding. Ionic bonding will affect the

strength of intermolecular forces.

Solubility

• Organic compounds may dissolve in solvents like mixture, ethyl

alcohol or white spirits.

Flammability and vapour pressure

• Flammability is a measure of how easy it would be for a substance to

catch alight and burn.

• When a substance is in the liquid or solid state there will be some

molecules in the gas state. The weaker the intermolecular forces within a

substance the higher the vapour pressure will be.

• Compounds with higher vapour pressures have lower flash points and are

therefore more flammable.

STEP 3: General properties of organic compounds (60 minutes).

|Activity: Small Group Discussion (20 minutes) |

| |

|DIVIDE students into small manageable groups. |

| |

|ASK students to discuss on the following question; |

|What are the general properties of organic compounds? |

| |

|ALLOW students to discuss for 15 minutes. |

| |

|ALLOW few groups to present and the rest to add points have not been |

|mentioned. |

| |

|CLARIFY and SUMMARIZE by using the contents below |

• An understanding of the various types of noncovalent forces allows us to

explain, on a molecular level, many observable physical properties of

organic compounds.

• Factors that influence physical properties of organic compound are:

o Intermolecular forces

o Type of function group

o Chain length

o Shape of the molecule

• Intermolecular forces are forces that exist between molecules. They

include;

o Van der waals forces –dipole-dipole forces

▪ -induced dipole-induced dipole forces (London dispersion

forces)

o Hydrogen bonding.

Flammability

• Flammability is a measure of how easy it would be for a substance to

catch alight and burn. The flash point of a substance is the lowest

temperature that is likely to form a gaseous mixture you could set

alight.

• If a liquid has a low enough flash point it is considered flammable (able

to be ignited easily) while those with higher flash points are considered

nonflammable.

• A substance that is classified as nonflammable can still be forced to

burn, but it will not ignite easily.

Vapor pressure

• When a substance is in the liquid or solid state there will be some

molecules in the gas state. These molecules have enough energy to

overcome the intermolecular forces holding the majority of the substance

in the liquid or solid phase

• These gas molecules exert a pressure on the liquid or solid (and the

container) and that pressure is the vapour pressure of that compound

• The weaker the intermolecular forces within a substance the higher the

vapour pressure will be

• Compounds with higher vapour pressures have lower flash points and are

therefore more flammable

Solubility

Solubility is a chemical property referring to the ability for a given

substance, the solute, to dissolve in a solvent.

• It is measured in terms of the maximum amount of solute dissolved in a

solvent at equilibrium. The resulting solution is called a saturated

solution.

Solubility of polar compounds in water

• When considering the solubility of an organic compound in a given

solvent, the most important issue to consider is how strong the

noncovalent interactions between the compound and the solvent molecules

are.

• If the solvent is polar, like water, then a smaller hydrocarbon component

and/or more charged, hydrogen bonding, and other polar groups will tend

to increase the solubility.

• If the solvent is non-polar, like hexane, then the exact opposite is

true.

• For example, table salt, or sodium chloride will dissolve almost

immediately in water, because water, as a very polar molecule, is able to

form many ion-dipole interactions with both the sodium cation and the

chloride anion, the energy from which is more than enough to make up for

energy required to break up the ion-ion interactions in the salt

crystal.

• The end result, then, is that in place of sodium chloride crystals, we

have individual sodium cations and chloride anions surrounded by water

molecules – the salt is now in solution.

• Charged species as a rule dissolve readily in water: in other words, they

are very hydrophilic (water-loving).

Solubility of non-polar compounds

• A compound called biphenyl, which in non-polar, will not dissolve in

water.

• This is because it is a very non-polar molecule, with only carbon-carbon

and carbon-hydrogen bonds.

o It is able to bond to itself very well through nonpolar van der Waals

interactions, but it is not able to form significant attractive

interactions with very polar solvent molecules like water.

o Thus, the energetic cost of breaking up the biphenyl-to-biphenyl

interactions in the solid is high, and very little is gained in terms

of new biphenyl-water interactions.

o Therefore, water is a terrible solvent for nonpolar hydrocarbon

molecules: they are very hydrophobic (water-fearing).

Solubility of alcohol in water

• Another example is alcohol compounds, starting with methanol (1 carbon)

and ending with octanol (8 carbons).

• Smaller alcohols – methanol, ethanol, and propanol – dissolve easily in

water, at any water/alcohol ratio that you try.

• This is because the water is able to form hydrogen bonds with the

hydroxyl group in these molecules, and the combined energy of formation

of these water-alcohol hydrogen bonds is more than enough to make up for

the energy that is lost when the alcohol-alcohol (and water-water)

hydrogen bonds are broken up.

• However, butanol is only sparingly soluble in water

• The longer-chain alcohols – pentanol, hexanol, heptanol, and octanol –

are increasingly non-soluble in water.

• This is because the larger alcohols have larger nonpolar, hydrophobic

regions in addition to their hydrophilic hydroxyl group.

• At about four or five carbons, the influence of the hydrophobic part of

the molecule begins to overcome that of the hydrophilic part, and water

solubility is lost.

Boiling point and melting point

• The observable melting and boiling points of different organic molecules

provides an additional illustration of the effects of noncovalent

interactions.

• Melting and boiling are processes in which noncovalent interactions

between identical molecules in a pure sample are disrupted.

• The stronger the noncovalent interactions, the more energy that is

required, in the form of heat, to break them apart

• As a rule, larger molecules have higher boiling (and melting) points.

Butane versus Octane

• Consider the boiling points of increasingly small versus larger

hydrocarbons.

• More carbons and hydrogens mean a greater surface area possible for van

der Waals interaction, and thus higher boiling points.

• Below zero degrees centigrade (and at atmospheric pressure) butane is a

liquid, because the butane molecules are held together by Van der Waals

forces.

• Above zero degrees, however, the molecules gain enough thermal energy to

break apart and enter the gas phase.

• Octane, in contrast, remains in the liquid phase all the way up to 128oC,

due to the increased van der Waals interactions made possible by the

larger surface area of the individual molecules.

• The strength of intermolecular hydrogen bonding and dipole-dipole

interactions is reflected in higher boiling points.

• Look at the trend for hexane (van der Waals interactions only), 3-

hexanone (dipole-dipole interactions), and 3-hexanol (hydrogen bonding).

• In all three molecules, van der Waals interactions are significant.

• The polar ketone group allows 3-hexanone to form intermolecular dipole-

dipole interactions, in addition to the weaker van der Waals

interactions. 3-hexanol, because of its hydroxyl group, is able to form

intermolecular hydrogen bonds, which are stronger yet.

• The effect of hydrogen bonding in water of particular interest.

• Because it is able to form tight networks of intermolecular hydrogen

bonds, water remains in the liquid phase at temperatures up to 100 OC

despite its small size.

STEP 4: Key Points (10 minutes).

• The physical properties of organic compounds include melting point,

boiling points, Solubility, flammability and vapor pressure

• When considering the solubility of an organic compound in a given

solvent, the most important issue to consider is how strong are the

noncovalent interactions between the compound and the solvent molecules

• Melting and boiling are processes in which noncovalent interactions

between identical molecules in a pure sample are disrupted.

STEP 5: Evaluation (10 minutes).

• What are physical properties of organic compounds?

• What is the solubility of polar compounds in water?

• What is the effect of larger molecules on melting and boiling points?

STEP 6: Take Home Assignment (10 minutes)

|Activity: Take home Assignment (10 minutes) |

| |

|DIVIDE students in groups or individual. |

| |

|ASK the students to work on the following assignment |

| |

|Write short notes on the solubility of glucose, benzoic acid and |

|acetic acid in water. |

| |

|ALLOCATE time for students to do the assignment and submit. |

| |

|REFER students to recommended references |

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.

Rama R. N. (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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