L3 6P

A104 Biology

Problem 03: To dissolve or not to

dissolve

6

Presentation

In Today’s Problem

Your task today was to:

•

Explore the differences between polar and

non-polar molecules.

•

Explore the different solubility of water

soluble and fat soluble vitamins in water.

What Do You Recognize?

• Vitamins that are fat soluble are non-polar and

vitamins that are water soluble are polar.

The solubility of a substance is related to its

molecular structure.

What is your approach?

• What is the shape of the H₂O molecule?

• What is the electronegativity of different atoms? • How to determine the polarity of bonds?

• Why there is unequal sharing of bonding electrons within a water molecule?

• How to determine the polarity of the molecule?

• What are the conditions to determine polar and non-polar molecules?

• What are fat soluble molecules? • What are water soluble molecules?

• What are the differences between fat soluble and water soluble molecules?

What is the shape of the H

O molecule?

• Figure 1 represents the molecular structure of a single H₂O molecule in which the bonds are 104⁰ apart.

• The Dot-cross diagram in Figure 2 shows how the electrons are distributed within water molecules.

Figure 1. Representation of molecular structure of H₂O molecule

What is the shape of the H

O molecule?

• Electrons are more attracted to oxygen atom than hydrogen atom. Hence the bond pair of electrons is not equally shared between H and O atom. The bond pair of electrons will be drawn nearer to oxygen atom than hydrogen atom as shown in Figure 2.

• This tendency of attracting electrons towards itself is known as “electronegativity”.

Figure 2: Dot-cross diagram of a single H₂O molecule

• Covalent bonding in many compounds is between atoms of different elements when the atoms share electrons.

Covalent bond is a type of intra-molecular bond.

• Since different atoms may have different attraction towards electrons, this also means that different atoms have

different electronegativity values.

• The atom with higher electronegativity have a tendency to pull the bonding pair of electrons towards itself.

• The electronegativity difference of the atoms in a bond determines whether the electrons are shared equally or unequally, and therefore the type of bond.

• If electrons are shared more or less equally, a non-polar covalent bond exists between the atoms (e.g. H-C). The difference in electronegativity values between the atoms will be less than 0.5.

• If electrons are shared unequally, a polar covalent bond exists between the atoms (e.g. H-O). The difference in electronegativity values between the atoms will be more than 0.5 and less than 2.0.

• In some cases, the difference in electronegativity is so great that the electrons are transferred completely. This results in the formation of an ionic bond (e.g. Na+_Cl−₎

with ions that have full charges. The difference in

electronegativity values between the atoms will be more than 2.0.

How to determine the polarity of bonds?

How to determine the polarity of bonds?

• For the single H₂O molecule, O atom has a higher electronegativity than H atom. The value of the electronegativity of O and H is 3.44 and 2.2 respectively.

• The intra-molecular bonding in a single H₂O molecule is polar

covalent since the difference in electronegativity between O atom and H atom is 1.24 (more than 0.5 but less than 2.0).

• The representation of partial charges on the H₂O molecule is shown below. Sharing of electrons is not equal on the H₂O molecule and leads to the formation of partial charges on molecules.

• As O has higher electronegativity, it has a tendency to pull the bonding pair of electrons towards itself. Hence, the H₂O molecule has a partial negative charge (δ-) near the oxygen atom and partial positive charges (δ+) near the hydrogen atoms.

Why there is unequal sharing of bonding

electrons within a water molecule?

Partial negative

Partial positive Partial positive

H H

O

How to determine the polarity of the

molecule?

• If a molecule does not have any polar bonds, this molecule must be a non-polar molecule.

Example: H₂

• The covalent bond between the two hydrogen atom is non-polar, thus H₂ is a non-polar molecule

H H

• For H₂O, determine if the molecule contains any polar bonds by calculating the difference in electronegativity between the O and H atoms in the bond.

• The difference in electronegativity between H and O is 1.24 (= 3.44 – 2.2). O-H bonds are polar bonds.

How to determine the polarity of the

molecule?

O

H H

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How to determine the polarity of the

molecule?

• For molecules with polar bonds, we determine if there is any net dipole moment.

• The arrows in the figure below represent the dipole moment, which indicates the shared electrons are being pulled towards O atom due to its higher

electronegativity.

• The polarity of covalent bond is measured in terms of individual dipole moment. • The electrons are attracted towards the oxygen which makes the side of the

molecule nearer to the oxygen being slightly negatively charged while the side nearer to the hydrogen being slightly positively charged.

The orange arrows

represent the dipole moment for the O-H bonds of the water

molecule.

O

H H

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How to determine the polarity of the

molecule?

• As the electron charge distribution is skewed toward one side, there exist a net positive charge on one end and a net negative charge on the other end of the water molecule as shown.

• This creates a net dipole moment for water molecule, making water molecule a polar molecule.

O

H H

δ−

δ

−

δ+

A partial negative charge exist at

this end A partial

positive charge exist at this

end

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How to determine the polarity of the

molecule?

• Consider the Carbon dioxide molecule as shown below:

• As the difference in electronegativity between the carbon

atom C and oxygen atom O is 0.89 (= 3.44 – 2.55), there is a dipole moment between the carbon and oxygen atom on

both sides.

• However, due to their opposite directions, the two dipole moments cancel out the effect of each other. Thus, there is no net dipole moment in CO₂.

Cancel each other, so no net dipole moment

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How to determine the polarity of the

molecule?

Cancel each other, so no net dipole moment

• Consider the Carbon dioxide molecule as shown below:

• The absence of any net dipole moment means that the electrons in that molecule are evenly distributed for most of the time.

• Hence, this molecule is a non-polar molecule, even though it has polar bonds.

Copyright © 2015

What are the conditions to determine polar

and non-polar molecules?

1. Non-polar molecules • Either no polar bonds OR

• Having polar bonds but no net dipole moment (dipole moments cancel out)

2. Polar molecules

What are fat soluble molecules?

• There are different types of fat soluble vitamins. • Example: Vitamin A

• Vitamin A has only one little hydroxyl (OH) group, attached to a long non-polar chain and even a big non-polar ring.

• The C-H and C-C bonds in vitamin A are non-polar, so the majority of the molecule becomes non-polar.

• Due to the non-polar bond between carbon and hydrogen (= 2.55 – 2.2), fat soluble vitamins are generally non-polar.

What are water soluble molecules?

• There are different types of water soluble vitamins. • Example: Vitamin C

• Vitamin C has a lot of hydroxyl (OH) groups. This group is very polar and can easily establish hydrogen bonds with the water. This makes vitamin C very well soluble in water.

What are the differences between fat

soluble and water soluble molecules?

• Generally molecules that have mainly carbon and

hydrogen are non-polar (fat-soluble) because

there is little to no difference in their

electronegativity.

• When there is a difference in electronegativity

between two atoms or molecules (between 0.5-2.)

and a net dipole moment, the molecule is polar

(water-soluble).

• An example is H

O since oxygen and hydrogen

electronegativity is greater than 0.5 and a net

dipole moment exists.

What are the differences between fat

soluble and water soluble molecules?

• Vitamin C is water-soluble which means it can

dissolve in water and will be able to pass through

the kidneys and will be able to urinate it out.

• Vitamin A is fat-soluble (non-polar) which means it

will not dissolve in water and therefore cannot be

excreted.

Learning points

• Most material properties are due to its molecular structure. • When two atoms share electrons, a covalent bond is formed.

Covalent bonding in many compounds is between atoms of different elements.

• Different atoms have different electronegativities. The difference in electronegativity of the atoms that make up the bonds in a

molecule determines whether the electrons are shared equally or unequally, and therefore the type of intra-molecular bond. If

electrons are shared unequally, a polar covalent bond exists between the atoms (e.g. H-O).

• The intra-molecular bonding in a single H₂O molecule is polar

covalent since the difference in electronegativity between O atom and H atom is 1.24 (more than 0.5 but less than 2.0).

• Sharing of electrons is not equal on the H₂O molecule.

What you have learnt

• Depending on the shape of the molecule and the direction of all the dipole moments, dipole moments can work together or cancel out.

• A polar molecule will have polar bonds and net dipole moment (that is, the dipole moments work together).

Discussion

The figure on the right shows the 3D

representation of a molecule.

Explain whether this is a polar molecule.

Element Electronegativity

Value

Nitrogen 3.04

Hydrogen 2.2

Chlorine 3.16

Fluorine 3.98

N F H Cl Resultant direction of dipole moment