Attractive Forces

Intramolecular Forces
- attractive forces within a molecule
(Example: H to O bond within a water molecule).

Intermolecular Forces
- attractive forces between molecules
(Example: water molecule to water molecule)
Types of Intermolecular Forces

1. Dipole-Dipole Interactions

2. Hydrogen Bonding (STRONGEST)

3. London Dispersion Forces (Weakest)

4. Ion-Dipole Interactions
-(Salt dissolving in solution; Na+ and Cl- )

5. Ion-ion interaction
 Types of Intermolecular
Forces of Attraction
1. Dipole-dipole interaction
- electrostatic forces between two permanent polar
molecules.
- generally, the positive end of one molecule is
attracted to the negative end of another molecule.
- as a result, the two molecules come closer, adding to
the stability of the substance.
- this interaction is different from a regular ionic or
covalent bond since there is no transfer or sharing of
electrons.
1. Dipole-Dipole Forces
- Attractive forces between polar molecules
Orientation of Polar Molecules in a Solid

-electrostatic forces results from the attraction of

the σ+ end of one polar molecule for the σ - end of
another polar molecule.
Dipole Forces occur between molecules containing a dipole
moment. The positive end of the dipole moment on one mole
is attracted to the Negative end of the dipole moment on a
nearby molecule.

2-methyl propane
(left) and acetone (right)
Both compounds are about
Equal in size and shape
What Causes a Dipole-dipole Interaction
➢ Dipole-dipole interaction arises due to
the uneven distribution of electrons in a
molecule.
➢ The electrons congregate at one end of the
molecule.
➢ Thus, the molecule acquires a partially
negative charge at one end and a partial
positive charge at the other end, thus
making them polar.
➢ Two polar molecules with opposite charges
will naturally attract one another.
Applications:

✓ Dipole-dipole forces play an essential role in

determining the properties of a substance.

✓ They are significant for the formation of stable

compounds, including biological organisms.

✓ Soft adhesives use dipole-dipole forces that allow

them to stick on to surfaces.
Some applications of the dipole-dipole interactions
are:

Medical industry: Adhesives are used widely in medical devices, allowing a patch to
stick to the skin. From wound dressings to transdermal patches, adhesives play a vital
role in designing products for end-users.

Construction materials: Construction materials like sand, gravel, rocks, cement, and
water have dipole-dipole forces that provide the desired strength. Aside, materials like
plywood and plastics are often glued together using appropriate glues.

Electronic devices: Electronic components are joined together using soft adhesives.
Besides, household gadgets consist of polar substances like diodes, metalloids, and
semiconductors capable of inducing dipole.

Sports equipment: Material like carbon fiber is used to build sports equipment like
racquets for tennis, badminton, and racquetball. These equipment are durable and require
strong intermolecular forces of attraction.
Some applications of the dipole-dipole interactions are:
1. Soil Moisture Retention and Water Movement:
•Water and Soil Interactions: Water molecules are polar and
interact with soil particles (especially clay and organic matter)
through dipole-dipole interactions. These forces help hold water in
the soil by creating hydrogen bonds between the water molecules
and soil particles, preventing water from evaporating too quickly.

2. Fertilizer Uptake:
•Nutrient Availability: Polar molecules like ammonium (NH₄⁺) or
phosphate ions (PO₄³⁻) in fertilizers interact with the water
molecules and soil particles through dipole-dipole interactions.
This can affect the solubility and bioavailability of nutrients to
plants. For instance, the polar nature of certain fertilizer
components helps them dissolve in water, making them available
for absorption by plant roots.
3. Pesticide Delivery and Efficiency:
•Formulation of Pesticides: Pesticides often rely on molecules
with polar functional groups that can form dipole-dipole
interactions with water or plant surfaces, improving their spread
and absorption. For example, surfactants or emulsifiers that are
used in pesticide formulations help the active ingredients to
disperse more effectively on plant surfaces, increasing efficacy.
•Herbicide and Insecticide Activity: The polarity of active
ingredients in herbicides and insecticides can enhance their
penetration through plant cell membranes or through insect
exoskeletons. These interactions can improve the selective action
of the pesticide while minimizing environmental harm.
2. Hydrogen Bond
- a special type of dipole-dipole interaction
- is formed between polar molecules that contain
hydrogen covalently bonded to a small, highly
electronegative atom: F, O, N.
- the hydrogen bond is essential in biology and
biochemistry.
- It is responsible for maintaining the structure of the
DNA molecule.

O—HN—H
F—H
• it will be attracted to another F, O, or N, on
another molecule.

• A dipole-dipole bond will be formed

between the two molecules which is
called a hydrogen bond.

hydrogen
bond
covalent bond

covalent bond
3. London Dispersion Forces(LDF) or
Van Der Waals Forces
- described as the attraction between two
temporarily polarized bonds

- arises due to inequality in the distribution of

electron in a nonpolar molecule as a result of
constant movement of electrons

- principal attractive force operating among

hydrocarbon molecules

- weakest intermolecular interaction

 London Dispersion Forces(LDF) or Van
Der Waals Forces
- A temporary attractive force that results when the electrons in two
adjacent atoms occupy positions that make the atoms form temporary
dipoles
- sometimes called an induced dipole-induced dipole

The larger the molecule the greater

it’s Dispersion Forces
4. Ion-Dipole Attraction
Attractive forces between an ion and a polar molecule

Ion-Dipole Interaction

The larger the charge the stronger the force

Olmsted Williams Fig 10-34

A molecular picture showing the ion-dipole

Interaction that helps a solid ionic crystal dissolve
in water. The arrows indicate ion-dipole interactions.
 How to Determine if a Molecule
Is Polar

1. Draw Lewis Structure

2. If all of the regions of electron density are

bound to the same thing (CCl4; CO2 ) than the
molecule is non-polar

3. If the regions of electron density are not bound to

the same thing (atom) than the molecule
is polar (HCN; SO2)
 Which of the following molecules are polar (have a
dipole moment)?H2O, CO2, SO2, and CH4

O S

dipole moment dipole moment

polar molecule polar molecule

H C H
O C O

no dipole moment H
nonpolar molecule no dipole moment
nonpolar molecule
10.2
 Some Properties of Organic Compounds and
How they are affected by Structural Effects

A. Melting Point – point wherein the

vapor pressure of solid is equal to the
vapor pressure of liquid

Melting – the breaking away of particles from

the powerful attractive force that keeps them
in place in the crystal lattice
B. Boiling Point –point wherein the vapor
pressure of liquid is equal to the atmospheric
pressure

Boiling- the breaking away of molecules from

the liquid phase
 Factors Affecting Boiling and Melting
Points
1. Strength of IFA : the stronger the IFA, the
higher the bp/mp
Strength of Attraction:
H-bond >> dipole-dipole > LDF

Which has a higher boiling point

a. ethanol or formaldehyde?
Ans. ethanol
b. formaldehyde or propane?
Ans. formaldehyde
2. Molecular Weight (or carbon chain length)
➢ the longer the C chain, the greater the
molecular weight, greater LDF interaction →
higher bp/mp

Which has the higher boiling point

a. n-octane or n-heptane?
Ans. n-octane
b. n-pentane or n-hexane?
Ans. n-hexane
3. Branching
- as the molecule become more branched, they
become more spherical, lesser extent of LDF
interaction, lower bp/mp

Which has the higher boiling point

a. n-hexane or isohexane?
Ans. n-hexane
b. isohexane or neohexane?
Ans. isohexane
4. Polarity
- the greater the polarity of the molecules,
the stronger the dipole-dipole interaction, the
higher the bp/mp

Which has the higher melting point

a. H2S or H2O?
Ans. H2O
b. HF or HCl?
Ans. HF
 Factors Affecting Solubility
1. Nature of Solute and Solvent
“ like dissolves like”
2. Carbon-chain Length
- as MW increases, the nonpolar R groups
becomes longer, the greater the LDF interaction
→ less soluble in water

• Which is more soluble in water

a. ethyl alcohol or propyl alcohol?
Ans. Ethyl alcohol
b. pentanoic acid or butyric acid?
Ans. Butyric acid
3. Branching
- as the molecule become more branched,
lesser extent of LDF interaction → more soluble
in water

• Which is more soluble in water

a. n-butyl alcohol or isobutyl alcohol?
Ans. Isobutyl alc.
b. isopentyl alcohol or neopentyl alcohol?
Ans. Neopentyl alc.
4. Effect of Functional Group
- functional group capable of forming H-
bond with water is soluble in water
Which is more soluble in water
a. acetic acid or acetone?
Ans. Acetic acid
b. ethanal or ethanol?
Ans. Ethanol
c. Propanoic acid or propanal
Ans. Propanoic acid
d. ethanethiol or ethylamine
Ans. ethylamine