Molecular Geometry & PolarityA. Substance structure will influence physical properties

• Lewis dot structure for molecules• Molecular Polarity (polar versus nonpolar)

A. Be able to draw Lewis dot structures

B. molecules have ideal or non-ideal geometry

C.electronegativity differencesdetermines bond polarity

D. Overall Molecular Geometry determines molecular polarity

These ideas are described on the next two slides

Symmetric versus Asymmetric

variable polarityammonia

gas water

OH

HNHH

H

••••

polar

C

H

HH

H

CFF

F

F

non-polar polar

CHH

F

FF

HH

H C

Symmetrical arrangement of polar bonds

bond polarities cancel

Asymmetrical arrangement of polar bonds

bond polarities don’t cancel

CHH

F

FF

HH

H C

TheseMoleculesHave aNet Dipole

NONetDipoleBondDipolesCancel

ideal non-ideal non-ideal

Molecular Substances (discrete units)• Molecular Geometry and Bond Polarity

– Ideal geometries show have variable polarity

– Non-ideal geometries are generally polar

••••

variable polarityammonia

gas water

OH

HNHH

H

••••

polar

C

H

HH

H

CFF

F

F

ideal non-polar polar

BUT, these idealgeometries hereare polar

CHH

F

FF

HH

H C

TheseMoleculesHave aNet Dipole

NONetDipoleBondDipolesCancel

TheseMoleculesHave aNet Dipole

C

H

CH

H

OH

H

C

H

CH COH

H

C

H

N

H

NHH

H

N N N

H

H

O H

H

C

H

O

C

N

HH

HH

HH

H

O HH

(unstable)nitrogen gasdiazene gasammonia gas

formaldehyde gaswater liquid

carbon dioxide gasacetylene gasethylene gasmethane gas

••••

••••

••••

••••

••••

••••

:

::

:

:

:::

:

:::

:

Draw in bond dipole for each bond

C

H

CH

H

OH

H

C

H

C

N

H C

H

NH

O

H

H

H

N N N

H

H

H

O

C

H

H

H

H

C

H

O

N N

H

H

C

Cl

H

C

Cl

H

C

Cl

Cl

C

H

C

H

H

Cl

H

H

C

H

Cl

C

Cl

trans

cis

transcis

:

:

:

::

:

::

:

:

DIPOLE MOMENTS & MOLECULAR POLARITYdetermining the bond polarity between two disimilar atoms in a chemical bond. Draw in all bond dipole moments and the overall dipole moment if the molecule is polar.

:

Molecular Polarity• Determining the polar nature of molecular substances

– Evaluating Substance Structure

– Physical Observations - Common Sense Approach

• Solubility (solute/solvent interactions) to gauge molecular polarity– “Like will dissolve Like”

• Polar solutes will have highest solubility in polar solvents

• Nonpolar solutes will have highest solubility in nonpolar solvents

• Polar solutes will have lowest solubility in nonpolar solvents

• Nonpolar solutes will have lowest solubility in polar solvents

• Sugar dissolves in water– Thus sugar molecules must be polar

• Methanol CH3OH dissolves in water

– Thus methanol molecules must be polar

• Gasoline -(CH 2)- does not dissolve in water

– Thus gasoline molecules must be NONPOLAR

Molecular substance solubility in water “Like dissolves Like ”

to gauge Molecular Polarity

δ+

δ−

OH

H

••

polar

H

H

C A gasoline molecule is a hydrocarbon made of repeating –( CH2 )– units is non-polar; NO net dipole

H H

H

H

CC

H

H H

H

CC

H

HH

H

CC

H

H H

H

CC

H

H

A gasoline hydrocarbon

All dipoles cancel

hydrocarbons are non-polar

Nonpolar does not dissolve Polar

δ+

δ−C

H

HH

H OH

H

••

methane does not dissolve in water

gasoline does not dissolve in water

n δ+

δ−C

H

HO

HH

••

Polar physically dissolves in Polar

methanol dissolves in water

δ+δ−

OH

•• ••

HHOH

H

H

Cδ−

δ+

sugar dissolves in water

n

δ+δ−

••

HHO

H

OH

Cδ−

δ+

opposites attractphysical bond

opposites attractphysical bonding

The highlighted area is an example of hydrogen bonding

methanol dissolves in water

δ+δ−

OH

•• ••

HHOH

H

H

Cδ−

δ+

sugar dissolve in water

n

δ+δ−

••

HHO

H

OH

Cδ−

δ+

“Like dissolves Like”• Polar dissolves Polar

– Vitamin B11 (folic acid)

– Vitamin C (ascorbic acid)

NH

NH2

HO

O

NH

HOO

N

NN

N

OOH

O

OHHO

OHO

HO

polar areas

δ+

δ−

OH

H

••

Water soluble vitamins

Where ever there are N and O atoms, there are polar areas

“Like dissolves Like”• Non-Polar dissolves Non-Polar

– Vitamin A, retinol (fat soluble; lipid soluble)

– triacylglycerine, a non-polar human body fat (lipid)

OH

C-O

C-O

O-C

O

O

O

Vitamins D, E, and K are fat soluble as wellWhat would their overall polarity be?

nonpolar areas

Predict whether the substance is Polar or Nonpolar

sugar C6H12O6

baby oil C20H42

candle wax C40H82

ethanol C2H5OH

oxygen O2

iodine I2

polar

nonpolar

nonpolar

polar

nonpolar

nonpolar