Chapter 2Structure and Properties

of Organic Molecules

Organic Chemistry, L. G. Wade, Jr.

Chapter 2 2

Molecular Shapes

• Bond angles cannot be explained with simple s and p orbitals. Use VSEPR theory.

• Hybridized orbitals are lower in energy because electron pairs are farther apart.

• Hybridization is LCAO within one atom, just prior to bonding.

CH2:4 Hybridization and Molecular Shape

Chapter 2 3

sp Hybrid Orbitals

• 2-VSEPR pair -1 S and 1 P orbital – 2 sp hybridized orbitals

• 180° bond angle

Chapter 2 4

sp2 Hybrid Orbitals• 3 VSEPR pairs – 1 s and 2 p orbitals• Trigonal planar e- pair geometry• 120° bond angle

Chapter 2 5

sp3 Hybrid Orbitals

• 4 VSEPR pairs – 1 s and 3 p orbitals• Tetrahedral e- pair geometry• 109.5° bond angle

Chapter 2 6

Sample Problems• Predict the hybridization, geometry,

and bond angle for each atom in the following molecules:

• Caution! You must start with a good Lewis structure!

• NH2NH2

• CH3-CC-CHO

CH3 C

OCH2

_

Chapter 2 7

• Single bonds freely rotate.

• Double bonds cannot rotate unless the bond is broken.

Rotation around Bonds

Chapter 2 8

Isomerism

• Same molecular formula, but different arrangement of atoms: isomers.

• Constitutional (or structural) isomers differ in their bonding sequence.

• Stereoisomers differ only in the arrangement of the atoms in space.

Chapter 2 9

Structural Isomers

CH3 O CH3 and CH3 CH2 OH

CH3

CH3

and

Chapter 2 10

Stereoisomers

C CBr

CH3

Br

H3CC C

CH3

Br

Br

H3Cand

Cis - same side Trans - across

Cis-trans isomers are also called geometric isomers.There must be two different groups on the sp2 carbon.

C CH3C

H H

H No cis-trans isomers possible

Chapter 2 11

Bond Dipole Moments• are due to differences in electronegativity.• depend on the amount of charge and

distance of separation.• In debyes, x (electron charge) x

d(angstroms)

Chapter 2 12

Molecular Dipole Moments• Depend on bond polarity and bond angles. • Vector sum of the bond dipole moments.

Chapter 2 13

Effect of Lone PairsLone pairs of electrons contribute to the dipole moment.

Chapter 2 14

Intermolecular Forces• Strength of attractions between

molecules influence m.p., b.p., and solubility, esp. for solids and liquids.

• Classification depends on structure.Dipole-dipole interactionsLondon dispersionsHydrogen bonding

Chapter 2 15

Dipole-Dipole Forces• Between polar molecules.• Positive end of one molecule aligns with

negative end of another molecule.• Lower energy than repulsions, so net force

is attractive.• Larger dipoles cause higher boiling points

and higher heats of vaporization.

Chapter 2 16

Dipole-Dipole

Chapter 2 17

London Dispersions• Between nonpolar molecules• Temporary dipole-dipole interactions• Larger atoms are more polarizable.• Branching lowers b.p. because of

decreased surface contact between molecules.

Chapter 2 18

Dispersions

Chapter 2 19

Hydrogen Bonding

• Strong dipole-dipole attraction.• Organic molecule must have N-H or O-H.• The hydrogen from one molecule is

strongly attracted to a lone pair of electrons on the other molecule.

• O-H more polar than N-H, so stronger hydrogen bonding.

Chapter 2 20

H Bonds

Chapter 2 21

Boiling Points and Intermolecular Forces

CH3 CH2 OHethanol, b.p. = 78°C

CH3 O CH3

dimethyl ether, b.p. = -25°C

trimethylamine, b.p. 3.5°C

N CH3H3C

CH3

propylamine, b.p. 49°C

CH3CH2CH2 N

H

H

ethylmethylamine, b.p. 37°C

N CH3CH3CH2

H

CH3 CH2 OH CH3 CH2 NH2

ethanol, b.p. = 78° C ethyl amine, b.p. = 17 ° C

Chapter 2 22

Solubility

• Like dissolves like.• Polar solutes dissolve in polar solvents.• Nonpolar solutes dissolve in nonpolar

solvents.• Molecules with similar intermolecular

forces will mix freely.

Chapter 2 23

Ionic Solute with Polar Solvent

Hydration releases energy.Entropy increases.

Chapter 2 24

Ionic Solute with Nonpolar Solvent

Chapter 2 25

Nonpolar Solute withNonpolar Solvent

Chapter 2 26

Nonpolar Solute with Polar Solvent

Chapter 2 27

Classes of Compounds• Classification based on functional group.• Three broad classes

HydrocarbonsCompounds containing oxygenCompounds containing nitrogen.

Chapter 2 28

Hydrocarbons

• Alkane: single bonds, sp3 carbons• Cycloalkane: carbons form a ring• Alkene: double bond, sp2 carbons• Cycloalkene: double bond in ring• Alkyne: triple bond, sp carbons• Aromatic: contains a benzene ring

Chapter 2 29

Compounds Containing Oxygen

• Alcohol: R-OH• Ether: R-O-R'• Aldehyde: RCHO

• Ketone: RCOR'

CH3CH2 C

O

H

CH3 C

O

CH3

Chapter 2 30

Carboxylic Acids and Their Derivatives

• Carboxylic Acid: RCOOH• Acid Chloride: RCOCl• Ester: RCOOR'• Amide: RCONH2

C

O

OH

C

O

Cl

C

O

OCH3C

O

NH2

=>

Chapter 2 31

Compounds Containing Nitrogen

• Amines: RNH2, RNHR', or R3N

• Amides: RCONH2, RCONHR, RCONR2

• Nitrile: RCN

N

O

CH3

CH3 C N

Chapter 2 32

End of Chapter 2