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

O

CH2

_

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

HNo 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 OH

ethanol, 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 classesHydrocarbonsCompounds 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