Chapter 2Molecular Representations

Structural Formulas and Condensed Structures

Because organic compounds are molecular in nature, the additional information conveyed in structural formulas is sometimes necessary.

Structural formulas show the connections between atoms based on their abilities to form covalent bonds.

HC

C

C

O

O

N

N

N

F

Cl

Br

Because certain carbon and hydrogen groupings are so common, some abbreviations are used.

Structural Formulas and Condensed Structures

C

H

H

H CH3 C

H

H

CH2

C

H

CH CH2CH2 (CH2)2

Abbreviations for Cyclic StructuresCH2

H2C

H2CCH2

CH2

CH2

CH2H2C

H2CCH2

CH

CH

CH2

CH2

CH2

CH2

C

C

CC

C

CH

H

H

H

H

H

Structural Formulas and Condensed Structures

C6H12

C10H18

C6H6

Ways of Depicting Formulas and Models of an Alkane

H3CCH3

CH3

H3C

H3CCH2

CH2CH3

CH3

CH2H3C

H3CCH

CHCH2

CH2CH3

CH3

CH2H3C

C9H20

C7H16

C9H10

C7H6

Give molecular formulas for the following compounds.

Line structure “Hidden” hydrogens revealed

H3CC

CH3

CH3

CH3

H H

H3C

HH

H

H

H

H

H

CH H

H

H H

H

Chemical Formulas and Unsaturation

C C C C C

C C C C C

H H H H H

H H H H H

H C C C C C H

H H H H H

H H H H H

H C C C C

H H H H

H H

C

H

H

H

H

H

Cn

CnH2n

CnH2n+2

CnH2n+2

Chemical Formulas and Unsaturation

C C C C C

Cn

C C C C C

Cn

C C C C C

Cn

CnH2n

H C C C C C H

H H H

H H H H H

C

C

C C

C

Cn

CnH2n

C

C

C C

C

HH

H

H

H

H H

H

H

H

Chemical Formulas and Unsaturation

Hydrocarbons

SaturatedHydrocarbons

UnsaturatedHydrocarbons

AlkanesCnH2n+2

CycloalkanesCnH2n

(one ring)

AlkenesCnH2n

(one C=C)

AlkynesCnH2n-2

(one C C)

BenzeneC6H6

CnH2n-6

C

C

C C C

H

H C

C

C C C C O

H

H HH

H

H H

H

HH

H

H

H

H

H

H

H

H H

H

H3C

CH

H3C

CH

HC CH

CH3

(CH2)4 OH

Example of a Condensed Structure: What does the molecular formula C11H22O represent ?

Structural Formulas and Condensed Structures

Rules concerning line structures:

1. Draw single-bonded carbons in a zig-zag manner.

2. Draw multiple bonds with correct geometry.

3. Direction of single bonds is irrelevant.

4. Heteroatoms and hydrogens attached to heteroatoms must be included.

5. Carbon can have no more than four bonds.

OH

OH

N

H

NH

O

N

Functional Groups

Functional group - A characteristic group of atoms and bonds that imparts particular physical and

chemical properties on a molecule.

Alkanes Aldehydes

Alkenes Ketones

Alkynes Carboxylic Acids

Aromatics Esters

Alcohols Amines

Ethers Amides

Families of Organic Compounds

C C

C C

C C

CC

CC

C

C

C O

H

C OC

CH

O

CC

O

C

CO

H

O

C

O

OC

N

CN

O

Functional Groups

Functional Groups

3. Identify the major functional groups in the following molecules.

CH2 CH

NH2

CO

O HHO

HO

alcohol (phenol)amine

carboxylic acid

esteramine

ester

amine

ether

Formal Charges on Atomsand

The Importance of Lone Pairs

Br Br

H Cl H Cl

F F

O O

O O

O O

Group # Charge Example # lone pairslone pairs revealed

7 Positive 3

Neutral 3

Negative 4

6 Positive 1

Neutral 2

Negative 3

Formal Charges on Atoms

N N

N N

N N

H

Group # Charge Example # lone pairslone pairs revealed

5 Positive 0

Neutral 1

Negative 2

4 Positive 0

Neutral 0

Negative 1

Formal Charges on Atoms

Representation of Bonds in Three Dimensions

C C HHHH

H HBonds in the plane of the screen

Bonds pointing backwardBonds pointing

toward the observer

Resonance Structures

Resonance structures - A series of structures which collectively represent a single entity with

“delocalized” electrons.

The Use of Curved Arrows to Show Movement of a Pair of Electrons

Source of Electronsa lone pair or a bond

An electron donor

Destination of Electronsa positive charge

or an atom lacking an octetAn electron acceptor

“Tail” to “Head” Movement

Formations of Allyl Cation

H2C CH

CH3 H2C CH

CH2+

C CH

HC

H

H

HH

C CH

HC+

H HH

H:-

Formations of Allyl Cation

CC

HH

CH

H

H

H

CC

HH

CH

H

H

H

CC

HH

C+H

H

H

or or

CC

HH

C+H

H

H

Resonance of Allyl Cation

CC

HH

C+H

H

H

C+CH

CH

H

H

H

Representing Resonance Structures

Moving Electrons

The three “legal” moves of resonance:

1) Two π bond electrons ➛ lone pair

2) Lone pair of electrons ➛ π bond

3) Two π bond electrons ➛ π bond

Moving Electrons

Two “illegal” moves for the formation of resonance structures:

1) Breaking single bonds

2) Exceeding octets for second-row elements

O C

H

H

H

O N H

H

O O

H

+ CH3+

Moving Electrons

Representing Resonance

allyl cation:

acetate anion:

benzene molecule:

resonance hybrid

H3CC

O

O H3CC

O

O H3CC

O

O

Resonance Structures

Formal Charges

Common Bonding Patterns

B C N O

C +

N +

O +

C −

N −

O −

B −

F

F +

− F

O O O OH H O O

H H

H H

O O O OO O

HH

O OO O

HH

1 bond + 3 lone pairsno net charge

2 bonds + 2 lone pair 3 bonds + 1 lone pair

Formal Charges on Oxygen

NH

NH

N NH

H

H

H N

NN N

H

N

H

N

N N N NH

H

N

2 bonds + 2 lone pairsno net charge

3 bonds + 1 lone pair 4 bonds + 0 lone pairs

Formal Charges on Nitrogen

C C C C

C C C C C

C C C C

3 bonds + 1 lone pairno net charge

4 bonds + 0 lone pairs 3 bonds + 0 lone pairs

Formal Charges on Carbon

Resonance Structures

Pattern Recognition

OO

O

O

OO

Allylic lone pair Allylic positive charge

Lone pair adjacent to

positive charge

pi bond between two atoms of

differing electronegativity

Conjugated pi bonds enclosed in

a ring

Two curved arrows

One curved arrow

One curved arrow

One curved arrow

Three curved arrows

Five Patterns for Resonance Structures

Resonance stabilization - The stabilization associated with the delocalization of electrons via

resonance.

Resonance Structures

Assessing the Importance of Structures

1. Number of charges should be minimized.

2. Highly electronegative atoms can bear a charge if they possess an octet of electrons.

3. Two adjacent carbons should not bear opposite charges.

0 chargessignificant

2 chargesinsignificant

Assessing the Importance of Structures

1 chargesignificant

1 chargesignificant

3 chargesinsignificant

3 chargesinsignificant

Assessing the Importance of Structures

O

O

O

O

O

O

O

O

O

O

1 chargesignificant

1 chargesignificant

3 chargesinsignificant

Assessing the Importance of Structures

OH

OH

NH

NHHH

Assessing the Importance of Structures

Both significant