Chapter 2 Molecular Representations - Home :...
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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