ORGANIC CHEMISTRY 3540
POLARITY, ELECTRONEGATIVITY, ACIDITY
POLARITY
ELECTRONEGATIVITY
ACIDITY
CURVED ARROW NOTATION
POLAR BONDS
A CONTINUUM OF BONDING
WHY?
DIFFERENCES IN
ELECTRONEGATIVITY
, THE ABILITY OF AN ATOM TO ATTRACT ELECTRONS IN A COVALENT BOND.
ELECTRONEGATIVITY DIFFERENCE:
IF ZERO, NONPOLAR COVALENT BOND
IF GREATER THAN ZERO BUT LESS THAN TWO, POLAR COVALENT BOND
IF GREATER THAN TWO, IONIC BOND
POLAR MOLECULES
DIPOLE MOMENT, AN EXPERIMENTAL MEASUREMENT (units of DEBYE {D})
NOTE; A PROPERTY OF THE MOLECULE
H
2
O = 1.85 D
CHCl
3
= 1.01 D
C
3
H
8
= 0.084 D
CO
2
= 0 D (??)
BF
3
= 0 D (??)
CCl
4
= 0 D (??)
HOW CAN YOU PREDICT THAT CHCl
3
IS POLAR BUT CCl
4
IS NOT?
DIPOLE MOMENT IS THE VECTOR SUM OF ALL OF THE BOND MOMENTS
NEED SHAPE!
HOW DO YOU GET SHAPE?
FROM HYBRIDIZATION!
HOW DO YOU GET HYBRIDIZATION?
FROM
LEWIS STRUCTURE
!!
CARBON DIOXIDE HAS POLAR BONDS BUT IS NONPOLAR!
WHAT IS THE BOND ANGLE IN CO
2
?
BORON TRIFLUORIDE HAS POLAR BONDS BUT IS NONPOLAR!
WHAT ARE THE BOND ANGLES IN BF
3
?
WHAT IS THE SHAPE OF THE MOLECULE?
ACIDITY - ACID-BASE THEORIES
BRONSTAD-LOWRY ACID-BASE THEORY
Terms: ACID, BASE, STRONGER, WEAKER, CONJUGATE
ACID: PROTON DONOR
BASE: PROTON ACCEPTOR
HCl + H
2
O ---> H
3
O
+1
+ Cl
-1
STRONGER ACID + STRONGER BASE ---> WEAKER ACID + WEAKER BASE
BUT! WEAKER ACID + WEAKER BASE ---> STRONGER ACID + STRONGER BASE
HOW? BOTH REACTIONS TAKING PLACE AT ONCE
ESTABLISHES AN EQUILIBRIUM
AT EQUILIBRIUM:
QUALITATIVE: HAVE MORE OF THE WEAKER PRESENT AND LESS OF THE STRONGER
QUANTITATIVE: USE EQUILIBRIUM CONSTANT pK
a
pK
a
= -Log K
a
HCl ..... K
a
= 10
7
, pK
a
= -7
CH
3
CO
2
H ..... K
a
= 1.7 X 10
-5
, pK
a
= 4.8
H
2
O ..... K
a
= 1.8 X 10
-16
, pK
a
= 15.7
NH
3
..... K
a
= 10
-36
, pK
a
= 36
WHICH IS THE STRONGER ACID?
HCN: pK
a
= 9.31; or HF: pK
a
= 3.5
NH
3
: pK
a
= 36; or H
2
O: pK
a
= 15.7
WHICH IS THE STRONGER BASE?
F
-1
; or OH
-1
STRONGER ACID HAS A WEAKER CONJUGATE BASE
STRONGER BASE HAS A WEAKER CONJUGATE ACID
WHAT IS THE CONJUGATE ACID OF NO
3
-1
?
WHAT IS THE CONJUGATE ACID OF SO
4
-2
?
WHAT IS THE CONJUGATE ACID OF CH
3
O
-1
?
WHAT IS THE CONJUGATE BASE OF H
2
O?
WHAT IS THE CONJUGATE BASE OF H
2
SO
4
?
WHAT IS THE CONJUGATE BASE OF CH
3
OH?
WHAT IS THE CONJUGATE ACID OF H
2
O?
LEWIS ACID-BASE THEORY
ACID: ELECTRON PAIR ACCEPTOR
MUST BE ELECTRON DEFICIENT: H
+1
, BH
3
BASE: ELECTRON PAIR DONOR
MUST BE ELECTRON RICH: OH
-1
, NH
3
YOU NEED A
REACTION:
H
+1
+ OH
-1
---> H
2
O
NO HYDROGENS!
BF
3
+ NF
3
---> F
3
BNF
3
MORE COMPLEX:
H
3
O
+1
+ OH
-1
---> 2H
2
O
REACTIONS ARE NOTHING MORE THAN CHANGING BONDS, THE MOVEMENT OF ELECTRONS!
CURVED ARROW NOTATION
DEMONSTRATE WITH A
CURVED ARROW:
REACTION:
H
+1
+ NH
3
---> NH
4
+1
REACTION:
BF
3
+ NF
3
---> F
3
BNF
3
MORE COMPLEX:
H
3
O
+1
+ OH
-1
---> 2H
2
O
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