Electronic Effects on the Strength of Group 13 Acids
Acidity
BF3 < BCl3 < BBr3 ~ BI3
Reason: π-donation from halide F > Cl > Br, I
B
B-X interaction: B-F > B-Cl > B-Br ~ B-I
F
F FB
F
F FB
F
F FB
D
BF F
F
Question
1/.Replacing F with Me (CH3) leads to a series of decreasing adduct stability: BF3 > BF2Me > BFMe2 ~ BMe3 when NMe3 is the donor. Why?
2/. The order of acidity wrt NMe3 is AlMe3 > GaMe3 > InMe3. Why?
CH3
SnH3C CH3
Cl + F- H3C SnCH3
CH3
F
Cl
F
SiF F
F+ F- F Si
F
F
F
F
-
-
Examples of Lewis Acids: Group 14Group 14 elements (except carbon) can accept electron pairs by using their empty d-orbitals
Examples of Lewis Acids: Group 15
This family affords many common examples of Lewis bases and a few less widely appreciate Lewis acids
e.g. PCl5 is a monomer in gas phase but an ionic compound in solid state
Cl PCl
Cl
Cl
ClGas phase
Cl
P
ClClCl
PCl
Cl Cl
Cl
Cl
Cl
Solid State
Examples of Lewis Acids: Group 15
F SbF
F
F
F
+ F Br
F
F
F SbF
FF
F
Br
F FF
Examples of Lewis Acids: Group 16
Most of the group 16 compounds are Lewis base but there are few examples of Group 16 Lewis acids
SO3 is a strong acid towards donors such as amines, alcohols
O
O O
S + NR3R3N S
O
OO
SF
F
F
F
+ R4N+F-
Examples of Lewis Acids: Group 16
SF
FF
F
R4N+ F
QuestionIt is known that dissolving SO3 in water leads to sulfuric acid. Could you express the reaction in term of an acid-base reaction between SO3 and water?
Cl
I
Cl
Cl + Cl- IClCl
ClCl-
I Cl +
I-
I ClCl -
Examples of Lewis Acids: Group 17
I I +
Cl-
I II-
triodide ion
QuestionGive the geometry of I3- and I3+ ions by using
VSEPR theory
Examples of Lewis Acids: Group 18
XeF6 + F- → [XeF7]-
XeO3 + F- → [XeO3F]-
CR
R
R
-
Structures of Some Lewis Bases: Group 14R = alkyl, aryl group or H
Alkyl ion
H H
H
HH
Example of aryl ion: phenyl ion
-
anionic
Structures of Some Lewis Bases: Group 15R = alkyl, aryl group or H
NR
R
R
PR
R
R
AsR
R
R
amine
phosphine
arsine
NR
R-
amide
PR
R-
AsR
R-
Anionic
phosphide
arsenide
Neutral
sp3 hybdridization sp3 hybdridization
Structures of Some Lewis Bases: Group 16
O H-
O R-
R = alkyl group i.e. CH3
alkoxide
S R-
thiolate
Se R-
hydroxide
selenoate
H-OR alcohol
H2O
H-SR thiol, H2S
H-SeR selenol, H2Se
O2-
S2-
Se2-
oxide
sulfide
selenide
hydrogen sulfide
hydrogen selenide
S H-
Se H-
dianionic anionic neutral
R-O-R ether
R-S-R thioether
R-Se-R dialkylselenider
Group 17 – halides are common Lewis bases
Structures of Some Lewis Bases: pseduohalides
N N N
C NC NS C NS Thiocyanide ion
Cyanide ion
Azide ion
- -
-
- -+
C NO C NO Isocyanate ion--
Structures of Some Lewis Bases: Heterocyclics
N O
SNH
pyridine
pyrrole
furan
thiophene
Structures of Some Lewis Bases Containing > 1 Donor Atoms
H2C CH2
NH2H2N
C C
OO
O O
O-
O ON
O-
OC
O-
ethylenediamine (en)
oxalate ion
nitrate ion
carbonate ion
C O
carbon monoxide
C NS-
thiocyanide ion
Factors that Control Lewis Basicity
Electronic effect
NF3 < NH3 < NMe3
Reason: F is more electron withdrawing than H and Me
Me is electron donating
Question1/. Identify the donor atoms in the following compounds
(i) (CH3)2SO (ii) SnCl2 (iii) SO2
2/. Interpret the order of adduct stability of the following bases with BMe3
NH3 < MeNH2 < Me3N < Me2NH
Me = methyl group CH3
1/. What Should be the order of basicity of the following pyridines with respect to BMe3?
N N CH3 N
CH3
Question
2/. Account for the basicity order of the oxygen atoms in the following molecules
MeC
Me2N
O >Me
C
Me
O >Me
C
MeO
O
Me = methyl group CH3
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