Electronic Effects on the Strength of Group 13 Acids

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


F SbF

F

F

F

+ F Br

F

F

F SbF

FF

F

Br

F FF


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-


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 -


I I +

Cl-

I II-

triodide ion

QuestionGive the geometry of I3- and I3+ ions by using

VSEPR theory


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

Electronic Effects on the Strength of Group 13 Acids

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