18Lewis Acids

8/13/2019 18Lewis Acids

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Chemistry 1000 Lecture 18: Lewis acids and bases

Marc R. Roussel


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Historical ideas about acids and bases

Arrhenius theory: based on the behavior of acids and bases inwaterArrhenius acid: dissociates in water, producing H+

Examples: HCl, CH3COOHArrhenius base: dissociates in water, producing OH

Examples: NaOH, Ba(OH)2Brnsted theory: puts the emphasis on proton transfer

Brnsted acid: proton donorExamples: HCl, CH3COOH, H2OBrnsted base: proton acceptor

Examples: OH , NH3, H2O


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Ammonia as a base

Ammonia is a Brnsted base:

HH H

H

H

NH H

N

+

+

..

+ H

Thinking of ammonia as a Brnsted base, we would say that it isaccepting a proton.

An alternative viewpoint is that ammonia is donating an electronpair to H+ .


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Now consider

B

F

F F FF

N:

H

HH

N

H

HH

B

F

+

This reaction and the reaction of NH3 with H+ are clearly of thesame kind, even though one is a Brnsted acid-base reaction, andthe other isnt.


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Lewis acids and bases

Lewis acid: electron pair acceptor

Lewis base: electron pair donor


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CO 2 as a Lewis acid

H:O:

C

:O:

C

:O:

OO

H

H

O

H

:O:

C

:O:

O

H

H

..

..

... . .

.

+

....

..

Note that this Lewis acid-base reaction make CO2 into theBrnsted acid H2CO3.


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Group 13 metal halides

In the gas phase, many of these exist as Lewis dimers:

:Cl

Al

Cl:

Al

Cl:

Al

Cl

AlCl

:Cl:

:Cl:

:Cl:

:Cl:

:Cl:

:Cl::Cl

..

..

....

....

..

..

.. ..

......

.. ..

..

.. ..


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Metal ions as Lewis acids

Metal ions often act as Lewis acids.

Example: hydrated Al3+ ion

H

H 2

OH 2

OH 2

O

O

2

H2O

H2OAl

..

3+

....

..

....

[Al(H2O)6]3+ is a Brnsted acid:

[Al(H2O)6]3+(aq) + H2O(l) [Al(H2O)5(OH)]2+(aq) + H3O

+(aq)

pK a = 4 .85


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Factors that affect the acidity of aqua ions

Acidity of aqua ions is due to the weakening of bonds betweenO and H when a new bond is formed between O and a metalion.What makes metal-water bonds strong?

Ion charge (z ): Lewis acidity is based on attraction of (in thiscase) an ion for one lone pair on the oxygenatom.

All other things being equal, we might thinkthat the force of attraction increases with z .

However, the polarization of the O-H bond alsoincreases with z , so the force between the ionand water molecule increases as z 2.

Ion radius (r ): A smaller radius increases the electrostaticpotential energy of the bond.

Overall: The acidity should increase with z 2/ r .


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0

2

4

6

8

10

12

14

16

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18

p K a

z 2 r -1 / m -1


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Consequence: Ions with small z and/or relatively large r have aquaions with little or no acidity in water (e.g. alkalimetal ions).

Additional consideration: All other things being equal, moreelectronegative metals tend to give more acidiccomplexes.Why?


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Acidity and solubility

We can sometimes think of the dissolution of ioniccompounds in terms of acid-base concepts, at least for simpleionic compounds.Take, e.g. oxides, i.e. compounds of O2 with metal ions.

We can think of these compounds as products of the reactionof a Lewis acidic metal ion with the Lewis basic oxide ion.The oxide ion is a strong Lewis base.If the metal ion is a strong Lewis acid, then the product ishard to break up and will not dissolve.Examples:

Na+ is a very weak Lewis acid so its compounds(including oxides) are very soluble.

Ti4+ is a very strong Lewis acid, so its oxide (in

particular) is insoluble in water.


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Acidity and solubility (continued)

Solubility can often be modulated by varying the pH.

Al3+

is an interesting case:Near neutral pH, Al2O3 is insoluble because Al3+ is a verystrong Lewis acid.At low pH, the oxide ion is removed by H+ , and Al3+ isobtained in solution (as the solvated ion).At high pH, the oxide is converted to [Al(OH)4]

, whichmakes it soluble.


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Summary of Lewis acids and bases

Categories of Lewis acids:Metal cations and H+

Metal-decient species such as BX3 and BeX2Molecules containing double or triple bonds between atomswith very different electronegativities such asCO2 and SO3

Lewis bases typically have lone pairs, as in NH3.

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18Lewis Acids