Lewis Acid and Base

In the 1920s, Gilbert Lewis proposed a new set of definitions for acids and bases. 

Understanding Lewis Acids and Bases

According to the Lewis definition an acid is an electron pair acceptor. 

Concept: Understanding Lewis Acids. 

5m
Video Transcript

Hey guys, in this new video, we're going to take a look at the last and final type of acids and bases – the Lewis acid and the Lewis base. So, let's take a look. We’re going to say in the 1920s, we had Gilbert Lewis, he proposed his own new definition of acids and bases. He basically did this almost to the same exact time as Bronsted and Lowry. Here he defined a Lewis acid as something that accepts electrons. So we’re going to say a Lewis acid is an electron acceptor. To understand a Lewis acid, we have to basically figure out what are the different types of Lewis acids. The first one, we say if we have H acts as a Lewis acid, when connected to a very electronegative element. The electronegative elements are oxygen, nitrogen, sulfur, phosphorus or the halogens, group 7A. I'm talking about fluorine, chlorine, bromine or iodine.
Think about it. Let’s think of H being connected to some of these elements. Since all of these elements are more electronegative, they're going to be partially negative. Since H is less electronegative, it's going to be partially positive. We tend to think of this type of Lewis acid when we have H single bonded to the one of these electronegative elements. That's the only time we really look at it and if we're talking about the bond between H and of these electronegative elements. Think about it. Electrons are negatively charged. Since they’re negatively charged, they’ll be attracted to something that's positive or in this case, partially positive. That's what that delta positive means. It means it’s partially positive, a little bit positive. In this case, we're going to say H can act as a Lewis acid.
The second type is anytime we have a positively, fully positively charged hydrogen or metals. Here H+ would be an example of a Lewis acid because it accepts electrons. Electrons are negative. It's positive, opposites attract. If we have positive metals, since they’re all positive, they’ll attract negative electrons. Then the last and final type of Lewis acid is anytime you're central element has less than eight valence electrons around it. Remember, this is the octet rule. Our central element, once they have eight electrons, so they could have a similar configuration as the noble gases. But we're going to say group 2A and 3A elements and transition metals when they're in the center tend to have less than eight valence electrons.
For example, we could have boron. It's in group 3A, so it makes three bonds. Remember, for every bond we make, we're sharing those electrons. Here, boron has six electrons around it. It still has room to accept two more. That's why boron trifluoride can act as a Lewis acid because the central element can accept more electrons. We could also have Be connected to two Cls. Here Be has four electrons around it, so it can accept more electrons, another electron pair to get closer to the octet rule. We could have aluminum bromide here. It has six around it as well. Then our transition metal, common ones, zinc. Here it has four electrons around it as well.
This last type Lewis acid is common with groups to 2A, 3A and the transition metals. When they're in the center, they tend to have less than eight electrons and as a result, they serve as Lewis acids. They're going to accept an electron pair.

When hydrogen is connected to an electronegative element such as P, O, N, S or halogens then it gains a partially positive charge, which makes hydrogen act as a Lewis acid. 

Since these metal ions accept negative electron pairs then some Lewis Acids may be positively charged. 

If the central element has less than 8 valence electrons around it then it is more likely to accept an electron pair to follow the octet rule. 

According to the Lewis definition a base is an electron pair donor. 

Concept: Understanding Lewis Bases. 

2m
Video Transcript

If the Lewis acid is an electron pair acceptor, that would have to mean that the Lewis base is an electron pair donor now. What makes Lewis bases? We’re going to say Lewis bases are compounds with lone pairs. See here, all of these, what they have in common is they all have lone pairs. They're all Lewis bases. Then we’re going to say also compounds with, what do all these have in common? They all have in common a negative charge. If you have a negative charge, that means you have excess of extra electrons, so excess of extra lone pairs. Remember, if you're an electron pair donor, you have to have lone pairs and/or a negative charge. Negative charge just means you have extra electrons hanging around. We may not see them but we know they’re there.
This definition is a little bit different than the others and honestly, I think it's the most difficult out of the three because when you get to organic chemistry, organic chemistry is predominantly the chemistry of Lewis acid and bases. That's why this type of theory seems a little bit different from the others because it resides more in the organic chemistry region than general chemistry. But you still need to know it. Remember, a Lewis acid is an electron pair acceptor. A Lewis base is an electron pair donor.

Compounds that have lone pair(s) on a central element can act as Lewis bases. 

Compounds that possess a negative charge can act as Lewis Bases. 

Example: Identify each of the compounds in the following chemical equation.

3m
Video Transcript

Hey guys, in this new video, we're going to continue with our discussion of Lewis acids and bases. If we take a look at this example, it says: Identify each of the following compounds in the following chemical equation. Here we have to identify who was a Lewis acid and who was a Lewis base. Remember, a Lewis acid has less than eight electrons around it or it's partially positive. It can accept more electrons to fulfill its octet rule. We're going to say that the Lewis base is when a central element has lone pairs or when the compound in general has a negative charge. If we take a look here, we have these two compounds somehow merging together to give us this one product here. What we're going to say here is we have aluminum which is a group 3A element. It has three valence electrons that it's using to make those bonds, so we know it doesn't have a lone pair.
We should also realize here that it is sharing an electron from each one of the bromines. In total, aluminum has six electrons around it. It's not fulfilling its octet rule so it has space to accept more electrons to get to eight. We're going to say that this compound here is the Lewis acid, which would mean that the other compound would be our Lewis base. But why? Just realize, remember, oxygen is in group 6A so it has six valence electrons. It's using two of them to make those bonds, which means it has two lone pairs that we don't see. What it does here is it uses its lone pair and shoots them out and shares them with the aluminum.
Remember, Lewis is different than Bronsted-Lowry. In Bronsted-Lowry, we're donating a proton. We stay behind. We throw the proton over to the other compound and we stay behind as a new compound. But here, we’re sharing electrons not protons. If I'm sharing my electrons with you, I'm going to come along for the ride. This whole compound is going to share its electrons with the aluminum but the whole compound comes along with it. That's why we get one massive product at the end. Because I'm adding them together, we call this an adduct because again, I'm adding them together. The name shows us the relationship that's going on. Lewis acid Lewis base combine together to give me one product. An addition of two things, an adduct.
Now that you've seen that, try to use your reasoning skills to attempt to do the practice questions below. Again, if you get stuck, don't worry. Come back. Watch a video of me explaining which compound is which.

Problem: Identify the Lewis acids and bases in the following reactions.

a) H+ + OH ⇌ H2O

b) Cl + BCl3 ⇋ BCl4

c) SO3 + H2O ⇌ H2SO4

4m

Problem: Identify each of the following compounds as either a Lewis acid, a Lewis base or neither.

a)  ZnCl2                                                                                                           b)  CN 

 

c)  NH4+                                                                                                           d) Co3+

4m

Lewis Acid and Base Additional Practice Problems

Classify each of the following as either a Lewis acid or a Lewis base:

H+ 

B(OH)3

Cl-

P(CH3)3

Watch Solution

Classify each of the following as a Lewis acid or a Lewis base.

H-, I-

Fe3+

NO2

SnCl4

CH3OH

(CH3)3N

Watch Solution

For Cu and CH4, which will behave as a Lewis acid toward OH - in water?

a. only Cu

b. only CH4

c. Cu and CH4

d. neither Cu nor CH4

Watch Solution

Which of the following could act as a Lewis base?

a. H2O

b. PCl3

c. Lu3+

d. FeCl3

e. Both (a) and (b) are Lewis bases

Watch Solution

Which of the following can act as a Lewis base? Cr3+, SO3, CH3NH2, BeCl2

A. SO3 only

B. SO3, CH3NH2

C. CH3NH2 only

D. CH3NH2, BeCl2

E. Cr3+, BeCl2

Watch Solution

Is SiF4 a lewis base or lewis acid?

Watch Solution

Which of the following can act as a Lewis base?

Cr3+, SO3, CH3NH2, BeCl

a. Cr3+, BeCl

b. SO3 only

c. CH3NH2, BeCl

d. SO3, CH3NH

e. CH3NH2 only

Watch Solution

Classify each of the following as a Lewis acid or a Lewis base: 

B(OH) 3

OF2

Cl-

SO3 

Na+

Watch Solution

Classify each of the following as either a Lewis acid or a Lewis base.?

1. B(OH)3

2. P(CH3)3

3. BeCl2

4. CN-

Watch Solution

Classify the following as a Lewis acid or Base:

BeCl2, OH-, B(OH)3, CN-

Watch Solution

Classify each of the following as a Lewis acid or a Lewis base:

Watch Solution

For the following reaction, indicate which reactant is the Lewis acid and which is the Lewis base.

4NH3 + Cu2+ ⇌ Cu(NH3)42+   

a. NH3 is the Lewis acid.

b. NH3 is the Lewis base.

c. Cu2+ is the Lewis acid.

d. Cu2+ is the Lewis base.

e. This not a Lewis acid-base reaction.

Watch Solution

For the following reaction, indicate which reactant is the Lewis acid and which is the Lewis base.

a. Benzene is the Lewis acid.

b. Benzene is the Lewis base.

c. Br+ is the Lewis acid.

d. Br+ is the Lewis base.

e. This not a Lewis acid-base reaction.

Watch Solution

For the following reaction, indicate which reactant is the Lewis acid and which is the Lewis base.

SO2 + OH- ⇌ HSO3-

a. SO2 is the Lewis acid.

b. SO2 is the Lewis base.

c. OH- is the Lewis acid.

d. OH- is the Lewis base.

e. This not a Lewis acid-base reaction.

Watch Solution

For the following reaction, indicate which reactant is the Lewis acid and which is the Lewis base.

a. BF3 is the Lewis acid.

b. BF3 is the Lewis base.

c. CH3OH is the Lewis acid.

d. CH3OH is the Lewis base.

e. This not a Lewis acid-base reaction.

Watch Solution

For the following reaction, indicate which reactant is the Lewis acid and which is the Lewis base.

Al3+ + 6H2O ⇌ Al(OH)3 + 3H3O+

a) Al3+ is the Lewis acid.

b) Al3+ is the Lewis base.

c) H2O is the Lewis acid.

d) H2O is the Lewis base.

e) This is not acid-base reaction.

Watch Solution

Identify the Lewis acid and Lewis base in each of the following reactions.

Watch Solution

Classify each of the following as a Lewis acid or a Lewis base.

Watch Solution

Classify each of the following as a Lewis acid or a Lewis base.

Watch Solution

Classify each of the following as a Lewis acid or a Lewis base.

BeCl2, NH3, O2-, SO3, Fe2+

Watch Solution

Classify each of the following as a Lewis acid or a Lewis base.

Watch Solution

Which of the following is a Lewis base?

A) AlF3

B) H2O

C) SiF4

D) C5H12

E) None of the above are Lewis bases.

Watch Solution

Identify the Lewis acid and Lewis base in the following reactions:

(a) CH3OH + H3O+ ⇌ CH3OH2+ + H2O

(b) BF3 + NH3 → BF3- + NH3+

(c) Cl2 + AlCl3 → AlCl5

 

Watch Solution

Which of the following are Lewis bases?

I)         BCl3

II)        H

III)       H2O

IV)       NH3

V)        AlCl3     

 

A) I), II), and IV)

B) I), II), and III)

C) II), III) and IV)

D) I), II), and V)

E) III), IV) and V)

Watch Solution

Which species in the following reaction acts as a Lewis acid?

            CuSO4(s) + 4NH3(aq) ⇌ [Cu(NH3)4]2+(aq) + SO42−(aq)      

A) Cu2+          

B) SO42−         

C) [Cu(NH3)4]2+(aq)              

D) NH3

E) [Cu(NH3)4]2+(aq) and SO42−

Watch Solution

Draw the product(s) of the following acid-base reaction. Include all lone pairs and formal charges. Use curved arrow(s) to show the electron movement that lead to the formation of the product(s). Finally, label (1) the acid and (2) the base.

Watch Solution

The second step in a synthesis of isoamyl acetate, an ester commonly used in the food and cosmetic industries to mimic banana flavor, is represented by the mechanism below. Identify the Lewis acid and the Lewis base.

Watch Solution

True or false: SO3 is a stronger Lewis acid than SO2.

Watch Solution

NCl3 would be described as:

A) Lewis acid because it can donate electrons

B) Lewis acid because it can receive electrons

C) Lewis base because it can donate electrons

D) Lewis base because it can receive electrons

E) This compound is neither acidic or basic

Watch Solution

In the reaction 

SnCl4 + 2Cl → SnCl62- 

identify the Lewis Acid.

1. Cl

2. SnCl4

3. SnCl62-

Watch Solution

Briefly describe the difference between Bronsted acid and bases and Lewis acid and bases. For example, are all Bronsted acids also Lewis acids; are all Lewis acids also Bronsted acids?

Watch Solution

Which of the following is a Lewis acid?

A) BBr3

B) CCl4

C) NH3

D) CHBr3

E) None of the above are Lewis acids.

Watch Solution

Which of the following characteristics is/are  potential identifiers of a Lewis acid?

I.  Unfilled octet 

II. Double bond present 

III. Compounds with metal and hydroxide 

A.  Only I      

B.  Only II      

C.  II and III      

D.  I and II      

E.  All three

Watch Solution

What is the most accurate and complete description of a Lewis Acid?

A. proton donor

B. proton acceptor

C. electron donor

D. electron acceptor

Watch Solution

Which of the following can be a potential Lewis base?

a. Ca +2

b. CH4

c. H +

d. NH3

e. H3+

Watch Solution

Which of the following is the definition of a base in the Lewis system?

a. A base is a substance that can accept an H + ion.

b. A base is a substance that, when dissolved in water, produces hydroxide ions.

c. A base is a substance that contains OH - in its chemical formula

d. A base is a substance that that can accept a pair of electrons

e. A base is a substance that can donate a pair of electrons.

Watch Solution

All are potential Lewis bases except:

 

A. Ammonia

B. Water

C. Methane (CH4)

D. Cyanide ion (CN-)

Watch Solution

All are examples of Lewis acid-base reactions except:

A. Cu2+(aq) + 4NH3(aq)   ⇌   [Cu(NH3)4] 2+(aq)

B. HCl(aq) + NH3(aq)   ⇌   NH4Cl(aq)

C. H+ (aq) + OH- (aq)   ⇌   H2O(l)

D. 2Na(s) + Cl2(g)   ⇌   2NaCl(s)

Watch Solution