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Learning Objective
- Use the definition of Lewis Acids and Bases to recognize electron movement in reactions
Acids and bases are an important part of chemistry. One of the most applicable theories is the Lewis acid/base motif that extends the definition of an acid and base beyond H+ and OH- ions as described by Brønsted-Lowry acids and bases.
Introduction
The Brønsted acid-base theory has been used throughout the history of acid and base chemistry. However, this theory is very restrictive and focuses primarily on acids and bases acting as proton donors and acceptors. Sometimes conditions arise where the theory doesn't necessarily fit, such as in solids and gases. In 1923, G.N. Lewis from UC Berkeley proposed an alternate theory to describe acids and bases. His theory gave a generalized explanation of acids and bases based on structure and bonding. Through the use of the Lewis definition of acids and bases, chemists are now able to predict a wider variety of acid-base reactions. Lewis' theory used electrons instead of proton transfer and specifically stated that an acid is a species that accepts an electron pair while a base donates an electron pair.
The reaction of a Lewis acid and a Lewis base will produce a coordinate covalent bond, as shown in Figure \(\PageIndex{1}\) above. A coordinate covalent bond is just a type of covalent bond in which one reactant gives it electron pair to another reactant. In this case the lewis base donates its electrons to the lewis acid. When they do react this way the resulting product is called an addition compound, or more commonly an adduct.
- Lewis Acid: a species that accepts an electron pair (i.e., an electrophile) and will have vacant orbitals
- Lewis Base: a species that donates an electron pair (i.e., a nucleophile) and will have lone-pair electrons
Lewis Acids
Lewis acids accept an electron pair. Lewis Acids are Electrophilic meaning that they are electron attracting. When bonding with a base the acid uses its lowest unoccupied molecular orbital or LUMO (Figure 2).
- Various species can act as Lewis acids. All cations are Lewis acids since they are able to accept electrons. (e.g., Cu2+, Fe2+, Fe3+)
- An atom, ion, or molecule with an incomplete octet of electrons can act as an Lewis acid (e.g., BF3, AlF3).
- Molecules where the central atom can have more than 8 valence shell electrons can be electron acceptors, and thus are classified as Lewis acids (e.g., SiBr4, SiF4).
- Molecules that have multiple bonds between two atoms of different electronegativities (e.g., CO2, SO2)
Lewis Bases
Lewis Bases donate an electron pair. Lewis Bases are Nucleophilic meaning that they “attack” a positive charge with their lone pair. They utilize the highest occupied molecular orbital or HOMO (Figure 2). An atom, ion, or molecule with a lone-pair of electrons can thus be a Lewis base. Each of the following anions can "give up" their electrons to an acid, e.g., \(OH^-\), \(CN^-\), \(CH_3COO^-\), \(:NH_3\), \(H_2O:\), \(CO:\). Lewis base's HOMO (highest occupied molecular orbital) interacts with the Lewis acid's LUMO (lowest unoccupied molecular orbital) to create bonded molecular orbitals. Both Lewis Acids and Bases contain HOMO and LUMOs but only the HOMO is considered for Bases and only the LUMO is considered for Acids (Figure \(\PageIndex{2}\)).
Complex Ion / Coordination Compounds
Complex ions are polyatomic ions, which are formed from a central metal ion that has other smaller ions joined around it. While Brønsted theory can't explain this reaction Lewis acid-base theory can help. A Lewis Base is often the ligand of a coordination compound with the metal acting as the Lewis Acid (see Oxidation States of Transition Metals).
\[Al^{3+} + 6 H_2O \rightleftharpoons [Al(H_2O)_6]^{3+} \label{1}\]
The aluminum ion is the metal and is a cation with an unfilled valence shell, and it is a Lewis Acid. Water has lone-pair electrons and is an anion, thus it is a Lewis Base.
The Lewis Acid accepts the electrons from the Lewis Base which donates the electrons. Another case where Lewis acid-base theory can explain the resulting compound is the reaction of ammonia with Zn2+.
\[ Zn^{2+} + 4NH_3 \rightarrow [Zn(NH_3)_4]^{4+} \label{2}\]
Similarly, the Lewis Acid is the zinc Ion and the Lewis Base is NH3. Note how Brønsted Theory of Acids and Bases will not be able to explain how this reaction occurs because there are no \(H^+\) or \(OH^-\) ions involved. Thus, Lewis Acid and Base Theory allows us to explain the formation of other species and complex ions which do not ordinarily contain hydronium or hydroxide ions. One is able to expand the definition of an acid and a base via the Lewis Acid and Base Theory. The lack of \(H^+\) or \(OH^-\) ions in many complex ions can make it harder to identify which species is an acid and which is a base. Therefore, by defining a species that donates an electron pair and a species that accepts an electron pair, the definition of a acid and base is expanded.
Amphoterism
As of now you should know that acids and bases are distinguished as two separate things however some substances can be both an acid and a base. You may have noticed this with water, which can act as both an acid or a base. This ability of water to do this makes it an amphoteric molecule. Water can act as an acid by donating its proton to the base and thus becoming its conjugate acid, OH-. However, water can also act as a base by accepting a proton from an acid to become its conjugate base, H3O+.
- Water acting as an Acid:
\[H_2O + NH_3 \rightarrow NH_4^+ + OH^- \label{3}\]
- Water acting as a Base:
\[H_2O + HCl \rightarrow Cl^- + H_3O^+ \label{4}\]
You may have noticed that the degree to which a molecule acts depends on the medium in which the molecule has been placed in. Water does not act as an acid in an acid medium and does not act as a base in a basic medium. Thus, the medium which a molecule is placed in has an effect on the properties of that molecule. Other molecules can also act as either an acid or a base. For example,
\[Al(OH)_3 + 3H^+ \rightarrow Al^{3+} + 3H_2O \label{5}\]
- where Al(OH)3 is acting as a Lewis Base.
\[Al(OH)_3 + OH^- \rightarrow Al(OH)_4^- \label{6}\]
- where Al(OH)3 is acting as an Lewis Acid.
Note how the amphoteric properties of the Al(OH)3 depends on what type of environment that molecule has been placed in.
Lewis Bases & Acids as Nucleophiles & Electrophiles
The emphasis on electron flow in the Lewis Theory of acids and bases is an important foundation for learning and predicting reaction mechanisms. The electron rich Lewis base can be described as a nucleophile. Nucleophiles are attracted to and can react with compounds or ions that have full or partial positive charge (like the nucleus). The electron poor Lewis acids can be described as electrophiles. Electrophiles attract nucleophiles until orbital overlap occurs between them triggering a reaction. At this point in the course, we can indicate electron flow using curved arrows when both the reactant(s) and product(s) are given.
Example
Exercises
For the following reactions,
a) add curved arrows to indicate the electron flow
b) label each reactant as the Nu (nucleophile) or E+ (electrophile).
Solutions
Outside Links
- Very Detailed review of Lewis Acids and Bases, covering all topics of this type of chemistry
- Very Complex and Detailed "Lewis Acid and Base Interaction Matrix"
- Youtube Video about Lewis Acids/Bases
References
- Cycloaddition on Ge(100) of the Lewis Acid AlCl3. Soon Jung Jung,, Young-Sang Youn,, Hangil Lee,,, Ki-Jeong Kim,,, Bong Soo Kim, and, Sehun Kim,. Journal of the American Chemical Society 2008 130 (11), 3288-3289
- Fluorescence Maxima of 10-Methylacridone? Metal Ion Salt Complexes: A Convenient and Quantitative Measure of Lewis Acidity of Metal Ion Salts. Shunichi Fukuzumi and, Kei Ohkubo. Journal of the American Chemical Society 2002 124 (35), 10270-10271.
- Harwood, William S., F. G. Herring, Jeffry D. Madura, and Ralph H. Petrucci. General Chemistry Principles and Modern Applications. 9th ed. New Jersey: Prentice Hall, 2007. 695-96.
Contributors and Attributions
- Adam Abudra (UCD), Tajinder Badial (UCD)
FAQs
What does Lewis say about acids and bases? ›
In the Lewis theory of acid-base reactions, bases donate pairs of electrons and acids accept pairs of electrons. A Lewis acid is therefore any substance, such as the H+ ion, that can accept a pair of nonbonding electrons. In other words, a Lewis acid is an electron-pair acceptor.
What are 5 examples of Lewis acids and bases? ›Examples of Lewis Acids: H+, K+, Mg2+, Fe3+, BF3, CO2, SO3, RMgX, AlCl3, Br2. Examples of Lewis Bases: OH-, F-, H2O, ROH, NH3, SO42-, H-, CO, PR3, C6H6.
Is H2O a Lewis acid or base? ›Water is an example of a Lewis base. Carbocations are examples of Lewis acids. When water reacts with a carbocation as shown below, one of the electron pairs from oxygen is used to form a new sigma bond to the central carbon in the carbocation.
How do you determine Lewis bases? ›A Lewis base compound has a pair of electrons that are 'donated' to an electron-acceptor forming a covalent bond. A Lewis Base will have lone-pair electrons. All anions are Lewis Bases since they donate electrons.
Is nh4+ a Lewis acid or base? ›Lewis acids are those acids which can accept a pair of electrons. For example, BF3,H+, and NH+4 are Lewis acids.
What is a Lewis base and examples? ›A Lewis base, then, is any species that has a filled orbital containing an electron pair which is not involved in bonding but may form a dative bond with a Lewis acid to form a Lewis adduct. For example, NH3 is a Lewis base, because it can donate its lone pair of electrons.
Which of the following is an example of a Lewis base? ›Lewis bases are electron pair donors. Thus NH3 is a Lewis base.
Which is not a Lewis base? ›Methane is not a Lewis base.
How do you identify Bronsted Lowry acids and bases? ›To determine whether a substance is an acid or a base, count the hydrogens on each substance before and after the reaction. If the number of hydrogens has decreased that substance is the acid (donates hydrogen ions). If the number of hydrogens has increased that substance is the base (accepts hydrogen ions).
What are the most common Lewis bases? ›Lewis bases usually have non-bonding electrons or lone pairs this makes oxygen and nitrogen compounds common Lewis bases. Lewis bases may be anionic or neutral.
Is NH3 a Lewis acid or base? ›
The lone pair on the nitrogen atom is transferred to the hydrogen ion, making the NH3 a Lewis base while the H+ is a Lewis acid.
Is co2 a Lewis acid or base? ›Carbon dioxide is a polar molecule whose positive center is on the carbon atom: This positive center is able to attract (and accept) the lone electron pairs present on the oxide ion (O2-). Thus, carbon dioxide is acting as a Lewis acid and the oxide ion is acting as a Lewis base.
Why is nh4 a Lewis acid? ›Ammonium ion that is NH4+ has a positive charge on it so it has a tendency to accept a pair of electrons from some lewis bases which are electron pair donor, to change into a more stable form that is without any charge. So ammonium ion are considered as lewis acid.
What are Lewis acids examples? ›Examples of Lewis Acids
For example, Cu+2, Zn+2, Fe+2, Fe3+ etc. are Lewis acids as they can accept electrons. H+ ions can be considered as Lewis acids. An atom or ion or molecule with an incomplete octet of electrons can act as a Lewis acid.
The nitrogen atom in ammonia has a lone pair of electrons which can be easily donated to suitable Lewis acid. Hence, ammonia can act as a Lewis base.
Is HCl a Lewis acid? ›HCl is not a Lewis acid as it cannot receive a pair of electron.
What is the Lewis base? ›Lewis Base
Atomic or molecular chemical species having a highly localized HOMO (The Highest Occupied Molecular Orbital) act as Lewis bases. These chemical species have the ability to donate an electron pair to a given Lewis acid in order to form an adduct, as discussed earlier.
Note that the + sign in the Lewis structure for H3O+ means that we have lost a valence electron. Therefore we only have 8 valence electrons for the H3O+ Lewis structure. H3O+ is an important compound in Acid-Base chemistry and is considered an acid.
Is SO3 a Lewis acid? ›SO3 and CO2 are not electron deficient, but the central atom in each has less than four electron regions (three around S and two around C), so they are Lewis acids. Their acidity is strengthened by positive formal charge.
Which are Lewis acids among H2O BF3 H+ NH4+? ›BF3,H+ and NH4+ are Lewis acids whereas H2O is a lewis base.
Is H+ a Lewis acid? ›
The proton (H +), which has no valence electrons, is a Lewis acid because it accepts a lone pair of electrons on the base to form a bond.
Are all anions Lewis bases? ›All anions are Lewis bases, but not all Lewis bases are anions. The lone pair is frequently, but not always, located on oxygen or nitrogen atoms. The strength of a base is increased by electron density.
What is an example of a Lewis acid base pair? ›The interaction between a magnesium cation (Mg+2) and a carbonyl oxygen is a common example of a Lewis acid-base reaction. The carbonyl oxygen (the Lewis base) donates a pair of electrons to the magnesium cation (the Lewis acid).
Is NaOH a Lewis base? ›NaOH can donate electron pair to electron-deficient species as it has lone pair present on and therefore it is also Lewis base.
Which is not a Lewis acid? ›In NH3, the octet of N is complete with 8 electrons. So, NH3 is not an Lewis acid.
Which of the following is Lewis acid or base? ›Lewis Acid is a species that accepts an electron pair (i.e., an electrophile) and will have vacant orbitals. In the given options BF3 is a strong electrophile and has vacant orbital to accept electron lone pair. Thus it is a Lewis acid. Other three H2O,Cl− and NH3 have lone pair of electrons and act as lewis base.
Is h2o a Bronsted base? ›As the proton acceptor, H2O is a Brønsted-Lowry base.
Is NaCl an acid or base? ›NaCl (Sodium chloride) is a salt that doesn't show acidic or basic nature. It is neutral with a pH of about 7.
How do I know if I have Bronsted Lowry or Lewis? ›A Bronsted-Lowry acid is a compound that can donate a hydrogen ion while a Bronsted-Lowry base is a compound that can accept a hydrogen ion. A Lewis base is a compound that can donate a pair of electrons while a Lewis acid is a compound that can accept a pair of electrons.
How do you know which Lewis base is strongest? ›Nitrogen, being smallest in size, can give up its lone pair of electrons most easily. Hence, the NH3 is the strongest Lewis base.
Which is strong Lewis base? ›
NH3 is the strongest Lewis base due to smallest size and high charge density on nitrogen atom.
Which behaves as Lewis base? ›An atom, ion, or molecule with a lone-pair of electrons can thus be a Lewis base.
Is NaCl a Lewis acid? ›In the first option, $ NaCl $ has no vacant orbitals. So, $ NaCl $ is not a Lewis acid.
Is NH3 and NH4+ an acid or base? ›NH3 and NH4+ are a conjugate acid-base pair. NH4+ is the conjugate acid of the base NH3. Every acid has a conjugate base, and every base has a conjugate acid.
Is NH3 or BF3 a Lewis acid? ›Lone pair of NH3 acts as a donor which makes it a lewis base where BF3 acts as a accepter of loan pair and hence act as a lewis acid.
Is SO2 a Lewis acid? ›SO2 acts as a Lewis acid as well as Lewis base.
Is bf3 a Lewis base? ›BF3 is a Lewis acid, but note it has no H to donate, it represents a new class of acids.
Is co2 H2O a Lewis acid? ›As a result, Carbon dioxide acts as a Lewis acid, whereas the Oxide ion acts as a Lewis base. When Carbon dioxide is dissolved in water to generate Carbonic acid, the water acts as a Lewis base, donating electron pairs while Carbon dioxide takes them.
Is nh2 a Lewis acid? ›NH2-MIL-101(Fe) contains abundant Lewis acid-base center. In NH2-MIL-101(Fe), -COOH as the Lewis acid center has strong ability of withdrawing electrons, while -NH2 as the Lewis base center has lone electron-pairs which could provide electrons forming chemical bonding [39].
Why is BF3 a Lewis acid? ›Reason behind BF 3 acting as a Lewis Acid: BF 3 has 6 electrons in the central metal atom, boron. That's why, it is electron deficient and hence is a Lewis acid.
Why is CCl4 a Lewis acid? ›
CCl4 can not keep more than 8 electrons in its outermost orbit due to the absence of d-orbitals. So, it not act as lewis acid.
Are all acids are Lewis acids? ›False. The acids that have the ability to donate protons are called Bronsted acids and the acids that have empty orbitals to accept a pair of electrons are called lewis acids. For example, HCl and H2SO4 can give protons but cannot accept a pair of electrons. Hence, they are Bronsted acids but not Lewis acids.
How do you identify acids and bases? ›To determine whether a substance is an acid or a base, count the hydrogens on each substance before and after the reaction. If the number of hydrogens has decreased that substance is the acid (donates hydrogen ions). If the number of hydrogens has increased that substance is the base (accepts hydrogen ions).
Which are strong Lewis acids? ›Hence, BBr3 has the highest acidic character and is the strongest lewis acid.
What is an example of a Lewis acid Lewis base reaction? ›The interaction between a magnesium cation (Mg+2) and a carbonyl oxygen is a common example of a Lewis acid-base reaction. The carbonyl oxygen (the Lewis base) donates a pair of electrons to the magnesium cation (the Lewis acid).
What is a Lewis acid What is a Lewis base quizlet? ›An electron-rich species that donates a lone pair to form a new covalent bond in a reaction, lewis bases. Electrophile. An electron-deficient species that accepts a lone pair of electrons from another reactant to form a new covalent bond, lewis acids. Examples of Nucleophiles/Lewis Bases. HO⁻, NH₃⁺, Cl⁻
How do you identify acid and base without test? ›If a colourless liquid is given, an acid or a base can be found out using indicators without tasting it. Litmus paper is one of such indicator which can determine an acid and a base.
What are two common ways to identify acids and bases 3? ›There are two ways to identify acids and bases, one way is sensory elevation which includes taste and touch. The other way to identify acids and bases are organic dyes which is a naturally occurring color pigment that changes color when exposed to acids or based. An example of the organic dye is the litmus test.
Which of the following is a Lewis acid base? ›Lewis bases are electron pair donors. Thus NH3 is a Lewis base.
How do you make a Lewis acid base reaction? ›A Lewis acid–base reaction happens when a pair of electrons are donated from one species to an electron-deficient species. The species which donates the electron pair is called a Lewis base, while the substance that accepts the lone pair of electrons is called the Lewis acid.