Ch. 3 - Acids and BasesWorksheetSee all chapters
All Chapters
Ch. 1 - A Review of General Chemistry
Ch. 2 - Molecular Representations
Ch. 3 - Acids and Bases
Ch. 4 - Alkanes and Cycloalkanes
Ch. 5 - Chirality
Ch. 6 - Thermodynamics and Kinetics
Ch. 7 - Substitution Reactions
Ch. 8 - Elimination Reactions
Ch. 9 - Alkenes and Alkynes
Ch. 10 - Addition Reactions
Ch. 11 - Radical Reactions
Ch. 12 - Alcohols, Ethers, Epoxides and Thiols
Ch. 13 - Alcohols and Carbonyl Compounds
Ch. 14 - Synthetic Techniques
Ch. 15 - Analytical Techniques: IR, NMR, Mass Spect
Ch. 16 - Conjugated Systems
Ch. 17 - Aromaticity
Ch. 18 - Reactions of Aromatics: EAS and Beyond
Ch. 19 - Aldehydes and Ketones: Nucleophilic Addition
Ch. 20 - Carboxylic Acid Derivatives: NAS
Ch. 21 - Enolate Chemistry: Reactions at the Alpha-Carbon
Ch. 22 - Condensation Chemistry
Ch. 23 - Amines
Ch. 24 - Carbohydrates
Ch. 25 - Phenols
Ch. 26 - Amino Acids, Peptides, and Proteins
Johnny Betancourt

Lewis acids and bases are electron acceptors and donors, respectively. Electrophiles are Lewis acids, and Nucleophiles are Lewis bases. 

Lewis acids and bases include Bronsted acids and bases, which in turn include Arrhenius acids and bases. That’s potentially a bit confusing to read, so please enjoy this colorful diagram:

Lewis Acid vs Bronsted Acid vs Arrhenius Acid DefinitionsLewis Acid vs Bronsted Acid vs Arrhenius Acid Definitions

Notice that the Lewis circle encompasses both Bronsted-Lowry and Arrhenius and that Bronsted encompasses only Arrhenius. 

Pro-tip: any time a proton is exchanged, electrons made it happen! That’s true for everything else in chemistry, btw—electrons are kind of a big deal. 

What exactly is the difference between an Arrhenius acid vs a Bronsted acid? By definition, Arrhenhius acids and bases are restricted to water while Bronsted acids and bases can be anything else—the only requirement is a proton exchange. 

Acids definitionsAcids definitions

Bases definitions

Bases definitions

Since Lewis acids are the broadest definition of acids, I'll be using that definition for the following examples of acids and bases.

1. Bronsted acids

Remember how I was saying that Bronsted acids are Lewis acids? Let’s check out the reaction between acetic acid and hydroxide. The curved arrows, as always, indicate the flow of electrons: 

Bronsted acid-base reactionBronsted acid-base reaction

The blue oxygen of the hydroxide anion donates a lone pair to grab the hydrogen. Hydrogen can only make one bond, so the green oxygen accepts the electron pair that was in its bond to hydrogen. The Bronsted acid is acting as a Lewis acid! And the Bronsted base is acting like a Lewis base!  

2. Lewis adducts

Lewis acids are electron acceptors, so they’re called electrophiles; Lewis bases are electron donors, and they’re called nucleophiles. Many, but not all, Lewis acids have empty p-orbitals. Nucleophiles can attack that electrophilic empty orbital to form an adduct (sometimes called a complex ion). 

The most common Lewis Acids are the "A, B, Cs" = Aluminum (AlX3), Boron (BX3), and Carbocations (C+)

Examples of Lewis AdductsExamples of Lewis Adducts

In this image, two reactions are happening. Ammonia's lone pair (non-bonding electrons) on the nitrogen is attacking the empty orbitals of the central atoms of BF3 and AlCl3. Notice that a new bond is formed between nitrogen and boron or nitrogen and aluminum. Since nitrogen donated its electrons, it’s a Lewis base (nucleophile); since boron and aluminum accepted electrons, it’s a Lewis acid (electrophile).

3. The Carbonyl Carbon in Nucleophilic addition

Molecules without empty orbitals can act as Lewis acids as well. How might that happen? Polar molecules like acetone have an electrophilic portion that loves to act as an electron-pair acceptor. Let’s use hydroxide as the electron-pair donor.

Nucleophilic additionNucleophilic addition

4. Lewis catalysts

Lewis acids are often used to lower the activation energy in electrophilic aromatic substitution reactions like halogenation. Benzene has a special kind of stability called aromaticity, so a Lewis acid catalyst needs to be present:EAS halogenation using FeBr3EAS halogenation using FeBr3

There’s a lot going on in this reaction, but what’s important for us right now is to see how the bromine in blue is making a bond to the empty orbital of the iron to form a complex. That makes the resulting FeBr4­ a good leaving group, so the aromatic molecule can now easily attack the electrophilic Br. 

Johnny Betancourt

Johnny got his start tutoring Organic in 2006 when he was a Teaching Assistant. He graduated in Chemistry from FIU and finished up his UF Doctor of Pharmacy last year. He now enjoys helping thousands of students crush mechanisms, while moonlighting as a clinical pharmacist on weekends.