Ch. 19 - Aldehydes and Ketones: Nucleophilic AdditionSee 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

Acetal and Hemiacetal

See all sections
Naming Aldehydes
Naming Ketones
Oxidizing and Reducing Agents
Oxidation of Alcohols
Alkyne Hydration
Nucleophilic Addition
Organometallics on Ketones
Overview of Nucleophilic Addition of Solvents
Acetal Protecting Group
Imine vs Enamine
Addition of Amine Derivatives
Wolff Kishner Reduction
Acid Chloride to Ketone
Nitrile to Ketone
Wittig Reaction
Ketone and Aldehyde Synthesis Reactions
Additional Practice
Physical Properties of Ketones and Aldehydes
Multi-Functionalized Carbonyl Nomenclauture
Catalytic Reduction of Carbonyls
Tollens’s Test
Fehling’s Test 
Alkyne Hydroboration to Yield Aldehydes
Nucleophilic Addition Reactivity
Strecker Synthesis
Synthesis Involving Acetals
Reduction of Carbonyls to Alkanes
Clemmensen vs Wolff-Kischner
Baeyer-Villiger Oxidation
Baeyer-Villiger Oxidation Synthesis
Weinreb Ketone Synthesis
Wittig Retrosynthesis
Horner–Wadsworth–Emmons Reaction
Carbonyl Missing Reagent
Carbonyl Hydrolysis
Carbonyl Synthesis
Carbonyl Retrosynthesis
Reactions of Ketenes
Ketene Synthesis
Additional Guides
Acetal and Hemiacetal
Johnny Betancourt

An acetal functional group is a geminal diether is produced from a reaction between a carbonyl and two equivalents of alcohol. A hemiacetal is very similar to an acetal structurally but has a hydroxyl group in place of one of the ethers. Acetals are often used as protecting groups for carbonyl groups.

Quick note:

Acetals vs ketalsAcetals vs ketals

Technically, there’s a difference between acetals and ketals. Aldehydes yield hemiacetals and acetals, and ketones yield hemiketals and ketals. From here on, though, we’re just going to refer to them collectively as acetals. 

Identifying hydrates, hemiacetals, and acetals:

All three are very similar as they all have a central carbon and two groups with oxygen, either as an alcohol or an ether. A hydrate’s oxygen-containing groups are both alcohols, a hemiacetal’s oxygen-containing groups are one alcohol and one ether, and an acetal’s oxygen-containing groups are both ethers. Check it out:

Hydrate, hemiacetal, and acetalHydrate, hemiacetal, and acetal

In acetals, the two R groups can be equivalent to each other (a "symmetric acetal") or not (a "mixed acetal"). Mixed acetals form when the solution has more than one type of alcohol. 

Formation of hemiacetals:

Heads up: hemiacetals can be formed in acid or base, but acetals can only be formed in acid. Let’s take a look at the mechanism to produce a hemiacetal from formaldehyde in acid:

Hemiacetal mechanismHemiacetal mechanism

The very first step is the protonation of the oxygen to make the carbonyl carbon more electrophilic. Once that’s done, the alcohol in solution attacks the carbonyl. This kicks the electrons in the carbonyl’s pi-bond up to the protonated oxygen to create an alcohol. Water or alcohol from the solution then deprotonates the positively charged oxygen to create an ether.

The acetal formation mechanism is extremely similar; in fact, it’s the same exact thing but starting from the hemiacetal. In other words, the acetal mechanism is the addition of alcohol and elimination of water twice. All hydrates, hemiacetals, and acetals are all susceptible to reversal through hydrolysis.

Cyclic acetals:

Cyclic acetals are formed when the two equivalents of alcohol are found on the same molecule. A great example of this is 1,2-ethanediol. The mechanism is almost identical to that of acetal synthesis; the only difference is that the other alcohol on the same alcohol performs the second nucleophilic addition.

Cyclic acetal​Cyclic acetal

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.