Organic Chemistry 101: Aldehydes and Ketones

It is an organic carbonyl compound. It consists of an alkane chain with a carbon-oxygen double bond on its terminal carbon.

Aldehydes are named by removing the “-e” from the end of the name of the parent alkane and replacing it with “-al.” The three-carbon chain above is called propanal.

This molecule is methanal.
Its common name is formaldehyde.

Aldehydes are named by removing the “-e” from the end of the name of the parent alkane and replacing it with “-al.” Know both the IUPAC and common name for this compound.

pentanal

To name an aldehyde, remove the “-e” from the end of the name of the parent alkane and replace it with “-al.” Since an aldehyde is a terminal group by definition, it is not necessary to write “1-pentanal.”

It is an organic carbonyl compound. It consists of an alkane chain with a carbon-oxygen double bond on one of the chain’s internal carbons.

Ketones are named by removing the “-e” from the end of the name of the parent alkane and replacing it with “-one.” The three-carbon chain above is called propanone, commonly known as acetone. Both names are valuable to know for this molecule.

butanone

Ketones are named by removing the “-e” from the end of the name of the parent alkane and replacing it with “-one.”

2-pentanone

To name an ketone, remove the “-e” from the end of the name of the parent alkane and replace it with “-one.” Since numbering begins from the end closest to the carbonyl, this structure is called 2-pentanone, not 4-pentanone.

Ketones and aldehydes have higher boiling points due to their larger intermolecular forces.

These molecules exert dipole-dipole forces due to their carbonyl groups, while hydrocarbons have only dispersion forces.

For example, propane has a boiling point of -42 °C, compared to propanal at 50 °C and propanone at 56 °C.

Ketones and aldehydes have lower boiling points due to their smaller intermolecular forces.

Alcohols can hydrogen bond, while ketones and aldehydes exhibit only the comparatively weaker dipole-dipole interactions.

For example, propanol has a boiling point of 97 °C, compared to propanal at 50 °C and propanone at 56 °C.

nucleophiles

Good nucleophiles have lone pairs or negative charges and are generally basic.

Organic chemistry involves a nucleophile attacking a carbonyl carbon (which possesses a partial positive charge).

The C=O (carbonyl) carbon is most easily attacked.

Oxygen is very electronegative and pulls electron density away from the carbon. This leaves the carbon with a partial positive charge, which is exactly what attracts nucleophiles.

• Hemiacetal is formed.

• Water is formed as a side product.

A hemiacetal, or “half acetal,” is a functional group that contains one -OR and one -OH group bound to the same carbon. Since they are derived from aldehydes, that carbon is also bound to a hydrogen atom.

• Hemiketal is formed.

• Water is formed as a side product.

A hemiketal, or “half ketal,” is a functional group that contains one -OR and one -OH group bound to the same carbon. Since they are derived from ketones, that carbon is also bound to two carbon chains.

It is an aldehyde derivative in which the carbonyl has been replaced by two -OR groups. The “R” can be identical or different.

An acetal is formed via the reaction of an aldehyde molecule with two equivalents of alcohol.

• Acetal is formed.

• Water is formed as the side product.

Note that an acetal is also formed when a hemiacetal is reacted with an additional equivalent of alcohol.

Itis a ketone derivative in which the carbonyl has been replaced by two -OR groups. The “R” can be identical or different.

A ketal is formed via the reaction of a ketone molecule with two equivalents of alcohol.

• Ketal is formed.

• Water is formed as the side product.

Note that a ketal is also formed when a hemiketal is reacted with an additional equivalent of alcohol.

hemiketal

In this reaction, the alcohol’s -OR group is the nucleophile, while the carbonyl carbon on the ketone is the electrophile. After the attack, the oxygen accepts the H instead of reforming the carbonyl. A basic workup keeps the -OH from further protonating and creating a leaving group.

hemiacetal

In this reaction, the alcohol’s -OR group is the nucleophile, while the carbonyl carbon on the aldehyde is the electrophile. After the attack, the oxygen accepts the H instead of reforming the carbonyl. A basic workup keeps the -OH from further protonating and creating a leaving group.

ketal

In this reaction, the alcohol’s -OR group is the nucleophile, while the carbonyl carbon on the ketone is the electrophile. After the attack, the oxygen accepts the H instead of reforming the carbonyl. An acidic workup protonates the -OH into H2O, creating a leaving group and allowing a second -OR substitution to occur.

acetal

In this reaction, the alcohol’s -OR group is the nucleophile, while the carbonyl carbon on the aldehyde is the electrophile. After the attack, the oxygen accepts the H instead of reforming the carbonyl. An acidic workup protonates the -OH into H2O, creating a leaving group and allowing a second -OR substitution to occur.

• Enamine is formed.

• Water is formed as a side product.

An enamine consists of a nitrogen group (amine) bound to a carbon that is participating in a C=C double bond.

imine

Imine nomenclature is sufficient to simply be able to identify this functional group.

An imine would form. Imines are functional groups that include a carbon-nitrogen double bond.

The details of imine formation are not likely to be tested, but it is important to be able to recognize the group.

Reactants in an aldol condensation can include either:

• two aldehydes

• two ketones

• aldehyde and a ketone

The final product is an enone, or a molecule with an alkene adjacent to (and conjugated with) a ketone.

self-condensation

The aldol condensation generally references self-condensations.

1. A base attracts an alpha hydrogen from one of the reactants, allowing an enolate to form.

2. The enolate attacks the carbonyl carbon of the other reactant. This forms an aldol (a β-hydroxyaldehyde or β-hydroxyketone).

3. Base is used to promote dehydration, or loss of water, which forms a conjugated enone.

An enone will form in this aldol condensation.

First, propanone is deprotonated, forming an enolate. This acts as a nucleophile to attack the intact propanone's carbonyl carbon, then protonates the original carbonyl oxygen. When a beta proton is shifted to protonate that atom again, it becomes H2O, a good leaving group. Condensation then occurs.

The exact product will be 4-methyl-3-penten-2-one. Simply know that this is an aldol condensation.

retro-aldol reaction

As its name implies, a retro-aldol reaction is simply the reverse of the forward reaction, aldol formation. The products will be two smaller carbonyl compounds (such as two aldehydes).

It involves the gain of bonds to oxygen or the loss of bonds to hydrogen.

This definition differs from the general chemistry idea of oxidation number. While both are valid ways to conceptualize redox reactions, thinking of the gain and loss of bonds can be much easier when dealing with organic compounds.

It involves the gain of bonds to hydrogen or the loss of bonds to oxygen.

This definition differs from the general chemistry idea of oxidation number. While both are valid ways to conceptualize redox reactions, thinking of the gain and loss of bonds can be much easier when dealing with organic compounds.

Common oxidizing agents includes:

• the chromium reagents (CrO3, CrO42-, Cr2O72-)

• permanganate (MnO4-) and manganate (MnO42-)

• nitric acid (HNO3)

• pyridinium chlorochromate (PCC)

Remember that oxidizing agents generally contain many oxygen atoms. Reducing agents usually contain many hydrogen atoms.

**carboxylic acid

The carbon on an aldehyde already contains two bonds to oxygen. To further oxidize, it would need to gain another one and become a carboxylic acid.

No product will form.

The carbon on a ketone already contains two bonds to oxygen and two bonds to carbon. In other words, it is maximally oxidized; it cannot be reacted further.

No difference will exist.

Since ketones already contain two bonds to oxygen and two to carbon, they are already fully oxidized. The strength of the oxidizing agent does not matter; both reactants will be unchanged.

These reagents both act as reducing agents.

LiAlH4 is a strong reducing agent, while NaBH4 is noticeably weaker.

• Reduction of a ketone will form a secondary alcohol.

• Reduction of an aldehyde will form a primary alcohol.

Reduction is the loss of bonds to oxygen and the gain of bonds to hydrogen. Aldehydes and ketones already contain two carbon-oxygen bonds, and must lose one during reduction.

• keto form

• enol form

The keto form contains a carbonyl group, while the enol form contains an alcohol adjacent to an alkene.

tautomerization

Keto-enol tautomerization includes the movement of an alpha hydrogen and the bonding electrons of the pi bond. Because atoms change their positions, it is not a type of resonance.

The keto form is generally more thermodynamically stable than the enol form. For this reason, it tends to predominate.

Exceptions include cases in which the enol form is conjugated. One example is phenol, which gains extra stability from conjugation that it would lose as a keto tautomer.

This molecule can form an enol that is stabilized by hydrogen bonding.

• Imines

• Enamines

Like the keto form, the imine form tends to predominate due to stability. However, the enamine form can be more prevalent if it allows the compound to be conjugated.

As in all examples of kinetic vs. thermodynamic control:

• the kinetic enolate is easier to form, with a lower activation energy. It is favored at low temperatures.

• the thermodynamic enolate is more energetically difficult to form, with a higher activation energy, but is more stable (more substituted). It is favored at high temperatures.

The structure on the right is the thermodynamic enolate. Since it is more substituted, it is more stable; however, its steric hindrance makes it difficult to form at low temperatures.

The structure on the left is the kinetic enolate. In contrast to above, it is less substituted, less stable, and easier to form.

It is a compound that contains an R group bound to a metal atom. Examples include n-BuLi and the Grignard reagents.

Organometallic reagents allow the R group to act as a nucleophile. They attack the carbonyl carbon, adding the R group to that position and protonating the carbonyl oxygen to an alcohol.

A ketone reacting with an organometallic reagent will produce a tertiary alcohol. This occurs because the metal allows the carbon chain to act as a nucleophile, adding to the carbonyl carbon and protonating the carbonyl oxygen.

It is a type of organometallic reagent with the formula R-MgX. Grignard reagents contain a magnesium halide bound to an R group, or carbon chain.

The Grignard reaction is a good way to add an alkyl chain to a carbonyl compound.

A Grignard, will form a secondary alcohol.
The R group acts like a nucleophile and attacks the carbonyl carbon, leading to the protonation of the carbonyl oxygen. The MgBr detatches and moves back into solution.

tertiary alcohol
The -OR group on the reactant is a good leaving group, so the overall molecule will react with two equivalents of Grignard reagent: first as a substitution of R' for -OR, and again as an addition of a second R' group.

The speed of attack, and reactivity of the compound in general, is decreased.

Large substituents near the carbonyl carbon cause steric hindrance and block the nucleophile's access to the electrophilic carbon.

The large, bulky groups decrease the reactivity.

In other words, the steric hindrance of the tert-butyl groups limits the nucleophile's access to the carbonyl carbon. This slows the reaction.

Alpha protons are more acidic than the other protons in a carbon chain.

When the alpha position is deprotonated, the resulting molecule can be stabilized by resonance. In other words, the conjugate base is especially stable, a characteristic that increases acidity.

The product is a resonance-stabilized carbanion, in both the keto and enol forms.

While carbanions are extremely reactive, the resonance here allows one to be stable enough to form. This explains the increased acidity of alpha protons.

Two resonance structures can form in addition to the original.

The structure on the left is an enolate, while that on the right has its double bond shifted to the beta-carbon.

The product will be a carbonyl with the nucleophile added to the beta position.

The nucleophile attacks at the beta position, causing the alpha, beta-unsaturated carbonyl to convert to the enol form. The enol then tautomerizes back to a carbonyl.

CH2-P-Ph3 is a ylide, specifically a triphenyl phosphonium ylide. Ylides are used in Wittig reactions.

The Wittig reaction allows a carbonyl to be converted into an alkene, a common process used in alkene synthesis. You need no outside knowledge about the reaction; just be able to recognize it if you see it in a passage.

cyanohydrin

It can be synthesized by reacting a carbonyl compound with a nitrile, such as CN-.

Cyanohydrins are involved in the Strecker synthesis, a procedure that can synthesize amino acids.