Aldehydes and Ketones Flashcards
Feeling a bit rusty with keto-enol tautomerization? Having trouble remembering the difference between acetals and hemiacetals? Use these cards to master aldehydes, ketones, and their reactions as they appear on the MCAT.
Define:
aldehyde
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.
What is the IUPAC name of the molecule below?
It is known as methanal, commonly referred to as formaldehyde.
Aldehydes are named by removing the “-e” from the end of the name of the parent alkane and replacing it with “-al.” For the MCAT, know both the IUPAC and common name for this compound.
What is the IUPAC name of the molecule below?
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.”
Define:
ketone
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. For the MCAT, both names are valuable to know for this molecule.
What is the IUPAC name of the molecule below?
butanone
Ketones are named by removing the “-e” from the end of the name of the parent alkane and replacing it with “-one.”
What is the IUPAC name of the molecule below?
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.
How do the boiling points of ketones and aldehydes compare to those of simple hydrocarbons?
Assume molecular masses of all compounds are similar.
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.
How do the boiling points of ketones and aldehydes compare to those of alcohols?
Assume all compounds have similar molecular masses, chain length, and branching.
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.
OH- tends to donate electrons and is attracted to electropositive atoms. What name is given to molecules that share these traits?
nucleophiles
Good nucleophiles have lone pairs or negative charges and are generally basic.
Most reactions in MCAT organic chemistry involve a nucleophile attacking a carbonyl carbon (which possesses a partial positive charge).
Which atom in a ketone or aldehyde is most susceptible to nucleophilic attack?
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.
What product is formed when an aldehyde and a single alcohol are reacted in the presence of an acid catalyst?
- 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.
What product is formed when a ketone and a single alcohol are reacted in the presence of an acid catalyst?
- 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.
Define:
acetal
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.
What product is formed when an aldehyde and excess alcohol are reacted in the presence of an acid catalyst?
- 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.
Define:
ketal
It is 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.
What product forms when a ketone and excess alcohol are reacted in the presence of an acid catalyst?
- 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.
What product will this reaction form?
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.
What product will this reaction form?
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.
What product will this reaction form?
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.
What product will this reaction form?
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.
What product forms when a ketone or aldehyde and a secondary amine undergo a condensation reaction?
- 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.
What is the name of the functional group that consists of a carbon double-bonded to a nitrogen atom?
imine
Imine nomenclature is rarely tested on the MCAT, so it is sufficient to simply be able to identify this functional group.
What product would form if the oxygen in the C=O bond of a carbonyl was replaced with an N-H 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 on the MCAT, but it is important to be able to recognize the group.
What types of reactants can take part in an aldol condensation, and what final product is formed?
- 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.
What term describes an aldol condensation in which the reactants are two equivalents of the same molecule?
self-condensation
The aldol condensation is not commonly seen on the MCAT, but when it does appear, it generally references self-condensations.
Briefly list the steps involved in an aldol condensation reaction.
- A base attracts an alpha hydrogen from one of the reactants, allowing an enolate to form.
- The enolate attacks the carbonyl carbon of the other reactant. This forms an aldol (a β-hydroxyaldehyde or β-hydroxyketone).
- Base is used to promote dehydration, or loss of water, which forms a conjugated enone.
What type of product will be formed from this reaction in the presence of heat?
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, but the MCAT would not ask for such detail. Simply know that this is an aldol condensation.
What name could be used to describe a reaction in which two carbonyl compounds are formed from a β-hydroxyaldehyde?
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).
In organic chemistry, how can an oxidation reaction be easily identified?
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.
In organic chemistry, how can a reduction reaction be easily identified?
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.
Name at least three compounds that are used as oxidizing agents in organic reactions.
- the chromium reagents (CrO3, CrO42-, Cr2O72-)
- permanganate (MnO4-) and manganate (MnO42-)
- nitric acid (HNO3)
- pyridinium chlorochromate (PCC)
For the MCAT, remember that oxidizing agents generally contain many oxygen atoms. Reducing agents usually contain many hydrogen atoms.
What product, if any, will be formed from the oxidation of an aldehyde group?
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.
What product, if any, will be formed from the oxidation of a ketone group?
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.
One ketone is reacted with KMnO4 (a very strong oxidizing agent), while another identical ketone is reacted with PCC (a considerably weaker oxidant). What difference will be seen between the two products?
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.
What is the main function of the hydride reagents NaBH4 and LiAlH4?
These reagents both act as reducing agents.
LiAlH4 is a strong reducing agent, while NaBH4 is noticeably weaker.
What products will form from the reduction of a ketone and an aldehyde, respectively?
Assume the reducing agent used was either NaBH4 or LiAlH4.
- 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.
Name the two isomeric forms of a carbonyl compound that can interconvert and exist in equilibrium.
- keto form
- enol form
The keto form contains a carbonyl group, while the enol form contains an alcohol adjacent to an alkene.
What name is given to the conversion between a compound’s keto and enol forms?
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. Remember this for the MCAT!
Which form tends to be more stable in aldehydes and ketones: keto or enol?
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.
What stable resonance structure can be formed by a 1,3 dicarbonyl structure?
This molecule can form an enol that is stabilized by hydrogen bonding.
In addition to the keto and enol forms, what other functional groups are involved in tautomerization?
- 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.
What is the difference between the kinetic and thermodynamic enolates?
- 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.
Is the circled structure on the right the kinetic or thermodynamic enolate?
It 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.
What is an organometallic reagent?
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.
What product will this reaction form?
2-methyl-butanol
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.
What atoms are included in a Grignard reagent?
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.
What product will this reaction form?
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.
What product will this reaction form after an acid work-up?
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.
How does the presence of large substituents near the carbonyl carbon affect the speed of a nucleophilic attack?
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.
Scientists are trying to perform a nucleophilic addition on the molecule below. How do the tert-butyl groups likely affect the reactivity?
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.
What chemical property is displayed by the alpha proton of a carbonyl compound?
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.
What product(s) exist immediately after the deprotonation of a carbonyl compound at the alpha position?
It 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.
How many resonance structure(s) can form from this alpha, beta unsaturated carbonyl?
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.
What is the product of a strong nucleophilic attack on an alpha, beta unsaturated carbonyl (as shown below)?
It 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.
What name is given to molecules like CH2-P-Ph3, and what reaction are they involved in?
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. For the MCAT, however, you need no outside knowledge about the reaction; just be able to recognize it if you see it in a passage.
What is the name of the molecule below?
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.
