A carbonyl group that is connected to a hydrogen atom is called an aldehyde. In methanal, the carbonyl group is connected to two hydrogen atoms, the most basic aldehyde (also known as formaldehyde). In other aldehydes, Carbonyl carbon is connected to one hydrogen atom and one carbon atom. A carbonyl group connected to two carbon atoms is known as a ketone.
Aldehyde or ketone functional groups are present in many natural substances. Ketones often have a pleasant fragrance whereas aldehydes have a strong smell.
Examples of naturally occurring aldehydes include vanillin and cinnamaldehyde. The strong aroma of vanillin by taking a sniff of vanilla extract is due to aldehyde. The distinctively pleasant scents of caraway seeds, spearmint leaves, and camphor tree leaves are due to the ketones camphor and carvone.
Two important biological ketones progesterone and testosterone show that structural variation can result in changes in biological activity. Both are sex hormones, testosterone is largely produced in the testes, while progesterone is primarily produced in the ovaries.
Nomenclature of Aldehydes and Ketones
Naming Aldehydes
An aldehyde systematic (IUPAC) name is given by putting “al” in place of the “e” at the end of the parent hydrocarbon name. Methanal is a one-carbon aldehyde, while ethanal is a two-carbon aldehyde. The carbonyl carbon always has the 1-position and does not require a specific position because it is always at the end of the parent hydrocarbon (otherwise the compound would not be an aldehyde).
Hexanedial does not eliminate the terminal “e” of the parent hydrocarbon chain. (The “e” is just omitted to prevent two consecutive vowels.) When an aldehyde group is connected to a ring, the name of the cyclic molecule is modified with the word “carbaldehyde”.
Naming Ketones
A ketone’s systematic name is given by putting “one” in place of the “e” at the end of the parent hydrocarbon name. The chain is numbered in a way that provides the carbonyl carbon with the lower number. Cyclic ketones do not require a number because the carbonyl carbon is at the 1-position. In a derived name, the carbonyl group’s substituents are listed alphabetically, followed by “ketone”.
Just a few ketones are known by their common names. Propanone, the smallest ketone, is referred to by its more widely used name, acetone.
Compared to an alcohol or an amino group, a carbonyl group has a greater nomenclature priority. Not all carbonyl compounds are given the same priority. The lower-priority functional group in a compound is denoted by a prefix, while the higher-priority functional group is denoted by a suffix.
The order of priority is;
X < ROR < RH < Alkyne < Alkene < R-NH2 < RCOR < RCHO < RCOOR < RCOOH
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In comparison to ketone, aldehyde has a higher priority. The prefix “oxo” is used to denote the carbonyl oxygen in a ketone or aldehyde name when a second functional group is present with greater naming priority.
if one of the functional groups is an alkene (or an alkyne) the suffix ends are used for both functional groups. The alkene (or alkyne) functional group is mentioned first, and its ending “e” is deleted to prevent two consecutive vowels.
When referring to the RC=O as a substituent, the term acyl (a-sil) group is used, along with the name suffix -yl. Formyl, acetyl, and benzoyl groups are represented by CHO, COCH3, and COC6H5, respectively.
Synthesis of Aldehydes and Ketones
Preparing Aldehydes
Using primary alcohols for oxidation is one of the greatest ways to form aldehydes. The reaction is frequently performed at room temperature using the Dess-Martin periodinane reagent in dichloromethane solvent.
Read more about; Swern Oxidation: Alcohols to Carbonyls
Aldehydes can be produced by partly reducing certain derivatives of carboxylic acids. A laboratory-scale technique of producing aldehydes is the partial reduction of an ester by diisobutylaluminum hydride (DIBAH, or DIBAL-H). Normally, the reaction takes place in a toluene solution at 278 °C (dry-ice temperature).
Preparing Ketones
By using a number of reagents, secondary alcohols are oxidized to produce ketones. The oxidant of choice is determined by the alcohol’s acid or basic sensitivity, as well as the scale of the reaction, cost, and other variables. The two most popular options are the Dess-Martin periodinane and a Cr(VI) reagent like CrO3.
Friedel-Crafts acylation of an aromatic ring with an acid chloride using AlCl3 catalyst as well as ozonolysis of alkenes, both reactions are useful in the synthesis of Ketones.
Reactions involving an acid chloride and a lithium diorganocuprate reagent are the most important reactions that are useful in the synthesis of Ketones.
Oxidation of Aldehydes and Ketones
Oxidation of Aldehydes to Carboxylic Acids
Several common oxidizing agents, such as chromic acid and molecular oxygen, can convert aldehydes into carboxylic acids. In fact, of all functional groups, aldehydes are one of the easiest to oxidize. Hexanal being converted into hexanoic acid serves as an example of oxidation by chromic acid;
Tollens’ reagent converts aldehydes into carboxylic acids by dissolving silver nitrate in water, precipitating silver ion as Ag2O with sodium hydroxide, and redissolving silver ion as the silver-ammonia complex ion with aqueous ammonia.
Aldehydes are oxidized to carboxylic anion when Tollens’ reagent is used, while Ag+ is reduced to metallic silver. The silver-mirror test is so named because, when this reaction is done correctly, silver precipitates as a mirror-like deposit.
Oxidation of Ketones to Carboxylic Acids
At greater temperatures and higher concentrations of nitric acid, HNO3, ketones are subjected to oxidative cleavage via their enol form by using potassium dichromate and potassium permanganate. Depending on the ketone’s substitution pattern, the carbon-carbon double bond of the enol is broken to form two carboxyl or ketone groups. The conversion of cyclohexanone to hexanedioic acid (adipic acid) is an industrial use of this reaction, Adipic acid is the monomer needed to form the polymer nylon 66.
Baeyer-Villiger Oxidation of Ketones
The Baeyer-Villiger oxidation involves the oxidative breakage of a carbon-carbon bond next to a carbonyl, which changes ketones into esters and cyclic ketones into lactones. Peracids, such as MCBPA, hydrogen peroxide, and a Lewis acid can be used in the Baeyer-Villiger oxidation.
The relative tendency of the substituents connected to the carbonyl to migrate determines the reaction’s regiospecificity. The order of substituents preference is tert. alkyl > cyclohexyl > sec. alkyl > phenyl > prim. alkyl > CH3 and substituent that can stabilize a positive charge migrate more quickly. Stereoelectronic or ring strain considerations also have an effect on the regiochemical result.
Reduction reactions of Aldehydes and Ketones
Ketones are converted into secondary alcohols and aldehydes into primary alcohols.
Read more about Primary vs Secondary Alcohols: The Key Differences
Catalytic Reduction
In the presence of a transition metal catalyst such as finely divided palladium, platinum, nickel, or rhodium, the carbonyl group of an aldehyde or ketone is reduced by hydrogen to a hydroxyl group. Reductions are typically conducted at temperatures between 25 and 100 °C and hydrogen pressures between 1 and 5 atm. Cyclohexanone is converted to cyclohexanol in these circumstances.
Aldehydes and ketones can be reduced catalytically, yields are frequently very high. One drawback is that under these cases, some other functional groups are also reduced. One example is carbon-carbon double bonds.
Metal Hydride Reductions
Sodium borohydride and lithium aluminum hydride are by far the most often used laboratory reagent for converting the carbonyl group of an aldehyde or ketone to a hydroxyl group. These substances act as a source of the nucleophile hydride ion. The formal negative charges on boron and aluminum can be seen in the structural formulas presented here for both reducing agents.
The carbonyl groups of aldehydes, ketones, and carboxylic acids as well as their functional derivatives are all reduced by the strong reducing agent lithium aluminum hydride. Only aldehydes and ketones are quickly reduced by sodium borohydride, which is a far more selective reagent. Most frequently, reductions with sodium borohydride take place in aqueous methanol, pure methanol, or ethanol. Tetraalkyl borate is the byproduct of reduction; when it is treated with water, it transforms into alcohol and sodium borate salts. Aldehyde or ketone can be reduced by one mole of sodium borohydride by four moles.
The important process in the metal hydride reduction of an aldehyde or ketone is the transfer of a hydride ion from the reducing agent to the carbonyl carbon to create a tetrahedral carbonyl addition intermediate. Just the hydrogen atom bound to carbon comes from the hydride-reducing agent when reducing an aldehyde or ketone to an alcohol, the hydrogen atom connected to oxygen comes from the water that is added to hydrolyze the metal alkoxide salt.
A carbon-carbon double bond can be reduced selectively when a carbonyl group is present. To achieve this, the carbonyl group must first be protected by an acetal. Using a protective group to reduce a carbon-carbon double bond selectively.
Reactions in the form of Enol Tautomers
Acid‐Catalyzed α‐Halogenation
Haloaldehydes and halo ketones are produced when aldehydes and ketones having at least one α-hydrogen react with bromine and chlorine near the carbon atom. Acetophenone, for example, forms a bromoketone when it interacts with the bromine in acetic acid.
Both acid and base act as catalysts for halogenation. The HBr or HCl produced by the reaction catalyzes other reactions in acid-catalyzed halogenation. Acid-catalyzed conditions result in a minor quantity of enol formation.
A new covalent bond is formed when an electrophile and a nucleophile react. Enol’s nucleophilic attack on the halogen molecule:
Haloketone is produced via proton transfer, which also produces HBr.
The benefit of “halogenation” is that it transforms the α-carbon atom into a center that has a good leaving group attached to it and which is replaceable by a range of excellent nucleophiles. An example of how diethylamine, a nucleophile, combines with bromoketone to produce diethylaminoketone is shown below.
Racemization at an α‐Carbon
The optical activity of 3-phenyl-2-butanone is unchanged when it is dissolved in ethanol, but decreases to zero when an acid is added. The achiral enol intermediate’s formation under acid catalysis can explain this result, as the R and S enantiomers are produced equally by tautomerism to the chiral keto form. A racemic mixture of 3 phenyl-2-butanone is discovered when it is separated from this solution.
Only carbon stereocenters with at least one hydrogen experience racemization via this process. As it is necessary to have an enantiomerically pure version of a molecule rather than a racemic mixture in medicine, for example, this process is typically an undesirable side effect of acid impurities in a sample.
Reactions that involve an addition-elimination mechanisms
Formation of Imines
In the presence of an acid catalyst, ammonia, primary aliphatic amines (RNH2), and primary aromatic amines (ArNH2) react with the carbonyl group of aldehydes and ketones to produce a compound that has a carbon-nitrogen double bond. An imine or, alternatively, a Schiff base is a compound that has a carbon-nitrogen double bond.
A tetrahedral carbonyl addition intermediate is produced when the nitrogen atom of ammonia or a primary amine, both strong nucleophiles, is added to the carbonyl carbon after a proton transfer.
OH2+, an excellent leaving group, is formed via protonation of the OH group.
The imine is produced by the loss of water and proton transfer to solvent. The proton transfer and water loss exhibit E2 reaction characteristics. In this dehydration, the carbon-nitrogen double bond develops, a base (in this example, a water molecule) takes a proton from N, and Nuleofuge (in this case, a water molecule) leaves.
Imine formation is reversible, just as hemiacetal and acetal-forming processes. When an imine is hydrolyzed by an acid, it produces a 1° amine as well as an aldehyde or a ketone. The 1° amine, a weak base, becomes an ammonium salt when one (or more) equivalents of acid are applied.
Reductive Amination of Aldehydes and Ketones
One of the main benefits of imines is the ability of hydrogen to convert the carbon-nitrogen double bond into a carbon-nitrogen single bond in the presence of a catalyst made of nickel or another transition metal. Reductive amination is a two-step process that turns a primary amine into a secondary amine via an imine, as by the transformation of cyclohexylamine into dicyclohexylamine.
Formation of hemiacetals
When an alcohol molecule is added to the carbonyl group of an aldehyde or ketone, a hemiacetal (also known as a half-acetal) is formed. Hemiacetal is a compound with an OH and an OR or OAr group attached to the same carbon. Both acid and base function as catalysts for this reaction. Oxygen and hydrogen both contribute to the carbonyl carbon and vice versa.
An OH group, an OR or OAr group, and carbon are bound together to form a hemiacetal’s functional group.
Mechanism (Base‐Catalyzed Formation of a Hemiacetal)
Alkoxide is produced when a proton is transferred from the alcohol to the base.
A carbonyl addition intermediate is formed when an alkoxide ion is added to the carbonyl.
Proton transfer from water to the intermediate of tetrahedral carbonyl addition leads to the formation of hemiacetal and the regeneration of hydroxide ion catalyst.
Acid‐Catalyzed Formation of a Hemiacetal
HA protonates the carbonyl group forming a resonance-stabilized cation. Positive charges are introduced to the carbon by the more significant resonance structure.
Alcohol is added to resonance-stabilized cations to form oxonium ions.
Proton transfer from oxonium to A generates hemiacetal, which regenerates acid catalyst.
Formation of Acetals
Hemiacetals can react with alcohol to produce acetals and water molecules. Acid catalyzes this reaction.
A carbon is linked to two OR or OAr groups to form an acetal’s functional group.
Acid‐Catalyzed Formation of an Acetal
A proton transfer from the acid HA to the hemiacetal OH group forms an oxonium ion.
Oxonium ion water loss results in a resonance-stabilized cation.
The conjugate acid of the acetal is produced by the reaction of the resonance-stabilized cation (an electrophile) with methanol (a nucleophile).
A proton is transferred from the protonated acetal to A, resulting in the formation of the acid catalyst HA. The synthesis of 5-hydroxy-5-phenylpentanal from benzaldehyde and 4-bromobutanal serves as an example of the application of acetals as carbonyl-protecting groups.
The Grignard reagent reacts rapidly with the 4-bromobutanal molecule’s, protecting it by converting it to an acetal. Given how simple it is to make, cyclic acetals are often employed.
Magnesium alkoxide is formed by treating protected bromoaldehyde with magnesium in diethyl ether and adding benzaldehyde.
Aqueous acid treatment of magnesium alkoxide results in two outcomes. Hydroxyl group is produced by protonating alkoxide anion and aldehyde group is restored by hydrolyzing cyclic acetal.
Reaction with Grignard Reagent
Using Grignard reagents is a great approach to creating new carbon-carbon bonds. Grignard reagents take on the characteristics of carbanions in their reactions. To create a tetrahedral carbonyl addition intermediate, a carbanion(great nucleophile) reacts with the carbonyl group of an aldehyde or ketone. Alkoxide ions produced in the Grignard reaction are strong bases and they react with an aqueous acid like HCl or aqueous NH4Cl to produce alcohols.
The addition reaction of Formaldehyde Gives a 1° Alcohol. Primary alcohol is produced by treating Grignard reagents with formaldehyde and hydrolyzing them in aqueous acid.
The addition reaction of an Aldehyde (Except Formaldehyde) Gives a 2° Alcohol
Secondary alcohol is produced by treating Grignard reagents with aldehydes (except formaldehyde) and hydrolyzing them in aqueous acid.
The addition reaction of a ketone Gives a 3° Alcohol. Tertiary alcohol is produced by treating Grignard reagents with ketones and hydrolyzing them in aqueous acid.
Wittig reaction (Formation of C=C bond)
A triphenylphosphonium ylide (Wittig Reagent) reacts with an aldehyde or ketone to produce an alkene and triphenylphosphine oxide. This Reaction has the name of its discoverer, German scientist Georg Wittig. Georg Wittig received the 1979 Chemistry Nobel Prize for this discovery. For synthesizing alkenes, this reaction is very useful. Alkene synthesis has the advantage of accurately defining the location of each double bond. A mixture of alkenes with double bonds at different positions are the outcome of alcohol dehydration.
Concepts Berg
Why are Aldehydes more Reactive toward Nucleophilic Substitutions than Ketones?
Aldehydes are more reactive than ketones towards nucleophilic substitutions due to lower steric hindrance and increased electrophilicity. The absence of bulky groups and the presence of a hydrogen atom in aldehydes allow nucleophiles to approach more easily.
Additionally, the absence of strong electron-donating groups in aldehydes makes their carbonyl carbon more electrophilic, enhancing their reactivity.
How to distinguish between aldehyde and ketone by the Tollens test?
The Tollens test can be used to distinguish between aldehydes and ketones. By adding a few drops of Tollens reagent to a solution of the unknown compound and heating it, you can observe the formation of a silver mirror if the compound is an aldehyde.
If no reaction occurs and the solution remains clear, the compound is a ketone. If the unknown compound is an aldehyde, a silver mirror will form on the inside of the test tube.
The silver mirror is a result of the reduction of silver ions (Ag+) to silver metal (Ag) by the aldehyde. If the unknown compound is a ketone, there will be no reaction, and the solution will remain clear.
Why are aldehydes more reactive than ketones?
Aldehydes are more reactive than ketones due to their electronic and steric properties. The presence of a hydrogen atom adjacent to the carbonyl group makes aldehydes more electrophilic, while the absence of this hydrogen atom in ketones reduces their reactivity.
Additionally, aldehydes have less steric hindrance, allowing nucleophiles to approach the carbonyl carbon more easily. These factors contribute to the higher reactivity of aldehydes.
How to distinguish between aldehyde and ketone IR?
Both aldehydes and ketones show a carbonyl absorption peak in the IR spectrum around 1700-1750 cm-1. Aldehydes generally have a slightly higher frequency absorption peak (1710-1740 cm-1) compared to ketones (1715-1735 cm-1).
The C=O stretch in aldehydes is typically more intense than in ketones due to the presence of a hydrogen atom attached to the carbonyl carbon. Aldehydes often exhibit an additional absorption band around 2700-2800 cm-1 due to the C-H stretching vibration of the aldehyde group, which is absent in ketones.
Considering these factors collectively can help differentiate between aldehydes and ketones using IR spectroscopy.
Which has a more pungent odor?
Aldehydes generally have a stronger and more pungent odor compared to ketones. The terminal carbonyl group in aldehydes makes them highly reactive and capable of interacting strongly with olfactory receptors.
Examples of aldehydes with intense odors include formaldehyde, acetaldehyde, and benzaldehyde. Ketones, on the other hand, have a carbonyl group in the middle of the carbon chain, reducing their reactivity and resulting in a less pungent odor.
Ketones like acetone and camphor may have noticeable smells, but they are generally less intense than some aldehydes. The specific odor of a compound can vary based on its chemical structure and individual sensitivity to smells.
What is the reaction of ketone with PCl5?
When a ketone reacts with PCl5 in the presence of a Lewis acid catalyst like AlCl3, it undergoes a Friedel-Crafts acylation reaction.
This reaction results in the formation of an acyl chloride, along with the release of phosphorus oxychloride (POCl3) and hydrogen chloride (HCl) as byproducts.
Reference Books
- Textbook of Organic Chemistry Book By M. Younas
- Textbook of Organic Chemistry Book By Frances A. Carry’s
- Textbook of Organic Chemistry Book By Paula Yurkanis Bruice
Reference links
- An article (chemistry.msu.edu)
