Carbohydrates, aldehydes and ketones

Carbohydrates

Carbohydrates are essential biomolecules in living organisms, acting primarily as energy sources and structural components. They range from monosaccharides (single-unit sugars) to polysaccharides (long chains of sugars).

Common examples include glucose, a key energy source, and fructose, found in fruits and table sugar.

Nomenclature and classification, common names

Monosaccharides: Single sugar units such as glucose
( $C_{6} H_{12} O_{6}$ ), galactose, and fructose.
Disaccharides: Composed of two monosaccharides linked by a glycosidic bond (e.g., sucrose, lactose, maltose).
Polysaccharides: Long chains of monosaccharides (e.g., starch, glycogen, cellulose).
Prefixes:
- deoxy indicates removal of an $- O H$ group, replaced by $- H$ .
- D/L denotes the absolute configuration relative to D-glyceraldehyde or L-glyceraldehyde.
- $α$ / $β$ specifies the anomeric configuration in cyclic sugars, referencing whether the anomeric $- O H$ group is oriented opposite or the same side as the $C H_{2} O H$ group.
All sugars end in –ose (e.g., glucose, fructose, galactose).

Absolute configuration

Monosaccharides often have multiple chiral centers, but the D/L notation focuses on the chiral carbon farthest from the carbonyl group in the open-chain form.

D sugars have the –OH on this reference carbon aligned similarly to D-glyceraldehyde.
L sugars mirror this configuration.
This system is independent of R/S designations, which use the Cahn–Ingold–Prelog priority rules.

Cyclic structure and conformations of hexoses

Monosaccharides like glucose commonly form cyclic hemiacetal or hemiketal rings in aqueous solutions. For six-carbon aldoses (e.g., glucose), a six-membered pyranose ring is typical, whereas for ketoses (e.g., fructose), a five-membered furanose ring may form.

Chair and boat conformations can arise, with chair generally being more stable due to minimized steric hindrance.

Epimers and anomers

Epimers differ in configuration at one chiral center, excluding the newly formed anomeric center in ring form.
Anomers specifically differ at the new chiral center (the anomeric carbon) created upon cyclization.
- $α$ anomer: The anomeric $- O H$ is oriented opposite the $C H_{2} O H$ group.
- $β$ anomer: The anomeric $- O H$ is oriented on the same side as the $C H_{2} O H$ group.
  Mutarotation is the interconversion between $α$ and $β$ forms in solution, changing the optical rotation.

Hydrolysis of the glycoside linkage

Glycosidic bonds join monosaccharides to form disaccharides and polysaccharides. Under acidic or enzymatic conditions, these bonds can be hydrolyzed, splitting the carbohydrate into its constituent sugars. Enzymes known as glycosidases facilitate this process in biological systems.

Keto-enol tautomerism of monosaccharides

Some monosaccharides, especially those with an aldehyde or ketone group in the open-chain form, can undergo keto-enol tautomerism. This involves temporarily transforming the carbonyl group into an enediol intermediate, allowing an aldose to rearrange into a ketose or vice versa under basic conditions. While not always prominent under physiological pH, this tautomerism contributes to sugar reactivity and interconversion.

Disaccharides

Sucrose: Formed by $α -glucose$ and $β$ -fructose linked at their anomeric carbons.
Lactose: Consists of β-galactose joined to glucose ( $α$ or $β$ ) through a 1→4 glycosidic bond.
Maltose: Composed of two glucose units connected via a dehydration reaction, typically $α -1,4$ linkage.

Polysaccharides

Starch: Predominantly $α -1,4$ glycosidic bonds linking glucose; used by plants for energy storage.
Glycogen: Similar to starch but with more frequent $α -1,6$ branches, making it a highly branched energy reserve in animals.
Cellulose (not mentioned above but commonly noted): Features $β -1,4$ linked glucose units, providing structural support in plant cell walls.

Aldehydes and Ketones

Aldehydes and ketones are characterized by a carbonyl group, in which a carbon is double-bonded to oxygen. Aldehydes have this group at the end of a carbon chain, while ketones have it within the chain. They serve as key intermediates in organic synthesis, playing central roles in forming more complex compounds.

Nomenclature

Aldehydes typically end in “-al,” as in methanal (formaldehyde) or ethanal (acetaldehyde). Ketones generally end in “-one,” as in propanone (acetone) or butanone. Common names are widely used, but the systematic IUPAC names specify the longest carbon chain containing the carbonyl.

Physical properties

These compounds often have higher boiling points than comparable alkanes due to polar $C = O$ bonds. However, without the ability to hydrogen-bond as donors, their boiling points are generally lower than those of alcohols. Many lower-molecular-weight aldehydes and ketones are water-soluble due to the carbonyl’s hydrogen-bond accepting ability.

Important reactions

The partial positive charge on the carbonyl carbon makes it susceptible to nucleophilic attack. This reactivity underlies much of aldehyde and ketone chemistry, ranging from simple additions to more complex condensation reactions.

Nucleophilic addition reactions at $C = O$ bond

A nucleophile attacks the electrophilic carbon, opening the double bond and subsequently yielding an alkoxide intermediate. Depending on the reaction conditions, this intermediate can be protonated or transformed into further derivatives.

Acetal, hemiacetal
When an alcohol reacts with an aldehyde or ketone, a hemiacetal initially forms. With excess alcohol under acidic conditions, this hemiacetal can convert into an acetal, in which two $- OR$ groups bond to the same carbon.
Imine, enamine
A primary amine reacts with an aldehyde or ketone to give an imine, formed by the replacement of the carbonyl oxygen with a double-bonded nitrogen. Secondary amines can yield an enamine, in which the α hydrogen migrates, generating a $C = C - N$ linkage.
Hydride reagents
Compounds such as sodium borohydride ( $N a B H_{4}$ ) or lithium aluminum hydride ( $L i A l H_{4}$ ) deliver hydride ions to the carbonyl, reducing aldehydes to primary alcohols and ketones to secondary alcohols.
Cyanohydrin
When hydrogen cyanide adds to a carbonyl, a cyanohydrin forms. This functional group contains both a $- CO$ and an $- O H$ attached to the former carbonyl carbon.

Oxidation of aldehydes
Aldehydes can be oxidized to carboxylic acids under relatively mild conditions. Ketones generally resist oxidation unless under extreme conditions that break carbon–carbon bonds.

Reactions at adjacent positions: enolate chemistry
An enolate arises when the α hydrogen is removed, generating a resonance-stabilized anion. This reactive species can participate in many transformations, notably aldol condensation.

Keto-enol tautomerism ( $α$ -racemization)
Aldehydes and ketones containing an $α$ hydrogen can interconvert between the keto form and the enol form through proton shifts. This equilibrium, known as tautomerism, can lead to racemization at the α carbon if it is chiral.
Aldol condensation, retro-aldol
Enolate ions formed from aldehydes or ketones can attack other carbonyl-containing molecules, yielding aldol products. Under certain conditions, the aldol can dehydrate to form an $α$ , $β$ -unsaturated carbonyl. The reverse, retro-aldol, breaks the bond formed in aldol reactions, regenerating smaller carbonyl compounds.
Kinetic versus thermodynamic enolate
The kinetic enolate forms rapidly under conditions favoring lower activation energy (often at low temperatures with a strong, bulky base), typically removing the most acidic proton. The thermodynamic enolate is more stable and forms at higher temperatures or with weaker bases, favoring the more substituted double bond.

Reaction mechanisms for base- and acid-catalysed aldol condensation

General principles
The polarity of the carbonyl bond, influenced by substituents, controls reactivity. Steric hindrance can slow nucleophilic approaches, whereas electron-withdrawing groups can increase the partial positive character of the carbon.

Effect of substituents on reactivity of $C = O$ ; steric hindrance
Electron-withdrawing substituents enhance electrophilicity of the carbonyl, making it more prone to nucleophilic addition. Bulky substituents hinder approach to the carbonyl center, reducing reaction rates.
Acidity of $α$ -H; carbanions
$α$ Hydrogens adjacent to the carbonyl are more acidic than typical alkyl $C - H$ bonds because deprotonation yields a resonance-stabilized carbanion (the enolate). This acidity enables many aldol-type and related transformations in synthetic chemistry.