Carbohydrates are essential biomolecules in living organisms. They mainly serve as energy sources and structural components. Carbohydrates range from monosaccharides (single sugar units) to polysaccharides (long chains of sugars).

Common examples include glucose, a major cellular fuel, and fructose, found in fruits and in table sugar.

Nomenclature and classification, common names

Monosaccharides: Single sugar units such as glucose ( $C_{6} H_{12} O_{6}$ ), galactose, and fructose.
Disaccharides: 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, based on whether the anomeric $- O H$ group is on the opposite side 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 contain multiple chiral centers, but the D/L system uses one specific reference point. In the open-chain form, look at the chiral carbon farthest from the carbonyl group; its configuration determines whether the sugar is D or L.

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

Cyclic structure and conformations of hexoses

In water, many monosaccharides exist mainly in cyclic form. Cyclization occurs when an internal alcohol group reacts with the carbonyl group to form a cyclic hemiacetal (from an aldose) or a hemiketal (from a ketose).

For six-carbon aldoses (e.g., glucose), a six-membered pyranose ring is common. For ketoses (e.g., fructose), a five-membered furanose ring may form.

Chair and boat conformations can occur, with the chair form generally more stable because it minimizes steric hindrance.

Epimers and anomers

Epimers differ in configuration at one chiral center, excluding the newly formed anomeric center in the ring form.
Anomers differ specifically at the new chiral center (the anomeric carbon) created during 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, which changes the observed optical rotation.

Hydrolysis of the glycoside linkage

Glycosidic bonds link monosaccharides to form disaccharides and polysaccharides. Under acidic or enzymatic conditions, these bonds can be hydrolyzed, splitting the carbohydrate into its constituent sugars. In biological systems, enzymes called glycosidases catalyze this process.

Keto-enol tautomerism of monosaccharides

Some monosaccharides (especially those that can adopt an open-chain form with an aldehyde or ketone) can undergo keto-enol tautomerism. In this process, the carbonyl group temporarily converts to an enediol intermediate. Under basic conditions, this can allow an aldose to rearrange into a ketose, or a ketose into an aldose.

This tautomerism is not always prominent at physiological pH, but it helps explain aspects of 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 defined by the carbonyl group ( $C = O$ ), where carbon is double-bonded to oxygen. In an aldehyde, the carbonyl is at the end of a carbon chain. In a ketone, the carbonyl is within the chain.

These functional groups are important intermediates in organic synthesis because the carbonyl carbon is often reactive and can be converted into many other functional groups.

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, while systematic IUPAC names are based on the longest carbon chain containing the carbonyl group.

Physical properties

Aldehydes and ketones often have higher boiling points than comparable alkanes because the $C = O$ bond is polar. However, because they cannot donate hydrogen bonds, their boiling points are generally lower than those of alcohols.

Many low-molecular-weight aldehydes and ketones are water-soluble because the carbonyl oxygen can accept hydrogen bonds from water.

Important reactions

The carbonyl bond is polarized, leaving the carbonyl carbon partially positive. That electrophilic carbon is a common site for nucleophilic attack, which explains many of the characteristic reactions of aldehydes and ketones.

Nucleophilic addition reactions at $C = O$ bond

In nucleophilic addition, a nucleophile attacks the electrophilic carbonyl carbon. The $π$ bond breaks, giving an alkoxide intermediate. Depending on conditions, that intermediate may be protonated or converted into other derivatives.

Acetal, hemiacetal
When an alcohol reacts with an aldehyde or ketone, a hemiacetal or hemiketal forms first. With excess alcohol under acidic conditions, the hemiacetal can convert into an acetal, where two $- OR$ groups are attached to the same carbon.
Imine, enamine
A primary amine reacts with an aldehyde or ketone to form an imine, where the carbonyl oxygen is replaced by a double-bonded nitrogen. Secondary amines can form an enamine, where an α hydrogen shifts to give a $C = C - N$ linkage.
Hydride reagents
Reagents such as sodium borohydride ( $N a B H_{4}$ ) or lithium aluminum hydride ( $L i A l H_{4}$ ) deliver hydride to the carbonyl carbon. This reduces aldehydes to primary alcohols and ketones to secondary alcohols.
Cyanohydrin
Addition of hydrogen cyanide to a carbonyl forms a cyanohydrin, which has both a $- CN$ 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 conditions are strong enough to break carbon-carbon bonds.

Reactions at adjacent positions: enolate chemistry
An enolate forms when an α hydrogen is removed, producing a resonance-stabilized anion. Enolates are key intermediates in many reactions, including aldol condensation.

Keto-enol tautomerism ( $α$ -racemization)
Aldehydes and ketones with an $α$ hydrogen can interconvert between keto and enol forms through proton shifts. This equilibrium can lead to racemization at the α carbon if that carbon is chiral.
Aldol condensation, retro-aldol
Enolates can attack other carbonyl compounds to form aldol products. Under some conditions, the aldol product dehydrates to form an $α$ , $β$ -unsaturated carbonyl. The reverse process, retro-aldol, breaks the bond formed in the aldol reaction and regenerates smaller carbonyl compounds.
Kinetic versus thermodynamic enolate
The kinetic enolate forms faster under conditions that favor the lowest activation energy (often low temperature with a strong, bulky base), typically by removing the most accessible proton. The thermodynamic enolate is more stable and is favored at higher temperatures or with weaker bases, typically giving the more substituted double bond.

Reaction mechanisms for base- and acid-catalysed aldol condensation

General principles
Carbonyl reactivity is largely controlled by the polarity of the $C = O$ bond and by how accessible the carbonyl carbon is to an incoming nucleophile. Substituents can change electron density at the carbonyl carbon, and steric bulk can slow nucleophilic approach.

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

Carbohydrates

Serve as energy sources and structural components
Range from monosaccharides to polysaccharides
Key examples: glucose (fuel), fructose (fruit sugar)

Nomenclature and classification, common names

Monosaccharides: single sugars (glucose, galactose, fructose)
Disaccharides: two sugars linked by glycosidic bond (sucrose, lactose, maltose)
Polysaccharides: long sugar chains (starch, glycogen, cellulose)
- Prefixes: deoxy (missing $- O H$ ), D/L (configuration), $ b $/$ \beta$ (anomeric orientation)
- All sugars end in -ose

Absolute configuration

D/L system based on chiral carbon farthest from carbonyl
- D: -OH matches D-glyceraldehyde
- L: mirror image of D
Independent of R/S system

Cyclic structure and conformations of hexoses

Monosaccharides cyclize in water to form hemiacetals/hemiketals
Glucose: pyranose (6-membered ring); fructose: furanose (5-membered ring)
Chair conformation more stable than boat

Epimers and anomers

Epimers: differ at one chiral center (not anomeric carbon)
Anomers: differ at anomeric carbon ( $α$ = opposite $C H_{2} O H$ , $β$ = same side)
- Mutarotation: interconversion between $α$ and $β$ forms

Hydrolysis of the glycoside linkage

Glycosidic bonds join sugars in di- and polysaccharides
Hydrolyzed by acid or glycosidase enzymes

Keto-enol tautomerism of monosaccharides

Aldoses/ketoses can interconvert via enediol intermediate
Enables aldose-ketose rearrangement under basic conditions

Disaccharides

Sucrose: $α$ -glucose + $β$ -fructose (anomeric linkage)
Lactose: $β$ -galactose + glucose (1 14 bond)
Maltose: two glucose units ( $α$ -1,4 linkage)

Polysaccharides

Starch: $α$ -1,4 glucose linkages (plant energy storage)
Glycogen: branched $α$ -1,4 and $α$ -1,6 linkages (animal energy storage)
Cellulose: $β$ -1,4 glucose linkages (plant structure)

Aldehydes and ketones

Defined by carbonyl group ( $C = O$ )
- Aldehyde: carbonyl at chain end
- Ketone: carbonyl within chain
Carbonyl carbon is electrophilic and reactive

Nomenclature

Aldehydes: end in -al (methanal, ethanal)
Ketones: end in -one (propanone, butanone)
IUPAC names based on longest chain with carbonyl

Physical properties

Higher boiling points than alkanes (polar $C = O$ )
Lower boiling points than alcohols (no H-bond donation)
Water-soluble if low molecular weight (H-bond acceptor)

Important reactions

Carbonyl carbon: site for nucleophilic attack

Nucleophilic addition reactions at $C = O$ bond

Alcohols: form hemiacetals/hemiketals, then acetals
Amines: primary forms imines, secondary forms enamines
Hydride reagents ( $N a B H_{4}$ , $L i A l H_{4}$ ): reduce to alcohols
Cyanohydrin formation: $- CN$ and $- O H$ added to carbonyl

Oxidation of aldehydes

Aldehydes oxidized to carboxylic acids
Ketones resist oxidation (require strong conditions)

Reactions at adjacent positions: enolate chemistry

Enolate: formed by deprotonation at $α$ -carbon
Key for aldol condensation and related reactions
- Keto-enol tautomerism: interconversion at $α$ -carbon (can cause racemization)
- Aldol condensation: enolate adds to carbonyl, forms aldol; dehydration yields $α, β$ -unsaturated carbonyl
- Retro-aldol: reverse of aldol, breaks C 13C bond
- Kinetic enolate: forms fastest, less substituted
- Thermodynamic enolate: more stable, more substituted

General principles

Carbonyl reactivity: controlled by $C = O$ polarity, substituent effects, and steric hindrance
- Electron-withdrawing groups increase reactivity
- Bulky groups hinder nucleophilic attack
$α$ -Hydrogen acidity: enhanced by resonance stabilization of enolate, enables many synthetic transformations

Carbohydrates, aldehydes and ketones

Carbohydrates

Common examples include glucose, a major cellular fuel, and fructose, found in fruits and in table sugar.

Nomenclature and classification, common names

Monosaccharides: Single sugar units such as glucose ( $C_{6} H_{12} O_{6}$ ), galactose, and fructose.
Disaccharides: 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, based on whether the anomeric $- O H$ group is on the opposite side or the same side as the $C H_{2} O H$ group.
All sugars end in -ose (e.g., glucose, fructose, galactose).

Absolute configuration

Cyclic structure and conformations of hexoses

For six-carbon aldoses (e.g., glucose), a six-membered pyranose ring is common. For ketoses (e.g., fructose), a five-membered furanose ring may form.

Chair and boat conformations can occur, with the chair form generally more stable because it minimizes steric hindrance.

Epimers and anomers

Epimers differ in configuration at one chiral center, excluding the newly formed anomeric center in the ring form.
Anomers differ specifically at the new chiral center (the anomeric carbon) created during 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, which changes the observed optical rotation.

Hydrolysis of the glycoside linkage

Keto-enol tautomerism of monosaccharides

This tautomerism is not always prominent at physiological pH, but it helps explain aspects of 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

These functional groups are important intermediates in organic synthesis because the carbonyl carbon is often reactive and can be converted into many other functional groups.

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, while systematic IUPAC names are based on the longest carbon chain containing the carbonyl group.

Physical properties

Many low-molecular-weight aldehydes and ketones are water-soluble because the carbonyl oxygen can accept hydrogen bonds from water.

Important reactions

Nucleophilic addition reactions at $C = O$ bond

Acetal, hemiacetal
When an alcohol reacts with an aldehyde or ketone, a hemiacetal or hemiketal forms first. With excess alcohol under acidic conditions, the hemiacetal can convert into an acetal, where two $- OR$ groups are attached to the same carbon.
Imine, enamine
A primary amine reacts with an aldehyde or ketone to form an imine, where the carbonyl oxygen is replaced by a double-bonded nitrogen. Secondary amines can form an enamine, where an α hydrogen shifts to give a $C = C - N$ linkage.
Hydride reagents
Reagents such as sodium borohydride ( $N a B H_{4}$ ) or lithium aluminum hydride ( $L i A l H_{4}$ ) deliver hydride to the carbonyl carbon. This reduces aldehydes to primary alcohols and ketones to secondary alcohols.
Cyanohydrin
Addition of hydrogen cyanide to a carbonyl forms a cyanohydrin, which has both a $- CN$ and an $- O H$ attached to the former carbonyl carbon.

Keto-enol tautomerism ( $α$ -racemization)
Aldehydes and ketones with an $α$ hydrogen can interconvert between keto and enol forms through proton shifts. This equilibrium can lead to racemization at the α carbon if that carbon is chiral.
Aldol condensation, retro-aldol
Enolates can attack other carbonyl compounds to form aldol products. Under some conditions, the aldol product dehydrates to form an $α$ , $β$ -unsaturated carbonyl. The reverse process, retro-aldol, breaks the bond formed in the aldol reaction and regenerates smaller carbonyl compounds.
Kinetic versus thermodynamic enolate
The kinetic enolate forms faster under conditions that favor the lowest activation energy (often low temperature with a strong, bulky base), typically by removing the most accessible proton. The thermodynamic enolate is more stable and is favored at higher temperatures or with weaker bases, typically giving the more substituted double bond.

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

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4. Chem/phys

4.9. Structure, function, and reactivity of bio-relevant molecules

Carbohydrates, aldehydes and ketones

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Carbohydrates

Common examples include glucose, a major cellular fuel, and fructose, found in fruits and in table sugar.

Nomenclature and classification, common names

Monosaccharides: Single sugar units such as glucose ( $C_{6} H_{12} O_{6}$ ), galactose, and fructose.
Disaccharides: 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, based on whether the anomeric $- O H$ group is on the opposite side or the same side as the $C H_{2} O H$ group.
All sugars end in -ose (e.g., glucose, fructose, galactose).

Absolute configuration

Cyclic structure and conformations of hexoses

For six-carbon aldoses (e.g., glucose), a six-membered pyranose ring is common. For ketoses (e.g., fructose), a five-membered furanose ring may form.

Chair and boat conformations can occur, with the chair form generally more stable because it minimizes steric hindrance.

Epimers and anomers

Epimers differ in configuration at one chiral center, excluding the newly formed anomeric center in the ring form.
Anomers differ specifically at the new chiral center (the anomeric carbon) created during 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, which changes the observed optical rotation.

Hydrolysis of the glycoside linkage

Keto-enol tautomerism of monosaccharides

This tautomerism is not always prominent at physiological pH, but it helps explain aspects of 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

These functional groups are important intermediates in organic synthesis because the carbonyl carbon is often reactive and can be converted into many other functional groups.

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, while systematic IUPAC names are based on the longest carbon chain containing the carbonyl group.

Physical properties

Many low-molecular-weight aldehydes and ketones are water-soluble because the carbonyl oxygen can accept hydrogen bonds from water.

Important reactions

Nucleophilic addition reactions at $C = O$ bond

Acetal, hemiacetal
When an alcohol reacts with an aldehyde or ketone, a hemiacetal or hemiketal forms first. With excess alcohol under acidic conditions, the hemiacetal can convert into an acetal, where two $- OR$ groups are attached to the same carbon.
Imine, enamine
A primary amine reacts with an aldehyde or ketone to form an imine, where the carbonyl oxygen is replaced by a double-bonded nitrogen. Secondary amines can form an enamine, where an α hydrogen shifts to give a $C = C - N$ linkage.
Hydride reagents
Reagents such as sodium borohydride ( $N a B H_{4}$ ) or lithium aluminum hydride ( $L i A l H_{4}$ ) deliver hydride to the carbonyl carbon. This reduces aldehydes to primary alcohols and ketones to secondary alcohols.
Cyanohydrin
Addition of hydrogen cyanide to a carbonyl forms a cyanohydrin, which has both a $- CN$ and an $- O H$ attached to the former carbonyl carbon.

Keto-enol tautomerism ( $α$ -racemization)
Aldehydes and ketones with an $α$ hydrogen can interconvert between keto and enol forms through proton shifts. This equilibrium can lead to racemization at the α carbon if that carbon is chiral.
Aldol condensation, retro-aldol
Enolates can attack other carbonyl compounds to form aldol products. Under some conditions, the aldol product dehydrates to form an $α$ , $β$ -unsaturated carbonyl. The reverse process, retro-aldol, breaks the bond formed in the aldol reaction and regenerates smaller carbonyl compounds.
Kinetic versus thermodynamic enolate
The kinetic enolate forms faster under conditions that favor the lowest activation energy (often low temperature with a strong, bulky base), typically by removing the most accessible proton. The thermodynamic enolate is more stable and is favored at higher temperatures or with weaker bases, typically giving the more substituted double bond.

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

Carbohydrates

Serve as energy sources and structural components
Range from monosaccharides to polysaccharides
Key examples: glucose (fuel), fructose (fruit sugar)

Nomenclature and classification, common names

Monosaccharides: single sugars (glucose, galactose, fructose)
Disaccharides: two sugars linked by glycosidic bond (sucrose, lactose, maltose)
Polysaccharides: long sugar chains (starch, glycogen, cellulose)
- Prefixes: deoxy (missing $- O H$ ), D/L (configuration), $ b $/$ \beta$ (anomeric orientation)
- All sugars end in -ose

Absolute configuration

D/L system based on chiral carbon farthest from carbonyl
- D: -OH matches D-glyceraldehyde
- L: mirror image of D
Independent of R/S system

Cyclic structure and conformations of hexoses

Monosaccharides cyclize in water to form hemiacetals/hemiketals
Glucose: pyranose (6-membered ring); fructose: furanose (5-membered ring)
Chair conformation more stable than boat

Epimers and anomers

Epimers: differ at one chiral center (not anomeric carbon)
Anomers: differ at anomeric carbon ( $α$ = opposite $C H_{2} O H$ , $β$ = same side)
- Mutarotation: interconversion between $α$ and $β$ forms

Hydrolysis of the glycoside linkage

Glycosidic bonds join sugars in di- and polysaccharides
Hydrolyzed by acid or glycosidase enzymes

Keto-enol tautomerism of monosaccharides

Aldoses/ketoses can interconvert via enediol intermediate
Enables aldose-ketose rearrangement under basic conditions

Disaccharides

Sucrose: $α$ -glucose + $β$ -fructose (anomeric linkage)
Lactose: $β$ -galactose + glucose (1 14 bond)
Maltose: two glucose units ( $α$ -1,4 linkage)

Polysaccharides

Starch: $α$ -1,4 glucose linkages (plant energy storage)
Glycogen: branched $α$ -1,4 and $α$ -1,6 linkages (animal energy storage)
Cellulose: $β$ -1,4 glucose linkages (plant structure)

Aldehydes and ketones

Defined by carbonyl group ( $C = O$ )
- Aldehyde: carbonyl at chain end
- Ketone: carbonyl within chain
Carbonyl carbon is electrophilic and reactive

Nomenclature

Aldehydes: end in -al (methanal, ethanal)
Ketones: end in -one (propanone, butanone)
IUPAC names based on longest chain with carbonyl

Physical properties

Higher boiling points than alkanes (polar $C = O$ )
Lower boiling points than alcohols (no H-bond donation)
Water-soluble if low molecular weight (H-bond acceptor)

Important reactions

Carbonyl carbon: site for nucleophilic attack

Nucleophilic addition reactions at $C = O$ bond

Alcohols: form hemiacetals/hemiketals, then acetals
Amines: primary forms imines, secondary forms enamines
Hydride reagents ( $N a B H_{4}$ , $L i A l H_{4}$ ): reduce to alcohols
Cyanohydrin formation: $- CN$ and $- O H$ added to carbonyl

Oxidation of aldehydes

Aldehydes oxidized to carboxylic acids
Ketones resist oxidation (require strong conditions)

Reactions at adjacent positions: enolate chemistry

Enolate: formed by deprotonation at $α$ -carbon
Key for aldol condensation and related reactions
- Keto-enol tautomerism: interconversion at $α$ -carbon (can cause racemization)
- Aldol condensation: enolate adds to carbonyl, forms aldol; dehydration yields $α, β$ -unsaturated carbonyl
- Retro-aldol: reverse of aldol, breaks C 13C bond
- Kinetic enolate: forms fastest, less substituted
- Thermodynamic enolate: more stable, more substituted

General principles

Carbonyl reactivity: controlled by $C = O$ polarity, substituent effects, and steric hindrance
- Electron-withdrawing groups increase reactivity
- Bulky groups hinder nucleophilic attack
$α$ -Hydrogen acidity: enhanced by resonance stabilization of enolate, enables many synthetic transformations

Carbohydrates, aldehydes and ketones

Carbohydrates

Common examples include glucose, a major cellular fuel, and fructose, found in fruits and in table sugar.

Nomenclature and classification, common names

Monosaccharides: Single sugar units such as glucose ( $C_{6} H_{12} O_{6}$ ), galactose, and fructose.
Disaccharides: 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, based on whether the anomeric $- O H$ group is on the opposite side or the same side as the $C H_{2} O H$ group.
All sugars end in -ose (e.g., glucose, fructose, galactose).

Absolute configuration

Cyclic structure and conformations of hexoses

For six-carbon aldoses (e.g., glucose), a six-membered pyranose ring is common. For ketoses (e.g., fructose), a five-membered furanose ring may form.

Chair and boat conformations can occur, with the chair form generally more stable because it minimizes steric hindrance.

Epimers and anomers

Epimers differ in configuration at one chiral center, excluding the newly formed anomeric center in the ring form.
Anomers differ specifically at the new chiral center (the anomeric carbon) created during 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, which changes the observed optical rotation.

Hydrolysis of the glycoside linkage

Keto-enol tautomerism of monosaccharides

This tautomerism is not always prominent at physiological pH, but it helps explain aspects of 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

These functional groups are important intermediates in organic synthesis because the carbonyl carbon is often reactive and can be converted into many other functional groups.

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, while systematic IUPAC names are based on the longest carbon chain containing the carbonyl group.

Physical properties

Many low-molecular-weight aldehydes and ketones are water-soluble because the carbonyl oxygen can accept hydrogen bonds from water.

Important reactions

Nucleophilic addition reactions at $C = O$ bond

Acetal, hemiacetal
When an alcohol reacts with an aldehyde or ketone, a hemiacetal or hemiketal forms first. With excess alcohol under acidic conditions, the hemiacetal can convert into an acetal, where two $- OR$ groups are attached to the same carbon.
Imine, enamine
A primary amine reacts with an aldehyde or ketone to form an imine, where the carbonyl oxygen is replaced by a double-bonded nitrogen. Secondary amines can form an enamine, where an α hydrogen shifts to give a $C = C - N$ linkage.
Hydride reagents
Reagents such as sodium borohydride ( $N a B H_{4}$ ) or lithium aluminum hydride ( $L i A l H_{4}$ ) deliver hydride to the carbonyl carbon. This reduces aldehydes to primary alcohols and ketones to secondary alcohols.
Cyanohydrin
Addition of hydrogen cyanide to a carbonyl forms a cyanohydrin, which has both a $- CN$ and an $- O H$ attached to the former carbonyl carbon.

Keto-enol tautomerism ( $α$ -racemization)
Aldehydes and ketones with an $α$ hydrogen can interconvert between keto and enol forms through proton shifts. This equilibrium can lead to racemization at the α carbon if that carbon is chiral.
Aldol condensation, retro-aldol
Enolates can attack other carbonyl compounds to form aldol products. Under some conditions, the aldol product dehydrates to form an $α$ , $β$ -unsaturated carbonyl. The reverse process, retro-aldol, breaks the bond formed in the aldol reaction and regenerates smaller carbonyl compounds.
Kinetic versus thermodynamic enolate
The kinetic enolate forms faster under conditions that favor the lowest activation energy (often low temperature with a strong, bulky base), typically by removing the most accessible proton. The thermodynamic enolate is more stable and is favored at higher temperatures or with weaker bases, typically giving the more substituted double bond.

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

Key points

Carbohydrates

Serve as energy sources and structural components
Range from monosaccharides to polysaccharides
Key examples: glucose (fuel), fructose (fruit sugar)

Nomenclature and classification, common names

Monosaccharides: single sugars (glucose, galactose, fructose)
Disaccharides: two sugars linked by glycosidic bond (sucrose, lactose, maltose)
Polysaccharides: long sugar chains (starch, glycogen, cellulose)
- Prefixes: deoxy (missing $- O H$ ), D/L (configuration), $ b $/$ \beta$ (anomeric orientation)
- All sugars end in -ose

Absolute configuration

D/L system based on chiral carbon farthest from carbonyl
- D: -OH matches D-glyceraldehyde
- L: mirror image of D
Independent of R/S system

Cyclic structure and conformations of hexoses

Monosaccharides cyclize in water to form hemiacetals/hemiketals
Glucose: pyranose (6-membered ring); fructose: furanose (5-membered ring)
Chair conformation more stable than boat

Epimers and anomers

Epimers: differ at one chiral center (not anomeric carbon)
Anomers: differ at anomeric carbon ( $α$ = opposite $C H_{2} O H$ , $β$ = same side)
- Mutarotation: interconversion between $α$ and $β$ forms

Hydrolysis of the glycoside linkage

Glycosidic bonds join sugars in di- and polysaccharides
Hydrolyzed by acid or glycosidase enzymes

Keto-enol tautomerism of monosaccharides

Aldoses/ketoses can interconvert via enediol intermediate
Enables aldose-ketose rearrangement under basic conditions

Disaccharides

Sucrose: $α$ -glucose + $β$ -fructose (anomeric linkage)
Lactose: $β$ -galactose + glucose (1 14 bond)
Maltose: two glucose units ( $α$ -1,4 linkage)

Polysaccharides

Starch: $α$ -1,4 glucose linkages (plant energy storage)
Glycogen: branched $α$ -1,4 and $α$ -1,6 linkages (animal energy storage)
Cellulose: $β$ -1,4 glucose linkages (plant structure)

Aldehydes and ketones

Defined by carbonyl group ( $C = O$ )
- Aldehyde: carbonyl at chain end
- Ketone: carbonyl within chain
Carbonyl carbon is electrophilic and reactive

Nomenclature

Aldehydes: end in -al (methanal, ethanal)
Ketones: end in -one (propanone, butanone)
IUPAC names based on longest chain with carbonyl

Physical properties

Higher boiling points than alkanes (polar $C = O$ )
Lower boiling points than alcohols (no H-bond donation)
Water-soluble if low molecular weight (H-bond acceptor)

Important reactions

Carbonyl carbon: site for nucleophilic attack

Nucleophilic addition reactions at $C = O$ bond

Alcohols: form hemiacetals/hemiketals, then acetals
Amines: primary forms imines, secondary forms enamines
Hydride reagents ( $N a B H_{4}$ , $L i A l H_{4}$ ): reduce to alcohols
Cyanohydrin formation: $- CN$ and $- O H$ added to carbonyl

Oxidation of aldehydes

Aldehydes oxidized to carboxylic acids
Ketones resist oxidation (require strong conditions)

Reactions at adjacent positions: enolate chemistry

Enolate: formed by deprotonation at $α$ -carbon
Key for aldol condensation and related reactions
- Keto-enol tautomerism: interconversion at $α$ -carbon (can cause racemization)
- Aldol condensation: enolate adds to carbonyl, forms aldol; dehydration yields $α, β$ -unsaturated carbonyl
- Retro-aldol: reverse of aldol, breaks C 13C bond
- Kinetic enolate: forms fastest, less substituted
- Thermodynamic enolate: more stable, more substituted

General principles

Carbonyl reactivity: controlled by $C = O$ polarity, substituent effects, and steric hindrance
- Electron-withdrawing groups increase reactivity
- Bulky groups hinder nucleophilic attack
$α$ -Hydrogen acidity: enhanced by resonance stabilization of enolate, enables many synthetic transformations

Carbohydrates, aldehydes and ketones

Carbohydrates

Nomenclature and classification, common names

Absolute configuration

Cyclic structure and conformations of hexoses

Epimers and anomers

Hydrolysis of the glycoside linkage

Keto-enol tautomerism of monosaccharides

Disaccharides

Polysaccharides

Aldehydes and ketones

Nomenclature

Physical properties

Important reactions

Nucleophilic addition reactions at C=O bond

Carbohydrates, aldehydes and ketones

Carbohydrates

Nomenclature and classification, common names

Absolute configuration

Cyclic structure and conformations of hexoses

Epimers and anomers

Hydrolysis of the glycoside linkage

Keto-enol tautomerism of monosaccharides

Disaccharides

Polysaccharides

Aldehydes and ketones

Nomenclature

Physical properties

Important reactions

Nucleophilic addition reactions at C=O bond

Carbohydrates, aldehydes and ketones

Carbohydrates

Nomenclature and classification, common names

Absolute configuration

Cyclic structure and conformations of hexoses

Epimers and anomers

Hydrolysis of the glycoside linkage

Keto-enol tautomerism of monosaccharides

Disaccharides

Polysaccharides

Aldehydes and ketones

Nomenclature

Physical properties

Important reactions

Nucleophilic addition reactions at C=O bond

Carbohydrates, aldehydes and ketones

Carbohydrates

Nomenclature and classification, common names

Absolute configuration

Cyclic structure and conformations of hexoses

Epimers and anomers

Hydrolysis of the glycoside linkage

Keto-enol tautomerism of monosaccharides

Disaccharides

Polysaccharides

Aldehydes and ketones

Nomenclature

Physical properties

Important reactions

Nucleophilic addition reactions at C=O bond

Nucleophilic addition reactions at $C = O$ bond

Nucleophilic addition reactions at $C = O$ bond

Nucleophilic addition reactions at $C = O$ bond

Nucleophilic addition reactions at $C = O$ bond