Aldehydes, Ketones, Carbohydrates, Carboxylic Acids, and Esters: Study Guide

Aldehydes and Ketones

Identification and Structure

Aldehydes and ketones are organic compounds containing the carbonyl group (C=O). The position of the carbonyl group distinguishes aldehydes from ketones.

• Aldehydes: The carbonyl group is at the end of the carbon chain. General formula: R-CHO.

• Ketones: The carbonyl group is within the carbon chain. General formula: R-CO-R'.

• Condensed structural formula: Shows all atoms in a molecule, e.g., CH3CHO for ethanal.

• Line-angle formula: Each vertex represents a carbon atom; lines represent bonds.

Example: Propanone (acetone): CH3COCH3

Nomenclature

• IUPAC Naming: Aldehydes use the suffix -al (e.g., ethanal), ketones use -one (e.g., propanone).

• Common Names: Often derived from historical sources (e.g., formaldehyde for methanal, acetone for propanone).

Physical Properties

• Boiling Points: Higher than alkanes/ethers due to dipole-dipole interactions, but lower than alcohols (no hydrogen bonding).

• Solubility: Small aldehydes and ketones are soluble in water due to hydrogen bonding with water molecules.

Oxidation and Reduction

• Oxidation: Aldehydes can be oxidized to carboxylic acids; ketones generally do not oxidize easily.

• Reduction: Both can be reduced to alcohols.

Example Equations:

• Oxidation of an aldehyde: $\mathrm{RCHO + [O] \rightarrow RCOOH}$

• Reduction of a ketone: $\mathrm{RCOR' + 2[H] \rightarrow RCH(OH)R'}$

Laboratory Tests

• Tollens' Test: Detects aldehydes (silver mirror forms); ketones do not react.

• Benedict's Test: Detects reducing sugars and aldehydes (red precipitate forms).

Hemiacetals and Acetals

• Hemiacetal Formation: Addition of one alcohol to an aldehyde or ketone.

• Acetal Formation: Addition of a second alcohol to a hemiacetal.

General Reaction:

• Hemiacetal: $\mathrm{RCHO + R'OH \rightarrow RCH(OH)(OR')}$

• Acetal: $\mathrm{RCH(OH)(OR') + R'OH \rightarrow RCH(OR')_2 + H_2O}$

Carbohydrates

Monosaccharides: Structure and Classification

Monosaccharides are the simplest carbohydrates, classified by the number of carbons and the type of carbonyl group.

• Aldose: Contains an aldehyde group (e.g., glucose).

• Ketose: Contains a ketone group (e.g., fructose).

• Number of Carbons: Triose (3C), tetrose (4C), pentose (5C), hexose (6C).

Chirality and Fischer Projections

• Chiral Carbon: A carbon atom bonded to four different groups.

• Achiral: Not chiral; at least two groups are the same.

• Fischer Projections: Two-dimensional representations to show stereochemistry.

• D and L Enantiomers: Determined by the position of the OH group on the chiral carbon farthest from the carbonyl group.

Example: D-glucose vs. L-glucose Fischer projections.

Common Monosaccharides

• Glucose: Aldohexose

• Galactose: Aldohexose

• Fructose: Ketohexose

Haworth Structures and Anomerism

• Haworth Structure: Cyclic form of monosaccharides (pyranose or furanose rings).

• α (alpha) and β (beta) Isomers: Differ in the position of the anomeric OH group.

Oxidation, Reduction, and Reducing Sugars

• Oxidation: Forms sugar acids.

• Reduction: Forms sugar alcohols.

• Reducing Sugar: Contains a free aldehyde or ketone group; can reduce Benedict's or Tollens' reagent.

Disaccharides and Glycosidic Bonds

• Disaccharide: Two monosaccharides linked by a glycosidic bond.

• Common Disaccharides: Maltose (glucose + glucose), lactose (glucose + galactose), sucrose (glucose + fructose).

Glycosidic Bond: Covalent bond joining two monosaccharides via an oxygen atom.

Polysaccharides: Structure and Hydrolysis

• Amylose: Unbranched chain of glucose units (α-1,4 linkages).

• Amylopectin: Branched chain (α-1,4 and α-1,6 linkages).

• Glycogen: Highly branched glucose polymer (animal storage).

• Cellulose: Unbranched β-1,4-linked glucose (plant cell walls).

• Hydrolysis: Polysaccharides break down to monosaccharides in the presence of acid or enzymes.

Carboxylic Acids and Esters

Nomenclature and Structure

• Carboxylic Acids: Contain the carboxyl group (-COOH). IUPAC names end in -oic acid (e.g., ethanoic acid).

• Esters: Derived from carboxylic acids and alcohols. IUPAC names: alkyl group from alcohol + acid name ending in -oate (e.g., ethyl ethanoate).

• Condensed and Line-Angle Formulas: Used to represent structures.

Acid-Base Reactions

• Dissociation in Water: Carboxylic acids partially ionize in water to form carboxylate ions and hydronium ions.

Equation:

• $\mathrm{RCOOH + H_2O \rightleftharpoons RCOO^- + H_3O^+}$

• Carboxylate Ions: Named by replacing -ic acid with -ate (e.g., acetate).

• Neutralization: Carboxylic acid reacts with a strong base (e.g., NaOH) to form a carboxylate salt and water.

Equation:

• $\mathrm{RCOOH + NaOH \rightarrow RCOONa + H_2O}$

Esterification and Hydrolysis

• Esterification: Carboxylic acid reacts with alcohol to form an ester and water (acid-catalyzed).

Equation:

• $\mathrm{RCOOH + R'OH \xrightarrow{H^+} RCOOR' + H_2O}$

• Hydrolysis: Esters can be hydrolyzed by acid or base to yield carboxylic acids and alcohols (acid hydrolysis) or carboxylate salts and alcohols (base hydrolysis).

Equations:

• Acid hydrolysis: $\mathrm{RCOOR' + H_2O \xrightarrow{H^+} RCOOH + R'OH}$

• Base hydrolysis (saponification): $\mathrm{RCOOR' + NaOH \rightarrow RCOONa + R'OH}$

Physical Properties

• Boiling Points: Carboxylic acids have higher boiling points than esters due to hydrogen bonding.

• Solubility: Lower members are soluble in water; solubility decreases with increasing chain length.

Laboratory Techniques

IR Spectroscopy

• Purpose: Identifies functional groups in organic molecules by measuring absorption of infrared light.

• Key Absorptions: Carbonyl group (C=O) around 1700 cm-1, O-H stretch (carboxylic acids) broad around 2500-3300 cm-1.

Carbohydrates Lab

• Tests: Benedict's and Barfoed's tests for reducing sugars; Seliwanoff's test for ketoses.

Aspirin Synthesis

• Reaction: Esterification of salicylic acid with acetic anhydride to form acetylsalicylic acid (aspirin).

Summary Table: Key Compounds and Reactions

Compound/Class	Functional Group	Key Reaction	Test/Property
Aldehyde	-CHO	Oxidation to carboxylic acid	Tollens', Benedict's positive
Ketone	RCOR'	Reduction to secondary alcohol	No Tollens' reaction
Carboxylic Acid	-COOH	Neutralization, esterification	High boiling point
Ester	-COOR	Hydrolysis	Pleasant odor, lower boiling point
Monosaccharide	Aldehyde/ketone + OH	Oxidation/reduction, glycosidic bond formation	Reducing sugar (if free carbonyl)

Additional info: Academic context and equations have been added to expand on the brief study guide points and ensure completeness.