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Aldehydes and Ketones Lecture Review

Chapter 15: Aldehydes and Ketones

This chapter introduces the fundamental concepts of aldehydes and ketones, including their structure, nomenclature, physical properties, common examples, and key reactions such as oxidation, reduction, and addition with alcohols.

15.1 The Carbonyl Group

  • Definition: A carbonyl group is a functional group characterized by a carbon atom joined to an oxygen atom by a double bond (C=O).

  • Carbonyl Molecule: Any molecule containing a carbonyl functional group.

  • Classification: Carbonyl molecules are classified based on the atoms bonded to the carbonyl carbon.

  • Polarity: The carbonyl group is strongly polarized because oxygen is significantly more electronegative than carbon. This polarity is responsible for its physicochemical properties and reactivity. The oxygen atom carries a partial negative charge (\delta-) and the carbon atom carries a partial positive charge (\delta+).

  • Shape: The three substituents associated with the carbonyl carbon atom form bond angles of approximately 120^\circ.

  • Aldehyde: A compound where the carbonyl carbon is bonded to one carbon atom and one hydrogen atom.

  • Ketone: A compound where the carbonyl carbon is bonded to two carbon atoms.

15.2 Naming Simple Aldehydes and Ketones

Aldehydes and ketones are known by both common names and IUPAC systematic names.

  • Naming Aldehydes:

    • Common Names: Most simple aldehydes end with the suffix "-aldehyde."

    • IUPAC Systematic Names: The final "-e" of the corresponding alkane name is replaced with "-al." When substituents are present, the carbon chain is numbered starting with the carbonyl carbon, which is assigned position 1.

  • Naming Ketones:

    • Common Names: Indicate the two alkyl groups bonded to the carbonyl carbon, followed by the word "-tone" or "ketone" (e.g., Ethyl Propyl Ketone).

    • IUPAC Systematic Names: The final "-e" of the corresponding alkane name is replaced with "-one." The numbering of the alkane chain begins from the end nearest the carbonyl group to ensure the carbonyl carbon receives the lowest possible number.

  • Worked Example 15.1 (3-Hexanone / Ethyl Propyl Ketone):

    • Analysis: A compound with a single carbonyl group bonded to two alkyl groups (ethyl and propyl) is identified as a ketone.

    • Common Name: Ethyl Propyl Ketone (listing alkyl groups alphabetically).

    • Systematic Name: 3-Hexanone (a 6-carbon chain with the carbonyl group at position 3).

15.3 Properties of Aldehydes and Ketones

  • Polarity: Aldehydes and ketones are moderately polar due to the polar carbonyl group (\delta- on oxygen, \delta+ on carbon).

  • Boiling Points:

    • They boil at higher temperatures than alkanes of similar molecular weight. This is attributed to stronger permanent dipole-dipole interactions between molecules and an increase in London dispersion forces due to greater mass/surface area of the carbonyl group.

    • They boil at lower temperatures than alcohols of similar molecular weight. This is because, unlike alcohols, aldehydes and ketones cannot form hydrogen bonds with each other.

  • Solubility:

    • They are soluble in common organic solvents (e.g., methanol, ethanol, propanol, butanol).

    • Those with fewer than 5-6 carbon atoms are soluble in water because they can form hydrogen bonds with water molecules (the carbonyl oxygen acts as a hydrogen bond acceptor).

  • Acetone (a simple ketone): An excellent solvent due to its ability to dissolve both polar and nonpolar compounds.

  • Summary of Properties:

    • Contain polar carbonyl groups, leading to higher boiling points than alkanes due to dipole-dipole interactions.

    • Cannot hydrogen bond with one another, but can hydrogen bond with other molecules like water or hydroxyl groups.

    • Water-soluble due to hydrogen bonding with water molecules.

    • Possess distinctive odors.

    • Ketones are generally less toxic than aldehydes.

15.4 Some Common Aldehydes and Ketones

Many natural aromas and flavors originate from aldehydes and ketones. Examples of naturally occurring aldehydes and ketones with distinctive odors include those released by stink beetles and stink bugs (trans-2-octenal and trans-2-decenal).

  • Formaldehyde (HCHO):

    • Properties: Colorless gas at room temperature, with a pungent and suffocating odor.

    • Toxicity: Low concentrations (0.1-1.1 ppm) can cause eye, throat, and bronchial irritation; higher concentrations can trigger asthma attacks. Skin contact can cause dermatitis. Can lead to kidney damage, coma, and death. It is a breakdown product of highly toxic methyl alcohol (methanol).

    • Sources: Formed during incomplete combustion of hydrocarbon fuels, contributing to smog.

    • Uses: Commonly sold as a 37\% aqueous solution called formalin. Used for disinfecting and sterilizing equipment because it kills viruses, fungi, and bacteria by reacting with amino groups in proteins. On standing, it polymerizes into a solid called paraformaldehyde.

  • Acetaldehyde (CH_3CHO):

    • Properties: Sweet-smelling, flammable liquid.

    • Toxicity: Less toxic than formaldehyde. Small amounts are produced during the normal catabolism (breakdown) of carbohydrates. However, it is a general narcotic, and large doses can cause respiratory failure.

    • Uses: Historically used in the production of acetic acid and acetic anhydride. Currently used in the preparation of polymeric resins and in the silvering of mirrors.

  • Acetone (CH3COCH3):

    • Properties: One of the most widely used organic solvents, dissolving most organic compounds and miscible with water. Volatile, posing a serious fire and explosion hazard if allowed to evaporate in closed spaces.

    • Health Risks: No chronic health risk associated with casual exposure.

    • Biological Relevance: Accumulates during excessive degradation of fats, such as during prolonged fasting.

  • Benzaldehyde:

    • Properties: Colorless liquid with a pleasant almond or cherry-like odor, first extracted from bitter almonds.

    • Uses: Used as a flavoring and fragrance in food, cosmetics, pharmaceuticals, and soap. Serves as a precursor (forerunner) to other organic molecules, including pharmaceuticals and plastic additives.

15.5 Oxidation of Aldehydes

  • General Principle: Alcohols can be oxidized to aldehydes or ketones. Aldehydes can be further oxidized to carboxylic acids.

  • Mechanism: Aldehyde oxidation involves the replacement of the hydrogen atom attached to the carbonyl carbon with an -OH group.

  • Ketones: Ketones cannot be further oxidized in this manner because they lack a hydrogen atom directly bonded to the carbonyl carbon atom; instead, they have two carbon substituents.

15.6 Reduction of Aldehydes and Ketones

  • General Principle: Aldehydes and ketones can be reduced back to alcohols, which is the reverse of oxidation.

  • Reaction: Reduction of a carbonyl group involves the addition of hydrogen (H_2) across the carbon-oxygen double bond, producing a hydroxyl (-OH) group.

  • Products:

    • Aldehydes are reduced to primary alcohols (the carbonyl carbon becomes bonded to one carbon and two hydrogens, with an -OH group).

    • Ketones are reduced to secondary alcohols (the carbonyl carbon becomes bonded to two carbons and one hydrogen, with an -OH group).

  • Mechanism: Reductions using hydrogen proceed through the formation of two new bonds: one to the carbonyl carbon using a hydride anion (:H^-) and another to the carbonyl oxygen using a hydrogen cation (H^+ or from H_3O^+).

    • The lone pair of electrons from the hydride anion (:H^-) forms a covalent bond with the electron-deficient carbonyl carbon (\delta+).

    • This causes the \pi electrons of the C=O bond to shift to the oxygen, resulting in a negative charge on the carbonyl oxygen.

    • Subsequently, an aqueous acid (H_3O^+ or H^+) donates a proton, forming a covalent bond with the negatively charged oxygen and yielding the alcohol.

  • Example: Cyclohexanone (a ketone) is reduced to cyclohexanol (a secondary alcohol).

  • Biological Systems: In biological contexts, the reduction of carbonyl groups is often facilitated by the coenzyme nicotinamide adenine dinucleotide (NADH).

15.7 Addition of Alcohols: Hemiacetals and Acetals

Aldehydes and ketones undergo addition reactions with alcohols, where the alcohol combines with the carbonyl carbon and oxygen.

  • Hemiacetal Formation:

    • Initial Product: Hemiacetals are the initial, usually unstable, products of addition reactions between aldehydes (or ketones) and alcohols.

    • Structure: A hemiacetal has both an alcohol-like group (-OH) and an ether-like group (-OR) bonded to the same carbon atom (the former carbonyl carbon).

    • Mechanism: The hydrogen atom from the alcohol bonds to the carbonyl oxygen, and the -OR portion of the alcohol bonds to the carbonyl carbon.

    • Stability: Hemiacetals may rapidly revert back to the original aldehyde or ketone and alcohol, establishing an equilibrium. They are often too unstable to be isolated.

    • Exception: A significant exception to their instability occurs when the -OH and CHO (aldehyde) functional groups are part of the same molecule, leading to the formation of stable cyclic hemiacetals. This is particularly important for simple sugars (carbohydrates), which primarily exist in cyclic hemiacetal forms.

    • Hemiacetal vs. Hemiketal: A hemiacetal is formed from an aldehyde (the carbon with -OH and -OR is also bonded to at least one H). A hemiketal is formed from a ketone (the carbon with -OH and -OR is bonded to two carbon groups).

  • Acetal Formation:

    • Process: If a small amount of acid catalyst is added to the reaction mixture of an alcohol with an aldehyde (or ketone), the initially formed hemiacetal can be converted into an acetal by reacting with a second molecule of alcohol.

    • Structure: An acetal is a compound that has two ether-like groups (-OR1 and -OR2) bonded to the same carbon atom (the former carbonyl carbon).

    • Difference from Hemiacetal: Instead of an -OH group, acetals have a second -OR group attached to the former carbonyl carbon.

    • Acetal vs. Ketal: An acetal is formed from an aldehyde (the central carbon has two -OR groups, an -R group, and an -H). A ketal is formed from a ketone (the central carbon has two -OR groups and two -R groups).

  • Worked Example 15.4 (Identifying Hemiacetals and Hemiketals):

    • Identification: Look for a carbon atom with single bonds to two oxygen atoms, where one is an -OH group and the other is an -OR group. The -OR group can be part of a ring structure.

    • Criteria: If the two remaining groups bonded to that carbon are carbons, it's a hemiketal. If one is a carbon and the other is a hydrogen, it's a hemiacetal.

    • Example (b): A cyclic hemiacetal (due to carbon bonded to -OH, -OR (in ring), -H, and -C).

    • Example (c): A hemiketal (due to carbon bonded to -OH, -OR, and two -C groups).

  • Worked Example 15.5 (Identifying Acetals and Ketals):

    • Identification: Look for a carbon atom with single bonds to two oxygen atoms, where both are -OR groups. The -OR group can be part of a ring structure.

    • Criteria: If the two remaining bonded groups are carbons, it's a ketal. If one is a carbon and the other is a hydrogen, it's an acetal.

    • Example (a): An acetal (due to carbon bonded to two -OR groups, one -CH_3, and one -H).

    • Example (c): A cyclic acetal (due to carbon bonded to two -OR groups (one in ring), one -CH2CH3, and one -H).

    • Example (d) Mannose: Specifically identified as a hemiacetal, not an acetal (due to having -OH and -OR).

  • Acetal Hydrolysis:

    • Definition: Hydrolysis is a reaction where a bond is broken, and the -H and -OH fragments of water add to the atoms of the broken bond.

    • Reversal: Acetal hydrolysis is the reverse of acetal formation. It requires an acid catalyst and a large quantity of water to drive the reaction back towards the aldehyde (or ketone) and the two alcohol molecules.

    • Products: The hydrolysis of an acetal yields the original aldehyde (or ketone) and two molecules of the alcohol from which it was formed.

Concepts to Review

  • Electronegativity and Molecular Polarity (Sections 4.9 and 4.10)

  • Oxidation and Reduction (Section 5.6)

  • Hydrogen Bonds (Section 8.2)

  • Functional Groups (Section 12.2)

  • Naming Alkanes (Section 12.6)

  • Types of Organic Reactions (Section 13.5)

Concept Map Summary of Organic Functional Groups

(A detailed concept map illustrates the classification of organic functional groups based on the presence of single/multiple bonds and specific atoms like O, N, S, distinguishing between alkanes, alkyl halides, alkenes, alkynes, aromatics, alcohols, phenols, ethers, thiols, disulfides, amines, aldehydes, ketones, carboxylic acids, esters, amides, and anhydrides.)