CHEM 1020 Final Exam Review: Fatty Acids and Sugars

Properties and Reactions of Fatty Acids

• Fatty Acid Structure and Unsaturation   - Polyunsaturated fatty acids contain multiple $C=C$ double bonds.   - Configuration (Cis vs. Trans):     - Cis: The hydrogen atoms associated with the double bond are on the same side of the carbon chain, resulting in a "kink" or bend in the chain.     - Trans: The hydrogen atoms are on opposite sides of the carbon chain, resulting in a straighter, linear chain.     - Neither: A double bond is neither cis nor trans if one of the carbons in the double bond is bonded to two identical groups (e.g., two hydrogens at the terminal end of a chain).

• Hydrogenation Reactions   - Complete Hydrogenation: To achieve complete saturation of a polyunsaturated fatty acid, the number of molecules of hydrogen ($H_2$) required is exactly equal to the number of $C=C$ double bonds present in the molecule. This process requires a suitable catalyst (e.g., $Pd$, $Pt$, or $Ni$).   - Partial Hydrogenation: This occurs when fewer molecules of $H_2$ are added than there are double bonds. In the reaction of a polyunsaturated fatty acid with just one molecule of $H_2$, multiple products are possible depending on which double bond is reduced. This can also lead to the isomerization of remaining cis bonds into trans bonds.

• Iodine Number   - The Iodine Number is a chemical value used to measure the degree of unsaturation in a fat or oil. It is defined as the mass of iodine ($I_2$) in grams that is consumed by $100\,g$ of a chemical substance. Higher iodine numbers indicate a higher degree of unsaturation (more double bonds).

• Reaction with Sodium Hydroxide (Saponification)   - When a fatty acid reacts with sodium hydroxide ($NaOH$, which is functionally $Na^+$ and $^-\text{O-H}$), an acid-base reaction occurs.   - The Equation: $\text{R-COOH} + NaOH \rightarrow \text{R-COO}^-Na^+ + H_2O$.   - The product is a fatty acid salt (soap) and water.

• Radical Abstraction by Hydroxyl Radical ($OH^\bullet$)   - The hydroxyl radical ($OH^\bullet$) is a highly reactive species that tends to abstract hydrogen atoms from fatty acids.   - Preferred Site: The hydrogen atoms most likely to be abstracted are those on the "bis-allylic" carbons—these are the methylene ($CH_2$) groups situated directly between two double bonds. These hydrogens have the lowest bond dissociation energy because the resulting radical is resonance-stabilized by both adjacent double bonds.

Triglyceride Metabolism and Synthesis

• Lipase-Catalyzed Metabolism   - Fats (triglycerides) are metabolized into their constituents through hydrolysis catalyzed by the enzyme lipase.   - Products: One molecule of triglyceride yields one molecule of glycerol (propane-1,2,3-triol) and three molecules of fatty acids. If the original fat was a mixed triglyceride, the resulting fatty acids will differ in structure based on the original ester chains.

• Triglyceride Synthesis and Nomenclature   - A triglyceride is an ester derived from glycerol and three fatty acids.   - Example Synthesis Task: Drawing a triglyceride derived from:     - Backbone: Triglycerol (propane-1,2,3-triol).     - Fatty Acid 1: Butanoic acid (a 4-carbon saturated acid; $CH_3CH_2CH_2COOH$).     - Fatty Acid 2: Deca-cis-7-enoic acid (a 10-carbon acid with a cis double bond at the 7th carbon from the carboxyl end).   - Process: The carboxyl group ($-COOH$) of the acids reacts with the hydroxyl groups ($-OH$) of the glycerol via esterification, releasing water ($H_2O$).

Radical Oxidation Mechanism

• Step-by-Step Mechanism for Radical Oxidation   1. Initiation: A hydroxyl radical ($OH^\bullet$) abstracts a hydrogen atom from a polyunsaturated fatty acid ($LH$), creating a carbon-centered fatty acid radical ($L^\bullet$) and water ($H_2O$). The hydrogen is typically abstracted from the bis-allylic position.   2. Oxygen Addition: The fatty acid radical ($L^\bullet$) reacts rapidly with an oxygen molecule ($O_2$) to form a peroxy radical ($LOO^\bullet$).   3. Propagation: The peroxy radical ($LOO^\bullet$) abstracts a hydrogen atom from a neighboring fatty acid molecule ($L'H$). This step produces a hydroperoxy-fatty acid ($LOOH$) and generates a new fatty acid radical ($L'^{\bullet}$), continuing the chain reaction.

• Vitamin E Involvement   - Vitamin E (alpha-tocopherol) acts as an antioxidant by reacting with the peroxy radical ($LOO^\bullet$).   - It donates a hydrogen atom to the radical, converting the peroxy radical into a hydroperoxide and forming a stable Vitamin E radical that does not continue the oxidation chain. This protects the triglyceride or fatty acid from further damage.

Solubility and Chemical Reactivity

• Water Solubility Criteria   - Solubility in water depends on the balance between hydrophilic (polar) groups and lipophilic (non-polar) carbon chains.   - Molecules A-E Analysis:     - Molecules with multiple hydroxyl ($-OH$) groups relative to a small carbon skeleton (like simple sugars) are typically water-soluble.     - Ionic species (like carboxylate salts $R-COO^-Na^+$) are generally water-soluble due to ion-dipole interactions.     - Molecules with large hydrocarbon rests (like Vitamin E with its lipophilic $R'$ tail) are insoluble in water but soluble in fats/lipids.

• Oxidation Reactions   - Alcohols to Carbonyls: Primary or secondary alcohols react with PCC (Pyridinium chlorochromate) or specific enzymes to form aldehydes or ketones, respectively.   - No Reaction (NR): If a molecule lacks a functional group susceptible to the reagent (e.g., a tertiary alcohol cannot be oxidized by PCC because it lacks an alpha-hydrogen), no reaction occurs.

Sugar Chemistry and Glycosylation

• Cyclic Hemiacetal Formation   - Open-form sugars (hydroxy-aldehydes) exist in equilibrium with cyclic hemiacetals.   - Mechanism: The oxygen of a hydroxyl group (usually on $C4$ or $C5$) performs a nucleophilic attack on the carbonyl carbon of the aldehyde group. Electrons move from the $C=O$ double bond to the oxygen, which then becomes protonated to form an $-OH$ group.   - Ring Size: A 4-carbon sugar typically forms a 5-membered cyclic ring (including the oxygen).

• Glycosylation Mechanism (Glucose + Flavanol)   - Step 1 (Protonation): The hemiacetal hydroxyl group ($-OH$) of glucose is protonated by an acid catalyst ($H^+$), turning into a good leaving group ($H_2O^+$).   - Step 2 (Leaving Group Loss): Water is lost, resulting in a resonance-stabilized carbocation (oxocarbenium ion) at the anomeric carbon.   - Step 3 (Nucleophilic Attack): The hydroxyl group ($-OH$) from another molecule (e.g., a Flavanol) attacks the carbocation.   - Step 4 (Deprotonation): Loss of a proton restores the catalyst and results in the glycosylated adduct (an acetal).

• Acetal Hydrolysis (Ethanol-Glucose Acetal)   - In the bloodstream, the acetal formed by ethanol and glucose can be hydrolyzed back to its components in the presence of water and acid.   - Mechanism: Protonation of the ether-like oxygen, followed by loss of ethanol to form a carbocation, which is then attacked by water to reform the glucose hemiacetal.

• Maltose Metabolism   - Maltose is a disaccharide consisting of two glucose units connected by an $\alpha(1\rightarrow 4)$ glycosidic bond.   - Hydrolysis: In the presence of trace acid and water, the glycosidic bond is cleaved via a mechanism similar to acetal hydrolysis, yielding two molecules of glucose.

• Ring-Opening Equilibria   - Sugar-like cyclic molecules can reach equilibrium with acyclic (open-chain) forms in the presence of a catalytic amount of $H^+$. This involves the protonation of the ring oxygen and subsequent cleavage of the bond at the anomeric carbon to reform a carbonyl and a hydroxyl group.

Amino Acids and Peptides

• Dipeptide Formation   - A dipeptide is formed by the condensation reaction between two amino acids, creating a peptide bond (amide linkage).   - Leucine (Leu) and Phenylalanine (Phe): Two distinct dipeptides can be formed depending on which amino acid provides the carboxyl group and which provides the amino group:     1. Leucyl-phenylalanine (Leu-Phe): The amino group of Phe reacts with the carboxyl group of Leu.     2. Phenylalanyl-leucine (Phe-Leu): The amino group of Leu reacts with the carboxyl group of Phe.