14 Detailed Notes on Esters and Lipids

Learning Outcomes

  • Recall the general structure of esters.
  • Recall the Fischer esterification reaction and the acid-catalyzed and base-promoted hydrolysis of esters.
  • Understand the different types and structures of lipids: glycerides (including triglycerides and phospholipids) and steroids.
  • Differentiate between triglycerides containing saturated and unsaturated fatty acids, their natural origins, and physical properties.
  • Recall the production processes of margarine and soap.
  • Recall the general structures and constituents of phospholipids.
  • Recall the common backbone structure of steroids.

Carboxylic Acid Derivatives: Esters

Overview

  • Occurrence
  • Functional group
  • Synthesis
  • Including mechanism
  • Hydrolysis
  • Acid-catalyzed hydrolysis
  • Base-promoted hydrolysis (saponification)

Functional Group

  • Esters are carboxylic acid derivatives.
  • General formula: R1COOR2 where R2 ≠ H (R2 is carbon-based).
  • Bond angles around ester carbon: ~120° (trigonal planar geometry).
  • Example: Ethyl butanoate (produced from butanoic acid and ethanol), which has the smell of pineapple.

Synthesis of Esters

Fischer Esterification Reaction

  • Requires a mineral acid catalyst.
  • The reaction is an equilibrium:
  • To increase the yield of ester, either water must be removed as it forms or an excess of one of the reactants used (applying Le Chatelier’s Principle).
Reaction Equation
  • Carboxylic Acid + Alcohol ⇌ Ester + Water

Mechanism of Fischer Esterification

  1. Protonation of the acid.
  2. Nucleophilic attack of alcohol onto the carbonyl.
  3. Deprotonation leading to ester formation.
  4. Reverse reaction is the mechanism of acid-catalyzed hydrolysis of an ester.

Hydrolysis of Esters

  • Hydrolysis occurs in both acidic and basic solutions, but is very slow in a neutral solution.
  • Acid-Catalyzed Hydrolysis: The reverse of Fischer esterification.
  • Base-Promoted Hydrolysis (Saponification): Used to convert fats (esters) to soap; this process is base-induced.

Saponification Reaction

  • Base (e.g., OH-) speeds up the reaction but is consumed and is not a true catalyst.

Mechanism for Base-Induced Hydrolysis of Esters

  1. Nucleophilic attack by OH- on the ester carbonyl (Rate Determining Step).
  2. Formation of a tetrahedral intermediate that quickly decomposes (producing carboxylic acid and alkoxide).
  3. Proton transfer gives the final products (carboxylate and alcohol).

Lipids

Overview

  • Types of lipids: Triglycerides, Phospholipids, Steroids, Eicosanoids.
  • Hydrolysable lipids: Contain esters.

Triglycerides

  • Hydrolysable lipids formed from glycerol and three fatty acids.
  • Simple triglycerides have the same fatty acid at all three positions. Mixed triglycerides contain different fatty acids.
Physical Properties
  • Saturated (solid at room temperature, high melting point, e.g., butter).
  • Unsaturated (liquid at room temperature, low melting point, e.g., olive oil).

Differences in Fatty Acids

  • Saturated Fatty Acids: Pack well, straight chains, high energy required to melt (higher melting point).
  • Unsaturated Fatty Acids: Kinks in chains, pack poorly, lower melting point due to weaker interactions.

Margarine and Shortening

  • Margarine: Typically lower in saturated fats than butter, made from partially hydrogenated vegetable oils.
  • Side-reaction during margarine production can lead to trans fats, which increase cardiovascular disease risk.

Soaps

  • Soaps are carboxylate salts of long-chain fatty acids, produced by hydrolysis of animal fats with alkali (saponification).
  • Reaction: NaOH + H2O + Fats → Soap + Glycerol

Phospholipids

  • Esters of glycerol with 2 carboxylic acids and 1 phosphoric acid derivative, critical for cell membranes.
  • Phosphoglycerides: Major components of cell membranes, exhibit amphipathic properties (hydrophilic head and hydrophobic tails).

Amphipathic Lipids

  • Form micelles or bilayers in water due to their structure, which enables them to exclude water from region of fatty tails.

Cholesterol

  • A component of cell membranes and a precursor for steroid hormones and bile acids.
  • Cholesterol helps to stabilize membrane fluidity and decrease permeability to small molecules.

Summary

  • Major takeaways: Structure and reactions of esters, significance and structure of lipids, differences between saturated and unsaturated fats, production of margarine and soap.