Topic 14: Acid-Base Equilibria

14A Strong and Weak Acids

The Brønsted–Lowry Theory

The Brønsted–Lowry theory defines acids as proton (H+) donors and bases as proton acceptors. This concept expands the definition of acids and bases beyond the limitations of the Arrhenius theory, allowing for reactions in both aqueous and non-aqueous environments.

• Acid: Substance that donates a proton.

• Base: Substance that accepts a proton.

• Example: In the reaction between hydrochloric acid and water:

Hydrogen Ion Concentration and the pH Scale

The pH scale is a logarithmic measure of hydrogen ion concentration in a solution. It is used to quantify the acidity or basicity of a solution.

• pH Formula:

• Acidic solutions: pH < 7

• Basic solutions: pH > 7

• Neutral solution: pH = 7

Ionic Product of Water, Kw

The ionic product of water, Kw, is the equilibrium constant for the self-ionization of water. It is important for calculating pH and pOH in aqueous solutions.

• Equation:

• At 25°C:

Analysing Data from pH Measurements

pH measurements are used to determine the concentration of hydrogen ions in a solution, which can be used to calculate equilibrium constants and assess acid/base strength.

• Application: Calculating acid dissociation constants (Ka) from pH data.

14B Acid-Base Titrations

Acid-Base Titrations, pH Curves and Indicators

Acid-base titrations involve the gradual addition of one solution to another to determine concentration. pH curves show the change in pH during titration, and indicators are used to signal the endpoint.

• Titration curve: Graph of pH vs. volume of titrant added.

• Indicator: Substance that changes color at a specific pH.

Buffer Solutions

Buffer solutions resist changes in pH when small amounts of acid or base are added. They are composed of a weak acid and its conjugate base, or a weak base and its conjugate acid.

• Buffer equation:

• Example: Acetic acid and sodium acetate buffer.

Buffer Solutions and pH Curves

Buffer solutions are often analyzed using pH curves to determine their effectiveness and capacity.

• Key point: Buffers maintain pH near the pKa of the acid.

Topic 15: Organic Chemistry – Carbonyls, Carboxylic Acids, and Chirality

15A Chirality

Chirality and Enantiomers

Chirality refers to the property of a molecule that is not superimposable on its mirror image. Enantiomers are pairs of chiral molecules that are mirror images of each other.

• Chiral center: Typically a carbon atom with four different substituents.

• Example: Lactic acid has a chiral center.

Optical Activity

Optical activity is the ability of chiral substances to rotate plane-polarized light. The direction and degree of rotation are characteristic of each enantiomer.

• Measurement: Using a polarimeter.

• Specific rotation formula:

Optical Activity and Reaction Mechanisms

Certain reaction mechanisms can lead to the formation or destruction of chirality, affecting optical activity.

• Example: SN1 reactions can produce racemic mixtures.

15B Carbonyl Compounds

Carbonyl Compounds and Their Physical Properties

Carbonyl compounds include aldehydes and ketones, characterized by the presence of a C=O group. Their physical properties are influenced by polarity and hydrogen bonding.

• Boiling points: Higher than alkanes, lower than alcohols.

• Solubility: Generally soluble in water due to hydrogen bonding.

Redox Reactions of Carbonyl Compounds

Carbonyl compounds undergo oxidation and reduction reactions, important in organic synthesis.

• Oxidation: Aldehydes can be oxidized to carboxylic acids.

• Reduction: Both aldehydes and ketones can be reduced to alcohols.

Nucleophilic Addition Reactions

Nucleophilic addition is a key reaction for carbonyl compounds, where nucleophiles attack the electrophilic carbon of the C=O group.

• Example: Addition of HCN to aldehydes and ketones.

15C Carboxylic Acids

Carboxylic Acids and Their Physical Properties

Carboxylic acids contain the -COOH functional group. They exhibit strong hydrogen bonding, leading to high boiling points and solubility in water.

• Boiling points: Higher than alcohols and carbonyls.

• Solubility: Soluble in water; solubility decreases with increasing chain length.

Preparations and Reactions of Carboxylic Acids

Carboxylic acids can be prepared by oxidation of primary alcohols or aldehydes and undergo reactions such as esterification and reduction.

• Preparation: Oxidation of ethanol to ethanoic acid.

• Reaction: Esterification with alcohols to form esters.

15D Carboxylic Acid Derivatives

Carboxylic Acid Derivatives: Acyl Chlorides

Acyl chlorides are highly reactive derivatives of carboxylic acids, used in acylation reactions.

• Preparation: Reaction of carboxylic acids with thionyl chloride.

• Reactivity: Reacts with alcohols, amines, and water.

Carboxylic Acid Derivatives: Esters

Esters are formed by the reaction of carboxylic acids with alcohols. They are commonly used as solvents and flavoring agents.

• Preparation: Esterification reaction.

• Properties: Pleasant odors, lower boiling points than acids.

Carboxylic Acid Derivatives: Polyesters

Polyesters are polymers formed from diacids and diols. They are used in fibers and plastics.

• Example: Polyethylene terephthalate (PET).

15E Spectroscopy and Chromatography

Simple Chromatography

Chromatography is a technique for separating mixtures based on differential adsorption. Simple chromatography includes paper and thin-layer chromatography.

• Application: Separation of pigments or organic compounds.

Determining Structures Using Mass Spectra

Mass spectrometry is used to determine molecular mass and structure by ionizing compounds and measuring mass-to-charge ratios.

• Key point: Fragmentation patterns help identify functional groups.

Chromatography: HPLC and GC

High-performance liquid chromatography (HPLC) and gas chromatography (GC) are advanced techniques for separating and analyzing organic compounds.

• HPLC: Used for non-volatile compounds.

• GC: Used for volatile compounds.

Chromatography and Mass Spectrometry

Combining chromatography with mass spectrometry (e.g., GC-MS) allows for separation and identification of complex mixtures.

• Application: Drug analysis, environmental testing.

Principles of NMR Spectroscopy

Nuclear magnetic resonance (NMR) spectroscopy is used to determine the structure of organic molecules by analyzing the magnetic properties of nuclei.

• Key nuclei: 1H and 13C.

• Chemical shift: Indicates environment of nuclei.

13C NMR Spectroscopy

13C NMR provides information about the carbon skeleton of organic molecules.

• Number of signals: Corresponds to unique carbon environments.

1H NMR Spectroscopy

1H NMR reveals information about hydrogen environments in molecules.

• Splitting patterns: Indicate neighboring hydrogens (n+1 rule).

Splitting Patterns in 1H NMR Spectra

Splitting patterns arise from spin-spin coupling between adjacent hydrogens, providing structural information.

• n+1 rule: A hydrogen with n neighbors splits into n+1 peaks.

Summary Table: Acid-Base and Organic Chemistry Topics