Chapter 3: Acids and Bases An Introduction to Organic Reactions and Their Mechanisms

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Notes on Acids and Bases

Chapter 3: Acids and Bases

An Introduction to Organic Reactions and Their Mechanisms

reaction mechanism – a description of the events that take place on a molecular level as reactants become products
intermediates – chemical species which intervene in a reaction that takes place in more than one step

  1. Acid-Base Reactions

  • Many of the reactions that occur in organic chemistry are either acid–base reactions themselves or they involve an acid–base reaction at some stage.

  • Acid-base reactions are simple fundamental reactions that will illustrate how chemists use curved arrows to represent mechanisms of reactions and how they depict the processes of bond breaking and bond making that occur as molecules react.

1A. Brønsted–Lowry Acids and Bases

  • Involve the transfer of protons

  • A Brønsted–Lowry acid is a substance that can donate (or lose) a proton.

  • A Brønsted–Lowry base is a substance that can accept (or remove) a proton.

Ex. Hydrogen chloride (HCl), is its pure form, is a gas. When HCl gas is bubbled into water, the following reaction occurs.

In this reaction hydrogen chloride donates a proton; therefore it acts as a Brønsted–Lowry acid. Water accepts a proton from hydrogen chloride; thus water serves as a Brønsted–Lowry base. The products are a hydronium ion (H3O+) and chloride ion (Cl−).
Reactants – acid or a base
Products – conjugate base or a conjugate acid

  • Conjugate base – the molecule or ion that forms when an acid loses its proton

  • Conjugate acid – the molecule or ion that forms when a base accepts a proton

  • Hydrogen chloride is a strong acid because transfer of its proton in water proceeds essentially to completion (as well as hydrogen iodide, hydrogen bromide, and sulfuric acid)

  • Acid strength – the extent to which an acid transfers protons to a base; therefore a measure of the percentage of ionization and not of concentration

  • Diprotic acid – acid that can transfer two protons (ex. Sulfuric acid)

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