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Organic Chemistry Review Part II
Organic Reactions Addition Elimination Substitution Rearrangement Condensation Esterification Hydrolysis Oxidation & Reductions Combustion
Addition Reactions • The components of an organic molecule A–B are added to the carbon atoms in a C=C bonds. • Involve the conversion of a π bond into 2 σ bonds. General form: A + B → C
Addition Reactions • Symmetrical alkenes produce one product. • Unsymmetrical alkenes produce racemic mixtures.
Addition Reactions • Alcohols are often produced by addition reactions. • Initial attack by the π bond of an alkene on a Hδ+ of H3O+ produces a carbocation. • The carbocation then undergoes nucleophilic attack by a lone pair of electrons from H2O. • This is followed by elimination of H+ to form the alcohol.
Addition Reactions • With symmetrical alkenes, addition of hydroxyl group produces one type of alcohol.
Addition Reactions • With unsymmetrical alkenes, addition of hydroxyl group produces different types of alcohols depending on the location of the double bond +
Addition Reactions Formation of hemiketals & hemiacetals: • Reactions between an acohol and either a ketone or aldehyde.
Elimination Reactions • The removal or “elimination” of adjacent atoms from a molecule. • Two σ bonds are lost, replaced by a new π bond. • The dehydration reaction of alcohols to generate alkene proceeds by heating the alcohols in the presence of a strong acid, such as sulfuric or phosphoric acid, at high temperatures.
Elimination Reactions • The required range of reaction temperature decreases with increasing substitution of the hydroxyl carbon: • 1° alcohols: 170° - 180°C • 2° alcohols: 100°– 140 °C • 3° alcohols: 25°– 80°C
Elimination Reactions • If the reaction is not sufficiently heated, the alcohols do not produce alkenes, but they react with one another to form ethers (Williamson Ether Synthesis).
Elimination Reactions General form: A → B + C
Elimination Reactions • 1⁰ Alcohols
Elimination Reactions • 2⁰ Alcohols
Elimination Reactions • In dehydration reactions of alcohols, hydride or alkyl shiftsrelocate the carbocation to a more stable position. • The dehydrated products are a mixture of alkenes, with and without carbocation rearrangement.
Elimination Reactions • Hydride or alkyl shifts are the result of hyperconjugation. The interaction between the filled orbitals of neighboring carbons and the singly occupied p orbital in the carbocation stabilizes the positive charge in carbocation. • The tertiary cation is more stable than a secondary cation, which is more stable than a primary cation.
Elimination Reactions • Hydride shift:
Elimination Reactions • Alkyl shift:
Substitution Reactions • Nucleophilic substitution reactions. • An electronegative atom is replaced by another more electronegative atom, called a stronger nucleophile. • The stronger nucleophile must possess a pair of electrons and have a greater affinity for the electropositive carbon atom than the original electronegative atom. • A σ bond is replaced by another σ bond .
Substitution Reactions General form: A + B → C + D • Non-polar reactions:
Substitution Reactions • Polar reactions:
Rearrangement Reactions • Are isomerisation reactions. • An organic molecule changes structure. • Constitutional change in carbon skeleton. • Reaction may involve changes in bond type. General form: A → B
Condensation Reactions • Two molecules combine to form one single molecule with the loss of a small molecule. • When this small molecule is water, it is known as a dehydration reaction. • Other possible small molecules lost include hydrogen chloride, methanol, or acetic acid.
Condensation Reactions • When two separate molecules react, their condensation is termed intermolecular. • The condensation of two amino acids to form a peptide bond (red) with expulsion of water (blue).
Condensation Reactions • When a condensation is performed between different parts of the same molecule, the reaction is termed intramolecular condensation. • In some cases this leads to ring formation.
Esterification Reactions • Esters are obtained by refluxing a carboxylic acid with an alcohol in the presence of an acid catalyst. • The reaction is driven to completion by using an excess of either the alcohol or the carboxylic acid, or by removing the water as it forms. • Alcohol reactivity order : CH3OH > 1o > 2o > 3o (steric effects).
Esterification Reactions • A carboxylic acid and an alcoholreact together under acidic conditions to form an ester and lose water.
Esterification Reactions • Esters can also be made from other carboxylic acid derivatives, especially acyl halides and anhydrides, by reacting them with the appropriate alcohol in the presence of a weak base. • If a compound contains both hydroxy- and carboxylic acid groups, then cyclic esters or lactones can form via an intramolecular reaction. Reactions that form 5- or 6-membered rings are particularly favorable.
Esterification Reactions Pericyclic esters
Hydrolysis • A reaction in which water is a reactant, and becomes part of the reaction product. • A number of organic compounds undergo hydrolysis with water, such as amides, esters, halogenoalkanes and acyl halides.
Hydrolysis • Reactions require a catalyst. • The catalyst is either an acid (H+ ions) or alkali (OH- ions). • Hydrolysis might involve refluxing in the presence of dilute hydrochloric acid or sodium hydroxide solution.
Hydrolysis • In the overall reaction, a bond in an organic molecule is broken. • A water molecule also breaks into ions. • The -OH group from water is added to one end of the organic molecule and the remaining H atom is added to the other.
Hydrolysis of an Ester: • The addition of a strong acid, such as dilute hydrochloric acid, is required to free the carboxylic acid molecule. • In the base-catalyzed, the carboxylic acid molecule loses a proton to a hydroxide ion.
Hydrolysis of Amides & Nitriles: • Amide acid catalyzed - HCl • Nitrile acid catalyzed – HCl or H2SO4
Summary of Hydrolysis Reactions • The hydrolysis of a primary amide: RCONH2+ H2O → RCOOH + NH3 • The hydrolysis of a secondary amide: RCONHR' + H2O → RCOOH + R'NH2
Summary of Hydrolysis Reactions • The hydrolysis of an ester: RCOOR' + H2O → RCOOH + R'OH • The hydrolysis of a halogenoalkane: RBr+ H2O → ROH + H+ + Br-
Reduction & Oxidation (REDOX) Reactions Oxidation States Oxidations Reductions
Definitions Oxidation-Reduction reactions: • Involve changes inoxidationstateatoneormoreatoms. • Can often be identified by changes in the number of oxygen atoms at a particular position in the hydrocarbon skeleton or in the number of bonds between carbon and oxygen at that position. • It is not consider anoxidationor reduction reaction: • Additionor lossof H+, H2O,HX.
Definitions • Oxidation: • Theoxidationstateincreases • Lossof H2 • Loss of a C-H bond • Additionof Oor O2 • Formation of a C-O bond or equivalent (C-Cl, CΞN, C-S) • Additionof X2 (halogens)
Definitions • Reduction: • Theoxidationstatedecreases • Additionof H2 or H- • Formation of a C-H bond • Lossof Oor O2 • Loss of a C-O bond or equivalent • Lossof X2. • An increase in the number of hydrogen atoms in a hydrocarbon is often an indication of a reduction.
Oxidation States • Carbon oxidation states are assigned on the basis ofthe electronegativity of attachedatoms. • Foreachbondtoamoreelectronegativeatomgive+1. • Foreachbondtoalesselectronegativeatom(evenH) give–1. • Foreachbondtocarbongive0.
Oxidation States • In nitrogen-containing compounds, the number of carbon–nitrogen bonds changes with the oxidation state of carbon.
Assign oxidation states to all atoms in the following structure: H C O H H C HO C H C H H H
