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Hofmann Elimination: (Also Known As: Hofmann Degradation Or Exhaustive Methylation)

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Hofmann Elimination: (Also Known As: Hofmann Degradation Or Exhaustive Methylation) Hofmann elimination: (Also known as: Hofmann degradation or exhaustive methylation) The Hofmann elimination is an organic reaction used to convert an amine with a β- hydrogen to an alkene using methyl iodide, silver oxide and water under thermal conditions. The mechanism begins with an attack of the amine on methyl iodide to form an ammonium iodide salt. The iodide then reacts with silver oxide to form silver iodide which is insoluble and precipitates out of solution and a silver oxide ion which deproto-nates water to form a hydroxide ion. Heating the mixture facilitates an elimination reaction where the hydroxide picks up the β- hydrogen from the ammonium ion and releases an amine to afford the final olefin product. Mechanism: Cope Elimination: Cope elimination is a reaction which is performed to identify the unknown tertiary amine. It involves treatment of a tertiary amine with hydrogen peroxide to obtain amine oxide which on heating gives N,N-diakylhydroxyl amine and an alkene. In other words a new variant of the Hofmann's elimination involves the pyrolysis of an amine oxide prepared by the action of H2O2 on a tertiary amine. This elimination which is called Cope elimination which occurs at rather mild temperature. In general the cope elimination reaction is written as Mechanism Cope elimination occurs under milder conditions than Hofmann elimination. It is particularly useful when a snesitive or reactive alkene must be synthesized by the elimination of an amine. It is to be noted that cope elimination is cis and needs lower temperatures than the pyrolysis of quatenary ammonium hydroxides. This type of reaction is also called a 1,2 nucleophilic addition. Nucleophilic Addition: a nucleophilic addition reaction is an addition reaction where a chemical compound with an electron-deficient or electrophilic double or triple bond, a π bond, reacts with electron-rich reactant, termed a nucleophile, with disappearance of the double bond and creation of two new single, or σ, bonds. Nucleophilic addition reactions are an important class of reactions that allow the interconversion of C=O into a range of important functional groups. What does the term "nucleophilic addition" imply ? A nucleophile, Nu-, is an electron rich species that will react with an electron poor species (here the C=O) An addition implies that two systems combine to a single entity. There are three fundamental events in a nucleophilic addition reaction: 1. formation of the new s bond between the nucleophile, Nu, to the electrophilic C of the C=O group 2. breaking of the p bond to the O resulting in the formation of an intermediate alkoxide 3. protonation of the intermediate alkoxide to give an alcohol derivative Depending on the reactivity of the nucleophile, there are two possible general scenarios: • Strong nucleophiles (anionic) add directly to the C=O to form the intermediate alkoxide. The alkoxideis then protonated on work-up with dilute acid. Examples of such nucleophilic systems are: RMgX, RLi, RC≡CM, LiAlH4, NaBH4 • Weaker nucleophiles (neutral) require that the C=O be activated prior to attack of the Nu. This can be done using a acid catalyst which protonates on the Lewis basic O and makes the system more electrophilic. Examples of such nucleophilic systems are: H2O, ROH, R-NH2 The protonation of a carbonyl gives a structure that can be redrawn in another resonance form that reveals the electrophilic character of the C since it is a carbocation. The reactions on the following pages are catalogued based on the nature of the nucleophilic atom involved. In most cases it is useful to indentify the mechanism in each case in terms of the two general schemes presented above. Reactivity: Overall a simple nucleophilic addition can be represented with curly arrows as follows: The reactivity of aldehydes and ketones can be easily rationalised by considering the important resonance contributor which has charge separation with a +ve C and -ve O. QUESTION Why is this resonance structure more important than the one with a -ve C and +ve O? ANSWER In general the reactivity order towards nucleophiles is : aldehydes > ketones (see below) The substituents have two contributing factors on the reactivity at the carbonyl C: 1. Size of the substituents attached to the C=O. Larger groups will tend to sterically hinder the approach of the Nu. 2. The electronic effect of the substituent. Alkyl groups are weakly electron donating so they make the C in the carbonyl less electrophilic and therefore less reactive towards nucleophiles. These trends are supported by the trends in the equilibrium data for the formation of hydrate (see later) Carbonyl K / M-1 % Hydrate methanal 41 99.96 -2 ethanal 1.8 x 10 50 Carbonyl 2,2-dimethylpropanal 4.1 x 10-3 19 Hydrate propanone 2.5 x 10-5 0.14 (K = [hydrate]/[C=O][H2O]) The following list is an overview of the reactions of aldehydes, RCHO, and ketones, RCOR', ordered by nucleophile, that are presented in the following pages. Reaction with "H": Reduction to Hydrocarbons H- : Hydride Reductions to Alcohols Reaction with C nucleophiles: -CN : Formation of Cyanohydrins RM : Reaction with Organometallics P ylide : Wittig Reaction Reaction with N nucleophiles: RNH2 : Primary amines and derivatives R2NH : Secondary amines Reaction with O nucleophiles: H2O : Formation of Hydrates ROH : Formation of acetals / ketals Oxidation Reactions Oxidation of Aldehydes Baeyer-Villager reaction Addition to Carbon-Carbon double bonds: .
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