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Chem 267: Chapter 23 Textbook Exam Review Notes

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CHEM 267
Monica Barra

Chem 267 Chapter 23 Notes Winter 2012 23.3 Properties of Amines  1° and 2° amines can form intermolecular H bonds and typically have higher boiling points than analogous alkanes but lower boiling points than analogous alcohols o 1° amines have higher boiling points than 2° amines b/c it can form more H-bonds Basicity of Amines  Amines are generally stronger bases than alcohols or ethers  High pK a amine is strongly basic  Low pK a amine is weakly basic Delocalization Effects  Ammonium ions of aryl amines are more acidic (lower pK )athan the ammonium ion of alkyl amines due to more resonance (due to delocalization of the lone pair on N around the ring) o Electron-donating groups (EDG) increase the basicity of aryl amines o Electron-withdrawing groups (EWG) decrease the basicity of aryl amines  More acidic (less basic) b/c the lone pair is extensively delocalized 23.4 Preparation of Amines: A Review Preparing an Amine from an Alkyl Halide  2-step process: the alkyl halide is converted to a nitrile, then undergoes reduction  NOTE: since the first step is aN S 2 reaction, 3° alkyl halides and vinyl halides CANNOT be used Preparing an Amine from a Carboxylic Acid  2-step process: the carboxylic acid is converted to an amide, which is then reduced to an amine  NH 2ssentially replaces the OH, then the C=O disappears Preparing Aniline and Its Derivatives from Benzene  Aryl amines (anilines) can be prepared from benzene  2-step process: nitration of the aromatic ring + reduction of then nitro group  NOTE: when reducing a nitro group in acidic conditions, the reaction MUST be followed up with a base (ex. NaOH) b/c the resulting amino group will be protonated under acidic conditions  NOTE: SnCl is used when other functional groups are present that would be susceptible to 2 hydrogenation (ex. carbonyl group) 23.5 Preparation of Amines via Substitution Reactions Alkylation of Ammonia  Ammonia is a good nucleophile and will undergo alkylation when treated with an alkyl halide  After the 1° amine is formed, it can undergo further alkylation to produce a 2°, 3°, and up to a quaternary (4°) amine salt  NOTE: monoalkylation is difficult to isolate b/c each successive alkylation renders the nitrogen atom more nucleophilic (since each alkyl group inductively donates electron density) o THEREFORE: NOT a good method for preparing 1° amines! The Azide Synthesis  Better method for preparing 1° amines than alkylation of ammonium  2-step process: treating an alkyl halide with sodium azide (Na3 ) + reduction - o Azide ion (N3) functions as a nucleophile and attacks the alkyl halide in aN S 2 process o Azide is then reduced using LAH (causing a loss of 2 ) o MECHANISM FOUND ON PG. 1103 The Gabriel Synthesis  Another method for preparing 1° amines  Hydroxide functions as a base and deprotonates phthalimide  Potassium phthalimide can then act as a nucleophile and attack an alkyl halide via N 2 o NOTE: Since it’s an N 2 process, the reaction works best with 1° alkyl halides!  Acid or base-catalyzed hydrolysis is performed to release the amine 23.6 Preparation of Amines via Reductive Amination  Reductive amination = Ketones/aldehydes are converted into imines when treated w/ ammonia (NH 3 in acidic conditions, and then into amines in the presence of a reducing agent (ex.2H ) o sodium cya
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