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Chemistry 110 Bettelheim, Brown, Campbell & Farrell Ninth Edition Introduction to General, Organic and Biochemistry Chapter 16 Amines Amine Molecules are Weak Bases Aliphatic & Aromatic Amines ¾Amines are relatives of ammonia, NH3, in which the hydrogen atoms on nitrogen have been replaced by one or more noncarbonyl substituent groups. ¾ Aliphatic amines in which alkyl groups replace hydrogen are slightly more basic than ammonia. ¾ Aromatic amines in which one or more of the substituent groups is an arene, are considerably less basic than ammonia. ¾ Heterocyclic amines have the nitrogen atom within a ring. The ring may be aliphatic or aromatic. ¾ Heterocyclic aliphatic amines are stronger bases than ammonia. ¾ Heterocyclic aromatic amines are considerably weaker bases than ammonia. In these rings the nitrogen atom contributes one or two electrons to the aromatic sextet of electrons . Aromatic versus Aliphatic Amines Aliphatic Amines have the amino- group attached by tetrahedral carbon: C C NH3 NH2 NH N C C C C primary, 1E Ammonia secondary, 2E tertiary, 3E Aromatic amines have the amino- group bonded to a benzene ring: NH2 aniline NHCH3 N-methylaniline N(CH3)2 N,N-dimethylaniline Aliphatic amines can have aromatic rings as long as the nitrogen is attached by tetrahedral carbon. 1 Heterocyclic Aliphatic/Aromatic Amines Heterocyclic aliphatic amines – the nitrogen is incorporated into a ring and is connected by two tetrahedral carbons. N CH3 N-methylpyrrolidine N piperidine 2E heterocyclic aliphatic amine 3E heterocyclic aliphatic amine Heterocyclic aromatic amines – the nitrogen is incorporated into an aromatic ring – N contributes to the aromatic sextet. N N N pyramidine N pyridine N N H imidazole N N N H purene Naming of Amines ¾ IUPAC names for aliphatic amines follow that for alcohols. Drop the final -e of the parent alkane and replace it with –amine. ¾ Use a number to locate the amino group on the parent chain. ¾ Name unsymmetrical secondary and tertiary amines as N-substituted primary amines. In this case take the largest group bonded to nitrogen as the parent amine. ¾ Name the smaller group(s) bonded to nitrogen, and show that their location is bonded to nitrogen by using the prefix Nbefore the listing of that substituent. N CH3 CH3CH2CH2NH2 CH3CH2CH2NHCH3 CH3 propanamine N,N-dimethylaniline N-methylpropanamine Naming of Amines ¾ IUPAC nomenclature retains the common name aniline for C6H5NH2, the simplest aromatic amine. ¾Name simple derivatives of aniline by using locator numbers starting with the amino group, or locate substituents by using the prefixes ortho (o), meta (m), and para (p). ¾Several derivatives of aniline have common names that are still widely used; among them is toluidine. ¾If another substituent overides with a special name locate the amine substituent with a locator number starting at the other substituent. NH2 NH2 NH2 H3C aniline 2-methylaniline o-toluidine COOH 2-aminobenzoic acid o-aminobenzoic acid 2 Naming of Amines Common Names ¾For most aliphatic amines, list the groups bonded to nitrogen in alphabetical order in one word ending in the suffix –amine. NH2 N N H isopropylamine cyclopentyldiethylamine ethylisopropylamine ¾Because amines are bases they react with strong acids to form ammonium salts. In these salts nitrogen has four bonds, has a positive charge and is associated with an anion. Cl NH3 Cl Cl N H H isopropylammonium chloride N H cyclopentyldiethylammonium chloride ethylisopropylammonium chloride Properties of Amines ¾Amines are polar, hydrogen bond with water, and 1Eand 2E with each other, but more weakly than alcohols ¾Amines have lower boiling points than alcohols of similar molecular weight, they are also soluble in water if the substituent group part is not too large and non-polar. C1-C7 = miscible to slightly soluble, C8 and higher are insoluble. ¾Amines are basic in water, the pH will go to higher values. ¾Amines readily react with acid. The resulting ammonium ion are much more soluble than the parent amines. H O O H N H H H H R N H O H H H R H O H H H OH OH Properties of Amines ¾The N-H---N hydrogen bond is weaker than an O-H---O hydrogen bond because the difference in electronegativity between N and H (3.0 - 2.1 = 0.9) is less than that between O and H (3.5 - 2.1 = 1.4). ¾The effect of hydrogen bonding between molecules of comparable molecular weight can be seen by comparing the boiling points of ethane, methanamine, and methanol. MW (am u) bp (°C) CH 3 CH 3 CH3 NH2 CH3 OH 30.1 -88.6 31.1 -6.3 32.0 65.0 ¾Like ammonia, low-molecular-weight amines have very sharp, penetrating odors; for example, rotting fish = trimethylamine. 3 Hydrogen Bonds – Amines in Water H O R H H N H H R N H H N H H O R H H H H O O H Hydrogen Bonds – Pure Amines – Except 3E H R N R H H N H H H N H H N R H R N R H R N H H R N H H Can You Name these Amines? H3C NH2 H3C HC NHCH3 H3C CH3 HC NH CH H3C CH3 Classify them as aromatic, aliphatic, and as primary, secondary or tertiary. O C OH H3C H3C NH2 CH2CH3 C NH CH H3C (CH3CH2)3N CH3 4 Amines Are Relatives of Ammonia Amines are basic: ( Aliphatic amines about as basic as ammonia) NH3 NH2 C primary H C C NH C secondary H N C C tertiary H H H NH2 C H NH C C And make ammonium salts in acidic solution. NH4 C H N C C Note: N has 3 bonds in amines, but 4 bonds in ammonium ions. Ammonium Ions React with Base Ammonium salts are weakly acidic: NH4 C H NH2 C H NH C OH OH OH NH3 + H2O NH2 C primary C H N C C OH C C NH C secondary N C C tertiary And react with hydroxide ion to regenerate the original amine + water Reaction of Amines with Water H H3C N H H H3C O + H H N H + O H H Reaction of Ammonium Ions with Water H3C H H N H + H O H H H H3C N H O + H H 5 What is the form of amines in blood plasma? pH of blood is 7.4, [OH] = 2.5 x 10 -7 Ethylamine: Kb = 4.3 x 10-4 = + + [CH3CH2NH3 ] Kb = [OH ] [CH3CH2NH2] - [CH3CH2NH3 ] [OH ] [CH3CH2NH2] = 4.3 x 10-4 2.5 x 10-7 = 1720 An aliphatic amine is mostly in the ammonium form in blood! + [C5H5NH ] [OH ] Pyridine: Kb = 1.5 x 10-9 = [C5H5N] + [C5H5NH ] Kb = [OH ] [C5H5N] = 1.5 x 10-9 2.5 x 10-7 = 0.0060 An aromatic amine is mostly in the amine form in blood! Carboxylic Acids & Amines Give Salts O CH3 H NCH2CH3 + CH3C OH Carboxylic Acid Acetic Acid Amine Ethylmethylamine 100% O CH3 H NCH2CH3 + CH3C O H Ammonium Ion Carboxylate Anion Ethylmethylammonium Acetate Aminoacids exist in vivo only in their ionized form! O O H2N CH C OH H3N CH C O OH CH2 CH2 Tyrosine OH isoelectric form What Products are Formed? If No Reaction write – NR CH3CH2CH2CH2NH3 Cl + NaOH CH3CH2CH2CH2NH2 + NaOH CH3CH2CH2CH2NH3 Cl + HCl CH3CH2CH2CH2NH2 + HCl 6