Amines & Amides. Classifying Amines. Nomenclature. Constitutional Isomerism. (Woods) Chem-131 Lec Amines 1

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1 (Woods) Chem-131 Lec Amines 1 Amines & Amides Amines and amides are derivatives of ammonia in which one or more hydrogen atoms have been replaced by organic groups. The organic groups can be aliphatic, aromatic, or both. Classifying Amines Primary (1 ), secondary (2 ), and tertiary (3 ). The hybridization about the nitrogen atom in singly bonded amines or amides is sp 3 or tetrahedral. Three of the four positions involve bonds, the last is a lone pair of electrons (non-bonding electrons). Bond angles: ~ (except in amides, ~120 ). Amines are also classified as aromatic (aryl) or aliphatic amines. Aromatic Amine: One or more aromatic group bonded to nitrogen. Aliphatic Amine: No aromatic group bonded to nitrogen. Constitutional Isomerism Nomenclature The rules are more varied than for other families. Amines with simple groups use the normal nomenclature system. Simple groups include: Unbranched alkyl: methyl, ethyl, propyl, butyl,., decyl Branched 3- and 4-carbon alkyl: isopropyl, isobutyl, s-butyl, t-butyl Unsubstituted cycloalkyl: cyclohexyl and cyclopentyl In the common system, groups (other than H) attached to nitrogen are named alphabetically followed by a space and the suffix amine. 1

2 (Woods) Chem-131 Lec Amines 2 Nomenclature C 6 H 5 NH 2 is called aniline. Substituted anilines are named as derivatives of aniline. The prefix N- is used to distinguish substituents directly attached to the nitrogen atom from those attached to the aromatic ring. If both an amine group and an oxygen containing functional group: the amine group is treated as a prefix the compound is named as a derivative of the molecule containing the oxygen group: -NH 2 amino -NHR N-alkylamino -NR 2 N,N-dialkylamino 2

3 (Woods) Chem-131 Lec Amines 3 Cyclic compounds containing a ring nitrogen (not a nitrogen attached to the ring) are called nitrogen heterocycles or heterocyclic amines or heterocyclic nitrogen bases. Physical Properties of Amines The physical properties of amines depend upon whether they are primary, secondary, or tertiary. Primary and secondary amines have melting and boiling points comparable to those of aldehydes d and ketones. Tertiary amines have melting and boiling points considerably lower than those of primary and secondary amines, comparable to those of ethers and hydrocarbons. All amines have water solubility comparable to those of aldehydes and ketones and slightly lower than that of alcohols. Physical Properties of Amines Physical Properties of Amines Primary amines contain two N-H bonds; secondary amines contain only one N-H bond. Both exhibit strong secondary forces (H-bonds), primary slightly more so than secondary amines. Tertiary amines do not possess an N-H bond and therefore exhibit only weak secondary forces. Hydrogen bonding involving nitrogen atoms is weaker than that involving oxygen atoms because nitrogen is less electronegative than oxygen. The secondary forces in primary and secondary amines are weaker than those in alcohols. The strength of N-H hydrogen bonding in amines is comparable to the dipole-dipole forces in aldehydes and ketones. The strength of the weaker secondary forces in tertiary amines is comparable to those in ethers and hydrocarbons. 3

4 (Woods) Chem-131 Lec Amines 4 Physical Properties of Amines Many amines have disagreeable odors; the smell of rotting meat is due to: Putrescine (1,4-butanediamine) Cadaverine (1,5-pentanediamine) Three- or four-carbon amines are soluble in water in all proportions. Solubility drops off until it becomes very low for amines with more than six or seven carbons. Tertiary amines are slightly less soluble than primary or secondary amines (less hydrogen bonding). The water solubility of amines is about the same as for aldehydes and ketones. Basicity of Amines In aqueous solutions, (alkyl)amines react with to form (alkyl)ammonium ions and hydroxide ions: The equilibrium of this reaction lies far to the left: only about 6% of the amine has reacted at equilibrium. Basicity of Amines Basicity of Amines The reaction of an amine with a strong acid such as HCl goes to completion: the amine is converted into an amine salt. Amines are more basic than water, but less basic than strong bases, such as sodium hydroxide. Amines are basic enough to turn red litmus paper blue. The only other type of organic compound sufficiently basic to turn red litmus paper blue is a carboxylate salt (RCOO - ). In the body, at physiological ph (~7) amines are almost entirely (>99%) in their amine salt form. Arylamines are much less basic than aliphatic amines. The aromatic ring has a greater electron withdrawing ability than an aliphatic carbon chain. This makes the lone electron pair less available for reaction with water. 4

5 (Woods) Chem-131 Lec Amines 5 Amine Salts Amine Salts Amine salts are named by naming the positive ion first and then naming the negative ion. Pharmaceutical companies often name amine salts by naming the parent amine followed by the name of the acid used to synthesize the salt: HCl: H 2 SO 4 : amine hydrochlorides amine hydrogen sulfates Properties of Amine Salts The melting and boiling points of amine salts are much higher than those of the corresponding amines. All amine salts are solids at room temperature. Methylammonium chloride MP 232 C, BP decomposes Methylamine MP -94 C, BP -6 C Amine salts are much more soluble in water than amines due to the strong interaction of water molecules with ions. Amines are soluble in neutral or lower ph where they are converted into their salt forms. Amines used as medicines are almost always used in their salt form: Amines undergo air oxidation resulting in a short shelf life. Water solubility is required for intravenous injection. Properties of Amine Salts An amine salt is a weak acid and undergoes an acid-base reaction with water and strong bases: Aqueous solutions of amine salts will turn blue litmus paper red. 5

6 (Woods) Chem-131 Lec Amines 6 Quaternary Ammonium Salts Quaternary Ammonium Salts Quaternary ammonium salts contain four alkyl or aryl groups attached to a central nitrogen atom. Quaternary Ammonium Salts: one of the alkyl groups is a long hydrocarbon chain (such as CH 3 (CH 2 ) 17 -) Used as soaps, germicides, id antiseptic soaps, and mouthwashes. h Quaternary ammonium salts are not acidic and do not turn blue litmus paper red. Quaternary ammonium hydroxides can be prepared by reacting a quaternary ammonium salt with a strong base: Used for disinfecting skin and hands prior to surgery and for sterilizing surgical instruments. Choline: a component of biological membranes (Phospholipids) and sometimes functions as a neuraltransmitter. Quaternary ammonium hydroxides are strong bases, as strong as NaOH and KOH. Amides Amides contain a carbonyl group attached directly to a nitrogen. They may be classified as primary, secondary, and tertiary: Synthesis of Amides Carboxylic acid with ammonia or a primary or secondary amine. At room temperature, an acid-base reaction occurs: At elevated temperatures (>100 C) the reactants form an amide: The most important biological amides are proteins. 6

7 (Woods) Chem-131 Lec Amines 7 Synthesis of Amides Nomenclature: Amides The reaction to form an amide is very similar to esterification. Dehydration between the carboxylic acid and the other reagent. Amide formation is reversible. Removal of water shifts the equilibrium towards the side of amide formation. Water is removed by running the reaction at a higher temperature than the boiling point of water. Tertiary amides cannot form amides, (No N-H bond). Tertiary amides undergo an acid-base reaction with carboxylic acids. Physical Properties of Amides Physical Properties of Amides The amide structure is best represented as a resonance hybrid of the following structures: Amides have the strongest secondary attractive forces and the highest melting and boiling points of any covalent organic compounds. The structure on the right is a dipolar ion (Zwitterion) atoms bearing opposite charges exist on the same molecule. The large charge separation in an amide molecule results in unusually large polarity and secondary attractive forces. Because the bond between the carbonyl carbon atom and the nitrogen atom has a partial double bond character, the bond angles about the carbonyl carbon atom and nitrogen atom are both close to 120 (sp 2 hybridization). The dipolar ion nature of the amide bond is very important in determining the structure and function of protein molecules. 7

8 (Woods) Chem-131 Lec Amines 8 Physical Properties of Amides In acidic and basic hydrolysis of amides, one of the products is converted into a salt, which drives the equilibrium to the right. NaOH Sodium Hydroxide ph (0.1M) 13.0 H 2 C H 3 C NH 2 Ethylamine 11.8 ph Aniline NH Properties: H 2 C H 3 C OH Ethanol 7.0 From Strong Bases The acid or base used to hydrolyze an amide is not a catalyst. It is consumed in the reaction and is required in equimolar amounts relative to the amide. The hydrolysis and synthesis of amide bonds occur continuously in living systems. These reactions are carried out using enzyme catalysts. OH 5.5 Phenol O H 3 C C 2.9 OH Acetic acid HCl 1.0 Hydrochloric acid To Strong Acids 8

UNIT (9) CARBOXYLIC ACIDS, ESTERS, AMINES, AND AMIDES

UNIT (9) CARBOXYLIC ACIDS, ESTERS, AMINES, AND AMIDES UNIT (9) CARBXYLIC ACIDS, ESTERS, AMINES, AND AMIDES 9.1 Carboxylic Acids The functional group in carboxylic acids is called the carboxyl group. A carboxyl group is a carbonyl group (C = ) with a hydroxyl