Functional Derivatives of Carboxylic Acids

Transcription

1 Functional Derivatives of Carboxylic Acids Derivatives of Carboxylic Acids are compounds in which the OH of a carboxyl group has been replaced by CI, OOCR, NH2, or OR'to convert acid chlorides,anhydrides, amides, and esters. Like the acid to which it is related, an acid derivative may be aliphatic or aromatic, substituted or unsubstituted; whatever the structure of the rest of the molecule, the properties of the functional group remain essentially the same. Nomenclature The names of acid derivatives are taken in simple ways from either the common name or the IUPAC name of the corresponding carboxylic acid. For example: 1

2 Physical properties 1- The presence of the C= O group makes the acid derivatives polar compounds. 2- Acid chlorides and anhydrides and esters have boiling points that are about the same as those of aldehydes or ketones of comparable molecular weight 3- Amides have quite high boiling points because they are capable of strong intermolecular hydrogen bonding 4-The border line for solubility in water ranges from three to five carbons for the esters to five or six carbons for the amides. The acid derivatives are soluble in the usual organic solvents. 5-Volatile esters have pleasant, rather characteristic odors; they are often used in the preparation of perfumes and artificial flavorings. Acid chlorides have sharp, irritating odors, at least partly due to their ready hydrolysis to HC1 and carboxylie acids 2

3 Nucleophilic acyl substitution. Role of the carbonyl group Acyl compounds carboxylic acids and their derivatives typically undergo nucleophilic substitution in which -OH, -Cl, OOCR, NH 2 or -OR' is replaced by some other basic group. Substitution takes place much more readily than at a saturated carbon atom; indeed, many of these substitutions do not usually take place at all in the absence of the carbonyl group, as, for example, replacement of-nh 2 by-oh. Carbonyl carbon is joined to three other atoms by a bonds; since these bonds utilize sp2 orbitals, they lie in a plane and are 120 apart. The remaining p orbital of the carbon overlaps a p orbital of oxygen to form a TT bond; carbon and oxygen are thus joined by a double bond. The part of the molecule immediately surrounding carbonyl carbon Is flat', oxygen, carbonyl carbon, and the two atoms directly attached to carbonyl carbon He in a plane 3

4 Nucleophilic substitution: alkyl vs. acyl nucieophilic substitution takes place much more readily at an acyl carbon than at saturated carbon. Thus, toward nucleophilic attack acid chlorides are more reactive than alkyl chlorides, amides are more reactive than amines (RNH2), and esters are more reactive than ethers 4

5 the carbonyl group that makes acyl compounds more reactive than alkyl compounds. Nucleophilic attack (SN 2)on a tetrahedral alkyl carbon involves a badly crowded transition state containing pentavalent carbon; a bond must be partly broken to permit the attachment of the nucleophile: Nucleophilic attack on a flat acyl compound involves a relatively unhindered transition state leading to a tetrahedral intermediate that is actually a compound; since the carbonyl group is unsaturated, attachment of the nucleophile requires breaking only of the weak Π ond, and places a negative charge on an atom quite willing to accept it; oxygen. 5

6 ACID CHLORIDES Preparation of acid chlorides Acid chlorides are prepared from the corresponding acids by reaction with hionyl chloride, phosphorus trichloride, or hosphorus pentachloride, REACTIONS OF ACID CHLORIDES 1. Conversion into acids and derivatives 6

7 2. Formation of ketones. Friedel-Crafts acylation 3. Formation of ketones. Reaction with organocadmium compounds 4. Formation of aldehydes by reduction 7

8 Preparation of acid anhydrides ACID ANHYDRIDES It is prepared by the reaction of cetic acid with ketene, CH2=C=O hich itself is prepared by high-temperature ehydration of acetic acid. 8

9 Reactions of acid anhydrides 9

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11 AMIDES Preparation of amides Reactions of amides 11

12 It involves nucleophilic substitution, in which the NH 2 group is replaced by H. Under acidic conditions hydrolysis involves attack by water on the protonated amide Under alkaline conditions hydrolysis involves attack by the strongly nucleophilic ydroxide ion on the amide itself: Imides Like other anhydrides, cyclic anhydrides react with ammonia to yield amides; inthis case the product contains both CONH2 and COOH groups. If thisacid-amide is heated, a molecule of water is lost, a ring forms, and a product isobtained in which two acyl groups have become attached to nitrogen; compoundsof this sort are called imides. Phthalic anhydride gives phthalamic acid and phthalimide: 12

13 PREPARATION OF ESTERS ESTERS 13

14 Reactions of esters Esters undergo the nucleophilic substitution that is typical of carboxylic acidderivatives. Attack occurs at the electron-deficient carbonyl carbon, and resultsin the replacement of the OR' group by OH, OR*, or NH 2 : REACTIONS OF ESTERS 14

15 Acidic hydrolysis of esters 15

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