Aromatic Compounds

1. Aromatic Compounds

2. Nomenclature of Benzene Derivatives • Benzene is the parent name for some monosubstituted benzenes; the substituent name is added as a prefix • For other monosubstituted benzenes, the presence of the substituent results in a new parent name

3. When two substituents are present their position may be indicated by the prefixes ortho, meta, and para (o, m and p) or by the corresponding numerical positions • Dimethyl substituted benzenes are called xylenes

4. Numbers must be used as locants when more than two substituents are present • The lowest possible set of numbers should be given to the substituents • The substituents should be listed in alphabetical order • If one of the substituents defines a parent other than benzene, this substituent defines the parent name and should be designated position 1

5. The C6H5- group is called phenyl when it is a substituent • Phenyl is abbreviated Ph or F • A hydrocarbon with a saturated chain and a benzene ring is named by choosing the larger structural unit as the parent • If the chain is unsaturated then it must be the parent and the benzene is then a phenyl substituent • The phenylmethyl group is called a benyl (abbreviated Bz)

6. The Kekule Structure for Benzene • Kekule was the first to formulate a reasonable representation of benzene • The Kekule structure suggests alternating double and single carbon-carbon bonds • Based on the Kekule structure one would expect there to be two different 1,2-dibromobenzenes but there is only one • Kekule suggested an equilibrium between these compounds to explain this observation but it is now known no such equilibrium exists

7. Other Aromatic Compounds • Benzenoid Aromatic Compounds • Polycyclic benzenoid aromatic compounds have two or more benzene rings fused together

8. Naphthalene can be represented by three resonance structures • The most important resonance structure is shown below • Calculations show that the 10 p electrons of napthalene are delocalized and that it has substantial resonance energy • Pyrene has 16 p electrons, a non-Huckel number, yet is known to be aromatic • Ignoring the central double bond, the periphery of pyrene has 14 p electrons, a Huckel number, and on this basis it resembles the aromatic [14]annulene

9. Nonbenzenoid Aromatic Compounds • Nonbenzenoid aromatic compounds do not contain benzene rings • Examples are cyclopentadienyl anion and the aromatic annulenes (except [6] annulene) • Azulene has substantial resonance energy and also substantial separation of charge, as shown in the electrostatic potential map

10. Fullerenes • Buckminsterfullerene is a C60 compound shaped like a soccer ball with interconnecting pentagons and hexagons • Each carbon is sp2 hybridized and has bonds to 3 other carbons • Buckminsterfullerene is aromatic • Analogs of “Buckyballs” have been synthesized (e.g. C70)

11. Aldehydes and Ketones

12. Nomenclature of Aldehydes and Ketones • Aldehydes are named by replacing the -e of the corresponding parent alkane with -al • The aldehyde functional group is always carbon 1 and need not be numbered • Some of the common names of aldehydes are shown in parenthesis • Aldehyde functional groups bonded to a ring are named using the suffix carbaldehyde • Benzaldehyde is used more commonly than the name benzenecarbaldehyde

13. Ketones are named by replacing the -e of the corresponding parent alkane with -one • The parent chain is numbered to give the ketone carbonyl the lowest possible number • In common nomenclature simple ketones are named by preceding the word ketone with the names of both groups attached to the ketone carbonyl • Common names of ketones that are also IUPAC names are shown below

14. The methanoyl or formyl group (-CHO) and the ethanoyl or acetyl group (-COCH3) are examples of acyl groups

15. Physical Properties • Molecules of aldehyde (or ketone) cannot hydrogen bond to each other • They rely only on intermolecular dipole-dipole interactions and therefore have lower boiling points than the corresponding alcohols • Aldehydes and ketones can form hydrogen bonds with water and therefore low molecular weight aldehydes and ketones have appreciable water solubility

16. Synthesis of Aldehydes • Aldehydes by Oxidation of 1o Alcohols • Primary alcohols are oxidized to aldehydes by PCC • Aldehydes by Reduction of Acyl Chlorides, Esters and Nitriles • Reduction of carboxylic acid to aldehyde is impossible to stop at the aldehyde stage • Aldehydes are much more easily reduced than carboxylic acids

17. Synthesis of Ketones • Ketones from Alkenes, Arenes, and 2o Alcohols • Ketones can be made from alkenes by ozonolysis • Aromatic ketones can be made by Friedel-Crafts Acylation • Ketones can be made from 2o alcohols by oxidation

18. Ketones from Alkynes • Markovnikov hydration of an alkyne initially yields a vinyl alcohol (enol) which then rearranges rapidly to a ketone (keto)

19. Ketones from Nitriles • Organolithium and Grignard reagents add to nitriles to form ketones • Addition does not occur twice because two negative charges on the nitrogen would result

20. Carboxylic Acids and Their Derivatives

21. Introduction • The carboxyl group (-CO2H) is the parent group of a family of compounds called acyl compounds or carboxylic acid derivatives

22. Nomenclature and Physical Properties • In IUPAC nomenclature, the name of a carboxylic acid is obtained by changing the -e of the corresponding parent alkane to -oic acid • The carboxyl carbon is assigned position 1 and need not be explicitly numbered • The common names for many carboxylic acids remain in use • Methanoic and ethanoic acid are usually referred to as formic and acetic acid • Carboxylic acids can form strong hydrogen bonds with each other and with water • Carboxylic acids with up to 4 carbons are miscible with water in all proportions

24. Acidity of Carboxylic Acids • The carboxyl proton of most carboxylic acids has a pKa = 4 - 5 • Carboxylic acids are readily deprotonated by sodium hydroxide or sodium bicarbonate to form carboxylate salts • Carboxylate salts are more water soluble than the corresponding carboxylic acid • Electron-withdrawing groups near the carboxyl group increase the carboxylic acid’s acidity • They stabilize the carboxylate anion by inductive delocalization of charge

25. Dicarboxylic Acids • Dicarboxylic acids are named as alkanedioic acids in the IUPAC system • Common names are often used for simple dicarboxylic acids

26. Esters • The names of esters are derived from the names of the corresponding carboxylic acid and alcohol from which the ester would be made • The alcohol portion is named first and has the ending -yl • The carboxylic acid portion follows and its name ends with -ate or -oate • Esters cannot hydrogen bond to each other and therefore have lower boiling points than carboxylic acids • Esters can hydrogen bond to water and have appreciable water solubility

28. Acid Anhydrides • Most anhydrides are named by dropping the word acid from the carboxylic acid name and adding the word anhydride • Acid Chlorides • Acid chlorides are named by dropping the -ic acid from the name of the carboxylic acid and adding -yl chloride

29. Amides • Amides with no substituents on nitrogen are named by replacing -ic acid in the name with amide • Groups on the nitrogen are named as substitutents and are given the locants N- or N,N- • Amides with one or two hydrogens on nitrogen form very strong hydrogen bonds and have high melting and boiling points • N,N-disubstituted amides cannot form hydrogen bonds to each other and have lower melting and boiling points

30. Hydrogen bonding between amides in proteins and peptides is an important factor in determining their 3-dimensional shape • Nitriles • Acyclic nitriles are named by adding the suffix -nitrile to the alkane name • The nitrile carbon is assigned position 1 • Ethanenitrile is usually called acetonitrile

31. Preparation of Carboxylic Acids • By Oxidation of Alkenes • By Oxidation of Aldehydes and Primary Alcohols • By Oxidation of Alkylbenzenes

32. By Oxidation of the Benzene Ring • By Oxidation of Methyl Ketones (The Haloform Reaction) • By Hydrolysis of Cyanohydrins and Other Nitriles • Hydrolysis of a cyanohydrin yields an a-hydroxy acid

33. Primary alkyl halides can react with cyanide to form nitriles and these can be hydrolyzed to carboxylic acids • By Carbonation of Grignard Reagents

34. Acid Chlorides • Synthesis of Acid Chlorides • Acid chlorides are made from carboxylic acids by reaction with thionyl chloride, phosphorus trichloride or phosphorus pentachloride • These reagents work because they turn the hydroxyl group of the carboxylic acid into an excellent leaving group

36. Carboxylic Acid Anhydrides • Synthesis of Carboxylic Acid Anhydrides • Acid chlorides react with carboxylic acids to form mixed or symmetrical anhydrides • It is necessary to use a base such as pyridine • Sodium carboxylates react readily with acid chlorides to form anhydrides

37. Cyclic anhydrides with 5- and 6-membered rings can be synthesized by heating the appropriate diacid • Reactions of Carboxylic Acid Anhydrides • Carboxylic acid anhydrides are very reactive and can be used to synthesize esters and amides • Hydrolysis of an anhydride yields the corresponding carboxylic acids

39. Esters • Synthesis of Esters: Esterification • Acid catalyzed reaction of alcohols and carboxylic acids to form esters is called Fischer esterification • Fischer esterification is an equilibrium process • Ester formation is favored by use of a large excess of either the alcohol or carboxylic acid • Ester formation is also favored by removal of water

40. The reverse reaction is acid-catalyzed ester hydrolysis • Ester hydrolysis is favored by use of dilute aqueous acid • Esters from Acid Chlorides • Acid chlorides react readily with alcohols in the presence of a base (e.g. pyridine) to form esters

41. Esters from Carboxylic Acid Anhydrides • Alcohols react readily with anhydrides to form esters

42. Base-Promoted Hydrolysis of Esters: Saponification • Reaction of an ester with sodium hydroxide results in the formation of a sodium carboxylate and an alcohol • The mechanism is reversible until the alcohol product is formed • This step draws the overall equilibrium toward completion of the hydrolysis

43. Amides • Synthesis of Amides • Amides From Acyl Chlorides • Ammonia, primary or secondary amines react with acid chlorides to form amides • An excess of amine is added to neutralize the HCl formed in the reaction • Carboxylic acids can be converted to amides via the corresponding acid chloride

44. Amides from Carboxylic Anhydrides • Anhydrides react with 2 equivalents of amine to produce an amide and an ammonium carboxylate • Reaction of a cyclic anhydride with an amine, followed by acidification yields a product containing both amide and carboxylic acid functional groups • Heating this product results in the formation of a cyclic imide

45. Amides from Carboxylic Acids and Ammonium Carboxylates • Direct reaction of carboxylic acids and ammonia yields ammonium salts • Some ammonium salts of carboxylic acids can be dehydrated to the amide at high temperatures • This is generally a poor method of amide synthesis • A good way to synthesize an amide is to convert a carboxylic acid to an acid chloride and to then to react the acid chloride with ammonia or an amine

46. Nitriles from the Dehydration of Amides • A nitrile can be formed by reaction of an amide with phosphorous pentoxide or boiling acetic anhydride

47. Amines

48. Nomenclature • Primary amines are named in systematic (IUPAC) nomenclature by replacing the -e of the corresponding parent alkane with -amine • In common nomenclature they are named as alkylamines • Simple secondary and tertiary amines are named in common nomenclature by designating the organic groups separately in front of the word amine • In systematic nomenclature, the smaller groups on the amine nitrogen are designated as substituents and given the locant N

49. In IUPAC nomenclature the substitutent -NH2 is called the amino group • Aryl Amines • The common arylamines have the following names: