Basic Organic Chemistry

Transcription

1 Basic rganic hemistry ourse code: EM (Pre-requisites : EM 11122) hapter 06 hemistry of Aldehydes & Ketones Dr. Dinesh R. Pandithavidana ffice: B1 222/3 Phone: (+94) (Mobile) dinesh@kln.ac.lk

2 arbonyl ompounds

3 arbonyl Structure arbon is sp 2 hybridized. = bond is shorter, stronger, and more polar than = bond in alkenes.

4 IUPA Names for Ketones Replace -e with -one. Indicate the position of the carbonyl with a number. Number the chain so that carbonyl carbon has the lowest number. For cyclic ketones the carbonyl carbon is assigned the number 1.

5 Examples methyl-2-butanone Br 3-bromocyclohexanone hydroxy-3-methyl-2-butanone

6 Naming Aldehydes Replace -e with -al. The aldehyde carbon is number 1. If - is attached to a ring, use the suffix - carbaldehyde methylpentanal 2-cyclopentenecarbaldehyde

7 Name as Substituent n a molecule with a higher priority functional group, = is oxo- and - is formyl. Aldehyde priority is higher than ketone methyl-4-oxopentanal 3-formylbenzoic acid

8 istorical ommon Names 3 3 acetone acetophenone 3 benzophenone

9 Aldehyde - ommon Names Use the common name of the acid. Drop -icacid and add -aldehyde. 1 : formic acid, formaldehyde 2 s: acetic acid, acetaldehyde 3 s: propionic acid, propionaldehyde 4 s: butyric acid, butyraldehyde.

10 Solubility Good solvent for alcohols. Lone pair of electrons on oxygen of carbonyl can accept a hydrogen bond from - or N-. Acetone and acetaldehyde are miscible in water. Formaldehyde Gas at room temperature. Formalin is a 40% aqueous solution. heat formaldehyde, b.p formalin trioxane, m.p. 62

11 xidation Synthesis Review 2 alcohol + Na 2 r 2 7 ketone 1 alcohol + P aldehyde zonolysis of alkenes. R R' R'' 1) 2) 3 ( 3 ) 2 S R + R' R'' ydration of terminal alkynes Use gs 4, 2 S 4, 2 for methyl ketone Use Sia 2 B followed by 2 2 in Na for aldehyde.

12 Synthesis Using 1,3-Dithiane Remove + with n-butyllithium. S S BuLi S _ S Alkylate with primary alkyl halide, then hydrolyze. S _ S 3 2 Br S S 2 3 +, gl

13 Ketones from 1,3-Dithiane After the first alkylation, remove the second +, react with another primary alkyl halide, then hydrolyze. S S BuLi S S _ 3 Br S S +, gl

14 Ketones from arboxylates rganolithium compounds attack the carbonyl and form a diion. Neutralization with aqueous acid produces an unstable hydrate that loses water to form a ketone. _ Li+ 3 Li _ + Li _ Li _ 2 3

15 Ketones from Nitriles A Grignard or organolithium reagent attacks the nitrile carbon. The imine salt is then hydrolyzed to form a ketone. 3 2 MgBr + N ether N MgBr

16 Aldehydes from Acid hlorides Use a mild reducing agent to prevent reduction to primary alcohol l LiAl(-t-Bu)

17 Ketones from Acid hlorides Use lithium dialkylcuprate (R 2 uli), formed by the reaction of 2 moles of R-Li with cuprous iodide Li ui ( ) 2 uli ( ) 2 uli l

18 Reactions of Nucleophilic Addition A strong nucleophile attacks the carbonyl carbon, forming an alkoxide ion that is then protonated. A weak nucleophile will attack a carbonyl if it has been protonated, thus increasing its reactivity. Aldehydes are more reactive than ketones.

19 Wittig Reaction Nucleophilic addition of phosphorus ylides. Product is alkene. = becomes =.

20 ow to Prepare Phosphorus Ylides Prepared from triphenylphosphine and an unhindered alkyl halide. Butyllithium then abstracts a hydrogen from the carbon attached to phosphorus. + _ Ph 3 P Br Ph 3 P 2 3 Br + Ph 3 P 2 3 BuLi _ + Ph 3 P 3 ylide

21 Mechanism for Wittig Reaction The negative on ylide attacks the positive of carbonyl to form a betaine. xygen combines with phosphine to form the phosphine oxide. + _ Ph 3 P + 3 Ph 3 P 3 3 Ph 3 Ph + _ Ph 3 P 3 3 Ph Ph 3 P 3 Ph 3 Ph 3 P 3 3 Ph

22 Addition of Water In acid, water is the nucleophile. In base, hydroxide is the nucleophile. Aldehydes are more electrophilic since they have fewer e - -donating alkyl groups. + 2 K = K = 0.002

23 N is highly toxic. Addition of N Use NaN or KN in base to add cyanide, then protonate to add. Reactivity formaldehyde > aldehydes > ketones >> bulky ketones. 3 2 N 3 + N 3 2 3

24 Formation of Imines Nucleophilic addition of ammonia or primary amine, followed by elimination of water molecule. = becomes =N-R RN 2 3 Ph R 2 N + 3 R 3 _ N Ph Ph R N 3 Ph R N 3 Ph

25 Addition of Alcohol

26 Must be acid-catalyzed. Mechanism Adding + to carbonyl makes it more reactive with weak nucleophile, R. emiacetal forms first, then acid-catalyzed loss of water, then addition of second molecule of R forms acetal. All steps are reversible.

27 Mechanism for emiacetal

28 emiacetal to Acetal

29 yclic Acetals Addition of a diol produces a cyclic acetal. Sugars commonly exist as acetals or hemiacetals

30 Acetals as Protecting Groups ydrolyze easily in acid, stable in base. Aldehydes more reactive than ketones

31 Selective Reaction of Ketone React with strong nucleophile (base) Remove protective group. + _ MgBr 3 3 MgBr 3 + 3

32 xidation of Aldehydes Easily oxidized to carboxylic acids.

33 Reduction Reagents Sodium borohydride, NaB 4, reduces =, but not =. Lithium aluminum hydride, LiAl 4, much stronger, difficult to handle. ydrogen gas with catalyst also reduces the = bond.

34 atalytic ydrogenation Widely used in industry. Raney nickel, finely divided Ni powder saturated with hydrogen gas. Pt and Rh also used as catalysts. Raney Ni

35 Deoxygenation Reduction of = to 2 Two methods: lemmensen reduction if molecule is stable in hot acid. Wolff-Kishner reduction if molecule is stable in very strong base.

36 lemmensen Reduction 2 3 Zn(g) l, 2 2 Zn(g) l, 2 2 3

37 Wolff-Kishner Reduction Form hydrazone, then heat with strong base like K or potassium t-butoxide. Use a high-boiling solvent: ethylene glycol, diethylene glycol, or DMS. 2 2 N N 2 2 NN 2 K heat 2 3