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HYDRATES

HYDRATES. Addition of Water. aldehyde or ketone favored. hydrates are unstable. and cannot be isolated in most cases. most hydrates revert to an aldehyde or ketone as soon as they form. ACID CATALYSIS. RECALL. +. . . . +. :. :. :. +. Acid catalysis enhances the reactivity

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HYDRATES

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  1. HYDRATES

  2. Addition of Water aldehyde or ketone favored hydrates are unstable and cannot be isolated in most cases most hydrates revert to an aldehyde or ketone as soon as they form

  3. ACID CATALYSIS RECALL + .. .. .. + : : : + Acid catalysis enhances the reactivity of the carbonyl group - nucleophilic addition proceeds more easily. :Nu weak nucleophiles can react Water is a weak nucleophile.

  4. for most compounds the equilibrium favors the starting materials and you cannot isolate the hydrate In a reaction where all steps are reversible, the steps in the reverse reaction are the same as those in the forward reaction, reversed! WATER ADDS TO THE CARBONYL GROUP OF ALDEHYDES AND KETONES TO FORM HYDRATES catalyzed by a trace of acid + .. .. .. .. + : : : : : + a hydrate .. .. .. .. .. .. + .. MICROREVERSIBILITY:

  5. ISOTOPE EXCHANGE REVEALS THE PRESENCE OF THE HYDRATE 18 an excess of H2O18 shifts the equilibrium to the right 18 H+ + excess -H2O +H2O18 18 exchange shows the presence of a symmetric intermediate

  6. SOME STABLE HYDRATES these also indicate that hydrates are possible d- d+ d- chloral hydrate chloral d- 120o expected 60o required 109o expected 60o required sp2 sp3 cyclopropanone cyclopropanone hydrate

  7. SOME ADDITIONAL STABLE HYDRATES glyoxal phenylglyoxal

  8. ACETALS AND HEMIACETALS

  9. ACID CATALYSIS RECALL + .. .. .. + : : : + Acid catalysis enhances the reactivity of the carbonyl group - nucleophilic addition proceeds more easily. :Nu weak nucleophiles can react Alcohols are weak nucleophiles.

  10. Addition of Alcohols TWO MOLES OF ALCOHOL WILL ADD addition of one mole H+ hemiacetal addition of second mole H+ an acetal The equilibria normally favor the aldehyde or ketone starting material, but we will show how they can be made.

  11. ACETALS AND HEMIACETALS aldehyde hemiacetal acetal ketone (hemiketal)* (ketal)* *older term *older term

  12. .. + Like a hydronium ion + + .. .. .. + : : : + .. .. .. .. ACID CATALYZED FORMATION OF A HEMIACETAL : first addition .. : Normally the starting material is favored - but a second molecule of alcohol can react if in excess (next slide) + + .. : .. hemiacetal

  13. FORMATION OF THE ACETAL ( from the hemiacetal ) remove + .. .. .. second addition .. .. : .. : + + : SN1 + : : : : .. .. hemiacetal Resonance stabilized carbocation .. .. .. : : + + : : : .. .. acetal

  14. Az WATER SEPARATOR AZEOTROPE Two miscible liquids that distill as a single substance with a boiling point that is lower than either of the original liquids. when cooled, the azeotrope separates benzene 80o C water 100o C benzene-water azeotrope benzene 69.4o C water benzene and water do not mix, but in the azeotrope the vapors (gases) mix and distill together benzene + water

  15. REMOVAL OF WATER SHIFTS THE EQUILIBRIUM ( Le Chatelier Principle ) + + 2 Removal of water shifts equilibrium starting materials are favored

  16. STABILITY OF ACETALS AND HEMIACETALS Most hemiacetals are not stable, except for those of sugars (see later). Acetals are not stable in aqueous acid, but they are stable to aqueous base. H2SO4 AQUEOUS ACID + H2O AQUEOUS BASE NaOH no reaction H2O

  17. CYCLIC ACETALS

  18. Formation of 2,2-Dimethoxypropane THIS IS A NON-CYCLIC ACETAL remove H2O Dry acid = HCl gas HCl in methanol HOTs dry acid = HCl gas or p-toluenesulfonic acid mp 106oC (TsOH)

  19. CYCLIC ACETALS Cyclic acetals can be formed if a bifunctional alcohol is used. 1,2-ethanediol H+ /benzene H2O acetophenone 1,3-propanedithiol H+ /benzene H2O

  20. TARGET NON-TARGET TARGET NON-TARGET PROTECTING GROUP STRATEGY Functional Group 1 Functional Group 2 Add Protecting Group React Unprotected Group NEW GROUP NON-TARGET Remove Protecting Group NEW GROUP NON-TARGET Unchanged Changed

  21. USE OF A CYCLIC ACETAL AS A PROTECTING GROUP The Grignard Reaction Takes Place in Basic Solution - The Acetal is Stable H3O+ Acetals Hydrolyze in Acidic Solution

  22. CARBOHYDRATES AND SUGARS Review Sections 5.14-5.17 Carbohydrate Structures Fischer Projections / D and L

  23. Cyclization of Monosaccharides only sugars seem to make stable hemiacetals a hemiacetal glucose glucopyranose

  24. furan pyran FURANOSE AND PYRANOSE RINGS 4 5 2 1 6 3 a pyranose ring two anomers are possible in each case 4 2 1 3 5 a furanose ring for clarity no hydroxyl groups are shown on the chains or rings

  25. ANOMERS anomeric carbon (hemiacetal) for clarity hydroxyl groups on the chain are not shown anomers differ in configuration at the anomeric carbon

  26. Glucose hemiacetals 34% 66% < 0.001% open chain

  27. HAWORTH PROJECTIONS It is convenient to view the cyclic sugars (glucopyranoses) as a “Haworth Projection”, where the ring is flattened. Standard Position HAWORTH PROJECTION upper-right back This orientation is always used for a Haworth Projection a-D-(+)-glucopyranose

  28. WE WILL LEARN HOW TO CONVERT FISCHER PROJECTIONS TO HAWORTH PROJECTIONS OF EITHER ANOMER GLUCOSE ENANTIOMERS FISCHER HAWORTH D-(+)-glucose L-(-)-glucose

  29. HAWORTH PROJECTIONS HERE ARE SOME CONVENTIONS YOU MUST LEARN 1) The ring is always oriented with the oxygen in the upper right-hand back corner. D 2) The -CH2OH group is placed UP for a D-sugar and DOWN for an L-sugar. L 3) a-Sugars have the -CH2OH group and the anomeric hydroxyl group trans. a 4) b-Sugars have the -CH2OH group and the anomeric hydroxyl group cis. b

  30. GLUCOPYRANOSES

  31. cis = b -CH2OH up = D trans = a -CH2OH up = D SOME HAWORTH PROJECTIONS D-SUGARS b-D ANOMERS a-D BOTH OF THESE ARE D-GLUCOSE

  32. trans = a -CH2OH down=L cis = b -CH2OH down=L SOME HAWORTH PROJECTIONS L-SUGARS a-L ANOMERS b-L BOTH OF THESE ARE L-GLUCOSE

  33. CONVERTING FISCHER PROJECTIONS TO HAWORTH PROJECTIONS

  34. cis = b trans = a on right = D CONVERTING TO HAWORTH PROJECTIONS D-(+)-glucose -CH2OH up = D D O W N U P 1 6 BOTH ANOMERS OF A D-SUGAR (D-glucose) 2 5 3 4 1 4 3 2 5 6 HAWORTH PROJECTIONS FISCHER PROJECTION

  35. cis = b trans = a CONVERTING TO ACTUAL CONFORMATIONS b-D-(+)-glucopyranose -CH2OH up = D HAWORTH CONFORMATION a-D-(+)-glucopyranose

  36. trans = a -CH2OH down=L cis = b HAWORTH PROJECTIONS OF L-SUGARS L-(+)-glucose D O W N U P BOTH ANOMERS OF A L-SUGAR (L-glucose) on left = L HAWORTH PROJECTIONS FISCHER PROJECTION

  37. CONVERTING FISCHER TO HAWORTH PROJECTIONS CAUTION ! Students often get the erroneous impression that all the Haworth rules are reversed for L-sugars - this is not the case! The only difference when converting D- and L- sugars is : These rules are the same for both D- and L- sugars LEFT = UP RIGHT = DOWN b = cis a = trans D-sugars -CH2OH = UP L-sugars -CH2OH = DOWN

  38. AN OPEN CHAIN CAN CONVERT TO EITHER ANOMER FISCHER HAWORTH a-ANOMER OPEN CHAIN b-ANOMER You can’t tell which anomer will result (predominate) when you look at the Fischer Projection. That information is not contained in Fischer Projection.

  39. FRUCTOFURANOSES

  40. FRUCTOSE standard position cis = b up = D 1 .. : anomeric carbon 2 6 3 5 2 .. 4 3 4 1 .. 5 6 b-D-(-)-Fructofuranose D-(-)-Fructose

  41. MUTAROTATION

  42. Glucose hemiacetals 34% 66% < 0.001% open chain

  43. MUTAROTATION +112o pure a-D-(+)-glucopyranose1 [a]D +57.2o 66% b 34% a pure b-D-(+)-glucopyranose2 +19o TIME (min) 1 Obtained by crystallization of glucose at room temperature. 2 Obtained by crystallization of glucose at 980 C.

  44. CONVERSION TO AN ACETAL

  45. any alcohol could be used hemiacetal acetal excess CH3OH dry HCl 3) + ROH 4) -H+ 1) +H+ conversion is via the carbocation 2) - H2O SN1

  46. THE ALCOHOL USED CAN BE ANOTHER SUGAR HO-Sugar a monosaccharide a disaccharide Since sugars have many -OH groups, this can continue on to make “polysaccharides”.

  47. DETOXIFICATION BY THE LIVER In mamalian metabolism, many molecules become glycosylated in the liver to become glycosides. The “glycosides” are more soluble than the original molecule and can be excreted because they are soluble in blood and urine. glucose MOLECULE-OH MOLECULE-O-Glu liver enzymes Glu = glucose a glycoside

  48. POLYSACCHARIDES

  49. enzyme mediated Cellobiose b-1,4-Glycosidic Linkage If continued, you get cellulose. Humans can’t digest b-1,4

  50. enzyme mediated Maltose a-1,4-Glycosidic Linkage Humans can digest a-1,4

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