Chemistry 304B Spring 1999 Lecture 24 1
Exam: Monday evening, 7:30-10 pm, McCosh 50 Open book, etc. Review sessions: Today: 5-6 pm Rm 124 Frick Sunday: 8-9:30 pm Rm 324 Frick Monday lecture 9 am, Rm 120 Frick NOTE: Extra copies of the handouts, problem sets, and lecture notes are in the Resource Center, Rm 323 Frick. Let me know if they run out.
Polymerization of Bifunctional Acyl Derivatives:
Homopolymer: O O O O O Y X Y Y Y Y
Alternating Copolymer: O Y X + X Y dielectrophile O dinucleophile
O O
Y Y Y Y Y
O O
Example: A Polyester Copolymer Dacron, Mylar
O O O OMe + HO OH O + H2O O O MeO O ethylene n Dimethyl terephthalate glycol
O OMe
+ H2N NH2 MeO O
O O H H N N N n NH2 MeO H O O Example: Polyamide 2 a. homopolymer of a -amino acids
O O H R O H R O N N NH2 NH HO HO N N 2
R R O H R O H R
O R H O R H O
Strong H-bonding between H N N N chains determines the structure 2 N N OH (fibers, helical, etc) e.g. R H O R H O R
O H R O H R O
N N HO N N NH2 R O H R O H R
The polymer of a -amino acids is called a polypeptide; this amide bond is a peptide bond. Large natural polypeptides are proteins
Alternating copolymer:
O NH2 + H2N Cl Cl hexamethylenediamine adipoyl chloride O
O H O
H2N N OH N N H O H O nylon 66
O H O
H2N N OH N N H O H O
O H O
H2N N OH N N H O H O Note: 3
H O caprolactam 2 N O catalyst H
O H O
H2N N O N H H O
nylon 6 Mechanism?
NYLON Synthesis: adipoyl chloride in hexane (non-polar, non-nucleophilic, lighter than water)
1,6-diaminohexane in water
O O Cl Cl Cl Cl O O Hexane POLYMER + HCl
H2N H2N NH2 NH2 H2O
Reaction at the interface of the two immiscible solutions gives a layer of polymer. One can mechanically remove the layer as a strand by hooking into it and pulling up. As the polymer is pulled away, more forms at the interface and a fairly continuous rope of nylon is obtained. Natural Carboxylic Acids: FATTY ACIDS 4
mp
OH 12 lauric acid 44 oC O OH 14 myristic acid 58 oC O OH 16 palmitic acid 63 oC O OH 18 stearic acid 69 oC O 20 OH 77 oC O O
OH 18 oleic acid 13
O
OH 18 -5 linoleic acid
O linolenic acid OH 18 -11
Always an even number of carbon atoms: biosynthesized from acetate (later)
Saturated fatty acids (no double bonds) can "pack" tightly in regular arrays through van der Waals interactions: relatively high mp, solids at room temperature Longer = higher mp
Unsaturated fatty acids pack together less well, lower mp. Mp depends on number and configuration of double bonds.
[Blow-up of fatty acid conformations: p 2 of handout 1 on fatty acids] FATS AND OILS: Triglyceride 5 O
R HO O a. R-CO2H 'R O a triglyceride HO b. R'-CO H 2 O O HO c. R"-CO2H "R glycerol O O
O
O O O
O
[structure of triglycerides: blow-up of structures, p 4 of handout 1 on fatty acids]
Triglycerides are a primary energy storage form.
carbohydrate: CnH2nOn + n O2 ® n CO2 + n H2O fats: approx: C10nH20nOn + (n+x) O2 ® 10n CO2 + 10n H2O More O2 consumed, more energy out
Unsaturated fatty acids are more reactive: the alkene unit is a reactive center. Good feature: easily metabolized more soluble
Oxidative instability of unsaturated fatty acids: easily removed to give a highly resonance stabilized radical 6
H H H H O O• O2 R R • CO2H CO2H
One possible mechanism for oxidative degradation of unsaturated fatty acids.
H R CO H R 2 CO2H
HO O O H
O R CO H R etc 2 OH O odor!
Artificial Fats: Taste like fat but are not broken down to fatty acids and stored. Prepared by synthesis: sucrose + fatty acids (natural ingredients)
O O O O O OO O O O O O O O O
O O O
Olestra O
Enzymes that normally break down fats cannot hydrolyze these esters. Fatty Acids in Biological Membranes: 7
Phospholipids: Polar "head groups" and hydrophobic (non-polar) "tail" (R1, R2)
O O O O R1 R1 O R1 O O O O O O R2 O R R2 O 2 O O O P NH O P N(Me) 3 O P NH3 3 O O O O O O O O a cehpalin a lecithin a phosphatidylserine
R1, R2 = fatty acids
In water, the polar head groups strongly associate with the water, but the non-polar ends groups are driven away, into interaction with themselves. A stable arrangement is a bilayer, with two polar surfaces and a non-polar inside of a "sandwich".
[Blow-up of simple membrane structure: 2 versions]
These provide structure for cell walls. More-or-less rigid depending on the frequence of unsaturated fatty acids in the mix. Also, cholesterol, a very rigid molecule, can be incorporated into the non-polar layer and add rigidity. The membranes can be modified by insertion of proteins which can provide "communication" through the cell wall.
[Blow-up of :Fig 17.4]
The bilayer can fold onto itself, forming a spherical structure with a large cavity = vesicle. The inner core of aqueous solution can carry collections of proteins and other molecules from one location to another inside the cell or between cells. The membranes are permeable and also can "fuse" with other bilayer membranes to spill the contents.