Dendrimers are fascinating highly branched macromolecules with tree-like structures. One method of p — Organic Chemistry Chemistry Question
Dendrimers: Tree-like Macromolecules
Dendrimers are fascinating highly branched macromolecules with tree-like structures. One method of preparation of these compounds makes use of the Michael addition reaction, a simple example of which is shown below:
[VISUAL]
A dendrimer can be obtained by the following sequence of reactions:
(1) NH3 is treated exhaustively with an excess of acrylonitrile to give a product that contains 3 cyanide groups.
(2) This product is reduced catalytically with H2 and a catalyst to produce a molecule with three primary amines.
(3) This primary amine is treated again with an excess of acrylonitrile.
(4) The product of step (3) is again hydrogenated with H2 and a catalyst to give a hexa-amine. This is the beginning of a branched macromolecule.
The sequence of treatment with acrylonitrile and subsequent reduction of the cyanide group can be repeated several times, leading finally to a spherical type of molecule with primary amino groups on the surface.
(a) Give the equation for the reaction of step (1).
(b) Give the equation for the reaction of step (2).
(c) Give the structure for the product from step (3).
(d) Give the structure of the hydrogenated product from step (4).
Model Answer
(a) NH3 + H2C═CHCN → N(CH2CH2C≡N)3
(b) N(CH2CH2C≡N)3 + 6 H2 → N(CH2CH2CH2NH2)3
(c) N[CH2CH2CH2N(CH2CH2CN)2]3
(d) [VISUAL] N[CH2CH2CH2N(CH2CH2CH2NH2)2]3
Calculate the amount of primary amine end groups that are present in the dendrimer after 5 full cycles. (The first cycle consists of steps 1 + 2.)
Model Answer
After the first cycle there are 3 amine groups. Then the number of amine groups is doubled after each cycle. Thus, after 5 full cycles the total number of amine end groups is 48.
(a) Calculate the amount of hydrogen (in moles per one mole of NH3) which is needed for 5 cycles.
(b) Calculate the number of moles of acrylonitrile which is needed for 5 cycles.
(c) The dendrimer increases in diameter by about 10 Å per cycle. Calculate the volume of the dendrimer after 5 cycles.
Model Answer
(a) After 5 full cycles 3 + (3 × 2) + (6 × 2) + (12 × 2) + (24 × 2) = 93 mol of H2 have been used.
(b) Idem for acrylonitrile (93 mol).
(c) Radius = diameter / 2 = 50 / 2 = 25 Å. Volume: 4/3 π r³.