There are few elements capable of forming elementary substances with long-chain molecules. — Organic Chemistry Chemistry Question
Inorganic polymers: polyphosphates and polysilicones
There are few elements capable of forming elementary substances with long-chain molecules.
Give 3 examples of elements, atoms of which can form elementary substance with linear (or close to linear) chain molecules (longer than 10 atoms).
Model Answer
Well known examples are: C (acethylenic carbon), S (various forms of polymeric sulfur), Se (grey selenium), P (red phosphorus), As (black arsenic), Sb (black antimony). Not all of these substances consist of perfectly linear chain molecules, but for sure these elements are capable of forming quite long polymers.
Write down the condensation reaction giving diphosphate from the orthophosphate precursor.
Model Answer
2 HPO4(2-) → P2O7(4-) + H2O (ionization state of the phosphate precursor depends on pH).
In general, condensation reactions are reversible. Write down the equilibrium constant of the condensation reaction between phosphate oligomers, provided that polyphosphate species of different polymerization degree (including monomers) are not kinetically distinguishable. Assume that each (poly)phosphate ion present in the system bears only a single bound proton (i.e. may be represented as Pi O3iOH(i+1)–).
Model Answer
With Pi standing for a polyphosphate with the degree of polymerization of i, for the reaction:
2 Pm-OH + Pn-OH → P_m-O-P_n + H2O
K = [Pm+n][H2O] / ([Pm-OH][Pn-OH])
As polyphosphates of various degrees of polymerization are not distinguishable, each of the concentrations [Pm], [Pn], [Pm+n] can be substituted with the total concentration of all phosphate species [P], thus:
K = [H2O] / [P]
Of the synthetic routes to long-chain polyphosphoric acids listed below, choose the most and the least energetically favorable. Take into account that the P–O bond is macroergic (for instance, ∆G°’ of adenosine triphosphate hydrolysis into adenosine diphosphate and inorganic phosphate is of about –31 kJ/mol).
i) H3PO4 condensation in 1 M aqueous solution at room temperature.
ii) H3PO4 condensation in concentrated solution at room temperature.
iii) H3PO4 condensation with dichlorophosphoric acid HPO2Cl2 at elevated temperature.
Model Answer
The following reasons should be taken into account. First, the free energy of hydrolysis is strongly negative, which means that the free energy of condensation (the reverse reaction) is positive. Thus, the equilibrium constant of an elementary condensation stage is low (less than 1), which is not consistent with the high-polymeric phosphate species. In general, lower equilibrium concentration of (poly)phosphate molecules means that more individual condensations have taken place, which is equivalent to the higher average degree of polymerization of the product. This is true for process ii): lower water concentration (at a certain equilibrium constant value) corresponds to lower equilibrium concentration of phosphate molecules (from the expression derived in i. 27.3). Thus, process ii) is more favorable than i). However, process iii) is the most favorable.
According to the equation, a highly volatile HCl is formed, which is efficiently removed from the reaction mixture by heating. As a result, the equilibrium is shifted rightwards.
Indeed, only route iii) can be applied in practice for the preparation of polyphosphoric acids. Condensation in concentrated solutions (process ii)) is quite slow, and yields significant amounts of polyphosphoric acids only upon heating (molten H3PO4, 230 - 250°C). Direct condensation in dilute solution (process i)) is so unfavorable that may come true only when coupled with a certain exoergic reaction (for instance, substrate phosphorylation in various biochemical processes) with the actual mechanism much more complicated than direct condensation.
Draw the structural formulae of isomeric compounds C2Cl3H5Si if none of these contains Si–H bonds. Write down a scheme of condensation of these compounds (in the presence of water) yielding a long-chain molecule. What are the atoms forming the main chain of the product?
Model Answer
The main chain of the polymer molecule is composed of Si and O atoms:
[VISUAL]
Which of the isomeric compounds C2Cl3H5Si from i. 5 gives the linear condensation product only? Draw the structure of the final condensation product provided all the reactions are by 100% complete. What functional groups may be additionally found in the product due to incomplete hydration or condensation reactions?
Model Answer
The Si-Cl bond is much more reactive than the C-Cl one in hydrolysis and condensation reactions. Thus, A2 can be considered bifunctional in polycondensation reaction, giving a non-branched polymer with the cyclic giant macromolecule of poly(chlorodimethylsiloxane) as the final product when absolutely all Si-Cl bonds are reacted:
[VISUAL]
If hydrolysis of Si-Cl bonds is incomplete, some Cl residues are present in the polymeric product. Incomplete condensation retains a number of OH–groups in the product.
Write down a reaction scheme illustrating appearance of branching in the main chain during condensation of another isomeric compound C2Cl3H5Si from i. 5 (that not chosen in i. 6).
Model Answer
A1 is trifunctional, thus giving rise to a branched polymer:
[VISUAL]