Question

1. Cyclopentadiene rapidly dimerizes at room temperature to dicyclopentadiene, and must be fractionally distilled immediately before use in this reaction. Explain in terms of thermodynamics (G, S and H) why the equilibrium for the dimerization of cyclopentadiene to dicyclopentadiene lies with the product at room temperature and with the starting materials at elevated temperatures. Consider the following in your answer: Given that two -bonds in the starting material are replaced with one-bond and two -bonds in the product, what is the expected sign (+ or -) for H for the forward reaction? Given that two molecules in the starting materials give one product molecule, what is the expected sign for S? How is G calculated from Hand S? In what position does the equilibrium of a reaction (magnitude of K) lie based on the sign (positive or negative) of G? What reaction condition will alter the equilibrium between cyclopentadiene and dicyclopentadiene?

Answer 1

1. The equilib the equilibroicum foro the dimerisation of Cyclopen-ladiene to heuyclopentadiene lieu coith the produet at room temperature at with starfing materials at elevated temperatures, 2 Elevated (47 + Qx]cycloadition temp. reaction (1) Enthalpy foctoro (1H): Two T-bands in the starting material diminish coith the foormation of two new Sigmabond (c-co-band) in the product, De > Newly genanated - bondes. Since the bond strength of Sigma bonds are highers due to direct ad on. Overlap of the two carbon orbitals than that of the A-interactions rence, relatively a highero amount of energy will Nelease due to the formation of 28-bonds in prooduct with nespect to the ñ bonds in neactant. 0; for the foron and reaction, AH 2-ve tü) Errtroopy factors (45): As two molecules of Cyclopenta. diene combiness to from one molecule of dieyclopenta- diene, the disorderness in the foroward reactions sie product fromatind deeneabel, Entropy change in the forward reaction will - be negative. Asa - ve, feas forrow and reaction cuí) from thermodynamico, the Gibbs prace energy Can be expressed in terms of Hands accondingly

G> H- TS , To temperature at a particular temperature the change in in Gibbs froce enenge 4G 2 AH-TAS If as become catre; AGo-k in foroward reaction, the forward reaction will occeer spontaneously and the equilibrium will lie toward product formation, • Proo forowand soeantimi: AH=-ve 45terve 493-AAH + TAS "AG will be negative if AH Y TAS Thereforre at room temperature cohen. I is no nelatively Small AH term can overcome the magnitude of TAS : term. Hence, at room tempenature product formation favowers.. Porno At elevated temperature the equilibrium lies towands neactant because in this case, AS = tve (dismodernese inen sanas) " TAS term is the majos contributing team to make AG? Ve. .