Question

Introduction Many reactions can take more than one course but the preferential formation of only one of the possible products may be attained by a judicial choice of the reaction conditions. In general, the 1 of 5 CHEM-2300 F19 Experiment 6 formation of a product is controlled by thermodynamic (differences in relative potential energies of products and reactants (translation: relative stabilities}) and kinetic factors (differences in activation energies of involved elementary reaction steps {translation: relative rates?) as well as specific reaction conditions (e.g. Le Châtelier's principle). For some reactions the thermodynamically more stable product is not the product that is generated the fastest. In this particular case, the outcome of the reaction can be controlled by choosing conditions that favour either the thermodynamically or the kinetically preferred product. Higher reaction temperatures generally favour the thermodynamically preferred product because all possible forward and reverse reactions can take place as sufficient activation energy is available (so an equilibrium is reached). The kinetic product is preferentially formed if the reaction temperature is kept so low that only the fastest formed product (lowest activation energy) is formed at a reasonable rate. In this experiment, a mixture of furan-2-carboxaldehyde (2-furaldehyde) and cyclohexanone is reacted with semicarbazide, a derivative of hydrazine (H2N-NH2), to form semicarbazones. These are imine-type functional groups, which will be seen in some detail in the 59-235 course. This type of reaction can occur at all pH's but is fastest at slightly acidic pH (ca. pH 5). Both reactions are fully reversible at sufficiently high temperature, which means the thermodynamically more stable product is formed preferentially. However, at sufficiently low temperature the more reactive carbonyl group (aldehyde or ketone) will react preferentially (kinetic control) but a too low temperature should be avoided as the rate of the reaction decreases by about 50% for every 10 Kelvin (or 10°C) the temperature is lowered. Tho. oo. How at thically Chain Ln. O witte + 440 The objective of this experiment is to determine if either the aldehyde or the ketone preferentially reacts at the three tested temperatures, which may indicate a change from kinetic to thermodynamic control as the temperature is increased. The melting point and colour of the crystalline product mixtures are used as first indicators since the semicarbazone of 2-furaldehyde is yellow and has a melting point of 202 C while the semicarbazone of cyclohexanone is colourless and has a melting point of 166 °C.

3) Which semicarbazone is colourless and which is yellow. Why do they differ in this property? In general, what gives a compound colour? (Isn’t chemistry cool?)

Answer 1

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