Organic chemistry
While it is possible that this reaction occurs by a direct mechanism involving the simultaneous collision of three molecules, this is extremely unlikely because ternary (three-body) collisions are extremely rare. So a multi-step mechanism needs to be sought. A possible mechanism that accounts for the known facts involves the formation of a dimer, N2O2, as is shown below: 2NO
fast
N2O2
rate = k1f [NO]2
N2O2
fast
2NO
rate = k1b [N2O2]
N2O2 + O2
slow
2NO2
rate = k2[N2O2][O2]
The fact that the overall reaction is first order in oxygen makes it inevitable that the third equation must be the rate-determining step, as it alone involves the concentration of oxygen. Details of this mechanism and how it can lead to an (experimental) rate equation lie well beyond the scope of the specification. However, an outline is given in the Notes, shown opposite.
3.3
Notes The first two reactions are in equilibrium, with forward ( f ) and backward (b) rate constants. Forward and backward rates are equal, so: k1f[NO]2 = k1b[N2O2] k1f and [N2O2] = ⎯⎯ [NO]2 k1b Substitution yields: k1f rate = ⎯⎯ k2[NO]2[O2] k1b as required by experiment.
Organic chemistry
3.3.7 Optical isomerism Structural isomerism was considered in (see Collins Student Support Materials: A-Level year 1 – Organic and Relevant Physical Chemistry, section 3.3.1.3). and includes chain isomerism, position isomerism and functional group isomerism:
Chain isomerism occurs when there are two or more ways of arranging the carbon skeleton of a molecule.
Position isomerism occurs when the isomers have the same carbon skeleton, but the functional group is attached at different places on the chain.
Functional group isomerism occurs when different functional groups are present in compounds which have the same molecular formula.
Stereoisomerism The two types of stereoisomerism are E–Z stereoisomerism (see Collins Student Support Materials: A-Level year 1 – Organic and Relevant Physical Chemistry, section 3.3.1.3) and optical isomerism.
Definition Stereoisomers are compounds which have the same structural formula but the bonds are arranged differently in space.
Notes When three or four different groups are attached to a C== C bond, Cahn–Ingold– Prelog (CIP) priority rules are used. These rules assign the priorities for use when naming such compounds. They involve looking at the two atoms attached directly to the left-hand carbon of a double bond, and giving priority to the atom with the highest atomic number. Similarly, for the right-hand carbon of the double bond, the atom with the highest atomic number is assigned the highest priority. CIP priority rules are discussed in more detail in Collins Student Support Materials: A-Level year 1 – Organic and Relevant Physical Chemistry, section 3.3.1.3.
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