OPTION H - Further organic chemistry

H.1 Stereoisomerism (3h)

Explain the reason for geometrical isomerism in non-cyclic alkenes. The existence of geometric (cis-trans) isomers as the result of restricted rotation around the C=C bond in organic chemistry. Explain the difference in physical and chemical properties of geometrical isomers. Different boiling points for, eg. cis-1,2-dichloroethene, and trans-1,2-dichloroethene, different reactions when heated of, eg. cis- and trans-but-2-ene- 1,4-dioic acid. Describe and explain the reason for geometrical isomerism in (C3 and C4) cyclo-alkanes. Use dichloro-derivatives of cyclopropane and cyclobutane as examples. Restricted rotation because the C-C bond is now part of a cyclic system. Explain the concept of plane-polarized light and describe how it interacts with enantiomers. This should build upon 11.3.3. Explain that optically active compounds exist as enantiomers. Define the term 'racemic mixture'. Describe the similarities and differences in the physical and chemical properties of enantiomers.

H.2 Free radical substitution reactions (3h)

Explain the gas phase reactions of alkanes and methylbenzene with halogens in terms of free radical reactions. Students should be able to recall the mechanism for the reaction of methane and methylbenzene with chlorine, and identify the initiation, propagation and termination steps. Explain how the gas phase reactions of chloroalkanes and other molecules affect the level of ozone in the atmosphere. This is an excellent opportunity to bring in how the free radical reactions in the atmosphere result in ozone depletion.
 

H.3 Electrophilic addition reactions (4h)

Describe and explain the electrophilic reactions of symmetrical alkenes.

A stepwise mechanistic approach is required. Reacting species should include halogens, mixed halogens and hydrogen halides. State and explain the relative stabilities of carbocations. Consideration should be restricted to the stability of CH3+, (CH3)2CH+, (CH3)3C+ and Ph+ only.
 

H.4 Electrophilic substitution reactions (4h)

Describe and explain the mechanism for the nitration of benzene. A stepwise electrophilic aromatic substitution mechanism is required. Students should be able to recall the formation of NO2+. Describe and explain the Friedel Crafts reactions of benzene and methylbenzene. Examples should include chlorination and alkylation with FeCl3 or AlCl3 as catalysts. Describe and explain the directing effects and relative rates of different substituents on a benzene ring.

Examples should be restricted to -CH3, -OH, -Cl, -NO2, -CO2CH3. The reaction of phenol with chlorine to form trichlorophenol (TCP) should be covered.
 

H.5 Nucleophilic addition reactions (1h)

Describe and explain the mechanism for the addition of hydrogen cyanide to alkanals and alkanones, followed by hydrolysis to give alkanoic acids.
 

H.6 Nucleophilic substitution reactions (1.5h)

Describe how the relative rate of nucleophilic substitution is affected by different nucleophiles. Eg. relative rate of reaction using hydroxide or water (polarity differences). Describe inductive and steric effects of substituents on substitution reaction. Describe and explain the relative rates of hydrolysis of halogenated benzene compounds. Compare the inertness of halogenated benzene compounds towards substitution relative to halogenoalkanes. Relate to 21.3
 

H.7 Elimination reactions (2h)

Describe and explain the mechanism for the elimination of water from alkanols. Describe and explain the mechanism for the elimination of HBr from bromoalkanes. It should be pointed out that under different conditions the same reactants can undergo either nucleophilic substitution or elimination, eg. 1-bromobutane with OH-.
 

H.8 Addition - elimination reactions (1.5h)

Describe and explain the reactions of 2,4-dinitro phenylhydrazine with alkanals and alkanones.
 

H.9 Acid-base reactions (2h)
 

Discuss and explain the acidic properties of phenol and substituted phenols in terms of bonding. Compare the acidity of phenol with other alkanols. Compare the acidity of 2,4,6 - trinitrophenol with phenol. Discuss and explain the acidic properties of substituted alkanoic acids in terms of bonding. Compare and explain the relative basicities of ammonia, amines and amides. Lone pair on nitrogen, effect of alkyl group (increased Kb). Formation of salts, liberation of the amine with NaOH. Cross reference to topic 19.


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