Sunday, August 19, 2007

Elimination reactions

Material relevant to main material blog

1. When heated with H-2SO-4, 2 degree alcohols undergo dehydration via an E1 mechanism.
The major product is the tri-substituted alkene, methylcyclohexene

2. When heated with strong bases such as NaOEt, alkyl bromides undergo E2 elimination.
The outcome of E2 reactions is dependent on the antiperiplanar arrangement of the C-H and C-LG bonds. For substituted cyclohexanes this requires that the LG be axial.
For the cis-isomer with the -Br axial, the more highly susbtituted alkene can be formed by removal of the H adjacent to the methyl group.
For the trans-isomer, when the -Br is axial the methyl group is also axial. Therefore the elimination must occur from the C3-H bond giving the anti-Zaitsev product.

The reactive conformation is an unfavourable diaxial conformer, therefore the reaction will be slower than that of the cis-isomer.

3. The lowest energy conformation of menthyl chloride has the chlorine atom in a equatorial postion.
In this position there is no antiperiplanar H , ring flip is difficult as it would require to formation of a triaxial conformer.
In contrast, in neomenthyl chloride, the lowest energy confromation has the chlorine atom axial with 2 H in the correct orientation to give the products. The major product is the more highly substituted alkene.

Question bank in chemistry

Thursday, August 16, 2007

Q.S. 2 Alkanes – Simple Questions (S.Q.S. 1)

1. What happens to boiling point of alkanes as molecular mass increases?
2. What happens to boiling point of branched isomers of alkanes?
3. Under what conditions does monochlorination of methane take place?
4. What is the `reaction in Kolbe Electrolytic method of preparing alkanes?
5. What are the reactants and products in hydrolysis of Grignard reagent?
6. In the case of halogenation of alkanes, what is order of reactivity among chlorine, fluorine, and bromine?
7. At ordinary temperatures, why is ultraviolet radiation required to initiate a reaction between halogens and alkanes?
8. What do you understand by 1 degree H, 2 degree H and 3 degree H?
9. What is the difference in reactions between an alkane and fluorine and an alkane and bromine?
10. What is the major product in reaction between bromine and n-butane?
11. What are the products of reaction between n-butane and chlorine at room temperature and sunlight?
12. How do you get iso-butane from n-butane?
13. When do you get aromatic compounds from alkanes?
14. What is the more common name of Methane?
15. What is % composition of Methane in coal gas?
16. How do you produce methane from aluminium chloride?
17. What are the reactants and products of Wurtz reaction?

Answer these questions. This will help you to answer other complex ways of asking questions like MCQs, multiple matching, passage based questions and fact & cause questions.