Typical problems with their Solutions


Problem 1 :

Carbon-Carbon bond length is 1.54A° in propane but 1.46A° in propene_ Account  for this statement.

Solution :

Carbon-carbon double bond is shorter than carbon-carbon, single bond because carbon-carbon, double bond is formed by the overlap of sp2 orbital of one carbon with sp2 -orbital of the another carbon. In other words, the carbon-carbon, double bond is an sp2-sp2 bond. On the other hand, carbon-carbon. single. bond is formed by the overlap of spa orbital of one carbon-atom with same orbital of the another C-atom, hence carbon-carbon, single bond may be considered as sp3-sp3 bond. Since sp2 orbital has less p-character(66%) and more s-character (33%) as compared to sp3orbital (s-character =25%, p-character =75%). it is therefore, a hybrid orbital with more s-character (sp2-orbital) would thus be smaller than a hybrid xbital with less s-character (spa-orbital). Because an s-orbital, compared with a p-orbital, lies closer to the nucleus and is smaller in size. Consequently, the bond formed by sp2-orbital would be shorter.

Due to the above reason, an sp2-spa bond (the carbon-carbon, single bond in propylene)should be shorter (1.501A°) than sp3-sp3 bond (say, the carbon-carbon bond in ethane 1.534A°) similarly, the sp-sp3 bond (says,the carbon carbon single bond) in propene should be still shorter(1.46A° ) as sp3-sp3 bond in propane(1.54A°)

 Note : Explanation is also based on hyperconjugation.

Problem 2 :

Identify the product ‘A’ and ‘B’ in the following reactions (DDUGU)

Solution :

Thus, compound A is Acetic acid & B is Acetaldehyde.

 Problem 3 :

How is the structure of an alkene elucidated by

(i) ozonolysis

(ii) oxidation with KMnO4 (DDUGU)

Solution :

Structure Elucidation of alkenes :

(a) By Ozonolysis Ozonolysis is the best method for locating the position of the double bonds in unknown alkenes. The oxygenated carbons in carbonyl compounds obtained by ozonolysis are the ones that were joined by a double bond in the original alkene. Let, an alkene on ozonolysis gives the following carbonyl compounds-

(ii) By Oxidation with KMnO4 :

(a) Alkenes when react with cold dilute KMnO4 solution form glycols. Glycols are compounds whose molecules contain two hydroxyl groups(-OH groups) on adjacent carbons. This reaction looks like the addition of two -OH groups of H202(HO-OH) to a double bond. Hence, by looking the position of -OH groups in glycols obtained by the oxidation of unknown alkene with cold dilute KMnO4; the position of double bonds can be located.

During the reaction , bright pink colour of KMnO4 disappears. The disappearance of the colour of KMnO4 indicates the presence of carbon-carbon, double bond in unknown-compound.

(b) When an alkene is treated with hot KMnO4 solution, splits at

Hence, on joining the oxygenated carbons of the products obtained by the oxidation with hot concentrated KMnO4 from an alkene, structure of unknown alkene can be determined.

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