Chapter 12 Organic Chemistry Some Basic Principles and Techniques Solutions
Question - 11 : - Draw the resonancestructures for the following compounds. Show the electron shift usingcurved-arrow notation.
(a) C6H5OH(b) C6H5NO2 (c) CH3CH = CH –CHO
(d) C6H5CHO(e)
(f) 
Answer - 11 : -
(a) The structure of C6H5OHis:
The resonatingstructures of phenol are represented as:
(b) The structure of C6H5NO2 is:
The resonatingstructures of nitro benzene are represented as:
(c) CH3CH =CH – CHO
The resonatingstructures of the given compound are represented as:.
(d) The structure of C6H5CHOis:
The resonatingstructures of benzaldehyde are represented as:
(e) C6H5CH2⊕
The resonatingstructures of the given compound are:
(f) CH3 CH= CH CH2⊕
The resonatingstructures of the given compound are:
Question - 12 : - What are electrophilesand nucleophiles? Explain with examples.
Answer - 12 : -
An electrophileis a reagent that takes away an electron pair. In other words, anelectron-seeking reagent is called an electrophile (E+).Electrophiles are electron-deficient and can receive an electron pair.
Carbocations
and neutral molecules having functional groups such as carbonyl group (
) are examples ofelectrophiles.A nulceophile is areagent that brings an electron pair. In other words, a nucleus-seeking reagentis called a nulceophile (Nu:).
For example: OH–,NC–, carbanions (R3C–), etc.
Neutral molecules suchas H2Ö and ammonia also act as nulceophiles because of the presenceof a lone pair.
Question - 13 : - Identify the reagentsshown in bold in the following equations as nucleophiles or electrophiles:
Answer - 13 : -
Electrophiles areelectron-deficient species and can receive an electron pair. On the other hand,nucleophiles are electron-rich species and can donate their electrons.
Here, HO– actsas a nucleophile as it is an electron-rich species, i.e., it is anucleus-seeking species.
Here, –CNacts as a nucleophile as it is an electron-rich species, i.e., it is anucleus-seeking species.
Here, 
acts as an electrophileas it is an electron-deficient species.
Question - 14 : - Classify the followingreactions in one of the reaction type studied in this unit.
(a) CH3CH2Br+ HS– → CH3CH2SH + Br–
(b) (CH3)2 C= CH2 + HCl → (CH3)2 ClC–CH3
(c) CH3CH2Br+ HO– → CH2 = CH2 + H2O+ Br–
(d) (CH3)3 C– CH2 OH + HBr → (CH3)2 CBrCH2CH3 +H2O
Answer - 14 : -
(a) It is an example ofsubstitution reaction as in this reaction the bromine group in bromoethane issubstituted by the –SH group.
(b) It is an example ofaddition reaction as in this reaction two reactant molecules combine to form asingle product.
(c) It is an example ofelimination reaction as in this reaction hydrogen and bromine are removed frombromoethane to give ethene.
(d) In this reaction,substitution takes place, followed by a rearrangement of atoms and groups ofatoms.
Question - 15 : - What is the relationshipbetween the members of following pairs of structures? Are they structural orgeometrical isomers or resonance contributors?
(a)
(b)
(c)
Answer - 15 : -
(a) Compounds havingthe same molecular formula but with different structures are called structuralisomers. The given compounds have the same molecular formula but they differ inthe position of the functional group (ketone group).
In structure I, ketonegroup is at the C-3 of the parent chain (hexane chain) and in structure II,ketone group is at the C-2 of the parent chain (hexane chain). Hence, the givenpair represents structural isomers.
(b) Compounds having thesame molecular formula, the same constitution, and the sequence of covalentbonds, but with different relative position of their atoms in space are calledgeometrical isomers.
In structures I and II,the relative position of Deuterium (D) and hydrogen (H) in space are different.Hence, the given pairs represent geometrical isomers.
(c) The givenstructures are canonical structures or contributing structures. They arehypothetical and individually do not represent any real molecule. Hence, thegiven pair represents resonance structures, called resonance isomers.
Question - 16 : - For the following bondcleavages, use curved-arrows to show the electron flow and classify each ashomolysis or heterolysis. Identify reactive intermediate produced as freeradical, carbocation and carbanion.
(a)
(b)
(c)
(d)
Answer - 16 : -
(a) The bond cleavage usingcurved-arrows to show the electron flow of the given reaction can berepresented as
It is an example ofhomolytic cleavage as one of the shared pair in a covalent bond goes with thebonded atom. The reaction intermediate formed is a free radical.
(b) The bond cleavageusing curved-arrows to show the electron flow of the given reaction can berepresented as
It is an example ofheterolytic cleavage as the bond breaks in such a manner that the shared pairof electrons remains with the carbon of propanone. The reaction intermediateformed is carbanion.
(c) The bond cleavageusing curved-arrows to show the electron flow of the given reaction can berepresented as
It is an example ofheterolytic cleavage as the bond breaks in such a manner that the shared pairof electrons remains with the bromine ion. The reaction intermediate formed isa carbocation.
(d) The bond cleavageusing curved-arrows to show the electron flow of the given reaction can berepresented as
It is a heterolyticcleavage as the bond breaks in such a manner that the shared pair of electronsremains with one of the fragments. The intermediate formed is a carbocation.
Question - 17 : - Explain the termsInductive and Electromeric effects. Which electron displacement effect explainsthe following correct orders of acidity of the carboxylic acids?
(a) Cl3CCOOH> Cl2CHCOOH > ClCH2COOH
(b) CH3CH2COOH> (CH3)2CHCOOH > (CH3)3C.COOH
Answer - 17 : -
Inductive effect
The permanentdisplacement of sigma (σ) electrons along a saturated chain, whenever anelectron withdrawing or electron donating group is present, is called inductiveeffect.
Inductive effect couldbe + I effect or – I effect. When an atom or group attracts electrons towardsitself more strongly than hydrogen, it is said to possess – I effect. Forexample,
When an atom or groupattracts electrons towards itself less strongly than hydrogen, it is said topossess + I effect. For example,
Electrometric effect
It involves the completetransfer of the shared pair of π electrons to either of the two atoms linked bymultiple bonds in the presence of an attacking agent. For example,
Electrometric effectcould be + E effect or – E effect.
+ E effect: When the electronsare transferred towards the attacking reagent
– E effect: When the electrons aretransferred away from the attacking reagent
(a) Cl3CCOOH> Cl2CHCOOH > ClCH2COOH
The order of acidity canbe explained on the basis of Inductive effect (– I effect). As the number ofchlorine atoms increases, the – I effect increases. With the increase in – Ieffect, the acid strength also increases accordingly.
]
(b) CH3CH2COOH> (CH3)2 CHCOOH > (CH3)3 C.COOH
The order of acidity canbe explained on the basis of inductive effect (+ I effect). As the number ofalkyl groups increases, the + I effect also increases. With the increase in + Ieffect, the acid strength also increases accordingly.
Question - 18 : - Give a brief descriptionof the principles of the following techniques taking an example in each case.
(a) Crystallisation (b)Distillation (c) Chromatography
Answer - 18 : -
(a) Crystallisation
Crystallisation is oneof the most commonly used techniques for the purification of solid organiccompounds.
Principle: It is based on thedifference in the solubilites of the compound and the impurities in a givensolvent. The impure compound gets dissolved in the solvent in which it issparingly soluble at room temperature, but appreciably soluble at highertemperature. The solution is concentrated to obtain a nearly saturatedsolution. On cooling the solution, the pure compound crystallises out and isremoved by filtration.
For example, pureaspirin is obtained by recrystallising crude aspirin. Approximately 2 – 4 g ofcrude aspirin is dissolved in about 20 mL of ethyl alcohol. The solution isheated (if necessary) to ensure complete dissolution. The solution is then leftundisturbed until some crystals start to separate out. The crystals are thenfiltered and dried.
(b) Distillation
This method is used toseparate volatile liquids from non-volatile impurities or a mixture of thoseliquids that have a sufficient difference in their boiling points.
Principle: It is based on thefact that liquids having different boiling points vapourise at differenttemperatures. The vapours are then cooled and the liquids so formed arecollected separately.
For example, a mixtureof chloroform (b.p = 334 K) and aniline (b.p = 457 K) can be separated by themethod of distillation. The mixture is taken in a round bottom flask fittedwith a condenser. It is then heated. Chloroform, being more volatile, vaporizesfirst and passes into the condenser. In the condenser, the vapours condense andchloroform trickles down. In the round bottom flask, aniline is left behind.
(c) Chromatography
It is one of the mostuseful methods for the separation and purification of organic compounds.
Principle: It is based on thedifference in movement of individual components of a mixture through thestationary phase under the influence of mobile phase.
For example, a mixtureof red and blue ink can be separated by chromatography. A drop of the mixtureis placed on the chromatogram. The component of the ink, which is less adsorbedon the chromatogram, moves with the mobile phase while the less adsorbedcomponent remains almost stationary.
Question - 19 : - Describe the method,which can be used to separate two compounds with different solubilities in asolvent S.
Answer - 19 : -
Fractionalcrystallisation is the method used for separating two compounds with differentsolubilities in a solvent S. The process of fractional crystallisation iscarried out in four steps.
(a) Preparation of the solution: The powdered mixture is taken in a flask and the solvent isadded to it slowly and stirred simultaneously. The solvent is added till thesolute is just dissolved in the solvent. This saturated solution is thenheated.
(b) Filtration of the solution: The hot saturated solution is then filtered through a filterpaper in a China dish.
(c) Fractional crystallisation: The solution in the China dish is now allowed to cool. The lesssoluble compound crystallises first, while the more soluble compound remains inthe solution. After separating these crystals from the mother liquor, thelatter is concentrated once again. The hot solution is allowed to cool andconsequently, the crystals of the more soluble compound are obtained.
(d) Isolation and drying: Thesecrystals are separated from the mother liquor by filtration. Finally, the crystalsare dried.
Question - 20 : - What is the differencebetween distillation, distillation under reduced pressure and steamdistillation ?
Answer - 20 : -
The differences amongdistillation, distillation under reduced pressure,
and steam distillation aregiven in the following table.
| Distillation | Distillation under reduced pressure | Steam distillation |
| 1. | It is used for the purification of compounds that are associated with non-volatile impurities or those liquids, which do not decompose on boiling. In other words, distillation is used to separate volatile liquids from non-volatile impurities or a mixture of those liquids that have sufficient difference in boiling points. | This method is used to purify a liquid that tends to decompose on boiling. Under the conditions of reduced pressure, the liquid will boil at a low temperature than its boiling point and will, therefore, not decompose. | It is used to purify an organic compound, which is steam volatile and immiscible in water. On passing steam, the compound gets heated up and the steam gets condensed to water. After some time, the mixture of water and liquid starts to boil and passes through the condenser. This condensed mixture of water and liquid is then separated by using a separating funnel. |
| 2. | Mixture of petrol and kerosene is separated by this method. | Glycerol is purified by this method. It boils with decomposition at a temperature of 593 K. At a reduced pressure, it boils at 453 K without decomposition. | A mixture of water and aniline is separated by steam distillation. |