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CBSE—XI----Chemistry-----U-12

ORGANIC CHEMISTRY – SOME BASIC PRINCIPLES AND TECHNIQUES

Solved problems of NCERT Text Book-2006

Problem 12.1
How many and bonds are present in
each of the following molecules?
(a) HC≡CCH=CHCH3 (b) CH2=C=CHCH3

Problem 12.2
What is the type of hybridisation of each
carbon in the following compounds?
(a) CH3Cl, (b) (CH3)2CO, (c) CH3CN,
(d) HCONH2, (e) CH3CH=CHCN

Problem 12.3
Write the state of hybridisation of carbon
in the following compounds and shapes
of each of the molecules.
(a) H2C=O, (b) CH3F, (c) HC≡N.

Problem 12.4
Expand each of the following condensed
formulas into their complete structural
formulas.
(a) CH3CH2COCH2CH3
(b) CH3CH=CH(CH2)3CH3

Problem 12.5
For each of the following compounds,
write a condensed formula and also their
bond-line formula.
(a) HOCH2CH2CH2CH(CH3)CH(CH3)CH3
(b)
(a)

Problem 12.6
Expand each of the following bond-line
formulas to show all the atoms including
carbon and hydrogen

(b)

(d)

Problem 12.7
Structures and IUPAC names of some
hydrocarbons are given below. Explain
why the names given in the parentheses
are incorrect.
2,5,6- Trimethyloctane
[and not 3,4,7-Trimethyloctane]
3-Ethyl-5-methylheptane
[and not 5-Ethyl-3-methylheptane]

Problem 12.8
Write the IUPAC names of the compounds
i-iv from their given structures.

Problem 12.9
Derive the structure of (i) 2-Chlorohexane,
(ii) Pent-4-en-2-ol, (iii) 3- Nitrocyclohexene,
(iv) Cyclohex-2-en-1-ol, (v) 6-Hydroxyheptanal.

Problem 12.10
Write the structural formula of:
(a) o-Ethylanisole, (b) p-Nitroaniline,
(c) 2,3 - Dibromo -1 - phenylpentane,
(d) 4-Ethyl-1-fluoro-2-nitrobenzene.

Problem 12.11
Using curved-arrow notation, show the
formation of reactive intermediates when
the following covalent bonds undergo
heterolytic cleavage.
(a) CH3–SCH3, (b) CH3–CN, (c) CH3–Cu

Problem 12.12
Giving justification, categorise the
following molecules/ions as nucleophile
or electrophile:

Problem 12.13
Identify electrophilic centre in the
following: CH3CH=O, CH3CN, CH3I.

Problem 12.14
Which bond is more polar in the following
pairs of molecules: (a) H3C-H, H3C-Br
(b) H3C-NH2, H3C-OH (c) H3C-OH,
H3C-SH

Problem 12.15
In which C–C bond of CH3CH2CH2Br, the
inductive effect is expected to be the
least?


Problem 12.16
Write resonance structures of CH3COO–
and show the movement of electrons by
curved arrows.

Problem 12.17
Write resonance structures of
CH2=CH–CHO. Indicate relative stability of
the contributing structures.

Problem 12.18
Explain why the following two structures,
I and II cannot be the major contributors
to the real structure of CH COOCH3.

Problem 12.19
Explain why (CH3)3C
+ is more stable than
+ +
3 2 3 CH CH andCH is the least stable
cation.

Problem 12.20
On complete combustion, 0.246 g of an
organic compound gave 0.198g of carbon
dioxide and 0.1014g of water. Determine
the percentage composition of carbon
and hydrogen in the compound.

Problem 12.21
In Dumas’ method for estimation of
nitrogen, 0.3g of an organic compound
gave 50mL of nitrogen collected at 300K
temperature and 715mm pressure.
Calculate the percentage composition of
nitrogen in the compound. (Aqueous
tension at 300K=15 mm)

Problem 12.22
During estimation of nitrogen present in
an organic compound by Kjeldahl’s
method, the ammonia evolved from 0.5
g of the compound in Kjeldahl’s
estimation of nitrogen, neutralized 10 mL
of 1 M H2SO4. Find out the percentage
of nitrogen in the compound.


Problem 12.23
In Carius method of estimation of
halogen, 0.15 g of an organic compound
gave 0.12 g of AgBr. Find out the
percentage of bromine in the compound.


Problem 12.24
In sulphur estimation, 0.157 g of an
organic compound gave 0.4813 g of
barium sulphate. What is the
percentage of sulphur in the compound?
.

EXERCISES
12.1 What are hybridisation states of each carbon atom in the following compounds ?
CH2=C=O, CH3CH=CH2, (CH3)2CO, CH2=CHCN, C6H6

12.2 Indicate the and bonds in the following molecules :
C6H6, C6H12, CH2Cl2, CH2=C=CH2, CH3NO2, HCONHCH3

12.3 Write bond line formulas for : Isopropyl alcohol, 2,3-Dimethyl butanal, Heptan-4-
one.

12.4 Give the IUPAC names of the following compounds :
(a) (b) (c)
(d) (e) (f) Cl2CHCH2OH

12.5 Which of the following represents the correct IUPAC name for the compounds
concerned ? (a) 2,2-Dimethylpentane or 2-Dimethylpentane (b) 2,4,7-
Trimethyloctane or 2,5,7-Trimethyloctane (c) 2-Chloro-4-methylpentane or
4-Chloro-2-methylpentane (d) But-3-yn-1-ol or But-4-ol-1-yne.
362 CHEMISTRY

12.6 Draw formulas for the first five members of each homologous series beginning with
the following compounds. (a) H–COOH (b) CH3COCH3 (c) H–CH=CH2

12.7 Give condensed and bond line structural formulas and identify the functional
group(s) present, if any, for :
(a) 2,2,4-Trimethylpentane
(b) 2-Hydroxy-1,2,3-propanetricarboxylic acid
(c) Hexanedial

12.8 Identify the functional groups in the following compounds
(a) (b) (c)

12.9 Which of the two: O2NCH2CH2O– or CH3CH2O– is expected to be more stable and
why ?

12.10 Explain why alkyl groups act as electron donors when attached to a system.

12.11 Draw the resonance structures for the following compounds. Show the electron
shift using curved-arrow notation.
(a) C6H5OH (b) C6H5NO2 (c) CH3CH=CHCHO (d) C6H5–CHO (e) 6 5 2 C H CH


(f) 3 2 CH CH CHCH



12.12 What are electrophiles and nucleophiles ? Explain with examples.

12.13 Identify the reagents shown in bold in the following equations as nucleophiles or
electrophiles:
(a) 3 3 2 CH COOH CH COO H O •– HO
(b) 3 3 32 CH COCH CH C CN OH •

CN
(c) 6 5 6 5 3 C H C H COCH •
+
3 CH CO

12.14 Classify the following reactions in one of the reaction type studied in this unit.
(a)
3 2 3 2 CH CH Br HS CH CH SH •
(b) 3 2 3 2 2 CH C CH HCl CH ClC •
(c)
3 2 2 2 2 CH CH Br HO CH CH H O •
(d) 3 2 3 2 3 2 CH C CH OH HBr CH CBrCH CH •

12.15 What is the relationship between the members of following pairs of structures ?
Are they structural or geometrical isomers or resonance contributors ?
(a)
363 ORGANIC CHEMISTRY – SOME BASIC PRINCIPLES AND TECHNIQUES
(b)
(c)

12.16 For the following bond cleavages, use curved-arrows to show the electron flow
and classify each as homolysis or heterolysis. Identify reactive intermediate
produced as free radical, carbocation and carbanion.
(a)
(b)
(c)
(d)

12.17 Explain the terms Inductive and Electromeric effects. Which electron displacement
effect explains the following correct orders of acidity of the carboxylic acids?
(a) Cl3CCOOH > Cl2CHCOOH > ClCH2COOH
(b) CH3CH2COOH > (CH3)2CHCOOH > (CH3)3C.COOH

12.18 Give a brief description of the principles of the following techniques taking an
example in each case.
(a) Crystallisation (b) Distillation (c) Chromatography

12.19 Describe the method, which can be used to separate two compounds with different
solubilities in a solvent S.

12.20 What is the difference between distillation, distillation under reduced pressure
and steam distillation ?

12.21 Discuss the chemistry of Lassaigne’s test.

12.22 Differentiate between the principle of estimation of nitrogen in an organic compound
by (i) Dumas method and (ii) Kjeldahl’s method.

12.23 Discuss the principle of estimation of halogens, sulphur and phosphorus present
in an organic compound.

12.24 Explain the principle of paper chromatography.

12.25 Why is nitric acid added to sodium extract before adding silver nitrate for testing
halogens?

12.26 Explain the reason for the fusion of an organic compound with metallic sodium
for testing nitrogen, sulphur and halogens.

12.27 Name a suitable technique of separation of the components from a mixture of
calcium sulphate and camphor.

12.28 Explain, why an organic liquid vaporises at a temperature below its boiling point
in its steam distillation ?

12.29 Will CCl4 give white precipitate of AgCl on heating it with silver nitrate? Give
reason for your answer.
364 CHEMISTRY

12.30 Why is a solution of potassium hydroxide used to absorb carbon dioxide evolved
during the estimation of carbon present in an organic compound?

12.31 Why is it necessary to use acetic acid and not sulphuric acid for acidification of
sodium extract for testing sulphur by lead acetate test?

12.32 An organic compound contains 69% carbon and 4.8% hydrogen, the remainder
being oxygen. Calculate the masses of carbon dioxide and water produced when
0.20 g of this substance is subjected to complete combustion.

12.33 A sample of 0.50 g of an organic compound was treated according to Kjeldahl’s
method. The ammonia evolved was absorbed in 50 ml of 0.5 M H2SO4. The residual
acid required 60 mL of 0.5 M solution of NaOH for neutralisation. Find the
percentage composition of nitrogen in the compound.

12.34 0.3780 g of an organic chloro compound gave 0.5740 g of silver chloride in Carius
estimation. Calculate the percentage of chlorine present in the compound.

12.35 In the estimation of sulphur by Carius method, 0.468 g of an organic sulphur
compound afforded 0.668 g of barium sulphate. Find out the percentage of sulphur
in the given compound.

12.36 In the organic compound CH2 = CH – CH2 – CH2 – C ≡CH, the pair of hydridised
orbitals involved in the formation of: C2 – C3 bond is:
(a) sp – sp2 (b) sp – sp3 (c) sp2 – sp3 (d) sp3 – sp3

12.37 In the Lassaigne’s test for nitrogen in an organic compound, the Prussian blue
colour is obtained due to the formation of:
(a) Na4[Fe(CN)6] (b) Fe4[Fe(CN)6]3 (c) Fe2[Fe(CN)6] (d) Fe3[Fe(CN)6]4

12.38 Which of the following carbocation is most stable ?
(a) (CH3)3C.
+
C H2 (b) (CH3)3
+
C (c) CH3CH2
+
C H2 (d) CH3
+
C H CH2CH3

12.39 The best and latest technique for isolation, purification and separation of organic
compounds is:
(a) Crystallisation (b) Distillation (c) Sublimation (d) Chromatography



12.40 The reaction:
CH3CH2I + KOH(aq) →CH3CH2OH + KI
is classified as :
(a) electrophilic substitution (b) nucleophilic substitution
(c) elimination (d) addition

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