VED ACCADEMY
STD 11 Science Chemistry Total Mark : 30
* Answer the following questions in detail. [18]
1. Write a short note on free radical.
Ans : Free radical:
i. A species with unpaired electron is called free radical.
OR
An uncharged species which is electrically neutral and which contains a single
electron is called free radical.
ii. A free radical is highly reactive, unstable and therefore has a transitory existence
(short-lived).
iii. Free radicals are formed as reaction intermediate which subsequently react with
another radical/molecule to restore stable bonding pair.
iv. In a carbon free radical, the carbon atom having unpaired electron is sp
hybridized and has planar trigonal geometry.
v. The alkyl free radicals are classified as primary, secondary or tertiary depending
upon the number of carbon atoms attached to the C-atom carrying the unpaired
electron.
vi. Stability of
alkyl free radicals decreases in the order 3° > 2° > 1° > methyl free radical.
2. Explain in short:
i. Positive resonance (+R) effect
ii. Negative resonance (-R) effect
Ans : i. Positive resonance (+R) effect or electron donating/releasing resonance
effect:
a. If the substituent group has a lone pair of electrons to donate to the attached K
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bond or conjugated system of π bonds, the effect is called +R effect.
b. The +R effect increases electron density at certain positions in a molecule.
e.g. +R effect in aniline increases the electron density at ortho and para positions.
c. Halogen, -OH, -OR, -O –, -NH2, -NHR, -NR2, – NHCOR, -OCOR, etc. are the groups
which show +R effect.
ii. Negative resonance (-R) effect:
a. If the substituent group has a tendency to withdraw electrons from the attached
π bond or conjugated system of π bonds towards itself the effect is called -R effect.
b. The -R effect results in developing a positive polarity at certain positions in a
molecule.
e.g. -R effect in nitrobenzene develops positive polarity at ortho and para positions.
c. -COOH, -CHO, – CO -, -CN, -NO 2, -COOR, etc., are the groups which represent -R
effect.
3. Explain chain isomerism in alkanes with two suitable examples.
Ans : Chain isomerism: When two or more compounds have the same molecular
formula but different parent chain or different carbon skeletons, it is referred to as
chain isomerism and such isomers are known as chain isomers.
e.g.
i. Butane (C4H10) exists in two isomeric forms:
Here, n-butane contains longest chain of four carbon atoms whereas isobutane
contains longest chain of three carbon atoms. Such isomers having different
carbon skeletons are called as chain isomers.
ii. Pentene (C5H12) exists in three isomeric forms:
* Select and write the correct answer for the following multiple-choice type of [3]
questions.
1. Which free radical from following is least stable?
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(A) CH 3⋅ (B) (CH 3 ) 3 C∙ (C) (CH
3 ) 2 CH∙
(D) CH 3 CH2 ⋅
Ans: (A) CH3 ⋅
2. Identify the compound that does not correctly match with its common name.
(A) (B) C H − NH
6 5 : (C)
2 (D) H − CHO :
Aniline Formaldehyde
Ans : (C)
3. Choose the INCORRECT statement from the following.
(A) Cyclohexane is an alicyclic compound.
(B) Pyridine is a heterocyclic compound.
(C) Piperidine is an aromatic compound.
(D) Tropone is a non-benzenoid compound.
Ans : (C) Piperidine is an aromatic compound.
* Answer the following in brief. [3 marks each] [9]
1. Find out the most stable species from the following. Justify.
Ans : a. The most stable species from the given species is ˙
(H3 C) 3 C i.e., tert-butyl
radical.
This is because it has greater number of alkyl groups attached to the C-atom
having unpaired electron. More the number of the alkyl groups, the greater will be
+1 inductive (electron releasing) effect, and thereby greater will be the stability of
the free radical.
b. The most stable species from the given species is CBr . −
3
This is because it contains 3 − Br atoms, which exhibits electron withdrawing
inductive effect. Carbanions are stabilized by -1 inductive (electron withdrawing)
effect. Larger the number of -I groups attached to the negatively charged
carbon atom, lower will be the electron density on the carbon atom and higher
will be its stability.
+
c. The most stable species from the given species is C H . 3
This because it does not contain Cl atom, which exhibits electron withdrawing
inductive effect. Carbocations are destabilized by -1 inductive (electron
withdrawing) effect. When more number of-I groups are attached to the
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positively charged carbon atom, the positive charge on the carbon atom
increases further, thus destabilizing the species. Hence, the species with no -I
groups will be most stable.
2. Identify the α-carbons in the following species and give the total number of α-
hydrogen in each.
Ans :
a.
In structure (i), C-2 and C-4 are α-carbon atoms.
Hydrogen atoms(s) attached to α-C atoms is a α-H atom. Thus, structure (i)
contains 4 α-H atoms.
b.
In structure (ii), carbon atoms adjacent to C-2 are α-carbon atoms (as shown in
the structure).
Thus, structure (ii) contains 6 α-H atoms.
c.
C-3 carbon atom, that is, C-atom next to (H2C=CH-) is a α-C atom.
Thus, structure (iii) contains 2 α-H atoms.
3. Explain the stability of tert-butyl cation, isopropyl cation, ethyl cation and methyl
cation on the basis of hyperconjugation.
Ans : i. Greater the number of alkyl groups attached to a positively charged carbon
atom, more is the number of α-hydrogens, more is the hyperconjugation
structures and more is the stability of the cation.
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ii. Thus, the relative stability of the cations decreases in the order:
3° carbocation > 2° carbocation > 1° carbocation > Methyl cation
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