Text Book Back Questions and Answers - Chapter 12 Basic Concepts of Organic Reactions 11th Chemistry Guide Samacheer Kalvi Solutions - SaraNextGen [2024-2025]

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On April 24, 2024, 11:35 AM

Basic Concepts of Organic Reactions
Textual Evaluation Solved
I. Choose The Best Answer:
Question 1.
For the Following reactions -
(A) $\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{CH}_2 \mathrm{Br}+\mathrm{KOH} \rightarrow \mathrm{CH}_3-\mathrm{CH}+\mathrm{KBr}+\mathrm{H}_2 \mathrm{O}$
(B) $\left(\mathrm{CH}_3\right)_3 \mathrm{CBr}+\mathrm{KOH} \rightarrow\left(\mathrm{CH}_3\right)_3 \mathrm{COH}+\mathrm{KBr}$

Which of the following statement is correct'?
(a) (A) is elimination, (B) and (C) are substitution
(b) (A) is substitution, (B) and (C) are elimination
(c) (A) and (B) are elimination and (C) is addition reaction
(d) (A) is elimination, B is substitution and (C) is addition reaction.
Answer:
(d) (A) is elimination, B is substitution and (C) is addition reaction.
Question 2.
What is the hybridisation state of benzyl carbonium ion?
(a) $\mathrm{sp}^2$
(b) $\mathrm{spd}^2$
(c) $\mathrm{sp}^3$
(d) $\mathrm{sp}^2 \mathrm{~d}$
Answer:
(a) $\mathrm{sp}^2$
Question 3.
Decreasing order of nucleophilicity is -
(a) $\mathrm{OH}^{-}>\mathrm{RNH}_2^{-}>-{ }^{-} \mathrm{OCH}_3>\mathrm{RNH}_2$
(b) $\mathrm{NH}_2>\mathrm{OH}^{-}>-{ }^{-} \mathrm{OCH}_3>\mathrm{RNH}_2$
(c) $\mathrm{NH},>\mathrm{CH}_3 \mathrm{O}>\mathrm{OH}^{-}>\mathrm{RNH}_2$
(d) $\mathrm{CH}_3 \mathrm{O}^{-}>\mathrm{NH}_2>\mathrm{OH}^{-}>\mathrm{RNH}_2$

Answer:
(b) $\mathrm{NH}_2>\mathrm{OH}^{-}>-{ }^{-} \mathrm{OH}_3>\mathrm{RNH}_{2+}$
Question 4.
Which of the tòllowing species is not eLectrophilic in nature?
(a) $\mathrm{Cl}^{+}$
(b) $\mathrm{BH}_3$
(c) $\mathrm{H}_3 \mathrm{O}^{+}$
(d) ${ }^{+} \mathrm{NO}_2$
Answer:
(c) $\mathrm{H}_3 \mathrm{O}^{+}$
Question 5.
Homolytic fission of covalent bond leads to the formation of -
(a) electrophile
(b) nucicophile
(c) Carbocation
(d) free radical
Answer:
(d) free radical
Question 6.
Hyper Conjugation is also known as -
(a) no bond resonance
(b) Baker - nathan effect
(c) both (a) and (b)
(d) none of these
Answer:
(c) both (a) and (b)
Question 7.
Which of the group has highest $+\mathrm{I}$ effect?
(a) $\mathrm{CH}_3^{-}$
(b) $\mathrm{CH}_3 \mathrm{CH}_2^{-}$
(c) $\left(\mathrm{CH}_3\right)_2-\mathrm{CH}-$
(d) $\left(\mathrm{CH}_3\right)_3-\mathrm{C}$ -
Answer:
(d) $\left(\mathrm{CH}_3\right)_3-\mathrm{C}-$

Question 8.
Which of the following species does not exert a resonance effect?
(a) $\mathrm{C}_6 \mathrm{H}_5 \mathrm{OH}$
(b) $\mathrm{C}_6 \mathrm{H}_5 \mathrm{Cl}$
(c) $\mathrm{C}_6 \mathrm{H}_5 \mathrm{NH}_2$
(d) $\mathrm{C}_6 \mathrm{H}_5 \mathrm{NH}_3$
Answer:
(d) $\mathrm{C}_6 \mathrm{H}_5 \mathrm{NH}_3$
Question 9.
-I effect is shown by -
(a) $-\mathrm{Cl}$
(b) $-\mathrm{Br}$
(c) both (a) and (b)
(d) $-\mathrm{CH}_3$
Answer:
(c) both (a) and (b)
Question 10.
Which of the following carbocation will be most stable?
(a) $\mathrm{PH}_3 \mathrm{C}^{+}-$
(b) $\mathrm{CH}_3-\mathrm{CH}_2$
(c) $\left(\mathrm{CH}_3\right)_2-\mathrm{CH}$
(d) $\mathrm{CH}_2=\mathrm{CH}-\mathrm{CH}_3$
Answer:
(a) $\mathrm{PH}_3 \mathrm{C}^{+}-$

Question 11.
Assertion : Tertiary Carbocations are generally formed more easily than primary Carbocations.
Reason : Hyper conjugation as well as inductive effect due to additional alkyl group stabilize tertiary carbonium ions.
(a) both assertion and reason arc true and reason is the correct explanation of assertion.
(b) both assertion and reason are true but reason is not the correct explanation of assertion.
(c) Assertion is true but reason is false
(d) Both assertion and reason are false
Answer:
(a) both assertion and reason are true and reason is the correct explanation of assertion.
Question 12 .
Heterolytic fission of $\mathrm{C}-\mathrm{C}$ bond results in the formation of -
(a) free radical
(b) Carbanion
(c) Carbocation
(d) Carbanion and Carbocation
Answer:
(e) Carbocation
Question 13.
Which of the following represent a set of nucleophiles?
(a) $\mathrm{BF}_3, \mathrm{H}_2 \mathrm{O}, \mathrm{NH}^{2-}$
(b) $\mathrm{AlCl}_3, \mathrm{BF}_3, \mathrm{NH}_3$
(c) $\mathrm{CN}^{-}, \mathrm{RCH}_2^{-}, \mathrm{ROH}$
(d) $\mathrm{H}^{+}, \mathrm{RNH}_2^{+}, \mathrm{CCl}_2$
Answer:
(c) $\mathrm{CN}^{-}, \mathrm{RCH}_2^{-}, \mathrm{ROH}$

Question 14.
Which of the following species does not acts as a nucleophile?
(a) $\mathrm{ROH}$
(b) ROR
(c) $\mathrm{PCl}_3$
(d) $\mathrm{BF}_3$
Answer:
(d) $\mathrm{BF}_3$
Question 15 .
The geometrical shape of carbocation is
(a) Linear
(b) tetrahedral
(c) Planar
(d) Pyramidal
Answer:
(c) Planar
II. Write brief answer to the following questions.
Question 16.
Write short notes on:
(a) Resonance
(b) Hyper conjugation
Answer:
(a) Resonance:
1. Certain organic compounds can be represented by more than one structure and they differ only in the position of bonding and lone pair of electrons. Such structures are called
resonance structure and this phenomenon is called as resonance. This phenomenon is also called as mesomerism or mesomeric effect.

2. For example, the structure of aromatic compounds such as benzene and conjugated system like 1,3 butadiene cannot be represent by a single structure and their observed properties can be explained on the base of a resonance hybrid.
3. Resonance structure of benzene.

(I) and (II) are called as resonance hybridš of benzenc.
4. For 1,3 butadiene:

(I), (II) and (III) are called as resonance hybrids of 1,3 butadiene.
(b) Hyper conjugation:
1. The denationalization of electrons of $\sigma$ bond is called as hyper conjugation. it is a special stabilizing effect that results due to the interaction of electrons of a $\sigma$ bond with the adjacent empty non-bonding P-orbitais resulting in an extended molecular orbital.
2. Hyper conjugation is a permanent effect.
3. For example, in propane. the $\sigma$ - electrons of $\mathrm{C}-\mathrm{H}$ bond of methyl group can be delocalised into the it- orbital of doubly bonded carbon as represented below.

Question 17.
What are electrophiles and nucleophiles? Give suitable examples for each.
Answer:
Electrophiles:
Electrophiles are reagents that are attracted towards negative charge or electron rich center.
They are either positively charged ions or electron deficient neutral molecules.
Example:
$
\mathrm{CO}_2, \mathrm{AlCl}_3, \mathrm{BF}_3, \mathrm{FeCl}_3, \mathrm{NO}^{+}, \mathrm{NO}_2^{+}, \text {, etc. }
$
Nucleophiles:
Nucleophiles are reagents that has high affinity for electropositive centers. They possess an atom has a unshared pair of electrons. They are usually negatively charged ions or electron rich neutral molecules.
Example.
$\mathrm{NH}_3, \mathrm{R}-\mathrm{NH}_2, \mathrm{R}-\mathrm{SH}, \mathrm{H}_2 \mathrm{O}, \mathrm{R}-\mathrm{OH}, \mathrm{CN}^{-} . \mathrm{OH}^{-}$etc.
Question 18.
Show the heterolysis of covalent bond by using curved arrow notation and complete the following equations. Identify the nucleophile In each case.
1. $\mathrm{CH}_3-\mathrm{Br}+\mathrm{KOH} \rightarrow$
2. $\mathrm{CH}_3-\mathrm{OCH}_3+\mathrm{HI} \rightarrow$
Answer:

(I)

In this case, $\stackrel{\ominus}{B r}$ and $\stackrel{\ominus}{O H}$ are nucleophiles.
Another mechanism:
Step - I

Step II

(ii)

In this case, $\mathrm{OCH}_3$ and -I are nucleophiles Another mechanism:
Step - I

Step II

Question 19.
Explain inductive effect with suitable example.
Inductive effect:
Answer:
1. it is defined as the change $i n$ the polarization of a covalent bond due to the presence of adjacent bonded atoms or groups in the molecule. It is denoted as I-effect.
2. Atoms or groups which lose electron towards a carbon atom are said to have a $+\mathrm{I}$ effect. Example:
$
\mathrm{CH}_3-,\left(\mathrm{CH}_3\right)_2 \mathrm{CH}-,\left(\mathrm{CH}_3\right)_2 \mathrm{C} \text { - etc. }
$
3. Atoms or groups which draw electrons away from a carbon atom are said to have a -I effect.
Example:
$
-\mathrm{NO}_2 .-\mathrm{I},-\mathrm{Br},-\mathrm{OH}, \mathrm{C}_6 \mathrm{H}_5 \text { etc. }
$
4. For example, consider ethane and ethyl chloride. The C-C bond in ethane is non polar while the $\mathrm{C}-\mathrm{C}$ bond in ethyl chloride is polar. We know that chlorine is more electronegative than carbon and hence it attracts the shared pair of electrons between $\mathrm{C}-\mathrm{Cl}$ in ethyl chloride towards it self.

This develops a slight negative charge on chlorine and a slight positive charge on carbon to which chlorine is attached. To compensate it, the $\mathrm{C}_1$ draws the shared pair of electron between itself and $\mathrm{C}_2$. This polarization effect is called inductive effect.
Question 20.
Explain electromeric effect.
Electromeric effect:
Answer:
1. The electromeric effect refers to the polarity produced in a multiple bonded the compound when it is attacked by a reagent when a double or a triple bond is exposed to an attack by an electrophile the two $\pi$ electrons which from the $\pi$ bond are completely transferred to one atom or the other.
2. When a nucleophile approaches the carbonyl compound, the $\pi$-electrons between $\mathrm{C}$ and $\mathrm{O}$ is instantaneously shifted to the more electronegative oxygen. This make the carbon electron deficient and thus facilitating the formation of a new bond between the incoming nucleophile and the carbonyl carbon atom.

3. When an electrophile such as $\mathrm{H}$ approaches an alkene molecule their electrons are instantaneously shifted to the electrophiie and a new bond is formed between carbon and hydrogen. This makes the other carbon electron deficient and hence it acquires a positive charge.

4. This effect denotes as E-effect.
Question 21.
Give examples for the following types of organic reactions
1. - elimination
2. Electrophilic substitution.
Answer:
1. $\beta$-elimination:
Elimination reactions involve the cleavage of a a bond and tórmation of air bond. A nucleophilic pair of electrons heads into a new it bond as a leaving group departs. This process is called 3-elimination because the bond Í to the nucleophilic pair of electrons breaks.
Example:
(a) n-propyl bromide on reaction with alcoholic $\mathrm{KOH}$ give propene. in this reaction hydrogen and $\mathrm{Br}$ are eliminated.

(b) Acid-catalysed dehydration of alcohols 

2. Electrophilic substitution:
Substitution reactions when are brought about by electrophiles are called electrophilic substitution reaction.
Example:
(a) Nitration of benzene

(b) Bromination of benzene: