Samacheer Kalvi 11th Chemistry Guide Chapter 12 Basic Concepts of Organic Reactions

Tamilnadu State Board New Syllabus Samacheer Kalvi 11th Chemistry Guide Pdf Chapter 12 Basic Concepts of Organic Reactions Text Book Back Questions and Answers, Notes.

Tamilnadu Samacheer Kalvi 11th Chemistry Solutions Chapter 12 Basic Concepts of Organic Reactions

11th Chemistry Guide Basic Concepts of Organic Reactions Text Book Back Questions and Answers

Textbook Evaluation:

I. Choose the best answer:

Question 1.
For the following reactions
(A) CH₃CH₂CH₂Br + KOH → CH₃ – CH = CH₂ + KBr + H₂O

(B) (CH₃)₃CBr + KOH → (CH₃)₃COH + KBr

(C)
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) sp²
b) spd²
c) sp³
d) sp²d
Answer:
a) sp²

Question 3.
Decreasing order of nucleophilicity is
a) OH^– > NH₂^– > -OCH₃ > RNH₂
b) NH₂^– > OH^– > -OCH3 > RNH₂
c) NH₂^– > CH₃O^– > OH^– > RNH₂
d) CH₃O^– > NH₂^– > OH^– > RNH₂
Answer:
b) NH₂^– > OH^– > -OCH3 > RNH₂

Question 4.
Which of the following species is not electrophilic in nature?
a) Cl⁺
b) BH₃
c) H₃O⁺
d) ⁺NO₂
Answer:
c) H₃O⁺

Question 5.
Homolytic fission of covalent bond leads to the formation of
a) electrophile
b) nucleophile
c) carbocation
d) free radical
Answer:
d) free radical

Question 6.
Hyper conjucation 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 + I effect?
a) CH₃–
b) CH₃ – CH₂ –
c) (CH₃)₂ – CH-
d) (CH₃)₃ – C –
Answer:
d) (CH₃)₃ – C –

Question 8.
Which of the following species does not exert a resonance effect?
a) C₆H₅OH
b) C₆H₅Cl
c) C₆H₅NH₂
d) C₆H₅NH₃
Answer:
d) C₆H₅NH₃

Question 9.
– I effect is shown by
a) – Cl
b) – Br
c) both (a) and (b)
d) – CH₃
Answer:
c) both (a) and (b)

Question 10.
Which of the following carbocation will be most stable?
a) Ph₃⁺C –
b) CH₃ – ⁺CH₂ –
c) (CH₃)₂ – ⁺CH
d) CH₂ = CH – ⁺CH₂
Answer:
d) CH₂ = CH – ⁺CH₂

Question 11.
Assertion:
Tertiary Carbocations are generally formed more easily than primary Carbocations ions.
Reason:
Hyper conjucation as well as inductive effect due to additional alkyl group stabilize tertiary carbonium ions.
a) both assertion and reason are 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 C – Br bond results in the formation of
a) free radical
b) Carbanion
c) Carbocation
d) Carbanion and Carbocation
Answer:
d) Carbanion and Carbocation

Question 13.
Which of the following represent a set of nucleophiles?
a) BF₃, H₂O, NH^2-
b) AlCl₃, BF₃, NH₃
c) CN, RCH₂^–, ROH
d) H⁺, RNH₃⁺, :CCl₂
Answer:
c) CN, RCH₂^–, ROH

Question 14.
Which of the following species does not acts as a nucleophile?
a) ROH
b) ROR
c) PCl₃
d) BF₃
Answer:
d) BF₃

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 (or) Mesomeric effect:
The resonance is a chemical phenomenon which is observed in certain organic compounds possessing double bonds at a suitable position. 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 structures (canonical structures) and this phenomenon is called resonance. This phenomenon is also called mesomerism or mesomeric effect.

For example, the structure of aromatic compounds such as benzene and conjugated systems like 1,3 – butadiene cannot be represented by a single structure, and their, observed properties can be explained on the basis of a resonance hybrid.

In 1,3 butadiene, it is expected that the bond between C¹ – C² and C³ – C⁴ should be shorter than that of C² – C³, but the observed bond lengths are of same. This property cannot be explained by a simple structure in which two n bonds localised between C¹ – C² and C³ – C⁴. Actually, the n electrons are delocalised as shown below.

The resonating structures are called canonical forms and the actual structure lies between these three resonating structures, and is called a resonance hybrid. The resonance hybrid is represented as below.

Similar to the other electron displacement effect, mesomeric effect is also classified into positive mesomeric effect (+M or +R) and negative mesomeric effect (-M or -R) based on the nature of the functional group present adjacent to the multiple bond.

b) Hyper Conjugation:
The delocalisation of electrons of a bond is called as hyperconjugation. It is a special stabilising effect that results due to the interaction of electrons of a σ -bond (usually C – H or C – C) with the adjacent, empty non-bonding p-orbital or an anti¬bonding σ* or π*-orbitals resulting in an extended molecular orbital. Unlike the electromeric effect, hyper conjugation is a permanent effect.

It requires an α-CH group or a lone pair on atom-like N, O adjacent to a π bond (sp² hybrid carbon). It occurs by the overlapping of the σ-bonding orbital or the orbital containing a lone pair with the adjacent π-orbital or p-orbital.

Example:
In propene, the σ -electrons of C-H bond of methyl group can be delocalised into the π -orbital of doubly bonded carbonas represented below.

In the above structure the sigma bond is involved in resonance and breaks in order to supply electrons for delocalization giving rise to 3 new canonical forms. In the contributing canonical structures: (Il), (III) & (IV) of propene, there is no bond between an α-carbon and one of the hydrogen atoms. Hence the hyperconjugation is also known as “no bond resonance” or “Baker Nathan effect”. The structures (II), (III) & (IV) are polar in nature.

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:
CO₂, AlCl₃, BF₃, FeCl₃, NO⁺, NO⁺₂,, etc.

Nucleophiles:
Nucleophiles are reagents that has high affinity for electropositive centers. They possess an atom that has an unshared pair of electrons. They are usually negatively charged ions or electron-rich neutral molecules.
Example.
NH₃, R-NH₂, R-SH, H₂O, R-OH, CN^–. OH^– etc.

Question 18.
Show the heterolysis of covalent bond by using curved arrow notation and complete the following equations. Identify the nucelophile in each case.
i) CH₃ – Br + KOH →
ii) CH₃ – O – CH₃ + HI →
Answer:
(i) CH₃ – Br + KOH →
CH₃ – Br + KQH → CH₃OH + KBr
Nucleophile is : OH^–

ii) CH₃ – O – CH₃ + HI →
H            I
CH₃ – O – CH₃ + HI → CH₃OH + CH₃ I
Nucleophile is : I^–

Question 19.
Explain inductive effect with suitable example.
Answer:
Inductive effect is defined as the change in the polarisation of a covalent bond due to the presence of adjacent bonds, atoms or groups in the molecule. This is a permanent phenomenon.

Let us explain the inductive effect by considering ethane and ethylchloride as examples. The C-C bond in ethane is non polar while the C-C bond in ethyl chloride is polar. We know that chlorine is more electronegative than carbon, and hence it attracts the shared pair of electron between C-Cl in ethyl chloride towards itself. This develops a slight negative charge on chlorine and a slight positive charge on carbon to which chlorine is attached.

To compensate for it, the C₁ draws the shared pair of electrons between itself and C₂ This polarisation effect is called the inductive effect. This effect is greatest for the adjacent bonds, but they also are felt farther away. However, the magnitude of the charge separation decreases rapidly, as we move away from C₂ and is observed maximum for 2 carbons and almost insignificant after 4 bonds from the active group.

It is important to note that the inductive effect does not transfer electrons from one atom to another but the displacement effect is permanent. The inductive effect represents the ability of a particular atom or a group to either withdraw or donate electron density to the attached carbon. Based on this ability the substituents are classified as +I groups and -I groups. Their ability to release or withdraw the electron through sigma covalent bond is called +I effect and -I effect respectively.

Highly electronegative atoms and atoms of groups which are cany a positive charge are electron-withdrawing or-I group.
Example :
-F , -Cl, -COOH, -NO₂ NH₂

Higher the electronegativity of the substituent, greater is the -I effect. The order of the -I effect of some groups are given below.
NH₃ > NO₂ > CN > SO₃H > CHO > CO > COOH > COCl > CONH₂ > F > Cl > Br > I > OH > OR > NH₂ > C₆H₅ > H

Highly electropositive atoms and atoms are groups which carry a negative charge are electron-donating or +1 groups.
Example:
Alkali metals, alkyl groups such as methyl, ethyl, negatively charged groups such as CH₃O^–, C₂H₅O^–, COO^– etc.

The lesser the electronegativity of the elements, the greater is the +1 effect. The relative order of +1 effect of some alkyl groups is given below.
-C(CH₃)₃ > -CH(CH₃)₂ > -CH₂CH₃ > -CH₃

Question 20.
Explain electromeric effect with suitable example.
Answer:
Electromeric is a temporary effect which operates in unsaturated compounds (containing > C = C <, > C = 0, etc…) in the presence of an attacking reagent.
Let us consider two different compounds.
(i) compounds containing carbonyl group ( > C = O)
(ii) unsaturated compounds such as alkenes ( > C = C < )

When a nucleophile approaches the carbonyl compound, the n electrons between C and O is instantaneously shifted to the more electronegative oxygen. This makes the carbon electron-deficient and thus facilitating the formation of a new bond between the incoming nucleophile and the carbonyl carbon atom.

On the other hand when an electrophile such as H⁺ approaches an alkene molecule, the π electrons are instantaneously shifted to the electrophile and a new bond is formed between carbon and hydrogen. This makes the other carbon electron-deficient and hence it acquires a positive charge.

The electrometric effect, is denoted as E effect. Like the inductive effect, the electrometric effect is also classified as + E and – E based on the direction in which the pair of electrons is transferred to form a new bond with the attacking agent.

When the π electron is transferred towards the attacking reagent, it is called + E (positive electrometric) effect.

The addition of H+ to alkene as shown above is an example of +E effect.
When the n electron is transferred away from the attacking reagent, it is called, -E (negative electrometric) effect.

The attack of CN^– on a carbonyl carbon, as shown above, is an example of – E effect.

Question 21.
Give examples for the following types of organic reactions
(i) β – elimination
(ii) Electrophilic substitution
Answer:
(i) β – elimination:
In this reaction two substituents are eliminated from the molecule, and a new C – C double bond is formed between the carbon atoms to which the eliminated atoms/groups are previously attached. Elimination reaction is always accompanied with change in hybridisation.
Example:
n – Propyl bromide on reaction with alcoholic KOH gives propene. In this reaction hydrogen and Br are eliminated.

(ii) Electrophilic substitution:

Here, Y⁺ is an electrophile.
Example :
Nitration of Benzene

11th Chemistry Guide Basic Concepts of Organic Reactions Additional Questions and Answers

I. Choose the best answer:

Question 1.
In organic reactions, the reactant is called ______.
a) intermediate
b) product
c) substrate
d) by product
Answer:
c) substrate

Question 2.
Heterolysis of C – Cl bond produces
a) Two free radicals
b) Two carbonium ions
c) Two carbanions
d) One cation and one anion
Answer:
d) One cation and one anion

Question 3.
Which one of the following is an example for free radical initiators?
(a) Benzoyl peroxide
(b) Benzyl alcohol
(c) Benzyl acetate
(d) Benzaldehyde.
Answer:
(a) Benzoyl peroxide.

Question 4.
Removal of hydride ion from a methane molecule will give a
a) Methyl radical
b) Carbonium ion
c) Carbanion
d) Methyl group
Answer:
b) Carbonium ion

Question 5.
The shape of carbonium ion is
a) Planar
b) Linear
c) Pyramidal
d) Tetrahedral
Answer:
a) Planar

Question 6.
Which of the following is correct order of the stability of carbocations?
(a) ⁺CH₃ > ⁺CH₂ CH₃> ⁺CH(CH₃)₂ > ⁺C(CH₃)₃
(b) ⁺CH₂CH₃ > ⁺CH₃ >⁺ CH(CH₃)₂ > ⁺C(CH₃)₃
(c) ⁺C(CH₃)₃ > ⁺CH(CH₃)₂ >⁺CH₂CH₃^–CH₃
(d) ⁺CH(CH₃)₂ > ⁺CH₃ > ⁺CH₂CH₃ > ⁺C(CH₃)₃
Answer:
(c) ⁺C(CH₃)₃ > ⁺CH(CH₃)₂ >⁺CH₂CH₃^–CH₃

Question 7.
The reaction, (CH₃)₃C – Br -» (CH₃)₃ C⁺ + Br is an example of
a) Homolytic fission
b) Heterolytic fission
c) Cracking
d) All the above
Answer:
b) Heterolytic fission

Question 8.
Which one of the following is not electrophile’?
(a) NH₃
(b) AlCl₃
(c) FeCl₃
(d) R-X
Answer:
(a) NH₃

Question 9.
Which of the following species is paramagnetic
a) A carbaonium ion
b) A free radical
c) A carbanion
d) All of these
Answer:
b) A free radical

Question 10.
Which one of the following are positively charged electrophiles?
(a) CO₂
(b) AlCl₃
(c) BF₃
(d) RX
Answer:
(d) RX

Question 11.
Among the following the true property about
is
a) non – planar
b) C⁺ is sp² – hybridized
c) Electrophile can attack on C⁺
d) Does not undergo hydrolysis
Answer:
b) C⁺ is sp² – hybridized

Question 12.
Which one of the following species has a tendency to show-I effect?
(a) -CH₃
(b) -CH₂-CH₃
(c) -CH(CH₃)₂
(d) -C₆H₅
Answer:
(d) -C₆H₅

Question 13.
Which of the following statements are correct for nucleophile?
a) All negatively charged species are nucleophiles
b) Nucleophiles are Lewis bases
c) Alkenes, alkynes, benzene and pyrrole are nucleophiles
d) All are correct
Answer:
d) All are correct

Question 14.
Which one of the following statement is not correct for electrophile?
a) Electron deficient species are electrophiles
b) Electrophiles are Lewis acids
c) All positively charged species are electrophiles
d) AlCl₃ and SO₃ are electrophiles
Answer:
c) All positively charged species are electrophiles

Question 15.
Electrophiles are
a) Lewis bases
b) Lewis acids
c) Amphoteric
d) All of these
Answer:
b) Lewis acids

Question 16.
Pick out the correct order of acid strength.
(a) CH₃-CH₂-COOH > CH₃COOH > CH₂CICOOH
(b) CH₃COOH > CH₃-CH₂-COOH >CH₂CICOOH
(c) CH₂ClCOOH > CH₃COOH > CH₃-CH₂-COOH
(d) CH₂ClCOOH > CH₃-CH₂-COOH > CH₃COOH
Answer:
(c) CH₂ClCOOH > CH₃COOH > CH₃—CH₂-COOH

Question 17.
Nucleophiles are
a) Electron loving
b) Electron hating
c) Nucleus loving
d) Nucleus hating
Answer:
c) Nucleus loving

Question 18.
Which of the following statement is false about an electrophile?
a) Electron – deficient species
b) An acidic reagent
c) A reagent which attacks an electron-deficient site in a molecule
d) A species which seeks a pair of electrons
Answer:
c) A reagent which attacks an electron-deficient site in a molecule

Question 19.
Which one of the following electrophiles used for nitration of benzene?
(a) \({ Br }^{ \oplus }\)
(b) \({ { NO }_{ 2 }^{ \ominus } }\)
(c) -NH₂
(d) \({ NO }^{ \ominus }\)
Answer:
(b) \({ { NO }_{ 2 }^{ \ominus } }\)

Question 20.
Ammonia molecule is
a) A nucleophile
b) An electron-deficient
c) A electrophile
d) An acid
Answer:
a) A nucleophile

Question 21.
Methyl carbanion is
a) Electrophile
b) Lewis acid
c) Both (a) and (b)
d) Nucleophile
Answer:
d) Nucelophile

Question 22.
Which of the following statements is correct about the inductive effect?
a) Implies the transfer of lone pair of electrons from more electronegative atom to lesser electronegative atom in a molecule.
b) Implies the transfer to lone pair of electrons from lesser electronegative atom to the more electronegative atom in a molecule
c) Increases with increase in distance
d) Implies the atoms ability to cause bond polarization
Answer:
d) Implies the atoms ability to cause bond polarization

Question 23.
The displacement of electrons in a multiple bond in the presence of attacking reagent is called
a) inductive effect
b) electromeric effect
c) resonance
d) Hyper-conjugation
Answer:
b) electromeric effect

Question 24.
The electromeric effect in organic compounds is a
a) Temporary effect
b) Permanent effect
c) Temporary or permanent effect
d) All of the above
Answer:
a) Temporary effect

Question 25.
Which of the following is not the correct condition for resonance?
a) The positions of all the atomic nuclei in the resonating structures may be differ
b) The resonating structures must have the same number of unpaired or paired electrons
c) The molecules exhibiting resonance must be planar in nature
d) The resonating structures must have nearly the same energies
Answer:
a) The positions of all the atomic nuclei in the resonating structures may be differ

Question 26.
Resonance is due to ______.
a) delocalization of sigma electrons
b) migration of H atoms
c) migration of proton
d) delocalization of pi electrons
Answer:
d) delocalization of pi electrons

Question 27.
Resonance in benzene is accompanied by delocalization of n – electrons. Each n electron is attached with
a) 4 carbons
b) 2 carbons
c) 3 carbons
d) 6 carbons
Answer:
d) 6 carbons

Question 28.
Reaction mechanism describes
a) Sequential account of each step describing the details of electron movement
b) energy changes during bond breaking and bond formation
c) kinetics of the reaction
d) all of these
Answer:
d) all of these

Question 29.
Substitution reaction may be
a) Free radical substitution
b) Nucleophilic substitution
c) Electrophilic substitution
d) All are correct
Answer:
d) All are correct

Question 30.
CH₃ – CH₂ – X + KOH_(alc) → CH₂ = CH₂ + KX + H₂O is
a) addition reaction
b) substitution reaction
c) elimination reaction
d) molecular rearrangement
Answer:
c) elimination reaction

Question 31.
CH₄ + Cl₂ CH₃Cl + HCl is an example
for
a) free radical substitution reaction
b) electrophilic substitution reaction
c) nucleophile substitution reaction
d) nueclophilic addition reaction
Answer:
a) free radical substitution reaction

Question 32.
C₆H₆ + 3 Cl₂ C₆H₆Cl
a) free radical substitution reaction
b) electrophilic addition reaction
c) free radical addition reaction
d) nueclophilic addition reaction
Answer:
c) free radical addition reaction

Question 33.
Which one of the following orders is correct regarding the – I effect of the substituents?
a) – NR₂ < – OR > – F
b) -NR₂ > – OR > – F
c) -NR₂ < – OR< – F
d) -NR₂ > – OR < – F
Answer:
c) -NR₂ < – OR< – F

Question 34.
Decreasing – I power of given groups is
(1) -CN
(2) -NO₂
(3) – NH₃⁺
(4) -F
a) 2 > 1 > 4 > 3
b) 2 > 3 > 4 > 1
c) 3 > 2 > 4 > 1
d) 3 > 2 > 1 > 4
Answer:
d) 3 > 2 > 1 > 4

Question 35.
Which of the following belongs to – I group?
a) – C₆H₅
b) – CH₃
c) – CH₂CH₃
d) – C (CH₃)₃
Answer:
a) – C₆H₅

Question 36.
Which of the following functional group shows + R effect?
a) – CHO
b) – NO₂
c) -CN
d) – NR₂
Answer:
d) – NR₂

Question 37.
Hyper conjugation is
a) σ – π conjugation
b) Due to delocalization of σ and π bonds
c) No bond resonance
d) All of these
Answer:
d) All of these

Question 38.
+ R power of the given groups
(1) -O^–
(2) – NH₂
(3) – OH
(4) – NHCOCH₃ in decreasing order is
a) 1 > 2 > 3 > 4
b) 4 > 3 > 2 > 1
c) 1 > 3 > 2 > 4
d) 1 > 4 > 3 > 2
Answer:
a) 1 > 2 > 3 > 4

Question 39.
In pyridine:

Number of conjugated electrons are:
a) 6
b) 8
c) Zero
d) 5
Answer:
a) 6

Question 40.
In hyperconjugation, the atom involved is
a) β – H atom
b) α – H atom
c) γ – H atom
d) All
Answer:
b) α – H atom

Question 41.
Hyper conjugation involves overlap of the following orbtials
a) σ – σ
b) σ – p
c) p – p
d) π – π
Answer:
b) σ – p

Question 42.
+ I effect is shown by
a) – NO₂
b) – Cl
c) – Br
d) – CH₃
Answer:
d) – CH₃

Question 43.
Zero inductive effect is exerted by
a) C₆H₅ –
b) – H
c) CH₃ –
d) – Cl
Answer:
b) – H

Question 44.
Hyper conjugation is most useful for stabilizing which of the following carbocations?
a) neo – Pentyl
b) tert – Butyl
c) iso – Propyl
d) Ethyl
Answer:
b) tert – Butyl

Question 45.
Hyper conjugation phenomenon is possible in
a) H₂C = CH₂
b) CH₃CH₂ – CH = CH₂
c) C₆H₅CH = CH₂
d) (CH₃)₃C – CH = CH₂
Answer:
b) CH₃CH₂ – CH = CH₂

Question 46.
Among the following carbocations which is more stable
a) (C₆H₅)₃C⁺
b)
c)
d)
Answer:
a) (C₆H₅)₃C⁺

Question 47.
The compound which gives the most stable carbonium ion on dehydration
a)
b)
c) CH₃ – CH₂ – CH₂ – CH₂OH
d)
Answer:
b)

Question 48.
Which carbocation is more stable?

Answer:
a)

Question 49.
Arrange the following groups in order of decreasing – R (Or – M) power
(1) NO₂
(2) SO₃H
(3) CF₃
(4) CHO
a) 1 > 3 > 2 > 4
b) 1 > 2 > 3 > 4
c) 1 > 4 > 3 > 2
d)4 > 3 > 2 > 1
Answer:
a) 1 > 3 > 2 > 4

Question 50.
The reaction intermediate produced by homolytic cleavage of bond is called
a) carbocations
b) carbanions
c) free radicals
d) carbenes
Answer:
c) free radicals

Question 51.
Most stable carbocation is
a) CH₃ – ⁺CH₂
b) ⁺CH₂CHCl₂
c) ⁺CH₂CH₂Cl
d) ⁺CH₂ – CH₂NO₂
Answer:
a) CH₃ – ⁺CH₂

Question 52.
Carbocation is a reaction intermediate in which of the following reactions
a) E₁ reactions
b) Electrophilic addition reactions of alkenes and alkynes
c) S_N¹ reactions
d) All of the above
Answer:
d) All of the above

Question 53.
Halogenation of an alkane takes place through the intermediate
a) Carbocation
b) carbanion
c) carbon free radical
d) carbene
Answer:
c) carbon free radical

Question 54.
Which free radical is the most stable?

Answer:
a)

Question 55.
The most stable carbonium ion is
a) Methyl carbonium ion
b) Primary carbonium ion
c) Secondary carbonium ion
d) Tertiary carbonium ion
Answer:
d) Tertiary carbonium ion

Question 56.
Stability of which intermediate is not govern by hyperconjugation?
a) Carbon cation
b) carbon anion
c) carbon-free radical
d) carboniumion
Answer:
b) carbon anion

Question 57.
Which of the following is an electrophilic reagent?
a) H₂O
b) OH^–
c) NO₂⁺
d) All of these
Answer:
c) NO₂⁺

Question 58.
CH₂ = CH₂ + Br₂ → CH₂Br – CH₂Br is an example for
a) addition reaction
b) substitution reaction
c) elimination reaction
d) molecular rearrangement
Answer:
a) addition reaction

Question 59.
R – X + OH^–_(aq) → R – OH + X⁺_(aq) is an example for
a) addition reaction
b) substitution reaction
c) elimination reaction
d) molecular rearrangement
Answer:
b) substitution reaction

Question 60.
C₆H₆ + Cl₂ C₆H₅Cl + HC1 is an example for
a) Free radical substitution reaction
b) Electrophilic substitution reaction
c) Nucleophilic addition reaction
d) Nucelophilic substitution reaction
Answer:
b) Electrophilic substitution reaction

Question 61.
Alkenes readily undergo
a) Substitution reactions
b) Addition reactions
c) Elimination reactions
d) Rearrangement reactions
Answer:
b) Addition reactions

Question 62.
Nitration of benzene is
a) nucleophilic substitution
b) nucleophilic addition
c) electrophilic substitution
d) free radical substitution
Answer:
c) electrophilic substitution

Question 63.
The reaction (CH₃)₃C – Br (CH₃)₃ C – OH is
a) elimination reaction
b) substitution reaction
c) free radical reaction
d) displacement reaction
Answer:
b) substitution reaction

Question 64.
Inductive effect involves
a) displacement of σ – electrons
b) delocalization of π – electrons
c) delocalization of σ – electrons
d) displacement of π – electrons
Answer:
b) delocalization of π – electrons

Question 65.
Compound which shows positive mesomeric effect
a) H₂C = CH – Cl
b) C₆H₅ – NO₂
c) H₂C = CH – CH₂Cl
d) both b & c
Answer:
a) H₂C = CH – Cl

Question 66.
The stability order, in the following carbocations,
CH₃CH₂⁺ (I), (CH₃)₂ ⁺CH (II), (CH₃)₃ C⁺ (III), ⁺CH₃ (IV)
a) I > IV > III > II
b) I > II > III > IV
c) III > IV > I > II
d) III > II > I > IV
Answer:
d) III > II > I > IV

Question 67.
Which is most stable carbocation?
a) n – propyl cation
b) iso – propyl cation
c) Ethyl cation
d) Triphenylmethyl cation
Answer:
d) Triphenylmethyl cation

Question 68.
Which one of the following carbanions is least stable?
a) CH₃CH₂^–
b) HC ≡ C^–
c) (C₆H₅)₃C^–
d) (CH₃)₃C^–
Answer:
d) (CH₃)₃C^–

Question 69.
Among the following, the strongest nucleophile is
a) C₂H₅SH
b) CH₃COO^–
c) CH₃NH₂
d) NCCH₂^–
Answer:
a) C₂H₅SH

Question 70.
Which of the following is least reactive in a nucleophile substitution reaction?
a) (CH₃)₃C – Cl
b) CH₂ = CHCl
c) CH₃CH₂Cl
d) CH₂ = CHCH₂Cl
Answer:
b) CH₂ = CHCl

II. Very short question and answers (2 Marks):

Question 1.
What are organic reactions?
Answer:
The substrate is an organic molecule that reacts with a reagent, which may be an organic, inorganic, or any agent like heat, photon, etc, that brings about the chemical change to form a product, this is known as organic reactions.

Question 2.
What is the Positive Mesomeric effect? Give example.
Answer:
A positive resonance effect occurs, when the electrons move away from substituents attached to the conjugated system. It occurs, if the electron releasing substituents are attached to the conjugated system. In such cases, the attached group has a tendency to release electrons through resonance. These electron releasing groups are usually denoted as +R or +M groups.
Example:
– OH , – SH , – OR , – SR, – NH₂, – O – etc…

Question 3.
Mention the types of fission of a covalent bond?
Answer:
There are three types of fission of a covalent bond. They are,

• Homolytic cleavage
• Heterolytic cleavage

Question 4.
Explain hyper conjugate effect of vinyl chloride molecule.
Answer:
Hyper conjugation effect is also observed when atoms/groups having lone pair of electrons are attached by a single bond, and in conjugation with a π bond. The lone pair of electrons enters into resonance and displaces π electrons resulting in more than one structure.

Question 5.
What are the types of substitution reactions?
Answer:
In this reaction, an atom or a group of atoms attached to a carbon atom is replaced by a new atom or a group of atoms. Based on the nature of the attacking reagent, these reactions can be classified as

• Nucleophilic substitution
• Electrophilic substitution
• Free radical substitution

Question 6.
Explain free radical substitution reaction with the suitable example.
Answer:

Aliphatic electrophilic substitution
A general aliphatic electrophilic substitution is represented as

Question 7.
Explain Nucleophilic Addition reaction with suitable example.
Answer:

Example:
Addition of HCN to acetaldehyde

Question 8.
Explain free radical addition reaction with suitable example.
Answer:
A general free radical addition reaction can be represented as below.


In the above reaction, Benzoyl perioxide acts as an radial initiator. The mechanism involves free radicals.

Question 9.
Write a note on Functional Group inter conversion.
Answer:
Organic synthesis involves functional group inter conversions. A particular functional group can be converted into other functional group by reacting it with suitable reagents. For example, The carboxylic acid group (-COOH) presents in organic acids can be transformed to a variety of other functional group such as – CH₂-OH, – CONH₂, – COCl by treating the acid with LiAlH₄, NH_3, and SOCl₂ respectively.

Question 10.
Differentiate the carbocation and carbanion.
Answer:
Carbocation:

• In a carbocation carbon-bearing positive charge.
• Carbon bearing positive charge has sp2 hybridization.
• It has a planar structure.
• Example: CH₃⁺ (CH₃)₂\({ C }^{ \ominus }\)H, (CH₃)₃C⁺ etc.

Carbanion:

• In a carbanion carbon bearing negative charge.
• Carbon bearing negative charge has sp³ hybridization.
• It has a pyramidal structure.
• Example: CH₃–\({ C }^{ \ominus }\)H₂, CH₃-CHO, (CH₃)₂\({ C }^{ \ominus }\)H etc.

III. Short answer questions (3 Marks):

Question 1.
Explain the hybridization of carbon in carbocation.
Answer:
In a carbocation, the carbon-bearing positive charge is sp² hybridized and hence it has a planar structure. In the reaction involving such a carbocation, the attack of a negatively charged species (nucleophiles) take place on either side of the carbocation as shown below.

Shape of carbocation

The carbanions are generally pyramidal in shape and the lone pair occupies one of the sp3 hybridized orbitais. An alkyl free radical may be either pyramidal or planar.

Shape of carbanion

The shape of Carbon radical

Question 2.
Write three types of electron movement in organic reaction.
Answer:

• lone pair becomes a bonding pair.
• bonding pair becomes a lone pair.
• a bond breaks and becomes another bond.

Type: 1
A lone pair to a bonding pair
Example:

Type: 2
A bonding pair to a lone pair
Example:

Type: 3
A bonding pair to an another bonding pair
Example:

Question 3.
Explain hyper conjugate effect of acrylonitrile.
Answer:
When electronegative atoms or group of atoms are in conjugation with a π – bond,they pull π – electrons from the multiple bond. In case of carbocations, greater the number of alky groups attached to the carbon bearing positive charge, greater is number of the hyper conjucate structure. thus the stability of various carbocations decreases in the order :
3° Carbocation > 2° Carbocation > 1° Carbocation.

Question 4.
What are functional group interconversions?
Answer:
Organic synthesis involves functional group interconversions. A particular functional group can be converted into other functional groups by reacting it with suitable reagents. For example, the carboxylic acid group (-COOH) present in organic acids can be transformed into -COCl by treating the acid with SOCl₂ reagent.

Question 5.
Explain nucleophilc substituion reaction with suitable example.
Answer:
This reaction can be represented as

Here Y^– is the incoming nuclophile or and attacking species and x^– is the leaving group.

Example:
Hydrolysis of alkyl halides by aqueous KOH.
CH₃Br  CH₃OH + Br^–
Aliphatic nucleophilic substitution reactions take place either by S_N¹ or S_N² mechanism.

Question 6.
Explain Electrophilic Addition Reaction with suitable example.
Answer:
A general electrophilic addition reaction can be represented as below.

Example:
Bromination of alkene to give bromo alkane

Question 7.
What are oxidation and reduction reactions? Give suitable example.
Answer:
Many oxidation and reduction reactions of organic compounds fall into one of the four types of reaction that we already discussed but others do not. Most of the oxidation reaction of organic compounds involves gain of oxygen or loss of hydrogen Reduction involves gain of hydrogen and loss of oxygen.
Example:

IV. Long answer questions (5 Marks):

Question 1.
Explain different types of Fission of a covalent bond.
Answer:
All organic molecules contain covalent bonds which are formed by the mutual sharing of electrons between atoms. These covalent bonds break in two different ways, namely homolytic cleavage (symmetrical splitting) and heterolytic cleavage (unsymmetrical splitting). The cleavage of a bond in the substrate is influenced by the nature of the reagent (attacking agent).

Homolytic Cleavage:
Homolytic cleavage is the process in which a covalent bond breaks symmetrically in such way that each of the bonded atoms retains one electron. It is denoted by a half headed arrow (fish hook arrow). This type of cleavage occurs under high temperature or in the presence of UV light in a compound containing non polar covalent bond formed between atoms of similar electronegativity.

In such molecules, the cleavage of bonds results into free radicals. They are short lived and are highly reactive. The type of reagents that promote holmolytic cleavage in substrate are called as free radical initiators. For example Azobisisobutyronitile (AIBN) and peroxides such as benzoyl peroxide are used as free radical initiators in polymerisation reactions.

As a free radical with an unpaired electron is neutral and unstable, it has a tendency to gain an electron to attain stability. Organic reactions involve homolytic fission of C – C bonds to form alkyl free radicals. The stability of alkyl free radicals is in the following order:
C(CH₃)₃ > CH(CH₃)₂ > CH₂CH₃ > CH₃

Heterolytic Cleavage:
Heterolytic cleavage is the process in which a covalent bond breaks unsymmetrically such that one of the bonded atoms retains the bond pair of electrons. It results in the formation of a cation and an anion. Of the two bonded atoms, the most electronegative atom becomes the anion and the other atom becomes the cation. The cleavage is denoted by a curved arrow pointing towards the more electronegative atom.

For example, in tert-butyl bromide, the C – Br bond is polar as bromine is more electronegative than carbon. The bonding electrons of the C – Br bond are attracted more by bromine than carbon. Hence, the C – Br undergoes heterolytic cleavage to form a tert-butyl cation during hydrolysis.

Let us consider the cleavage in a carbon-hydrogen (C-H) bond of aldehydes or ketones We know that the carbon is more electronegative than hydrogen and hence the heterolytic cleavage of C-H bonds results in the formation of carbanion (carbon bears a negative charge). For example in aldol condensation the OH- ion abstracts a a-hydrogen from the aldehyde, which leads to the formation of the below mentioned carbanion.

Question 2.
Explain the acidic nature of carboxylic acid.
Answer:
When a halogen atom is attached to the carbon which is nearer to the carboxylic acid group, its -I effect withdraws the bonded electrons towards itself and makes the ionisation of H easy. The acidity of various chioro acetic acid is in the following order. The strength of the acid increases with increase in the -J effect of the group attached to the carboxyl group.

Trichioro acetic acid > Dichloro acetic acid > Chloro acetic acid > acetic acid.

Similarly, the following order of acidity in the carboxylic acids is due to the +I effect of alkyl group.