Chapter 8: Organic Chemistry – Some Basic Principles and Techniques Short Questions | chemistry Class 11 CBSE

Chapter 8: Organic Chemistry – Some Basic Principles and Techniques Short Questions | chemistry Class 11 CBSE | ByteNotes.in

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Question 1

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State the hybridisation and geometry of carbon in (a) methane (b) ethene (c) ethyne.

In methane, carbon is sp3 hybridised with tetrahedral geometry and bond angles of 109.5 degrees.
In ethene, carbon is sp2 hybridised with trigonal planar geometry; all six atoms lie in the same plane.
In ethyne, carbon is sp hybridised with linear geometry and a bond angle of 180 degrees.

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Question 2

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How does the s-character of a hybrid orbital affect bond length and electronegativity of carbon?

Greater s-character means the orbital is closer to the nucleus, resulting in shorter and stronger bonds; sp bonds are shorter than sp2 which are shorter than sp3.
Greater s-character also increases the electronegativity of carbon; sp carbon (50% s) is more electronegative than sp2 (33% s) and sp3 (25% s).
This difference in electronegativity affects bond polarity and the physical and chemical properties of the resulting organic compounds.

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Question 3

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Why is rotation about a carbon-carbon double bond restricted?

In a C=C double bond, the pi bond is formed by lateral (side-by-side) overlap of unhybridised p orbitals on adjacent carbons, requiring parallel orientation of both p orbitals.
Rotation of one CH2 fragment relative to the other would break this lateral overlap, destroying the pi bond and requiring significant energy input.
Therefore, unlike sigma bonds, C=C double bonds are rigid; this gives rise to geometrical isomerism in appropriate molecules.

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Question 4

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What is a bond-line structural formula? How is 3-methyloctane represented in bond-line notation?

Bond-line structural formula is a simplified representation where C and H atoms are not shown; C-C bonds are drawn as zig-zag lines, terminals represent CH3 groups, and junctions represent carbon atoms.
Heteroatoms such as O, N, Cl and functional groups are written explicitly; the number of H atoms on each carbon is inferred from its valency.
3-Methyloctane is drawn as a nine-carbon zig-zag chain with a one-carbon branch at the third junction from either end.

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Question 5

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Distinguish between alicyclic and aromatic compounds with one example each.

Alicyclic compounds contain carbon atoms joined in a ring (carbocyclic) and exhibit properties similar to aliphatic compounds; example: cyclohexane.
Aromatic compounds are special cyclic compounds with delocalized pi electrons and exhibit distinctive stability and substitution reactivity; example: benzene.
Aromatic compounds may also contain heteroatoms in the ring (heterocyclic aromatic), such as pyridine or furan, while alicyclic compounds can similarly have heteroatoms, as in tetrahydrofuran.

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Question 6

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Define a functional group and a homologous series. Give one example of each.

A functional group is an atom or group of atoms bonded to the carbon chain in a unique manner that determines the characteristic chemical properties of the compound; example: -OH (hydroxyl group) in alcohols.
A homologous series is a group of organic compounds each containing the same characteristic functional group, representable by a general molecular formula, with successive members differing by a -CH2 unit.
Example of a homologous series: alkanes (CH4, C2H6, C3H8...) where each successive member has one additional CH2 unit and shares similar chemical properties.

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Question 7

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Write the IUPAC name of CH3CH2CH(OH)CH2CH(CH3)2.

First, identify the functional group: -OH (alcohol), which uses the suffix -ol.
The longest chain containing the -OH group has 6 carbon atoms (hexane as parent). Numbering from the -OH end gives -OH at C-3 and a methyl branch at C-5.
The correct IUPAC name is 5-methylhexan-3-ol, with numbering done to give the functional group the lowest possible locant.

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Question 8

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State the rules for numbering carbon atoms in a branched-chain alkane with two examples.

The longest carbon chain is identified as the parent chain and carbon atoms are numbered from the end that gives substituents the lowest possible locant numbers.
If two substituents are at equivalent positions, the one that comes first alphabetically receives the lower number.
Example: 2-methylbutane (not 3-methylbutane); 2,4-dimethylpentane (not 2,4 from the other end since this gives the same sum but 2,4 is the set with lower locants).

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Question 9

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What is chain isomerism? Give two chain isomers of C5H12.

Chain isomerism arises when two or more compounds have the same molecular formula but different carbon skeletons (arrangements of the carbon chain).
First chain isomer of C5H12: pentane (CH3CH2CH2CH2CH3), an unbranched straight chain.
Second chain isomer: 2-methylbutane (CH3CH(CH3)CH2CH3); a third isomer, 2,2-dimethylpropane, also exists for this formula.

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Question 10

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Differentiate between heterolytic and homolytic cleavage of a covalent bond.

In heterolytic cleavage, the shared electron pair remains with one fragment, forming a carbocation (electron-deficient) and an anion such as Br-; this leads to ionic or polar reactions.
In homolytic cleavage, one electron of the shared pair goes to each bonded atom, forming neutral free radicals with unpaired electrons; this leads to free radical reactions.
Heterolytic cleavage is favoured by polar solvents and polar reagents, while homolytic cleavage is favoured by heat or light and nonpolar conditions.

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Question 11

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What is the stability order of carbocations? Explain with inductive effect.

The stability order is: tertiary carbocation > secondary carbocation > primary carbocation > methyl carbocation.
Alkyl groups attached to the positive carbon are electron-donating due to the inductive effect, which disperses the positive charge and stabilises the carbocation.
More alkyl groups on the positive carbon means greater electron donation and better charge dispersal, hence tertiary carbocations are most stable; the methyl cation has no alkyl groups and is least stable.

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Question 12

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Define nucleophile and electrophile with two examples of each.

A nucleophile is an electron pair donor that attacks the electron-deficient (electrophilic) centre of a substrate; examples include OH- (hydroxide ion) and CN- (cyanide ion).
An electrophile is an electron pair acceptor that attacks the electron-rich (nucleophilic) centre of a substrate; examples include carbocations (CH3+) and BF3.
Neutral molecules with lone pairs like NH3 and H2O can act as nucleophiles, while neutral molecules with partial positive charges like alkyl halides (R-X) can act as electrophiles.

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Question 13

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Explain the inductive effect with reference to chloroethane.

In chloroethane (CH3CH2Cl), the C-Cl bond is polar because Cl is more electronegative, developing a partial negative charge (delta-) on Cl and a partial positive charge (delta+) on C-1.
C-1 with partial positive charge draws electron density from the adjacent C-C bond, causing a smaller positive charge (delta-delta+) to develop on C-2; this is the inductive effect.
The inductive effect is transmitted through the sigma bond framework but diminishes rapidly with increasing number of intervening bonds, becoming negligible beyond three bonds.

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Chapter 8: Organic Chemistry – Some Basic Principles and Techniques | Short Questions

State the hybridisation and geometry of carbon in (a) methane (b) ethene (c) ethyne.

How does the s-character of a hybrid orbital affect bond length and electronegativity of carbon?

Why is rotation about a carbon-carbon double bond restricted?

What is a bond-line structural formula? How is 3-methyloctane represented in bond-line notation?

Distinguish between alicyclic and aromatic compounds with one example each.

Define a functional group and a homologous series. Give one example of each.

Write the IUPAC name of CH3CH2CH(OH)CH2CH(CH3)2.

State the rules for numbering carbon atoms in a branched-chain alkane with two examples.

What is chain isomerism? Give two chain isomers of C5H12.

Differentiate between heterolytic and homolytic cleavage of a covalent bond.

What is the stability order of carbocations? Explain with inductive effect.

Define nucleophile and electrophile with two examples of each.

Explain the inductive effect with reference to chloroethane.

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