Unit 11: Hydrocarbons — Short Questions

(i) Cycloalkanes: Saturated hydrocarbons with carbon atoms arranged in a ring, having the general formula CₙH₂ₙ.
(ii) Isomerism: The phenomenon where compounds have the same molecular formula but different structures or spatial arrangements.
(iii) Conjugated dienes: Dienes in which two double bonds are separated by one single bond, allowing electron delocalization (e.g., 1,3-butadiene).
(iv) Inductive effect: The shifting of electrons in a σ-bond due to electronegativity differences, transmitted through the chain of atoms.

(i) Aliphatic and Aromatic hydrocarbons
• Aliphatic: The compound that are not aromatic are called aliphatic hydrocarbons. (e.g. alkanes, alkenes, etc.).
• Aromatic: It is a special class of cyclic hydrocarbons having high carbon to hydrogen ratio and based on benzene and their derivatives (e.g. benzene, naphthalene etc.).
(ii) Homolytic and Heterolytic Fission:
• Homolytic: Bond breaks evenly, each atom gets one electron (forms free radicals).
• Heterolytic: Bond breaks unevenly, one atom gets both electrons (forms ions).
(iii) Electrophile and Nucleophile:
• Electrophile: Electron-deficient species that accepts electrons (c.g H⁺).
• Nucleophile: Electron-rich species that donates electrons (e.g OH⁻).

Alkanes do not undergo addition reactions because they are saturated hydrocarbons with strong C–C and C–H sigma bonds, which are relatively non-reactive.

In an elimination reaction, atoms/groups are removed from a molecule, forming a double bond. It is the reverse of addition reactions, where atoms/groups are added to a molecule, breaking a double bond.

2-butene forms a more stable secondary carbocation, while propene forms a less stable primary carbocation. Thus, the carbocation in 2-butene is more stable due to greater alkyl group stabilization.

Structural isomers of C₅H₁₀ (alkenes):
(i) Pent-1-ene
(ii) Pent-2-ene
(iii) 2-Methylbut-1-ene
(iv) 2-Methylbut-2-ene
(v) 3-Methylbut-1-ene

2-bromopropane is favored due to the Markovnikov's rule. In HBr addition, the more stable secondary carbocation intermediate forms (at C-2), leading to 2-bromopropane.

Conjugated alkenes show different reactivity due to electron delocalization over alternating double bonds, making them more stable and reactive in specific conditions like Diels-Alder reactions.

Inductive effect of alkyl groups stabilizes carbocations by pushing electron density toward the positively charged carbon atom, reducing its charge intensity.

Reaction equations (i) CH₃CH₂CH=CH₂ + H₂ → CH₃CH₂CH₂CH₃ (Ni catalyst, 250°C)
(ii) CH₃CH=CH₂ + Cl₂ → CH₃CH(Cl)CH₂Cl (CCl₄)
(iii) CH₃CH₂CH=CHCH₂CH₃ + H₂O → CH₃CH₂CH(OH)CH₂CH₂CH₃ (H₂SO₄)

Structural formulas (i) Isobutylene: CH₂=C-CH₃ with CH₃ below
(ii) 2,3,4,4-Tetramethyl-2-pentene: H₃C-C=C-C-CH₃ with CH₃ groups
(iii) 2,5-Heptadiene: CH₃-CH=CH-CH₂-CH=CH-CH₃
(iv) 4,5-Dimethyl-2-hexene: CH₃-CH=CH-CH-CH-CH₃ with CH₃ groups
(v) Vinylacetylene: CH₂=CH-C≡CH
(vi) 1,3-Pentadiene: CH₂=CH-CH=CH-CH₃
(vii) 1-Butyne: CH≡C-CH₂-CH₃
(viii) 3-n-Propyl-1,4-pentadiene: CH₂=CH-CH-CH=CH₂ with propyl chain
(ix) Vinyl bromide: CH₂=CH-Br
(x) But-1-en-3-yne: CH₂=CH-C≡CH
(xi) 4-Methyl-2-pentyne: CH₃-C≡C-CH-CH₃ with CH₃
(xii) Isopentane: CH₃-CH-CH₂-CH₃ with CH₃ branch

(i) H₃C-CH=CH(CH₂)₂CH₃: Hex-2-ene
(ii) (CH₃)C=CH₂: 2-Methylpropene
(iii) CH₃-CH₂-CH₂-C=CH₂ with CH(CH₃)₂: 2-Iso-propyl-1-pentene
(iv) CH₂=CH-CH=CH₂: 1,3-Butadiene
(v) CH₃=C-CH₂CH₂CH₃ with C₂H₅: 2-Ethyl-1-pentene
(vi) (CH₃CH₂)₂CH: 3-Ethylpentane
(vii) CH₃CH₂C(CH₃)₂CH(CH₂CH₃)CH₃: 3,3,4-Trimethylhexane
(viii) CH₂=CH-C=C-CH=CH₂: 1,5-Hexadien-3-yne
(ix) (CH₃)₂CH-CH₂-C(CH₃)₃: 2,2,4-Trimethylpentane
(x) CH₂=CH-C≡CH: 1-Buten-3-yne
(xi) CH₃C(CH₃)₂(CH₂)CH₃: 2,2-Dimethylbutane
(xii) H₃C-CH₂-CH₂-CH₂-CH₂CH₃ with CH₃: 3-Ethylhexane
(xiii) (CH₃)₂CH-CH-CH(CH₃)₂ with CH₃: 2,3,4-Trimethylpentane
(xiv) H₃C-CH₂-CH₂-CH with CH₃: 2-Methylpentane

Hydrocarbons are organic compounds composed only of carbon and hydrogen atoms. They are classified into:
• Alkanes (saturated hydrocarbons)
• Alkenes (unsaturated with one or more double bonds)
• Alkynes (unsaturated with one or more triple bonds)
• Aromatic hydrocarbons (contain benzene rings)

Alkanes are saturated hydrocarbons with single covalent bonds between carbon atoms. Their names are based on the number of carbon atoms using Greek/Latin prefixes and end in "-ane".
Example
• CH₄ – Methane
• C₂H₆ – Ethane
• C₃H₈ – Propane

Alkanes show chain isomerism, where compounds have the same molecular formula but different carbon chain arrangements.
Example
• C₄H₁₀ → n-butane and isobutane (2-methylpropane)

Alkanes are called paraffins (Latin: parum affinis meaning "little affinity") because they are less reactive due to strong C-C and C-H bonds and absence of functional groups.

Halogenation is the substitution reaction where hydrogen atoms are replaced by halogens in the presence of heat or UV light.
Example
CH₄ + Cl₂ → CH₃Cl + HCl (in UV light)

Alkenes are unsaturated hydrocarbons with at least one double bond between carbon atoms.
General formula: CₙH₂ₙ
Example
• Ethene: C₂H₄
• Propene: C₃H₆

Alkenes show • Chain isomerism
• Position isomerism (location of double bond)
• Geometrical (cis-trans) isomerism due to restricted rotation of the double bond
Example
But-2-ene exists as cis-but-2-ene and trans-but-2-ene

Electrophilic addition is the reaction where an electrophile adds to the double bond of alkenes.
Example
CH₂=CH₂ + HBr → CH₃-CH₂Br

Markovnikov's Rule states that in the addition of HX to an alkene, the hydrogen attaches to the carbon with more hydrogens, and halogen attaches to the carbon with fewer hydrogens.
Example
CH₃-CH=CH₂ + HBr → CH₃-CHBr-CH₃ (2-bromopropane)

Ozonolysis involves breaking the double bond of an alkene by ozone (O₃) to form aldehydes or ketones.
Example
CH₂=CH₂ + O₃ → 2HCHO (formaldehyde)

Alkynes are unsaturated hydrocarbons containing one or more triple bonds between carbon atoms.
General formula: CₙH₂ₙ₋₂
Example
• Ethyne (C₂H₂)
• Propyne (C₃H₄)

Due to the presence of a triple bond with high electron density, alkynes are more reactive in addition reactions than alkanes.

Ethyne + H₂ → Ethene → Ethane
In presence of Ni or Pd catalyst

C₆H₆ + Br₂ → C₆H₅Br + HBr (in FeBr₃ catalyst)

• Terminal alkyne: Triple bond at the end (e.g., ethyne) these are acidic alkynes
• Non-terminal alkyne: Triple bond in the middle (e.g., but-2-yne) these are non-acidic alkynes.

Aromatic hydrocarbons are compounds that contain benzene ring(s) with delocalized π electrons, following Hückel's rule (4n+2 π electrons).

Benzene (C₆H₆) has a hexagonal ring structure with alternating double bonds. It shows resonance, where electrons are delocalized over the ring.

Benzene avoids addition to preserve aromatic stability. Substitution maintains the delocalized π system.

In this reaction, an electrophile replaces a hydrogen atom in benzene.

• Benzene (C₆H₆)
• Toluene (C₆H₅CH₃)
• Naphthalene (C₁₀H₈)

Boiling point increases with molecular size due to greater van der Waals forces. Branching lowers the boiling point.
Example
• n-butane > isobutane

Hydrocarbons are non-polar, thus insoluble in water but soluble in organic solvents like ether and benzene.

No, most hydrocarbons are less dense than water and float on it. They have more empty space than water.

Combustion is the oxidation of hydrocarbons producing CO₂ and H₂O with energy release.
Example
CH₄ + 2O₂ → CO₂ + 2H₂O + heat

A reaction where an atom/group is replaced by another. Common in alkanes.
Example:
CH₄ + Cl₂ → CH₃Cl + HCl (UV light)

By heating sodium acetate with soda lime (NaOH + CaO):
CH₃COONa + NaOH → CH₄ + Na₂CO₃ (CaO)

By dehydration of ethanol with concentrated H₂SO₄:
C₂H₅OH → C₂H₄ + H₂O (heat) (H₂SO₄)

Unsaturated compounds decolorize bromine water (reddish brown → colorless) or alkaline KMnO₄ (purple → brown).

• Fuel (natural gas)
• Production of hydrogen, methanol
• Source of energy in homes

Incomplete combustion produces carbon monoxide (CO) and soot, contributing to air pollution, greenhouse effect, and acid rain (from sulfur/nitrogen compounds).

Butane is a fuel used in lighters. Isobutene is used as a propellant in products such as shaving gel.

See Q.6 from theory.

See Q.8 from theory.

See Q.11,12 from theory.

See Q.11,13 from theory.