“Organic Chemistry: Hydrocarbons”

🟢 Lite — Quick Review (1h–1d)

Rapid summary of hydrocarbons for NABTEB chemistry.

Hydrocarbons are compounds containing only carbon and hydrogen atoms. They are the basis of organic chemistry and are classified as:

1. Saturated Hydrocarbons (Alkanes):

• General formula: $C_nH_{2n+2}$
• Single bonds only (C–C)
• Methane ($CH_4$), Ethane ($C_2H_6$), Propane ($C_3H_8$), Butane ($C_4H_{10}$)
• undergo substitution reactions

2. Unsaturated Hydrocarbons — Alkenes:

• General formula: $C_nH_{2n}$
• At least one carbon-carbon double bond (C=C)
• Ethene ($C_2H_4$), Propene ($C_3H_6$)
• Undergo addition reactions

3. Unsaturated Hydrocarbons — Alkynes:

• General formula: $C_nH_{2n-2}$
• At least one carbon-carbon triple bond (C≡C)
• Ethyne/Acetylene ($C_2H_2$), Propyne ($C_3H_4$)
• Undergo addition reactions

IUPAC Naming:

1. Find the longest carbon chain (parent name)
2. Number from end to give substituents lowest numbers
3. Name substituents with prefixes (methyl, ethyl, chloro, etc.)
4. Use prefixes (di-, tri-, tetra-) for multiple identical substituents

Key Reactions:

Alkanes — Substitution: $$CH_4 + Cl_2 \xrightarrow{UV} CH_3Cl + HCl$$

Alkenes — Addition: $$C_2H_4 + H_2 \xrightarrow{Pt} C_2H_6 \text{ (hydrogenation)}$$ $$C_2H_4 + Br_2 \rightarrow C_2H_4Br_2 \text{ (bromine test — decolourises in alkenes)}$$

Alkynes — Addition (forms alkene first, then alkane): $$C_2H_2 + H_2 \xrightarrow{Pt} C_2H_4 \xrightarrow{H_2/Pt} C_2H_6$$

⚡ NABTEB Exam Tip: Bromine water test distinguishes alkenes from alkanes — alkenes decolourise bromine (brown to colourless) due to addition reaction. Alkanes do not react with bromine water.

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🟡 Standard — Regular Study (2d–2mo)

For NABTEB students who want solid understanding of hydrocarbon chemistry.

Structure and Bonding in Alkanes:

Alkanes have $sp^3$ hybridised carbon atoms. Each carbon forms four single bonds arranged tetrahedrally with bond angles of approximately 109.5°.

Properties:

• Boiling points increase with molecular size (more carbons = stronger London dispersion forces)
• Alkanes are non-polar and insoluble in water
• They are less dense than water
• First four alkanes (methane to butane) are gases at room temperature
• Pentane to heptadecane are liquids; octadecane and above are solids

Isomerism in Alkanes:

Butane ($C_4H_{10}$) has two isomers:

• n-Butane: straight chain $CH_3-CH_2-CH_2-CH_3$
• Isobutane: branched $CH_3-CH(CH_3)-CH_3$

Pentane ($C_5H_{12}$) has three isomers.

Structure and Bonding in Alkenes:

Alkenes have $sp^2$ hybridised carbon atoms. Each carbon in the double bond forms three bonds in a trigonal planar arrangement (120° bond angles).

The double bond consists of:

• One sigma ($\sigma$) bond: formed by head-on overlap of orbitals
• One pi ($\pi$) bond: formed by sideways overlap of unhybridised p orbitals

The $\pi$ bond is weaker than the $\sigma$ bond and breaks first during reactions.

Addition Reactions of Alkenes:

• Hydrogenation: Addition of $H_2$ (needs catalyst: Pt, Pd, or Ni)
• Halogenation: Addition of $Br_2$ or $Cl_2$
• Hydration: Addition of $H_2O$ (needs acid catalyst)
• Hydrohalogenation: Addition of HX (HCl, HBr, HI)

Markovnikov’s Rule: In addition of HX to an unsymmetrical alkene, the hydrogen attaches to the carbon with more hydrogen atoms already.

Structure and Bonding in Alkynes:

Alkynes have $sp$ hybridised carbon atoms. Each carbon in the triple bond forms two bonds in a linear arrangement (180° bond angles).

The triple bond consists of:

• One $\sigma$ bond
• Two $\pi$ bonds

Chemical Properties Comparison:

Property	Alkanes	Alkenes	Alkynes
Bonding	C–C single only	C=C double	C≡C triple
Hybridisation	$sp^3$	$sp^2$	$sp$
Reaction type	Substitution	Addition	Addition
Bromine test	No reaction	Decolourises Br₂	Decolourises Br₂
Baeyer’s reagent	No reaction	Decolourises (purple)	Decolourises

Aromatic Hydrocarbons (Benzene):

Benzene ($C_6H_6$) has a ring structure with alternating double bonds, but the bonds are delocalised (equal bond lengths of 1.40 Å). This special stability is called aromaticity.

Benzene undergoes electrophilic substitution reactions rather than addition reactions due to its aromatic stability:

• Nitration: $C_6H_6 + HNO_3 \xrightarrow{H_2SO_4} C_6H_5NO_2 + H_2O$
• Halogenation: $C_6H_6 + Cl_2 \xrightarrow{FeCl_3} C_6H_5Cl + HCl$

⚡ NABTEB Exam Tip: Always identify the functional group first. If there’s a C=C or C≡C bond, it’s unsaturated — expect addition reactions. If only C–C single bonds, it’s saturated — expect substitution reactions.

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage of hydrocarbons for thorough NABTEB preparation.

Homologous Series:

A homologous series is a family of compounds with:

• Same general formula
• Same functional group
• Similar chemical properties
• Gradual change in physical properties with increasing molecular size

Alkanes ($C_nH_{2n+2}$):

Name	Formula	Boiling Point (°C)	State at RTP
Methane	$CH_4$	-162	Gas
Ethane	$C_2H_6$	-89	Gas
Propane	$C_3H_8$	-42	Gas
Butane	$C_4H_{10}$	0	Gas
Pentane	$C_5H_{12}$	36	Liquid
Hexane	$C_6H_{14}$	69	Liquid

Mechanism of Substitution in Alkanes — Chlorination:

1. Initiation: UV light homolytically splits chlorine molecules $$Cl_2 \xrightarrow{UV} 2Cl^\bullet$$

2. Propagation: Chlorine radicals abstract hydrogen from methane $$Cl^\bullet + CH_4 \rightarrow CH_3^\bullet + HCl$$ $$CH_3^\bullet + Cl_2 \rightarrow CH_3Cl + Cl^\bullet$$

3. Termination: Two radicals combine $$Cl^\bullet + Cl^\bullet \rightarrow Cl_2$$ $$CH_3^\bullet + Cl^\bullet \rightarrow CH_3Cl$$ $$CH_3^\bullet + CH_3^\bullet \rightarrow C_2H_6$$

Mechanism of Addition in Alkenes:

Electrophilic addition mechanism:

1. The $\pi$ electrons attack the electrophile ($H^+$ from HBr)
2. A carbocation intermediate forms on the more substituted carbon
3. The nucleophile ($Br^-$) attacks the carbocation

This explains Markovnikov’s rule — the carbocation forms on the more substituted (stable) carbon.

Isomerism in Alkenes — E/Z System:

For alkenes where each carbon of the double bond has two different groups attached:

• E (Entgegen): Higher priority groups on opposite sides
• Z (Zusammen): Higher priority groups on the same side

Priority determined by Cahn-Ingold-Prelog rules (atomic number of directly attached atoms).

Testing for Unsaturation:

1. Bromine water test: Alkenes and alkynes decolourise bromine water (brown to colourless)
2. Baeyer’s reagent test: Alkenes decolourise alkaline potassium manganate(VII) (purple to brown precipitate of $MnO_2$) $$CH_2=CH_2 + [O] + H_2O \rightarrow CH_2OH-CH_2OH \text{ (ethanediol)}$$

Environmental Chemistry:

Sources of hydrocarbon pollution:

• Incomplete combustion of fuels (produces CO, unburnt hydrocarbons)
• Vehicle exhaust
• Industrial processes

Cracking:

Large alkanes can be broken down (cracked) into smaller, more useful alkanes and alkenes: $$C_{12}H_{26} \xrightarrow{heat} C_6H_{14} + C_6H_{12}$$

Catalytic cracking uses zeolite catalysts at ~500°C. Thermal cracking uses high pressure without catalyst.

Combustion Reactions:

Complete combustion: $$CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$$

Incomplete combustion (limited oxygen): $$2CH_4 + 3O_2 \rightarrow 2CO + 4H_2O$$

Produces CO (toxic) and soot (carbon particles).

⚡ NABTEB Quick Reference:

• Alkanes: $C_nH_{2n+2}$, $sp^3$, single bonds, substitution reactions
• Alkenes: $C_nH_{2n}$, $sp^2$, double bond, addition reactions
• Alkynes: $C_nH_{2n-2}$, $sp$, triple bond, addition reactions
• Benzene: $C_6H_6$, aromatic, electrophilic substitution
• Markovnikov: H adds to carbon with MORE H atoms already
• Bromine test: alkenes/alkynes decolourise bromine water
• Cracking: large alkanes → smaller alkanes + alkenes