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Questions and Answers
Which method involves the reduction of unsaturated hydrocarbons using hydrogen and a nickel catalyst?
Which method involves the reduction of unsaturated hydrocarbons using hydrogen and a nickel catalyst?
What is a necessary condition for the halogenation of alkanes?
What is a necessary condition for the halogenation of alkanes?
Which of the following is a product formed when CH4 reacts with Cl2 under halogenation conditions?
Which of the following is a product formed when CH4 reacts with Cl2 under halogenation conditions?
Which alkane has the molecular formula C5H12?
Which alkane has the molecular formula C5H12?
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In the Wurtz reaction, what is the expected outcome when two alkyl halides are reacted with sodium?
In the Wurtz reaction, what is the expected outcome when two alkyl halides are reacted with sodium?
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Which of the following steps in the free radical mechanism of halogenation is considered the rate determining step?
Which of the following steps in the free radical mechanism of halogenation is considered the rate determining step?
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Which of the following IUPAC names indicates a branched alkane?
Which of the following IUPAC names indicates a branched alkane?
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What is the main reason branched isomers have lower boiling points compared to straight-chain isomers?
What is the main reason branched isomers have lower boiling points compared to straight-chain isomers?
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When distinguishing between two competing parent chains of equal length, what factor is prioritized?
When distinguishing between two competing parent chains of equal length, what factor is prioritized?
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Which of the following is the correct order of alkanes based on boiling points from highest to lowest for C1 to C5 alkanes?
Which of the following is the correct order of alkanes based on boiling points from highest to lowest for C1 to C5 alkanes?
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Study Notes
Alkanes (Paraffins)
- General formula: CnH2n+2
- Methane (CH4), Ethane (CH3CH3), Propane (CH3CH2CH3), Butane (CH3(CH2)2CH3), Pentane (CH3(CH2)3CH3), Hexane (CH3(CH2)4CH3), Heptane (CH3(CH2)5CH3), Octane (CH3(CH2)6CH3), Nonane (CH3(CH2)7CH3), Decane (CH3(CH2)8CH3) are examples of alkanes.
- Alkyl groups: Methane → Methyl, Ethane → Ethyl, etc.
IUPAC Nomenclature
- Select the longest carbon chain.
- Number the chain from the end closest to the substituent to give the lowest possible numbers.
- Name any substituents using prefixes (di, tri, etc.)
- Number the position of the substituents for correct nomenclature.
Nomenclature Rules
- Use prefixes (di-, tri-, tetra-) for multiple substituents.
- When branches are equidistant, choose the name that gives the lower number at the difference point.
Types of alkyl groups based on carbon substitution
- Primary, secondary, tertiary, and quaternary carbons.
- Primary alkyl group, Secondary alkyl group, Tertiary alkyl group (examples shown in the notes)
- Special cases like sec-butyl, isopropyl, isobutyl, tert-butyl, and neopentyl.
Physical Properties
- Alkanes with 1 to 4 carbons are gases.
- Alkanes with 5 to 17 carbons are liquids.
- Alkanes with 18 or more carbons are solids.
- Branched isomers have lower boiling points than straight-chain isomers.
General Methods for Preparation
- From compounds with the same number of carbons:
- Catalytic reduction of unsaturated hydrocarbons (R-C≡C-R → R-CH2-CH2-R using H2/Ni catalyst)
- From ketones:
- Clemmensen reduction (Zn/HCl/Hg)
- Wolff-Kishner reduction (NH2NH2/NaOH)
- From alkyl halides (reduction and using metals and acids):
- Zn/HCl
Wurtz Reaction
- Used to prepare symmetrical alkanes.
- 2RX + 2Na → R-R + 2NaX (R can be any alkyl group and X is a halogen).
Reactions of Alkanes
- Substitution reactions (using X2 like Cl2, Br2, F2, I2)
- Conditions: ultraviolet light (UV) or high temperature (250-400°C).
- Mechanism: Free radical
Mechanism of Free Radical Halogenation
- Initiation, propagation, and termination steps.
Factors Affecting Halogenation Rate
- Type of halogen (Reactivity: F2 > Cl2 > Br2 > I2). Fluorination is difficult/explosive, whereas iodination doesn't typically take place with alkanes.
- Ease of hydrogen abstraction (rate determining step): Tertiary > Secondary > Primary.
Stability of Free Radical Formed
- Increasing stability for free radicals: Methyl < Primary < Secondary < Tertiary (due to hyperconjugation).
Selective Halogenation
- Br2 is more selective than Cl2 (due to Br2 being less reactive).
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Description
Test your knowledge on alkanes and their IUPAC nomenclature rules. This quiz covers the general formula of alkanes, examples, and nomenclature rules. Understand the classification of alkyl groups as you tackle this challenge!
