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Alkanes formulas

3 formulas, 4 reference tables and 5 common traps for MHT-CET Chemistry Alkanes, grouped by subtopic.

Full notes with worked examples

The Alkane Series: Formula, Isomers, Carbon Types and Boiling Points

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The Homologous Series and Its Molar Masses

Alkane molar mass

CnH2n+2:M=14n+2 g mol−1\mathrm{C_nH_{2n+2}}:\quad M = 14n + 2\ \text{g mol}^{-1}

Structural Isomers and Primary, Secondary, Tertiary Carbons

IsomersAlkaneCarbon moles in n mol
2C₄H₁₀4nQ
3C₅H₁₂5nQ
5C₆H₁₄6nQ

Boiling Points and Uses

QuestionAnswer
Highest bp: 2-methoxypropane, n-butane, 2-methylbutane, n-pentanen-PentaneQ
Highest bp among the C₆H₁₄ isomersn-HexaneQ
Alkanes used for road surfacingMore than 35 C atomsQ

Common traps

Starting the count at ethane

The third alkane is propane, 44 — count from methane. 58 is butane, the fourth.

Neopentane as the IUPAC name

Neopentane is the common name, and it is offered. The IUPAC name of (CH₃)₄C is 2,2-dimethylpropane.

Making Alkanes: Wurtz, Grignard Reagents and Decarboxylation

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The Wurtz Reaction and Mixed Halides

Wurtz reaction

2R−X+2Na→dry etherR−R+2NaX\mathrm{2R{-}X + 2Na \xrightarrow{\text{dry ether}} R{-}R + 2NaX}

Grignard Reagents Meeting an Active Hydrogen

Grignard protonation

R−MgX+H−Z→R−H+Mg(Z)X(Z=OH, OR, NH2)\mathrm{R{-}MgX + H{-}Z \rightarrow R{-}H + Mg(Z)X}\quad (Z = OH,\ OR,\ NH_2)

Decarboxylation and Hydrogenation of CO

ReactionProduct
Sodium propanoate + soda-lime, ΔEthane + Na₂CO₃ (paper keys propane)Q
Decarboxylation loses a carbon, so C₂H₅COONa gives ethane. The 9 May 2023 key is propane; learn the rule, and expect the paper's answer.
CO + H₂ over NiMethane and waterQ

Common traps

Forgetting the self-coupled products

A mixed Wurtz does not only give the cross product. Both halides also couple with themselves, so check all three alkanes before picking the one that cannot form.

Expecting an amine or an alcohol

NH₃ and CH₃OH look like reagents that would add their group. With a Grignard reagent they only give up a proton: the product is the alkane every time.

Free-Radical Halogenation: Reactivity and Selectivity

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Halogen Reactivity and Bromine's Selectivity

Alkane + Br₂, UVMajor productWhy
2-Methylpropane (isobutane)2-Bromo-2-methylpropane3° HQ
(CH₃)₃CH(CH₃)₃CBr, 99%3° HQ
Propane2-Bromopropane2° HQ

Common traps

Counting hydrogens instead of ranking them

Isobutane has nine primary H and one tertiary H, which would favour the 1-bromo product by number. Bromine's selectivity overrides the count: the tertiary bromide is 99%.

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