Why are alkanes described as saturated?

Alkanes contain only single carbon–carbon bonds. Each carbon already forms four single bonds, so no more atoms can be added to the molecule.

Why is cyclohexane saturated even though it does not fit the formula CₙH₂ₙ₊₂?

Cyclohexane, C₆H₁₂, contains only single bonds, so the molecule is saturated. A ring closure removes two hydrogen atoms, giving CₙH₂ₙ.

What are the names and molecular formulae of the unbranched alkanes with one to six carbon atoms?

Methane is CH₄, ethane C₂H₆ and propane C₃H₈. Butane is C₄H₁₀, pentane C₅H₁₂ and hexane C₆H₁₄.

Why do the fractions of crude oil separate by chain length?

Longer alkane molecules have stronger London forces and so higher boiling points. Long chains condense low in the column, while short chains rise to the top as gases.

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Organic Chemistry

Alkanes from crude oil, combustion and halogenation

3 things to know
A2 2

Overview

Alkanes and their formula, fractional distillation of crude oil, cracking and reforming with equations, complete and incomplete combustion, and the free-radical chlorination of methane.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

How is crude oil separated by fractional distillation?

Process

Describe the free-radical mechanism for the chlorination of methane.

Key term

What is the general formula of the alkanes?

Questions from this subtopic

Each one with the model answer underneath.

1
What is the general formula of the alkanes?
Model answer
The alkanes have the general formula CₙH₂ₙ₊₂. Hexane, with six carbons, is therefore C₆H₁₄.
2
Why are alkanes described as saturated?
Model answer
Alkanes contain only single carbon–carbon bonds. Each carbon already forms four single bonds, so no more atoms can be added to the molecule.
3
Why is cyclohexane saturated even though it does not fit the formula CₙH₂ₙ₊₂?
Model answer
Cyclohexane, C₆H₁₂, contains only single bonds, so the molecule is saturated. A ring closure removes two hydrogen atoms, giving CₙH₂ₙ.
4
What are the names and molecular formulae of the unbranched alkanes with one to six carbon atoms?
Model answer
Methane is CH₄, ethane C₂H₆ and propane C₃H₈. Butane is C₄H₁₀, pentane C₅H₁₂ and hexane C₆H₁₄.
5
How is crude oil separated by fractional distillation?
Model answer
Crude oil is heated until most of it vaporises. The vapour rises up a column that is hot at the bottom and cool at the top. Each fraction condenses where the temperature falls below its boiling point.
6
Why do the fractions of crude oil separate by chain length?
Model answer
Longer alkane molecules have stronger London forces and so higher boiling points. Long chains condense low in the column, while short chains rise to the top as gases.

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