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Chapter 6: Problem 26

From their molecular formulas, classify each of these straight-chain hydrocarbons as an alkane, an alkene, or an alkyne. (a) \(\mathrm{C}_{21} \mathrm{H}_{44}\) (b) \(\mathrm{C}_{4} \mathrm{H}_{6}\) (c) \(\mathrm{C}_{8} \mathrm{H}_{16}\)

Short Answer

Expert verified
(a) Alkane, (b) Alkyne, (c) Alkene.

Step by step solution

01

Identify the General Formula for Alkanes

Alkanes have the general molecular formula \(\mathrm{C}_n\mathrm{H}_{2n+2}\). This means the number of hydrogen atoms is always twice the number of carbon atoms plus two.
02

Identify the General Formula for Alkenes

Alkenes have the general formula \(\mathrm{C}_n\mathrm{H}_{2n}\). This means the number of hydrogen atoms is exactly twice the number of carbon atoms.
03

Identify the General Formula for Alkynes

Alkynes have the general formula \(\mathrm{C}_n\mathrm{H}_{2n-2}\). This means the number of hydrogen atoms is twice the number of carbon atoms minus two.
04

Classify Hydrocarbon (a) \(\mathrm{C}_{21} \mathrm{H}_{44}\)

Check against the alkane formula: \(2 \cdot 21 + 2 = 44\). The molecular formula fits the alkane general formula, so \(\mathrm{C}_{21} \mathrm{H}_{44}\) is an alkane.
05

Classify Hydrocarbon (b) \(\mathrm{C}_{4} \mathrm{H}_{6}\)

Check against the alkyne formula: \(2 \cdot 4 - 2 = 6\). The molecular formula fits the alkyne general formula, so \(\mathrm{C}_{4} \mathrm{H}_{6}\) is an alkyne.
06

Classify Hydrocarbon (c) \(\mathrm{C}_{8} \mathrm{H}_{16}\)

Check against the alkene formula: \(2 \cdot 8 = 16\). The molecular formula fits the alkene general formula, so \(\mathrm{C}_{8} \mathrm{H}_{16}\) is an alkene.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Alkane
Alkanes are the simplest type of hydrocarbon, often known as saturated hydrocarbons. This is because they contain only single bonds between carbon atoms, leading to a straightforward linear or branched structure. These single bonds allow for maximum hydrogen saturation.
  • General formula: \(\mathrm{C}_n\mathrm{H}_{2n+2}\)
  • Structure: Single bonds only (\(-\))
  • Example: Methane (\(\mathrm{CH}_4\)) is the simplest alkane.
If you encounter a hydrocarbon with the number of hydrogen atoms equal to twice the number of carbon atoms plus two, it is an alkane.
For instance, with \(\mathrm{C}_{21} \mathrm{H}_{44}\), applying the formula \(2 \times 21 + 2\) gives 44. Thus, this molecule is an alkane. They are mostly used as fuels and lubricants, including substances like propane and butane.
Alkene
Alkenes are a group of hydrocarbons characterized by at least one carbon-carbon double bond, making them unsaturated.
This double bond gives rise to unique chemical properties that make alkenes quite reactive.
  • General formula: \(\mathrm{C}_n\mathrm{H}_{2n}\)
  • Structure: At least one double bond (\(=\))
  • Example: Ethene (or ethylene, \(\mathrm{C}_2\mathrm{H}_4\)), is the simplest alkene.
Given a hydrocarbon where the number of hydrogen atoms is exactly twice the number of carbon atoms, it's classified as an alkene.
For example, \(\mathrm{C}_{8} \mathrm{H}_{16}\) fits this formula \(2 \times 8 = 16\). These compounds are often used in the production of polymers, like polyethylene, which is used to make plastic items.
Alkyne
Alkynes are hydrocarbons containing at least one carbon-carbon triple bond, making them unsaturated and quite reactive.
The presence of the triple bond makes alkynes distinctive, both in structure and reactivity.
  • General formula: \(\mathrm{C}_n\mathrm{H}_{2n-2}\)
  • Structure: At least one triple bond (\(\equiv\))
  • Example: Acetylene (\(\mathrm{C}_2\mathrm{H}_2\)) is the simplest alkyne.
If you find a hydrocarbon whose hydrogen atoms are equal to twice the number of carbon atoms minus two, it is an alkyne.
For instance, the compound \(\mathrm{C}_{4} \mathrm{H}_{6}\) satisfies this condition since \(2 \times 4 - 2 = 6\).
Alkynes are used in organic synthesis and as a fuel, such as in welding torches.

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Most popular questions from this chapter

Consider a series of molecules in which the \(\mathrm{C}\) atom is bonded to atoms of second-period elements: \(\mathrm{C}-\mathrm{O}\), \(\mathrm{C}-\mathrm{F}, \mathrm{C}-\mathrm{N}, \mathrm{C}-\mathrm{C},\) and \(\mathrm{C}-\mathrm{B}\). Place these bonds in order of increasing bond length.

Write correct Lewis structures and assign a formal charge to each atom. (a) \(\mathrm{CH}_{3} \mathrm{CHO}\) (b) \(\mathrm{N}_{3}^{-}\) (c) \(\mathrm{CH}_{3} \mathrm{CN}\)

Write a Lewis structure for each molecule. Which formulas represent alkanes? Which are probably aromatic? Which fall into neither category? (a) \(\mathrm{C}_{8} \mathrm{H}_{10}\) (b) \(\mathrm{C}_{10} \mathrm{H}_{8}\) (c) \(\mathrm{C}_{6} \mathrm{H}_{12}\) (d) \(\mathrm{C}_{6} \mathrm{H}_{14}\) (e) \(\mathrm{C}_{8} \mathrm{H}_{18}\) (f) \(\mathrm{C}_{6} \mathrm{H}_{10}\)

Tetrasulfur tetranitride reacts with disulfur dichloride to form \(\mathrm{S}_{4} \mathrm{~N}_{3} \mathrm{Cl}\), a salt. $$ 3 \mathrm{~S}_{4} \mathrm{~N}_{4}+2 \mathrm{~S}_{2} \mathrm{Cl}_{2} \longrightarrow 4 \mathrm{~S}_{4} \mathrm{~N}_{3} \mathrm{Cl} $$ Write a plausible Lewis structure for the two reactants and the cation of the salt.

In each case, tell whether cis and trans isomers exist. If they do, write structural formulas for the two isomers and label each cis or trans. For those that cannot have cistrans isomers, explain why. (a) \(\mathrm{Br}_{2} \mathrm{CH}_{2}\) (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}=\mathrm{CHCH}_{2} \mathrm{CH}_{3}\) (c) \(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CHCH}_{3}\) (d) \(\mathrm{CH}_{2}=\mathrm{CHCH}_{2} \mathrm{CH}_{3}\)
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