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Naming Cycloalkanes and Naming Alkenes

SECTION 20 - Intro to Organic Chemistry


Besides forming chains, alkanes can form rings and become cycloalkanes.

Cycloalkanes

➞  Cycloalkanes are also called "cyclic alkanes."

➞  They have the general formula:  " CnH2n "

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Cycloalkane Examples

ex:  Cyclopropane,  C3H6

c3h6 Name and Structure

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ex:  Cyclobutane,  C4H8

c4h8 Cyclobutane Name and Structure

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The above two cycloalkanes (cyclopropane and cyclobutane) are very unstable due to angle strain, so the bonds are very weak.  

The angle strain occurs because the sp3 hybrid orbitals want to have 109.5° bond angles.

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ex:  Cyclopentane,  C5H10

c5h10 Cyclopentane Name and Structure

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ex:  Cyclohexane,  C6H12

C6H12 Cyclohexane Name and Structure

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Naming Cycloalkanes

The naming process for cycloalkanes is very similar to the rules we learned for naming straight-chain alkanes.

Here's three examples:

ex:  Name the following cyclic alkanes.

a.     Naming Cycloalkanes➞  answer:  methylcyclopropane

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b.  1-isopropyl-3-methylcyclohexane Name and Structure➞  answer:  1-isopropyl-3-methylcyclohexane

The numbering pattern was chosen because it places the branches in alphabetical order.

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c.  1-ethyl-2-propylcyclobutane Name and Structure➞  answer:  1-ethyl-2-propylcyclobutane

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Alkenes and Alkynes

Alkenes  =  unsaturated hydrocarbons having at least 1 carbon-carbon double bond.  See below:

Alkene Double Bond

Alkynes  =  unsaturated hydrocarbons having at least 1 carbon-carbon triple bond.  See below:

Alkyne Triple Bond

Today we'll just focus on Alkenes...

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Alkenes

➞  Alkenes used to also be called by an old-fashioned name, "olefins."

➞  They have the general formula:  " CnH2n "

The simplest alkene is ethene ("ethylene"), with the formula:  C2H4

Structure of Ethylene C2H4

Each carbon atom is is sp2-hybridized.

The C—C sigma bond (σ-bond) is formed by sharing an electron-pair in a line directly between the sp2 hybrid orbitals.

The C—C pi bond (π-bond) is formed by sharing an electron-pair between the p-orbitals on each carbon atom.

Here's what I mean:

Hybridization Structure of Ethene

Sigma and Pi Bonds of Ethylene, Ethene

As you can see, a C=C double bond contains 4 shared electrons: 1 σ-bond and 1 π-bond.

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NOTE - Because the two 2p atomic orbitals must be aligned (parallel) to form the π-bond, rotation about the C=C double bond is not possible.

This rigidity allows for stereoisomerism:

ex  2-butene

Here's the structure of 2-butene...

Cis Trans Isomers of 2-butene

Upon closer inspection however, there are two stereoisomers for 2-butene:

cis-2-butene  and  trans-2-butene

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In the next video blog post from SECTION 20 - Introduction to Organic Chemistry,

We'll learn How to Name Alkenes and Alkynes...