S20E3 - Naming Cycloalkanes and Naming Alkenes

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

Cycloalkanes

➞  Cycloalkanes are also called "cyclic alkanes."

➞  They have the general formula:  " C_nH_2n "

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Cycloalkane Examples

ex:  Cyclopropane,  C₃H₆

(Ignore the cyclopentane typo -- this is cyclopropane. Duh!)

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ex:  Cyclobutane,  C₄H₈

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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

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ex:  Cyclohexane,  C₆H₁₂

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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.     

➞  answer:  methylcyclopropane

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b.  

➞  answer:  1-isopropyl-3-methylcyclohexane

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

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c.  

➞  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:

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

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:  " C_nH_2n "

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

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...

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...