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Naming Alkanes: Straight-Chain and Branched Alkanes

SECTION 20 - Intro to Organic Chemistry

Rules for Naming Alkanes

There are 4 Rules for naming straight-chain and/or branched alkanes. Let's discuss the rules, then we'll do some practice problems.

RULE #1 - The name of the unbranched alkane is obtained by adding "-ane" to the end of the root name for the number of carbon atoms (pent- , hex- , etc...).

For a branched alkane, the longest continuous chain determines the root name for the alkane.

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RULE #2 - When alkane groups appear as branches off of the main (longest) chain, they're named by dropping the "-ane" and replacing with "-yl" suffix.  See below:

   Common Alkyl Branches

Common Alkyl Branches
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RULE #3 - Positions of branches (substituents) are specified by numbering the longest carbon chain, starting at the end closest to the branching.

Hyphens are used between the branch number and the branch name.

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RULE #4 - The location and name of each branch is followed by the root (parent) alkane name. Branches are listed in alphabetical order, and prefixes (di-, tri-, etc.) are used to indicate multiple identical substituents (branches).

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If reading the 4 rules above made absolutely no sense, that's okay!! That's what examples are for.  Here we go...

Practice Naming Alkanes

ex:  Draw the 5 structural isomers for C6H14 and give their systematic (IUPAC) name.

  ➞ Start with the straight-chain alkane: n-hexane.
 Practice Naming Alkanes and Hexanes

  ➞ Next, we'll move one carbon into a branching position. We can get two structural isomers out of this: 2-methylpentane, and 3-methylpentane.

2-Methylpentane and 3-Methylpentane

➞ Lastly, we can now move two carbons into branching positions.  We can get two structural isomers out of this: 2,3-dimethylbutane, and 2,2-dimethylbutane.

Structure of 2,3-Dimethylbutane

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ex:  Determine the structure for the following hydrocarbons.

  a.  4-ethyl-3,5-dimethylnonane 

Example of Naming Alkanes and Hydrocarbons

  b.  4-tert-butylheptane

4-tertbutylheptane Structure

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Next up, video #3 from my blog series covering SECTION 20 - An Introduction to Organic Chemistry

In that video post, we'll discuss Naming Cycloalkanes, and we'll begin talking about Alkenes and Alkynes.