Organic Chemistry

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9. Alkenes and Alkynes

Acetylide

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Now it’s time to introduce some of the most important nucleophiles in all of organic chemistry, alkynides

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concept

Understanding how to convert terminal alkynes to alkynides.

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Recall that most hydrocarbons are terrible acids, having pKas from 40-50. However, terminal alkynes (alkynes with a hydrogen) are uniquely acidic due to the hybridization effect, having a pKa of 25.

This means they can be easily deprotonated using a strong base (typically NaH or NaNH2). 

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The resulting conjugate base is known as a sodium alkynide, and these are often used to lengthen carbon chains to through SN2 reactions.

  • For now, this will be the only way to increase the number of carbons in your product.

Let's take a look at the mechanism of akynide synthesis.

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concept

The mechanism of akynide synthesis.

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Beware: You can only use this reaction with primary alkyl halides. Secondary alkyl halides will favor E2!

Let's put it all together!

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concept

Using double dehydrohalogenation to perform alkynide synthesis.

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example

Predict the final product of the following reaction sequence.

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