What alkynes would you start with to prepare the following...

Key Concepts

Structural Analysis of Alkynes for Ketone Synthesis

Selecting the appropriate alkyne as a starting material requires careful examination of its structure, particularly the substitution pattern around the triple bond. The position and symmetry of substituents will dictate the regiochemistry of the hydration reaction and ultimately determine the placement of the carbonyl group in the ketone product.

Alkyne Hydration

This reaction involves the addition of water to a carbon–carbon triple bond under acidic and often mercuric ion–catalyzed conditions. The process converts an alkyne into an enol, which subsequently tautomerizes to form a ketone. It is a cornerstone reaction in synthetic organic chemistry for constructing carbonyl compounds from alkynes.

Markovnikov’s Rule in Alkyne Addition

When hydrating unsymmetrical alkynes, the addition of the components of water follows Markovnikov’s rule, meaning that the proton adds to the carbon with more hydrogen substituents, while the hydroxyl group is added to the more substituted carbon. This regioselectivity is key in determining the structure of the resulting ketone.

Tautomerization Process

After the initial formation of an enol from the hydration of an alkyne, tautomerization occurs, shifting the double bond and hydrogen atom to form the more thermodynamically stable ketone. This keto-enol tautomerism is fundamental in converting transient reaction intermediates into stable carbonyl compounds.

Transcript

00:01 For assigning systematic names to alkynes, we look for the longest continuous carbon chain that contains both carbons of the triple bond.

00:11 So in this first example, there is a carbon chain of six carbons with the triple bond between carbons three and four and methyl substituents on the second and fifth carbons.

00:25 So the methyl substituents get prefixes.

00:31 So the name is going to be 2 ,5 -dimethyl -hex -3 -ine, the locant three immediately in front of the yne indicating the position of the triple bond.

00:45 A lot of people would also name that 2 ,5 -dimethyl -3 -hex -ine because the locant three could only be locating the triple bond in that case.

00:59 Triple bond is the only piece of the name that requiring a locant that follows the three.

01:07 So either one would be considered unambiguous.

01:16 On the second example, the parent chain is four carbons and will number from the end closer to the triple bond.

01:27 So that means the triple bond starts at carbon one.

01:30 So the two methyl substituents are both attached to carbon three.

01:36 So the full name is 3 ,3 -dimethyl -but -1 -ine.

01:40 Again, 3 ,3 -dimethyl -1 -but -ine could be considered acceptable.

01:49 Much in the same way, this third example, we treat it in a similar fashion.

01:58 The parent chain is eight carbons and the triple bond is between carbons four and five regardless of which end we number from.

02:08 So we're going to number from the end closer to the methyl substituents so they get the lower locant.

02:15 So if we number left to right, the methyl groups are on carbon three.

02:20 Triple bond starts at carbon four.

02:23 So 3 ,3 -dimethyl -oct -4 -ine or 3 ,3 -dimethyl -4 -oct -ine would work for that.

02:33 And a similar situation, this fourth example, although the carbon chain now is only seven carbons, and we want to number from the end closer to where the triple bond starts...

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