Chapter 11 O Lecture

8/7/2019 Chapter 11 O Lecture

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The physical properties of alkynes resemble those ofhydrocarbons of similar shape and molecular weight.

Alkynes have low melting points and boiling points.

Melting point and boiling point increase as the number

of carbons increases.

Alkynes are soluble in organic solvents and insoluble inwater.

Physical Properties


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Recall that alkynes are prepared by eliminationreactions. A strong base removes two equivalents of HXfrom a vicinal or geminal dihalide to yield an alkyne

through two successive E2 elimination reactions.

Preparation of Alkynes


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Like alkenes, alkynes undergo addition reactionsbecause they contain relatively weak T bonds.

Two sequential reactions can take place: addition of one

equivalent of reagent forms an alkene, which can then

add a second equivalent of reagent to yield a producthaving four new bonds.

Introduction to Alkyne ReactionsAdditions


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Figure 11.5

Fou

r addition reactionsof 1-butyne


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Two equivalents of HX are usually used: addition of onemole forms a vinyl halide, which then reacts with asecond mole of HX to form a geminal dihalide.

HydrohalogenationElectrophilic Addition of HX

Alkynes undergo hydrohalogenation, i.e the, addition ofhydrogen halides, HX (X = Cl, Br, I).


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Electrophilic addition of HX to alkynes is slower than

electrophilic addition of HX to alkenes, even though alkynes aremore polarizable and have more loosely held T electrons thanalkenes.

Markovnikov addition in step [3] places the H on the terminal

carbon to form the more substituted carbocation A, rather than

the less substituted carbocation B.


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Carbocation A is stabilized by resonance, but B is not.

Two resonance structures can be drawn for carbocationA, but only one Lewis structure can be drawn forcarbocation B.

Markovnikovs rule applies to the addition of HX to vinylhalides because addition of H+ forms a resonance-

stabilized carbocation.


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Halogens X2 (X = Cl or Br) add to alkynes just as they doto alkenes. Addition of one mole of X2 forms a transdihalide, which can then react with a second mole of X2

to yield a tetrahalide.

HalogenationAddition of Halogen


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In the presence of strong acid or Hg2+ catalyst, theelements of H2O add to the triple bond, but the initialaddition product, an enol, is unstable and rearranges toa product containing a carbonyl groupthat is, a C=O. A

carbonyl compound having two alkyl groups bonded tothe C=O carbon is called a ketone.

HydrationElectrophilic Addition of Water


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Internal alkynes undergo hydration with concentrated

acid, whereas terminal alkynes require the presence ofan additional Hg2+ catalystusually HgSO4to yieldmethyl ketones by Markovnikov addition of water.


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Consider the conversion of a general enol A to the carbonylcompound B. A and B are tautomers: A is the enol form and B is the

keto form of the tautomer.

Equilibrium favors the keto form largely because the C=O is muchstronger than a C=C. Tautomerization, the process of converting

one tautomer into another, is catalyzed by both acid and base.


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HydroborationOxidation

Hydroborationoxidation is a two step reaction sequencethat converts an alkyne to a carbonyl compound.


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Hydroborationox

idation of an internal alkyne forms aketone.

Hydroboration of a terminal alkyne adds BH2 to the lesssubstituted, terminal carbon. After oxidation to the enol,tautomerization yields an aldehyde, a carbonyl compound

having a hydrogen atom bonded to the carbonyl carbon.


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Because sp hybridized CH bonds are more acidic thansp2 and sp3 hybridized CH bonds, terminal alkynes arereadily deprotonated with strong base in a Brnsted-Lowry acid-base reaction. The resulting ion is called the

acetylide ion.

Introduction to Alkyne ReactionsAcetylide anions


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Reactions of Acetylide Anions

Acetylide anions react with unhindered alkyl halides to yieldproducts of nucleophilic substitution.

Because acetylides are strong nucleophiles, the mechanismof substitution is SN2, and thus the reaction is fastest with

CH3X and 10 alkyl halides.


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Steric hindrance around the leaving group causes 2 and 3

alkyl halides to undergo elimination by an E2 mechanism, asshown with 2-bromo-2-methylpropane.

Thus, nucleophilic substitution with acetylide anions formsnew carbon-carbon bonds in high yield only with unhindered

CH3X and 1 alkyl halides.


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Acetylide anions are strong nucleophiles that open epoxiderings by an SN2 mechanism.

Backside attack occurs at the less substituted end of theepoxide.


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Synthesis

You can now begin to consider (for example) how to prepare afive-carbon product from three smaller precursor molecules

using the reactions you have learned.

To plan a synthesis of more than one step, we use the processof retrosynthetic analysisthat is, working backwards from a

desired product to determine the starting materials fromwhich it is made.


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To write a synthesis working backwards from the products to thestarting materials, an open arrow () is used to indicate that theproduct is drawn on the left and the starting material on the right.

The product of the synthesis is called the target compound.

In designing a synthesis, reactions are often divided into twocategories:

1. Those that form new carbon-carbon bonds.

2. Those that convert one functional group into anotherthat is,

functional group interconversions.

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Chapter 11 O Lecture

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