Alkynes contain a C≡C triple bond

Acetylene: H-C≡C-H is the common name for ethyne,

used as a torch fuel

Number from the end closest to the triple bond1.

If it's a tie between the ends to two triple bonds,

use branching

2.

Double bonds take priority over triple bonds when

they are equal distance to the end of a chain.

3.

Triple bonds are linear so there is no cis/trans or

E/Z, although any double bonds in the molecule

can still have stereochemical designators.

4.

Alkyne nomenclature follows normal hydrocarbon

naming rules:

Chapter 8: Alkynes: an introduction to organic synthesis

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Recall from 7.1 that an alkene can be prepared by

elimination of H and Br from an alkyl bromide using KOH.

The same can be done (twice) to make an alkyne.

This is useful in making an alkyne (triple bond) from an

alkene (double bond).

As we'll see later, an alkyne can be reduced selectively to

either a cis or trans alkene, so this can be a way of

converting between cis and trans alkenes.

8.2 Preparation of alkynes: elimination of dihalides

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Alkynes contain linear (180o bond angle), sp-hybridized

carbons.

The triple bond is made of one σ and two perpendicular

π bonds. Those π bonds can react very similarly to the π

bond in an alkene.

Addition of HX to an alkyne takes place via a vinylic

carbocation (+ is on a double-bonded C)

8.3 Reactions of alkynes: addition of HX

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Two equivalents of HBr or HCl can add to an alkyne -

both with Markovnikov regiochemistry. H and X usually

have trans stereochemistry.

Halogens can also add twice:

Addition of HX and X2

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Hydration of alkynes has many ideas in common with

hydration of alkenes, but there are some very important

differences.

Aqueous acid alone will allow Markovnikov hydration of

alkenes, but will not react with alkynes.

Aqueous acid + catalytic HgSO4 will readily hydrate

alkenes, but the enol produced will rapidly isomerize to a

more stable ketone.

Tautomerism: rapid isomerization between enol and

keto isomers by proton transfer.

(enol intermediate)

8.4 Hydration of alkynes and tautomerism

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Electrophilic addition of Hg2+ to make vinylic cation.1.

Nucleophilic addition of H2O with loss of proton.2.

Proton from aqueous acid replaces Hg+3.

Proton transfer from O to C in 2 steps converts enol

to more stable keto tautomer.

4.

Hydration is most useful on terminal alkynes (R-C≡C-H)

and symmetrical alkynes because it produces only one

product. Unsymmetrical internall alkynes make mixtures.

Mechanism of mercury(II)-catalyzed hydration

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BH3 adds to alkynes in a non-Markovnikov orientation to

give a vinylic borane. Oxidation makes an enol that

tautomerizes to a more stable ketone.

Again, unsymmetrical internal alkynes produce a mixture

of products.

Terminal alkynes produce an aldehyde because addition

is non-Markovnikov, opposite of Hg(II)-catalyzed

hydration.

Hydroboration/oxidation of alkynes

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Catalytic hydrogenation with Pd/C gives complete

reduction to an alkane just like hydrogenation of alkenes.

Reduction with a deactivated catalyst (Lindlar catalyst =

"poisoned" Pd on CaCO3) occurs by a syn addition to give

a cis alkene.

A trans alkene can be formed with a dissolving-metal

reduction. Addition of H's is stepwise so the more stable

trans alkene is formed.

8.5 Reduction of alkynes

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Alkynes can be cleaved by KMnO4 or O3 to make

carboxylic acids. This is not so useful synthetically, but

can be used to determine the position of triple bonds in

alkynes.

Ozonolysis of an unknown alkyne gives one 6-carbon

dicarboxylic acid HOOC-(CH2)4-COOH and 2 equivalents

of acetic acid, CH3COOH. What is the structure of the

unknown alkyne?

8.6 Oxidative cleavage of alkynes

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Terminal alkynes are weak acids (pKa ~25).

Which direction will the following equilibria lie?

HC≡CH + OH- � HC≡C:- + H2O

HC≡CH + NH2- � HC≡C:- + NH3

NaNH2 is Na+ NH2- (sodium amide) - a base strong enough

to deprotonate terminal alkynes. The conjugate base of

an alkyne is called an acetylide ion.

(Hydrogens attached to sp carbons are more acidic than

sp2 or sp3 because the sp carbon anion is more stable)

Molecule pKa

CH4 60

CH2=CH2 44

NH3 35

HC≡CH 25

H2O 16

8.7 Alkyne acidity: formation of acetylide anions

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Our first carbon-carbon bond forming reaction is the

alkylation of acetylide ions.

The acetylide ion is a very strong nucleophile. It can add

to electrophilic carbons, displacing one of the other

bonds on the carbon. This works best with a 1o alkyl

bromide or iodide.

Careful, this does not work with 2o or 3o alkyl halides!

8.8 Alkylation of acetylide anions

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Organic synthesis is the process of building complex

molecules from simpler ones, one reaction at a time,

through a multistep sequence.

Synthesis of isolated natural products�

Synthesis of modified natural products�

Synthesis of new molecules�

For drug/material design�

To better understand chemsitry�

To be successful in mulitstep organic synthesis, you must

know the reactions!

Starting functional group�

Functional group produced (regiochemistry,

stereochemistry)

�

Reagents used�

Limitations�

8.9 An introduction to multistep organic synthesis

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Retrosynthetic analysis is the best strategy for planning

syntheses: it's working backwards - first looking at the

complex final product you need to make, and considering

what reactions could make that product.

The retrosynthetic analysis uses a two-bar arrow.

Carbon-carbon bonds that are "broken" in the

retrosynthetic direction are actual carbon-carbon bond

forming reactions in the forward direction.

Retrosynthetic direction:

Forward direction:

Retrosynthetic analysis

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Plan a retrosynthesis for the following synthesis problem,

then write the full forward sequence of synthetic

reactions with reagents.

Synthesis practice

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Plan a retrosynthesis for the following synthesis problem,

then write the full forward sequence of synthetic

reactions with reagents.

Synthesis practice

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Plan a retrosynthesis for the following compound,

starting with compounds with no more than five

carbons, then write the full forward sequence of

synthetic reactions with reagents.

Synthesis practice

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Synthesis practice

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Synthesis practice

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