An alkyne is a hydrocarbon that contain a Carbon carbon triple bond. Acetylene, the simplest alkyne, widely used in industry for the synthesis of

Transcription

1 Alkynes

2 An alkyne is a hydrocarbon that contain a Carbon carbon triple bond. Acetylene, the simplest alkyne, widely used in industry for the synthesis of acetaldehyde, acetic acid, vinyl chloride O O H C C H H 3 C C H H 3 C C OH H 2 C HCl

3 Alkynes naming follows the general rules for hydrocarbon, suffix yne is used and the position of triple bond is indicated by giving the number of the first alkyne carbon, numbering begin at the end near to the triple bond giving it as much as possible the lowest number. If there is a double bond and triple bond in a compound then start numbering from the end near to any multiple bond.

4 As with alkyl and alkenyl substituents derived from alkane and alkenes, respectively alkynyl groups are also possible.

5 Alkynes can be prepared by elimination of HX from alkylhalides same manner as alkenes. Treatment of a 1,2-dihaloalkane ( a vicinal dihalide) with excess strong base such as KOH or NaNH2 result in twofold elimination of HX and formation of alkyne. Vicinal dihalides are prepared by addition of Br2 or Cl2 to alkenes. Halogenation/dehydrohalogenation sequence convert alkene to alkyne. KOH Ethanol

6 The twofold dehydrohalogenation take place through a vinylic halide intermediate, this means vinylic halides themselves should give alkynes when treated with strong base.

7 Acetylene is sp hybridize with linear shape of an angle 180 C, the triple bond made of sp-sp (σ) and two p orbital overlapping (2π). Electrophiles undergo addition reaction with alkynes much as they do with alkenes.

8 Example reaction of 1-hexyne with 2 equivalents of HBr yield 2,2-dibromohexane. The regiochemistry of addition follows Markovnikov s rule, halogen adds to the more substituted side of alkyne bond and hydrogen adds to the less substituted side with trans steriochemistry of H and X.

9 Bromine and chlorine also add to alkynes to give addition products, and trans stereochemistry again results.

10 The mechanism of alkyne addition is similar but not identical to alkene addition. The electrophile such HBr addition to alkene take place in two step through carbocation intermediate. Addition of HBr to alkyne by the same above mechanism form vinylic carbocation as intermediate which then add bromine ion.

11 A vinylic carbocation is formed less readily, secondary vinylic carbocation forms about same as a primary carbocation, thus primary vinylic carbocation is so difficult to form and even there is no evidence it even exist

12 Alkynes can be hydrated by either of two methods: 1) direct addition of water catalysed by mercury (II) ion to yield Markovnikov product 2) indirect addition of water by hydroboration /oxidation (antimarkovnikov). Alkynes don t react directly with aqueous acid but it undergo hydration in the presence of mercuric sulphate as Lewis acid catalyst. Reaction occur according to Markovnikov rule OH added to the most substituted carbon.

13 CH 3 CH 2 CH 2 CH 2 C O H CH 2 CH 3 CH 2 CH 2 CH 2 C O CH 3 Tautomerize The product isolated from alkyne hydration is not vinylic alcohol or enol (ene + ol), but is ketone, although the enol is an intermediate in the reaction. The enol immediately rearranges to ketone by process called ketoenol tautomerism. The individual keto and enol forms are said to be tautomers (constitutional isomers rapidly interconvert into each other) The equilibrium lies to the side of the ketone.

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16 This reaction of hydration of alkyne in presence og Hg++ is useful when terminal alkyne is used, as the hydration of unsymmetrical substituted internal alkyne leads to mixture of ketones

17 Borane adds rapidly to an alkyne as it does in an alkene resulting in vinylic borane that can be oxidize by hydrogen peroxide to yield an enol, that tautomerize into ketone or aldehde depending on the structure of the alkyne. Internal alkyne such as 3-hexyne give ketone whereas terminal alkyne an aldehyde.

18 The relatively unhindered terminal alkyne undergo two additions giving a double hydroborated intermediate. Oxidation with H2O2 replace both boron atoms with OH which give aldehyde

19 Comparison of hydration in presence of mercurry ion to that of hydroboration oxidation

20 Alkynes are reduced to alkanes by addition of H2 over a metal catalyst. The reaction occurs insteps through an alkene intermediate. The first step is more exothermic than the second.

21 Alkynes like alkenes can be cleaved by reaction with a powerful agents such as ozone or KmnO4. The triple bond is less reactive and yield of product is low. Cleave of internal alkyne is carboxylic acids and terminal alkyne give CO2.

22 The most important difference between alkene and alkynes is that terminal alkynes are weakly acidic, pka 25. When terminal alkyne is treated with strong base such as sodium amide Na + - NH 2 the terminal hydrogen is removed and an acetylide anion is formed.

23 The acetylide anion is stabilized by the great s character sp orbital unlike sp2 or sp3 in case of alkenyl or alkyl ion.

24 The presence of a negative charge and unshared electrons on the carbon makes acetylide anion strong nucleophile they can react with many different kinds of electrophiles.

25 These reaction are called alkylation is limited to primary alkyhalide as in secondary alkylhalides elimination will take place because acetylide anion is strong base can eliminate HX.