(5) Free-radical addition of hydrogen halides to alkenes - Anti-Markovnikov formation of alkyl halides:

Carbon free radicals are very reactive intermediates with one unpaired electron in the carbon valence shell. They will be studied in depth in Module CM3001.

Free radical reactions usually require a small amount of an initiator - a compound which readily generates free radicals - in order to get started:


Mechanism:

Note the use of single-headed 'fishook' arrows to indicate movement of a single electron.

(6) Oxidation of alkenes with per-acids - formation of epoxides:

The reaction is a concerted syn- addition of an oxygen atom to the double bond - the C-C p-bond is broken and the two C-O bonds are formed simultaneously - so that the stereochemistry of the alkene is preserved in the epoxide.

(7) Oxidation of alkenes with potassium permanganate - formation of cis- 1,2-diols (glycols):

(8) Oxidation of alkenes with ozone - cleavage of the double bond via the formation of ozonides and their subsequent reduction:

Notice that in the reduction step of ozonolysis:

(i) an alkene with an unsubstituted carbon atom yields formaldehyde

(ii) an alkene with a monosubstituted carbon atom yields an aldehyde

(iii) an alkene with a disubstituted carbon atom yields a ketone

Hence these reactions can be useful for the characterisation of molecules of unknown structure which contain carbon-carbon double bonds.


(9) Catalytic reduction of alkenes to alkanes:

Alkenes are not spontaneously reduced to alkanes when treated with hydrogen. However in the presence of a transition metal catalyst the reaction proceeds efficiently - and usually with cis- stereospecificity - at room temperature and with either 1 At or elevated pressures of hydrogen.

Catalysts can either be soluble in the reaction mixture (homogeneous) or insoluble (heterogeneous).

One of the most important of the homogeneous catalysts is (PPh3)3RhCl, tris(triphenylphosphine)rhodium(I) chloride ('Wilkinson's catalyst').

Typical heterogeneous catalysts are palladium on charcoal (Pd/C) or platinum oxide (PtO2, 'Adam's catalyst').

Reduction of alkenes with the aid of homogeneous catalysts such as Wilkinson's Catalyst will be studied in depth as part of the Module CM4104.


THE CARBON-CARBON TRIPLE BOND - THE CHEMISTRY OF ALKYNES

Text references: McMurry (5th Edition) Chapter 8.

Electronic structure of the carbon-carbon triple bond:

Nomenclature - the systematic rules for naming alkynes:

(1) Find the longest chain of carbon atoms that includes the triple bond - then name as for the corresponding saturated hydrocarbon (alkane) but use the termination -yne rather than -ane:

The Preparation of Alkynes:

Remember how we generated carbon-carbon double bonds by -elimination of HX from an alkyl halide:

A similar double b-elimination of HX from a 1,2- or vicinial alkyl dihalide generates a triple bond:

If we remember that 1,2-dihalides are themselves prepared by the electrophilic addition of a halogen to an alkene we have a sequence of reactions for converting double to triple bonds:

The double dehydrohalogenation of an 1,2-dihaloalkene proceeds via a vinyl halide - hence vinyl halides themselves are useful precursors for alkynes:

Reactions of Alkynes:

(1) C-H Acidity:

Unlike p- and d-orbitals, s-orbitals do not have a node (i.e. a region of zero electron density) at the nucleus. In fact s-orbital electron density actually penetrates right into the atomic nucleus. The amount of s-electron density involved is tiny but enough to produce observable effects. One significance of this effect is that s-electrons - or electrons in orbitals with a high % s-character - are attracted to and stabilised by the positively charged nucleus more strongly than electrons in orbitals with little or no s-character. This, in turn, influences the acidity of C-H bonds.

Of all hydrocarbons, the sp terminal C-H bonds in alkynes are the most easily deprotonated:

Acetylide anions are both good nucleophiles and strong bases: