CHAPTER 4: ALKYNES

Contents 4.1 Nomenclature 4.2 Preparation of Alkynes 4.3 Structure 4.4 Physical Properties 4.5 Chemical Reactivity

Alkynes are hydrocarbons that have at least one triple bond, CC, between two carbon atoms, with the formula CnH2n-2 The alkynes are traditionally known as the acetylene series. Disubstituted alkynes, R-C  C-R', are described as "internal" alkynes. Monosubstituted alkynes, R-C  C-H, are described as "terminal" alkynes.

 An internal or a terminal alkyne? (a) hex-1-yne  (c) cyclooctyne  (b) oct-3-yne  (d) propyne 

4.1 Nomenclature Alkynes are named by general rules similar to those used for alkanes and alkenes. The suffix -yne is used in the parent hydrocarbon name to denote an alkyne. The position of the triple bond is indicated by its number in the chain. Numbering begin at the chain end near the triple bond so that the triple bond as low a number as possible.

Compounds containing both double and triple bonds are called enynes. Numbering of the hydrocarbon chain starts from the end nearer the first multiple bond, whether double or triple. If there is a choice in numbering, double bonds receive lower numbers than triple bonds. Hept-1-en-6-yne 4-methylnon-7-en-1-yne

4.2 Preparation of Alkynes Alkynes are generally prepared by a. Dehydrohalogenation of either  geminal (1,1-) or -CX2-  vicinal (1,2-) alkyl dihalides -CHX-CHX- (X: usually Br or Cl),  using a strong base, usually NaNH2 or (CH3)3CO-K+ These reactions are typically E2 reactions

 Reactions occur via an alkenyl halide. b. The reaction of metal acetylides with primary alkyl halides. + + + NaX

 Practice Problem What is the alkyne product from the reactions of the following with NaNH2: (a) 2,2-dibromopropane (b) 1,1-dibromooctane (c) 1,2-dibromohexane (d) 2,3-dibromohexane

4.3 Structure The functional group alkyne, CC consists of two sp hybridised C atoms bonded to each other via one σ and two  bonds. The 2π bonds are produced by the side-to-side overlap of the two pairs of p-orbitals not utilised in the hybrids. The substituents are attached to the C atoms in triple bonds CC via σ bonds.

The 2 C of the CC and the 2 atoms attached directly to the -C  C- are linear.  They cannot exist as cis- / trans- isomers. The two separate perpendicular Combined p p molecular orbitals molecular orbitals

4.4 Physical Properties In general, alkynes are non-polar insoluble in water but soluble in non-polar organic solvents.

4.5 Chemical Reactivity The  bonds are a region of high electron density; it allows an acetylinic carbon to have a greater amount of electronegative character. Alkynes are typically nucleophiles.

Terminal alkynes, R-CC-H, are quite acidic; pKa = 26. A terminal alkyne with a strong base  sodium, sodium amide, n-butyllithium or a Grignard reagent,  gives the anion of the terminal alkyne (a metal acetylide): 2 RC≡CH + 2 Na → 2 RC≡CNa + H2 R-C≡C-H R-C≡C:- + B-H B:-

The acetylide ion is a good nucleophile and can be alkylated to give higher alkynes. The alkyne CC undergoes a variety of addition reactions  in which one or both of the π-bonds are converted to new σ bonds.

4.5.1 Alkylation of Alkynes 1. R-C≡C-H + NaNH2  R-C≡C:-Na+ + NH3 Reaction Type: Acid / Base  and  Nucleophilic Substitution 1. R-C≡C-H + NaNH2  R-C≡C:-Na+ + NH3 2. R-C≡C:-Na+ + R’-X  R-C≡C-R’ + NaX

The acetylide carbanion is a good C nucleophile and can undergo substitution reactions with 1o or 2o alkyl halides (Cl, Br or I) to produce an internal alkyne. One or both of the terminal H atoms in ethyne (acetylene) H-C  C-H can be susbtituted providing access to monosubstituted (R-C  C-H) and symmetrical (R=R') or unsymmetrical (R≠R') disubstituted alkynes R-CC-R'

MECHANISM FOR ALKYLATION OF ALKYNES Step 1: Attacks of the amide ion removes the terminal H to generate the acetylide ion, a carbon nucleophile. Step 2: The carbanion reacts with the alkyl halide to form a new C-C bond.

 Practice Problems What is the product of the reactions of CH3-CC-Na+ with each of the following: (a) 2-bromopropane (b) 1-iodooctane (c) (R)-2-bromohexane (d)  ethyl tosylate (e)   bromobenzene

4.5.2 Addition Reactions of Alkynes Notes: The high electron density of the  bonds makes them nucleophilic. Alkynes undergo addition reactions in an analogous fashion to those of alkenes.

Types of Reactions 1. Hydrogenation 2. Dissolving Metal reduction 3. Hydrohalogenation 4. Hydration 5. Halogenation 6. Ozonolysis

1. Hydrogenation of Alkynes Alkynes can be partially reduced to cis-alkenes with H2 in the presence of Lindlar's catalyst  Pd / CaCO3 / quinoline (poisoned catalysts). Alkynes can be reduced to alkanes with H2 in the presence of catalysts  Pt, Pd, Ni etc.

The reaction is stereospecific giving only the syn-addition product. The new C-H σ bonds are formed from H atoms absorbed onto the metal surface.

2. Dissolving Metal Reduction of Alkynes Alkynes can be reduced to trans-alkenes using Na in NH3 (liquid). This reaction is stereospecific giving only the trans-alkene via an anti addition.

The reaction proceeds via single electron transfer from the Na with H coming from the NH3. These reaction conditions do not reduce alkenes

3. Reaction of Alkynes with Hydrogen Halides The reaction of alkynes with HX forms vinyl halides.  Hydrogen halide reactivity order : HI > HBr > HCl > HF.

Regioselectivity predicted by Markovnikov's rule:  the H adding to the C with the most H already present. Reaction proceeds via protonation to give the more stable carbocation intermediate.

In the presence of excess HX,  a second addition can occur to the product alkene giving a geminal dihalide.  Can you suggest a reason why this regioselectivity is observed ? 

 Practice Problems What would be the product from the reaction of 2-butyne with excess HBr?  For HBr, in the presence of radicals the addition occurs with opposite regiochemistry Why does this reaction have the opposite regiochemistry ? 

4. Hydration of Alkynes Alkynes can be hydrated to form enols that immediately tautomerise to ketones Reagents: aq. H2SO4 with a mercury salt. Regioselectivity predicted by Markovnikov's rule.

Reaction proceeds via protonation to give the more stable carbocation intermediate. Not stereoselective since reactions proceeds via planar carbocation.

5. Halogenation of Alkynes Overall transformation :  C  C to  X-C=C-X or finally to  X2C-CX2 Reagent : the halogen (e.g. Br2) in a solvent methylene chloride, CH2Cl2.

Reaction proceeds via cyclic halonium ion. Stereoselectivity : anti since the two C-X bonds form in separate steps one from X2 the other X-.

6. Ozonolysis of Alkynes Ozonolysis implies that ozone causes the alkyne to break (-lysis). Overall transformation :  C  C to 2 x CO2H Reagents : ozone followed by aqueous work-up. The C  C bond becomes two C=O bonds (Reagent: KMnO4/HO-) R1-C≡C-R2 R1COOH + R2COOH 1. + O3 2. + H2O/Zn

 Practice Problem What would be the products of the ozonolysis reactions of:  (a) ethyne ?   (b) but-1-yne ?  (c) but-2-yne ?   (d) cyclooctyne ?  -The End- 

The End of Alkynes