Alkynes Unit 9

Introduction Alkynes contain a triple bond. General formula is CnH2n-2. Two elements of unsaturation for each triple bond. Some reactions resemble the reactions of alkenes, like addition and oxidation. Some reactions are specific to alkynes.

Nomenclature: IUPAC Find the longest chain containing the triple bond. Change -ane ending to -yne. Number the chain, starting at the end closest to the triple bond. Give branches or other substituents a number to locate their position.

Examples of Nomenclature All other functional groups, except ethers and halides have a higher priority than alkynes.

Nomenclature IUPAC: use the infix -yn- to show the presence of a carbon- carbon triple bond Common names: prefix the substituents on the triple bond to the word “acetylene” IUPAC name: 2-Butyne 1-Buten-3-yne Common name: D imethylacetylene V inylacetylene

Cycloalkynes Cyclononyne is the smallest cycloalkyne isolated it is quite unstable and polymerizes at room temp the C-C-C bond angle about the triple bond is approximately 155°, indicating high angle strain

Physical Properties Nonpolar, insoluble in water. Soluble in most organic solvents. Boiling points are similar to alkane of same size. Less dense than water. Up to four carbons, gas at room temperature.

Physical Properties Similar to alkanes and alkenes of comparable molecular weight and carbon skeleton

Acetylene Acetylene is used in welding torches. In pure oxygen, temperature of flame reaches 2800C. It would violently decompose to its elements, but the cylinder on the torch contains crushed firebrick wet with acetone to moderate it.

Acidity of Alkynes Terminal alkynes, are more acidic than other hydrocarbons due to the higher s character of the sp hybridized carbon. Terminal alkynes can be deprotonated quantitatively with strong bases such as sodium amide (-NH2). Hydroxide and alkoxide bases are not strong enough to deprotonate the alkyne quantitatively.

Acidity The pKa of acetylene and terminal alkynes is approximately 25, which makes them stronger acids than ammonia but weaker acids than alcohols terminal alkynes react with sodium amide to form alkyne anions

Acidity Terminal alkynes can also be converted to alkyne anions by reaction with sodium hydride or lithium diisopropylamide (LDA) Because water is a stronger acid than terminal alkynes, hydroxide ion is not a strong enough base to convert a terminal alkyne to an alkyne anion

Alkylation of Alkyne Anions Alkyne anions are both strong bases and good nucleophiles They participate in nucleophilic substitution reactions with alkyl halides to form new C-C bonds to alkyl groups; they undergo alkylation

Preparation from Alkenes Treatment of a vicinal dibromoalkane with two moles of base, most commonly sodium amide, results in two successive dehydrohalogenation reactions (removal of H and X from adjacent carbons) and formation of an alkyne

Allene Allene: a compound containing a C=C=C group the simplest allene is 1,2-propadiene, commonly named allene

Allenes most allenes are less stable than their isomeric alkynes, and are generally only minor products in alkyne-forming dehydrohalogenation reactions

Addition of X2 Alkynes add one mole of bromine to give a dibromoalkene addition shows anti stereoselectivity

Addition of HX Alkynes undergo regioselective addition of either 1 or 2 moles of HX, depending on the ratios in which the alkyne and halogen acid are mixed B r B r H B r H B r C H 3 C H C H 3 = 2 C H 3 B r Propyne 2-Bromopropene 2,2-Dibromopropane

Mechanism of Hydrogen Halide Addition The triple bonds abstract a proton from the hydrogen halide forming a vinyl cation. The proton adds to the least substituted carbon. The second step of the mechanism is the attack by the halide.

Hydroboration Addition of borane to an internal alkyne gives a trialkenylborane addition is syn stereoselective

Hydroboration Treating an alkenylborane with H2O2 in aqueous NaOH gives an enol enol: a compound containing an OH group on one carbon of a carbon- carbon double bond an enol is in equilibrium with a keto form by migration of a hydrogen from oxygen to carbon and the double bond from C=C to C=O keto forms generally predominate at equilibrium keto and enol forms are tautomers and their interconversion is called tautomerism

Hydroboration O 1 . B H 2 . H O , N a 3-Hexyne 3-Hexanone hydroboration/oxidation of an internal alkyne gives a ketone hydroboration/oxidation of a terminal alkyne gives an aldehyde O 1 . B H 3 2 . H O , N a 3-Hexyne 3-Hexanone

Addition of H2O: hydration In the presence of sulfuric acid and Hg(II) salts, alkynes undergo addition of water O H O H 2 S O 4 C H 3 C H + H 2 O C H 3 = 2 C H 3 H g S O 4 Propyne 1-Propen-2-ol (an enol) Propanone (Acetone)

Reduction Treatment of an alkyne with hydrogen in the presence of a transition metal catalyst, most commonly Pd, Pt, or Ni, converts the alkyne to an alkane

Reduction With the Lindlar catalyst, reduction stops at addition of one mole of H2 this reduction shows syn stereoselectivity *Lindlar catalyst – Pd deposited on CaCO3 then poisoned with various forms of Pb or S

Ozonolysis Ozonolysis of alkynes produces carboxylic acids (alkenes gave aldehydes and ketones). Used to find location of triple bond in an unknown compound. H O C 2 3 (2) (1) +