Alkynes

1. Alkynes .1Introduction .2Nomenclature of Alkynes .3Physical Properties of Alkynes .4Preparation of Alkynes .5Reactions of Alkynes Dr Manal F. AbouTaleb

2. Structure Of Alkynes • They are unsaturated hydrocarbons – made up of C and H atoms and contain one or more C≡Ctriple bond somewhere in their structures. • Their general formula is CnH2n-2. • The alkyne triple bond is composed of one σ and two 2  covalent bonds, the triple bond can be terminal or internal. • Terminal alkyneshave the triple bond at the end of the carbon chain so that a hydrogen atom is directly bonded to a carbon atom of the triple bond. • Internal alkynes have a carbon atom bonded to each carbon atom of the triple bond. 2

3. 2p 2p sp 2s 1s 2p 1s 2s 1s Structure of Alkynes Acetylene (ethyne): spHybrid orbitals sp-hybridized state Exited state Ground state Hybridization Promotion of electron The shape of an sp hybrid obital: Each sp hybrid orbital: 50% s character 50% p character

4. 180° An sp-hybridized carbon atom: • Each carbon is sp hybridized with a linear geometry and bond angles of 1800, perpendicular to the two remaining porbitals. • Geometric structure of sp-hybridized C atoms is linear. • In the mole. of acetylene, the formation of C-Cσbond : sp-sp overlap; • The formation C-H σ bonds: sp-1s overlap. The formation of two C-C π bonds: 2py-2py overlap and 2pz-2pz overlap.

6. Nomenclature of Alkynes Nomenclature of Alkenes 1. In the IUPAC system, change the –aneending of the parent alkane name to the suffix –yne. 2.Determine the stem nameby selecting the longest possible straight chain containing the C ≡ C triple bond and number the chain to give the triple bond the lower number. 3. Compounds with two triple bonds are named as diynes, those with three are named as triynes and so forth. 3. Designate the position of the C ≡ C triple bond by using the number of the first atom of the double bond

7. Nomenclature of Alkynes Examples: 5-bromo-2-pentyne Propyne IUPAC :2,6-Dimethyl-3-heptyne Common :Isobutylisopropylacetylene 1-Butyne 7

8. Nomenclature of Alkynes • 4. Double and triple bonds are considered to have equal priority: thus in a molecule with both a double and triple bond, whichever is close to the end of the chain determines the direction of numbering. • 5. In case where double and triple bonds would have the same position number, the double bond takes the lower number. • 6. Compounds with both a double and triple bond are named as enynes ( ‘’ene’’ comes before ‘’yne’’). • 7. The simplest alkyne, H-CC-H, named in the IUPAC system as ethyne, is more often called acetylene, its common name. • 8. The two-carbon alkyl group derived from acetylene is called an ethynyl group. 1-penten-4-yne 3-Penten-1-yne

10. Common Nomenclature Of Alkynes • The simplest alkyne its common name is acetylene • Therefore the common names of alkynes are derived from acetylene ( e.g. Methyl acetylene) Examples: Common : Methyl acetylene Common : Isobutyisopropylacetylene Common: Isopropylmethylacetylene

11. Exercise 11 1. Give the IUPAC and common names of the following compounds:

12. Alkynes are non polar compounds. Insoluble in water. Soluble in non polar organic solvents. They are less dense than water. Alkynes have low melting points and boiling points. Melting point and boiling point increase as the number of carbons increases. Terminal alkynes, R-CC-H, are more acidic than other hydrocarbons. Physical Properties of Alkynes

13. Preparation of Alkynes Elimination Reactions 1. Dehydrohalogenation of dihaloalkanes (-2HX) • Dehydrohalogenationis the elimination of (-2HX) from a dihaloalkane in presence strong base

14. Preparation of Alkynes 2. From reaction of sodium Acetylide with Primary Alkyl Halides

15. Reactions of Alkenes Addition Reactions • Like alkenes, alkynes undergo addition reactions because they contain relatively weak  bonds. • Two sequential reactions can take place: addition of one equivalent of reagent forms an alkene, which can then add a second equivalent of reagent to yield a product having four new bonds. • Reactivity: Alkynes < Alkenes

16. 1. Addition of hydrogen ( Hydrogenation) Reactions of Alkenes • Alkynes can be partially reduced to cis-alkenes with H2 in the presence of poisoned catalysts. • Lindlar catalyst: Pd/ CaCO3, Pb(Ac)2-quinoline 16

17. Alkynes can be reduced totrans-alkenes using Na or Li in liquid NH3 (meaning Metal-Ammonia ) 17

18. 2. Addition of halogen Excess HX Reactions of Alkenes 3. Addition of hydrogen halide Markovnikov’s rule applies to the addition of HX to vinyl halides Alkyl dihalide Alkenyl halide 18

19. 4. Addition of water: Hydration

20. Keto-enol tautomerism Transformation of Functional groups:

21. Hydroboration—Oxidation • Hydroboration—oxidation is a two step reaction sequence that converts an alkyne to a carbonyl compound.

22. Hydroboration—oxidation of an internal alkyne forms a ketone. • Hydroboration –oxidation of a terminal alkyne form an aldehyde.

23. Reactions of Alkenes Examples: Four addition reactions of 1-butyne

24. Exercise • 1. An alkyne’s name ends with • (a) –ane (b) -ene • (c) –yne (d) diene • 2. An alkyne function has …….. pi bond(s). • (a) one (b) two • (c) three (d) four • 3. Alkynes react with HCl by a mechanism called • (a) elimination (b) Markovnikov addition • substitution 4. Alkynes react with water in the presence of a catalyst to give (a) a dialcohol (diol) (b) an alkane (c) an enol (d) a dibromide 5. The conversion of alkynes to alkanes is an example of (a) oxidation (b) reduction (c) chlorination (d) dehydration