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Alkynes & Dienes

Alkynes & Dienes. Alkynes. Hydrocarbons that have at least one triple bond between two adjacent carbons Contain the general formula of C n H 2n-2 Contains carbon atoms with sp hybrid orbital (2 p orbitals & 2 sp hybrid orbitals ) Undergoes electrophilic addition

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Alkynes & Dienes

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  1. Alkynes & Dienes

  2. Alkynes • Hydrocarbons that have at least one triple bond between two adjacent carbons • Contain the general formula of CnH2n-2 • Contains carbon atoms with sp hybrid orbital (2 p orbitals & 2 sp hybrid orbitals) • Undergoes electrophilic addition • The major type reaction are the addition type reaction

  3. Physical Properties • They are insoluble in water • Has low polarity thus still insoluble in water and quite soluble in solvents of low polarity like ligroin, ether, benzene and CCl4 • They are less dense than water • There boiling points show the usual increase with increasing carbon number

  4. Chemical Properties • Alkynes are unstable and most reactive compared to alkanes and alkenes because of their shorter bond length & increased electron density. • Electrons in an s orbital benefit from closer proximity to the positively charged atom nucleus, and therefore lower in energy • The acidity is based to be stabilized as a result of high character of sp orbital

  5. Terminal & Internal alkynes • Terminal alkyneshave a hydrogen atom bonded to at least one of the sp hybridized carbons (those involve in the triple bond). ex. Methyl acetylene • Internal alkyneshave something other than hydrogen attached to the sp hybridized carbons, usually carbon atom, but could be a hetero atom. ex. 2- pentyne

  6. Preparation of Alkynes 1. a) From coal and limestone

  7. b) ( CO and H2 are important side products and needed in the production of alcohol)

  8. Alkynes Preparation 2. Dehydrohalogenation of Alkyl Dihalides ( Elimination reaction) • This reaction is particularly useful since the dihalides are readily obtained from the corresponding alkenes by the addition of halogen

  9. Dehydrohalogenation of Alkyl Dihalides

  10. 3. Displacement or substitution reaction a.)Reaction of sodium acetylides with primary alkyl halides • This reaction involves substitution of acetylide ion for halide ion • It results from the attack by the acetylide ion on carbon • The reaction is limited to used of primary halides because of the general tendency for secondary and tertiary halides to undergo a side reaction, elimination

  11. Reaction of sodium acetylides with primary alkyl halides Example: HC C:-Na+ + CH3CH2CH2CH2Br HC C(CH2)3CH3

  12. b.) using HMPT( hexamethylphosphorictriamide)

  13. 4. Dehalogenation of tetrahalides • This reaction is to eliminate the halogens to form a tetrahalides • The groups are eliminated and the reagent used are essentially the same as the preparations of alkenes

  14. Dehalogenation of tetrahalides Example: Br Br CH3 C CH Zn CH3 C CH Br Br

  15. Reaction of Alkynes 1. Addition of Hydrogen The addition of hydrogen atoms react to the triple bond carbon that can easily form a C-H bond formation generating the alkenes ex. Alkynes convert to alkenes by the addition of Hydrogen H+ H -

  16. 2. Addition Halogens( halogenation) ex. Acetylene was attacked by halogens resulting to form 1-2, dibromoethene Br+ Br - Br+ Br -

  17. 3. Addition of Hydrogen Halides ex. The hydrogen halides reacts to the alkynes to form bromopropene

  18. The reaction of an excess hydrogen halides, a second addition will occur to the product of alkene giving a giminaldihalide.

  19. 4. Addition of water. Hydration The addition of water to acetylene to form acetyldehyde, which can be oxidized to acetic acid , is an extremely important industrial process ex. By the addition of water the alkyne was reacted to form a acetyldehyde

  20. 5. Formation of Heavy metal acetylides Formation of Heavy metal acetylides The acidic acetylene will react with certain heavy metals such as the silver (Ag+) to form insoluble acetylides ex. H HAg C C  + 2Ag alcohol C C  + 2H+ Ag Acetylene will react to a heavy metal to form silver acetylides Ag+ Ag -

  21. 6. Formation of alkali metal acetylides The acetylene will react to a alkali metal which is the sodium enable liberate the hydrogen gas to form sodium acetylides ex. HC C- H + Na HC C: -Na+ + ½H2 Or metal acetylide can react with ketone and further with an acid to form alcohol

  22. Dienes

  23. Dienes are hydrocarbons which contain two double bonds • Intermediate between alkanes and polyenes

  24. Classes • Cumulated dienesdienes have the neighboring double bonds ex. 1,2 Butadiene • Conjugated dieneshave conjugated double bonds separated by one single bond. ex. 2- methyl-1,3- butadiene • Isolated dieneshavedouble bonds that are separated by more than one single bond ex. 1,3-Butadiene

  25. Properties of Dienes • The chemical properties of diene depend upon the arrangement of its bonds • Isolated diene are identical with the simple alkenes • In conjugated dienes they differ from simple alkenes in three ways: (a) they are more stable, (b) they favor 1,4-addtion than 1,2-addition, (c) toward free radical addition, they are more reactive

  26. Preparation of Dienes • Dienes are usually prepared by adaptations of the methods used to make simple alkenes 1. By catalytic cracking ( dehydrogenation) CH3CH2CH2CH3heat, catalyst CH3CH=CH=CH2

  27. 2. Dehydrationloss of water by chemical compound: the process by which a chemical compound loses water molecules ex. CH2CH2CH2CH2HEAT /ACID CH2=CH-CH=CH2 1, 3-Butadiene OH OH

  28. By dehydration process, simultaneously happened. OH will dissociate, become OH-, forming H2O. Since C1 and C4 is electron deficient, the two carbon will be pulled an electron. Therefore the C1-C2 and C3-C4 forming double bonds. That have a product of 1,3-Butadiene

  29. ELECTROPHILIC ADDITION TO CONJUGATED DIENES-1,4-ADDITION • In addition to conjugated dienes; a rearrangement may attach itself not only to a pair of adjacent carbons (1,2-addition), but also to the carbons at the two ends of the conjugated system (1,4-addition).Electrophilic addition to conjugated dienes yields a mixture of 1,2- and 1,4- addition products

  30. Mechanism

  31. Stability of Conjugated Dienes • More stable than non conjugated dienes 1. Stability observed as less heat release by hydrogenation than non conjugated dienes • Stability observed in more substituted compounds either conjugated or not 2. Bond length also indicates stability due to overlap of hybrid orbital ∆H= 60.8 Kcal/mol ∆H = 54.1 kcal/mol

  32. Stability of Conjugated Dienes 3. Structurally a. Delocalization b. Hyperconjugation

  33. Reactions of Dienes Hydrogenation Ozonolysis Glycol formation Halogenation

  34. the end!!

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