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Topic 5D Alkenes and alkynes

Topic 5D Alkenes and alkynes. 42. 5D Alkenes and alkynes. Structure of double bond :. A sigma (  ) bond between two sp 2 hybridised carbons A pi (  ) bond formed by sideways overlap of two unhybridized p z orbitals. 43. Pi bonding. Pi  bonds weaker than sigma bonds

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Topic 5D Alkenes and alkynes

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  1. Topic 5DAlkenes and alkynes

  2. 42 5D Alkenes and alkynes Structure of double bond : • A sigma () bond between two sp2 hybridised carbons • A pi () bond formed by sideways overlap of two unhybridized pz orbitals

  3. 43 Pi bonding • Pibonds weaker than sigma bonds • Alkenes are more reactive than alkanes. • Electrons in p-orbitals further from the nucleus than s-electrons and are less tightly bound • Reactivity of alkenes involves the -bond, not the -bonds. Bond energy: Total — Sigma = Pi681 397 284 kJmol–1

  4. 43 Structural consequences of -bonding Rotation means loss of  overlap Movie from Saunders General Chemistry CD-ROM

  5. 43 Structural consequences of -bonding • Whereas in alkanesthere is free rotationabout C—C bonds,Alkenes are rigid • For rotation the -bond must be broken requiring 285 kJmol–1

  6. 43 Rotation in C—C bonds • Staggered and eclipsed conformations possible • Eclipsed higher in energy by 11 kJmol–1

  7. 44 Restricted rotation in C=C bonds • Alkenes are however rigid and can have two configurations • Groups on the double bond may be cis or transeg 2-butene

  8. Top of p -orbital viewed from above 45 Geometrical isomers • These configurations require different groups on each end of the double bond • Different forms are called geometrical isomers Two faces B A B A A B A B C D C C C D C C Equivalent isomers Geometrical isomers

  9. 45 Shapes of alkynes • Because each carbon is sp hybridised (hybrid orbitals 180° apart), ethyne is a linear molecule. • Pi bonds form a barrel of electron density around the CC bond.

  10. 46 Naming alkenes • Suffix "ene" is used to denote alkenes. • Number from end nearest double bond(giving first carbon of alkene the lowest number) • The stem is based on the longest chain containing the double bond • Geometrical isomers are identified using the E–Z nomeclature system

  11. 4 3 2 1 CH CH CH CH 3 2 2 4 3 2 1 2-Butene CHCH3 CH CH 3 CH CH CH 3 2 2 C CHCH 3 3 2 1 CH CH CH CH 3 2 2 2 7 6 5 4 46 Naming alkenes Examples: 1-Butene 3-propyl-2-heptene

  12. 46 Naming alkynes • Suffix "yne" is used to denote alkynes. • Number from end nearest triple bond(giving first carbon of alkyne the lowest number) • The stem is based on the longest chain containing the triple bond • Linear — no geometrical isomerism

  13. 46 Naming alkynes Examples: • ethyne H–CC–H • propyne CH3–CC–H • 1-butyne CH3–CH2–CC–H • 2-butyne CH3–CC–CH3

  14. CH CH CH 3 3 3 CH 3 47 The E–Z system H • Distinguishes cisandtrans geometrical isomers • A group on each end is given preference using rules • If these are on the same side — cis — we have the Z-form. "Z" stands for "zusamen" • If these are on opposite side — trans— we have the E-form. "E" stands for "entgegen" C C C C and H H H cis trans

  15. H CH CH H 3 3 CH CH CH CH 3 3 3 3 47 The E–Z system Must have different substituents at eachend There is only one form of 2-methyl-2-butene 2-methyl-2-butene

  16. 48 Prioritising groups Priorities based on atomic NUMBER of attached atoms If same atoms are attached compare the attached atoms next along the chain Double and triple bonds are treated as follows:

  17. CH H CH H 3 3 H CH CH H 3 3 Z-2-butene E-2-butene (cis) (trans) 48 E/Z designation • Groups assigned priority based on atomic number of attached atom — CH3 has priority over H • Z—priority groups on the same face • E—priority groups on the opposite face

  18. Oxygen and carbon have priority over hydrogen Chlorine and carbon have priority over hydrogen H OH HO H CH CH H CH CH H 3 2 3 2 H CH H CH H Cl Cl H 3 3 Trans(E) Cis(Z) 1-propenol 48 The E–Z system Trans(E) Cis(Z) 1-chloro-1-butene

  19. HO has priority over methyl and carbon has priority over hydrogen Chlorine and ethyl have priority over hydrogen and methyl CH CH CH CH 3 2 3 2 CH3 OH CH3 HO CH3 CH3 H CH H CH H Cl Cl H 3 3 (E) (Z) (E) (Z) 1-chloro-2-methyl-1-butene 2-buten-2-ol 48 The E–Z system

  20. The double bond has priority over methyl Methyl has priority over hydrogen Priority groups are on the same side (Z)-alkene 48 The E–Z system

  21. 49 Cycloalkenes • Alkene carbon is numbered "one" • Number along double bond around ring to the nearest substituent

  22. 50 Seeing is believing Movies from Saunders General Chemistry CD-ROM Note the isomerisation is actually from cis to trans

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