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NOMAN CLATURE OF ALKENE,ALKYNE,ARENES Unsaturated Hydrocarbons

L O V E L Y P R O F E S S I O N A L U N I V E R S I T Y TEARM PAPER ROLL.NO-69 REG.NO-10801207 CLASS-DIPP B.TECH CSE-B SUB-CHEMISTRY SUB.TEACHER-GAGANDEP MADAM. NOMAN CLATURE OF ALKENE,ALKYNE,ARENES Unsaturated Hydrocarbons.

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NOMAN CLATURE OF ALKENE,ALKYNE,ARENES Unsaturated Hydrocarbons

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  1. L O V E L Y P R O F E S S I O N A L U N I V E R S I T YTEARM PAPERROLL.NO-69REG.NO-10801207CLASS-DIPP B.TECH CSE-BSUB-CHEMISTRYSUB.TEACHER-GAGANDEP MADAM

  2. NOMAN CLATURE OF ALKENE,ALKYNE,ARENESUnsaturated Hydrocarbons • Unsaturated hydrocarbon:Contains one or more multiple bonds. • Alkene:Contains a carbon-carbon double bond and has the general formula CnH2n. • Alkyne:Contains a carbon-carbon triple bond and has the general formula CnH2n-2.

  3. Unsaturated Hydrocarbons • Arenes:Benzene and its derivatives (Ch 21-22) • The phenyl group is not reactive under any of the conditions we describe in Ch 6.

  4. Structure of Alkenes • A double bond consists of • one sigma bond formed by the overlap of sp2 hybrid orbitals and one pi bond formed by the overlap of parallel 2p orbitals. • the two carbon atoms of a double bond and the four atoms bonded to them lie in a plane, with bond angles of approximately 120°.

  5. Structure of Alkenes • it takes approximately 264 kJ (63 kcal)/mol to break the pi bond in ethylene; that is, to rotate one carbon by 90° with respect to the other so that there is no overlap between 2p orbitals on adjacent carbons.

  6. Cis,Trans Isomerism in Alkenes • Cis,trans isomers: Isomers that have the same connectivity but a different arrangement of their atoms in space due to the presence of either a ring or a carbon-carbon double bond.

  7. IUPAC Nomenclature 1. Number the longest chain of carbon atoms that contains the double bond in the direction that gives the carbons of the double bond the lowest numbers. 2. Locate the double bond by the number of its first carbon. 3. Name substituents. 4. Number the carbons, locate and name substituents, locate the double bond, and name the main chain.

  8. Common Names • Despite the precision and universal acceptance of IUPAC nomenclature, some alkenes, particularly low-molecular-weight alkenes, are known almost exclusively by their common names.

  9. Common Names • the common names methylene, vinyl, and allyl are often used to show the presence of the following alkenyl groups:

  10. The Cis,Trans System • Configuration is determined by the orientation of atoms of the main chain.

  11. The E,Z System • Uses priority rules (Chapter 3). • If groups of higher priority are on the same side, the configuration is Z (German, zusammen). • If groups of higher priority are on opposite sides, the configuration is E (German, entgegen).

  12. The E,Z System • Example:Name each alkene and specify its configuration by the E,Z system.

  13. Cis,Trans Isomerism • Cycloalkenes • In small-ring cycloalkenes, the configuration of the double bond is cis. • These rings are not large enough to accommodate a trans double bond.

  14. Cis,Trans Isomerism • Trans-cyclooctene is the smallest trans cycloalkene that has been prepared in pure form and is stable at room temperature. • The cis isomer is 38 kJ (9.1 kcal)/mol more stable than the trans isomer. • The trans isomer is chiral even though it has no chiral center.

  15. Dienes, Trienes, and Polyenes • For alkenes containing two or more double bonds, change the infix -en- to -adien-, -atrien-, etc. • Those containing several double bonds are often referred to more generally as polyenes.

  16. Dienes, Trienes, and Polyenes • For alkenes with n double bonds, each of which can show cis,trans isomerism, 2n stereoisomers are possible.

  17. Physical Properties • Alkenes are nonpolar compounds. • The only attractive forces between their molecules are dispersion forces. • The physical properties of alkenes are similar to those of alkanes.

  18. 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”.

  19. 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.

  20. Physical Properties • Similar to alkanes/alkenes of comparable molecular weight and carbon skeleton.

  21. Acidity • The pKa of acetylene and terminal alkynes is approximately 25, which makes them stronger acids than ammonia but weaker acids than alcohols (Section 4.1). • Terminal alkynes react with sodium amide to form alkyne anions.

  22. Acidity • Terminal alkynes can also react with sodium hydride or lithium diisopropylamide (LDA). • Hydroxide ion is not a strong enough base to convert a terminal alkyne to an alkyne anion.

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