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Understanding the Nomenclature and Stability of Alkynes and Alkenes in Organic Chemistry

This chapter explores the nomenclature of alkynes and alkenes, focusing on carbon-carbon triple and double bonds. It explains key concepts such as terminal alkynes, acetylenic protons, and the significance of hybridization and stability of alkenes. The chapter covers cis/trans and E/Z configurations, the influence of substituent groups on stability, and the synthesis of alkenes through dehydrohalogenation and dehydration reactions. Mechanisms such as E2 and E1 are discussed, emphasizing their stereochemistry and product outcomes.

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Understanding the Nomenclature and Stability of Alkynes and Alkenes in Organic Chemistry

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  1. Organic Chemistry ReviewsChapter 7 Cindy Boulton November 9, 2008

  2. Nomenclature of Alkynes • Carbon – Carbon triple bond • Ending –yne • sp hybridized • Linear and angle = 1800 • Number the bond with the carbon that has the lower number • Terminal alkyne • a triple bond at a terminal carbon • Has a acetylenic proton • Acetylenic Proton • Proton at the end attached to a Carbon with a triple bond • Easily pulled off with a pKa value = 25

  3. Nomenclature of alkenes • Carbon – Carbon double bond • Ending –ene • sp2 hybridized • All atoms are coplanar and angle = 1200 • Double bond cannot rotate • Number the bond with the carbon that has the lower number • Cis: same groups on SAME side • Trans: same groups on OPPOSITE side • Diasteromers • Same molecular formula, same connectivity, not mirror images

  4. E-Z Nomenclature • If no 2 groups are the same, cannot use cis or trans • Identify the highest priority (highest mass) group attatched to each Carbon. • (Z)- SAME side (Zame Zide) • (E)- OPPOSITE side

  5. Vinyl and Allyl Groups • Vinyl Group • CH2 = CH – • Allyl Group • CH2 = CH – CH2 –

  6. Stability of Alkenes • R- alkyl groups provide electron density to stabilize the alkene • Hydrogen does not provide electron density • Electronics: more electron donors, more stable • Sterics: more crowding, less stable • The greater number of attached alkyl groups or the more highly substituted the carbon atoms of the double bond, the greater is the alkene’s stability

  7. Stability of Alkenes cont. • Tetrasubstituted: 4 alkyl groups attached • Trisubstiuted: 3 alkyl groups attached • Disubstitued: 2 alkyl groups attached • On same carbon (3o Carbon) • Trans • Cis • Monosubstituted: 1 alkyl group attached • Unsubstitued: no alkyl groups attached

  8. Stability of Cycloalkenes and Cycloalkynes • Angle Strain • 8 is the magic number • Cycloalkenes: • Cyclopropene to Cycloheptene • Angle strain • Must be in cis form (not stable in trans form) • Cyclooctene • First stable cycloalkene • Tans at double bond • Cycloalkynes • Cyclooctyne: can isolate at room temperature • Unstable due to angle strain • Wants to be linear (180o) but is 145o

  9. Synthesis of Alkenes Dehydrohalogenation reaction (E2) • α Carbon: Carbon with Halide/Leaving Group attached to • β Carbon: Carbon directly attached to α Carbon, has βHydrogens attached • E2 mechanism: • Leaving group leaves, Nucleophile/Base attacks β Hydrogen, double bond forms between α and β carbons • Transition step, no carbocation intermediate • Two Products: • Zaitsev: small bases lead to more stable/substituted alkenes due to electronics • Hoffman: big, bulky bases lead to less stable/substitued alkenes due to sterics and crowding • SN2 Product also forms

  10. Stereochemistry of E2 reaction • Anti periplanar transition state • β Hydrogen needs to be oppostie the leaving group • Enantiomers will have the same E-Z nomenclature after dehyrdohalogenation reaction • Diastereomes will have opposite E-Z nomenclature after dehydrohalogenation reaction

  11. Dehydration of Alcohols • Hydroxyl group becomes protonated by an acid forming H-O-H+ to make a good “leaving group” • Acid is a catalyze • E1 mechanism • H-O-H leaves and to form Carbocation intermediate • H-O-H acts as “nucleophile” attacking β Hydrogen forming alkene • Two Products: • Hoffman Product: less stable/substituted with bulky base • Zaitsev Product: more stable/substituted with small base

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