Organic Chemistry Reviews Chapter 7

1. Organic Chemistry ReviewsChapter 7 Cindy Boulton November 16, 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

12. Dehydration of Alcohols cont. • Alkyl and hydride migration • A hydride (H-) or alkyl group will migrate from a β Carbon to the α Carbocation to form a more stable Carbocation • Skeletal rearrangement • Multiple products will be formed

13. Debromination of vic-dibromides • Gem- halides on same carbon (twins) • Vic- halides on adjacent carbons • Reacts with Zn/H2O or 2NaI to form alkenes • Vic-dibromide must be in anitperi planar form • I- acts as nucleophile pulling off one of the Br as the other Br leaves forming an alkene

14. Debromination of vic-dibromides cont. • Why 2 NaI? • The second I pulls the I off the I-Br complex forming I-I • With only 1 the reaction will stall • Enanitomers have same product, same E/Z nomenclature • Diastereomers have different products, different E/Z nomenclature • A racemic mixture of vic-dibromide would have a single product

15. Terminal Alkynes • Terminal Alkynes have an acetylenic proton with a pKa = 25 • Reacts with a strong base • LDA • NaNH2 • Forms an acetylide • Carbonanion • Use acetylide as a nucleophile to attack alkyl halides to make alkynes bigger • Hard to add 3o RX because the elimination product will be major

16. Hydrogenation of alkenes • Use Pt as catalyst • Can use Ni, Pd, Rh or others • Lowers the activation energy to speed up the reaction • H are added to the same face of the alkene • Stereochemistry: Syn Addition • Z -> RS and SR • E -> RR and SS • Forms a racemic mixture of enantiomers • Regiochemistry: 1, 2 addition • Carbons of double bond are side by side

17. Hydrogenation of Alkynes • Pt as catalyst: • Forms an alkene but can not stop so continues to form alkane • Lindlar’s Catalyst: • H2/Pd/CaCO3 • Stops as alkene, • Ca prevents alkene from being hydrogenated • Stereochemistry: syn addition • Hydrogens added to same side • Form Z or Cis alkene

18. Hydrogenation of Alkynes cont. • H2/Ni2B as Catalyst • Stops as alkene, • B prevents alkene from being hydrogenated • Stereochemistry: syn addition • Hydrogens added to same side • Form Z or Cis alkene • 1) Li, C2H5NH2 2) NH4Cl • Sterochemistry: anit addition • Hydrogens added to opposite side • Form E or Trans alkene