1. Organic Chemistry Reviews Chapter 7 Cindy Boulton November 2, 2009

2. Nomenclature of Alkynes  Carbon – Carbon triple bond  Ending –yne  sp hybridized  Linear and angle = 180 0  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  sp 2 hybridized  All atoms are coplanar and angle = 120 0  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 attached to each Carbon.  (Z): SAME side  Zame Zide  (E): OPPOSITE side

5. Vinyl and Allyl Groups  Vinyl Group  CH 2 = CH –  Allyl Group  CH 2 = CH – CH 2 –

6. Stability of Alkenes  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 (3 o Carbon)  Trans  Cis  Monosubstituted: 1 alkyl group attached  Unsubstitued: no alkyl groups attached (In order of decreasing stability)

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 (180 o ) but is 145 o

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 β Hydrogens 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/H 2 O or 2NaI to form alkenes  Vic-dibromide must be in antiperi 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  NaNH 2  Forms an acetylide  Carbonanion  Use acetylide as a nucleophile to attack alkyl halides to make alkynes bigger  Hard to add 3 o 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:  H 2 /Pd/CaCO 3  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.  H 2 /Ni 2 B 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, C 2 H 5 NH 2 2) NH 4 Cl  Sterochemistry: anti-addition  Hydrogens added to opposite side  Form E or Trans alkene