Organic Chemistry ReviewsChapter 7

Cindy Boulton

November 2, 2009

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

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

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

Vinyl and Allyl Groups Vinyl Group

CH2 = CH –

Allyl Group CH2 = CH – CH2 –

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

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

(In order of decreasing stability)

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

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

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

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

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

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 antiperi planar form I- acts as nucleophile pulling off one of the Br

as the other Br leaves forming an alkene

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

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

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

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

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: anti-addition

Hydrogens added to opposite side Form E or Trans alkene