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dienes & alkynes...LECTURE ELEVEN

PREVIOUSLY...

radicals...Br

➓dienes12

CH

H HH

dienediene

isolated conjugateddiene

cumulated

allene

dienescumulatedallenes

CH

Ph HPh

CH

PhHPh

mirror

CH

H

Ph

PhC C

CH

PhHPh

π(2py+2py)

orbitals

CH

H

Ph

PhC C

CH

PhHPh

π(2py+2py)

π(2pz+2pz)

sp2

sp

90°shape

dienesconjugated

stabilisation

stabilisationresonance

ObservedkJmol–1

ExpectedkJmol–1

127

253 254

254 254

254239

stabilisationresonance

to conjugated dienes➓electrophilic addition13

HX

X

X

H

H

H

HX

X

regioisomers

HX

XH

XH

HX

HH

XX

H

X?regioisomershow many

?

XH

XH

HX

HH

XX

H

X

regioisomershow many

?

XH

XH

HX

HH

XX

H

X

regioisomershow many

?

favouredleast

to conjugated dienes➓electrophilic addition13

Br2

2 x Br2 BrBr

BrBr

to conjugated dienes➓electrophilic addition13

regioisomers

Br2

Br2

BrBr

BrBr

mechanism

BrBr

Br

Br

Br

BrBr

BrBr

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alkynes

➓acidity of alkynes14

R H Na NH2

R Na NHH2

base

very strong

XR

H

H R

H

HH

notacidic

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sp3

(25% s)sp2

(33% s)sp

(50% s)

molecularorbitals

HCl >R OH

O>

OH

>HO

H>

HO

R>R

H

> >HN

HH

H

R

Cl <R O

O<

O

< OH

< OR

<R

< <HNH R

decreasing acid strength

increasing base strength

strong acid strong

weaker

base

weaker

+

acid + base

strong acid

strong

weak

base

weakacid

base

R

OH

R

HO

strong strong

weakbaseweak

acidbase

R

H O

R

OH

Xacid

http://www2.lsdiv.harvard.edu/labs/evans/

best pKa table I know

nucleophile

R H Na NH2 RBr

R

good

SN2

DANGER

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R

strong base

RBr

R

SN2 E2X

side reaction

H3O...Hg(OAc)2

HgOAc

HgOAc

OHH2O

➓hydration of alkynes15

additionMarkvonikov

tautomerisation

OH O

ketoneenol

hydroboration/oxidation

BH3H

BH2

H2O2NaOH

HOH

O

additionMarkvonikovanti

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goodluck