Organic Chemistry – The Functional Group...

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Organic Chemistry – The Functional Group Approach

alkane(no F.G.)

non-polar (grease, fats)

tetrahedral

OH

alcohol

polar (water soluble)

tetrahedral

Br

halide

non-polar (water insoluble)

tetrahedral

alkene


trigonal

alkyne


linear

aromatic


flat

aldehyde/ketone


trigonal

imine


trigonal

O NH

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alkane(no F.G.)


tetrahedral

OH

alcohol


tetrahedral

Br

halide


tetrahedral

alkene


trigonal

alkyne


linear

aromatic


flat

aldehyde/ketone


trigonal

imine


trigonal

O NH

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alkane(no F.G.)


tetrahedral

OH

alcohol


tetrahedral

Br

halide


tetrahedral

alkene


trigonal

alkyne


linear

aromatic


flat

aldehyde/ketone


trigonal

imine


trigonal

O NH

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Carey Chapter 6 – Addition Reactions of Alkenes

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Carey Chapter 6 – Addition Reactions of Alkenes

R

R

R

R+ X Y X C C

R

R

R

R

Y

Involves addition of atoms or groups to adjacent carbons

X often = H; Y = good nucleophile

Examples of both stepwise and concerted mechanisms

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6.1 Hydrogenation of Alkenes

Needs a precious metal catalyst such as Pt or Pd

(Not covering Mechanism 6.1)

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6.2 Heats of Hydrogenation of Isomeric C4H8 Alkenes

Figure 6.1 (KJ/mol)

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6.2 Heats of Hydrogenation of Alkenes

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6.3 Stereochemistry of Alkene Hydrogenation

H atoms have added to the same face of the alkene ‐ syn addition

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H atoms have added to the same face of the alkene ‐ syn addition

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Figure 6.2

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6.4 Electrophilic Addition of Hydrogen Halides to Alkenes

Stronger acids react faster :

H‐I > H‐Br > H‐Cl >> H‐F

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6.4 Electrophilic Addition Mechanism

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6.4 Electrophilic Addition Mechanism

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6.5 Regioselectivity in Electrophilic Addition

Since the reaction involves formation of cations, the major product arises from

the more stablized intermediate carbocation

Markovnikov’s Rule

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6.6 Mechanistic Basis for Markovnikov's rule

Figure 6.4

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Examples of H-X Addition to Alkenes

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6.7 Cation Rearrangements in H-X Addition to Alkenes

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6.7 Cation Rearrangements in H-X Addition to Alkenes

2o cation undergoes intramolecular rearrangement to more stable 3o cation

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6.8 Free Radical Addition of H-Br to Alkenes

Peroxides = HOOH, ROOR (R = Ph, t‐Bu, etc.)

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6.9 Electrophilic Addition of Sulfuric Acid

Outcome depends upon concentration of H2SO4 used

Sulfonate ester isolated using concentrated H2SO4

Alcohol isolated directly with dilute H2SO4

Markovnikoff Rule applies for nonsymmetrical alkenes

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6.10 Acid-catalyzed Hydration of Alkenes

Note the Markovnikoff regioselectivity

Reaction is exothermic; products favoured (stronger single bonds vs. pi bond)

Use of catalytic acid can avoid decomposition of more complicated substrates

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6.10 Acid-catalyzed Hydration of Alkenes

Related to relative stabilities of intermediate carbocations

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6.11 Thermodynamics of Addition-Elimination

If the alkene is less stable, how can this reaction be useful?

Recall: G = H ‐ TS

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6.11 Thermodynamics of Addition-Elimination

How do you get one product over the other?

In dehydration (elimination) – remove alkene, run at high temperature

In hydration (addition) – use excess water, run at low temperature

Taking advantage of both Le Châtelier’s principle and G = H ‐ TS

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6.12 Hydroboration - Oxidation of Alkenes

Addition

Oxidation

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6.12 Hydroboration – Modern Reagents

H.C. BrownBH BH

“parachute borane”

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6.12 Hydroboration – Regioselective and Stereoselective

Two possible transition states for concerted addition of BH3 to an unsymmetrical alkene

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Left T.S. is more favourable since it avoids the larger CH3 groups and the BH2

group interacting – results in regioselectivity

6.12 Hydroboration – Regioselective and Stereoselective

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6.13 Stereochemistry of Hydroboration

Addition of BH3 is a concerted syn addition – evidence for mechanism

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6.13 Stereochemistry of Hydroboration

H H

OH

NaOH/H2O2BH

B

CH3CH3 CH3

Oxidation step retains the stereochemistry from first step

Important evidence for the mechanism being concerted

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6.14 Mechanism of Hydroboration/Oxidation

CH31. H-BR2, THF

2. NaOH, H2O2

H

OH

CH3

HH

H

H

Step 1 Syn Addition

HBR

R

H

HHH

R2B

H

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Step 2 Oxidation

H O O H + NaOH H O O Na-H2O

CH3

HR2B

H O O

CH3

R2BO

O H

CH3

HOR2B

CH3

HOHH

OH

CH3

OH-

then hydrolyze

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6.15-6.17 Addition of Halogens - Anti addition via cations

No syn addition product formed

Stepwise or concerted mechanism?

Anti addition outcome is easily seen in cycles

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Two possibilities for stepwise mechanism, second explains stereochemistry

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Two possibilities for stepwise mechanism, second explains stereochemistry

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6.18 Addition of “X-OH” - Halohydrin Formation

Reaction can also result in regioselective outcome:

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6.19 Epoxides from Alkenes

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6.19 Epoxides – Essential Synthetic Intermediates

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6.20 Ozonolysis of Alkenes - Cleavage of the Double Bond

H3C

CH3

1. O3

2. Zn, H2O

CH3

H

CH3

H3C OO

CH3

H

H3C

H3C OO

O

O

OO

CH3

H3C

malozonide ozonide

CH3H3C

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6.21 Synthesis

OH

HH

H

OH

Br

Br2

H

Br

Br

H

Br

or hv

base (E2)

HBr (addition)

neutral (E1)

H2O (SN1)

H+, H2O

H3PO4or H2SO4

heat

HBrperoxides

1. B2H62. NaOH, H2O2

H2, Pd(also BrOH)

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6.22 Reactions of Alkenes with Alkenes - Polymerization200oC

ethylene polyethylene

or peroxides

tetrafluoroethylene teflon

F

F

F

F

FF

FF

FF

FF

FF

FF

H

H

H

H

200oC

ethylene plexiglass

or peroxides

CO2CH3

CH3H

H

CH3O2CCO2CH3

CO2CH3

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6.22 Radical Polymerization

Organic Chemistry – The Functional Group...

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