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8/12/2019 Reaksi Kimia Organik - 2

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Heterolysis of CZ Bonds

Heterolysis of CZ Bonds Generates Ionic Species

Carbocation: Postively Charged C Atom

Carbanion: Negatively Charged C Atom

C Z

C + Z

C Z

C + Z


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Nucleophiles and Electrophiles

Carbocations:

Electrophiles

Seek Electrons in Reaction to Fill/Stabilize Valence

Carbanions:

Nucleophiles

Seek Proton or Some Other Positive Center

Nucleo From Nucleus (Where Positive Charge Resides)


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Acids & Bases in Organic Chemistry

Many of the organic reactions you willstudy involve acid-base reactions.

Understanding these reactions will require

you to: recognize organic compounds that canserve as acids or bases

throw away the idea that anything with anOH is a base and anything with an H is anacid!!!


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Common definitions of acids and bases: Arrhenius acids and bases Bronsted-Lowry acids and bases Lewis acids and bases

Bronsted-Lowry Acid any substance that can donate a proton(H+ion)

Bronsted-Lowry Base any substance that can accept a proton


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Acid-base reactions always produce a newacid and a new base: conjugate acid:

The new acid formed when the base

gains a proton always found on the product side

conjugate base: The new base formed by removing a

proton from an acid always found on the product side

CH3CO

2H + NaOH

CH3CO

2Na + H

2O

Conj. base Conj. acid


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The strength of an acid or base isdetermined by the extent to which itionizes:

Ka= acid dissociation constant

Kb= base dissociation constant

HA + H2O H3O

+

+ A

-

Ka = [H3O

+

][A

-

][HA]

B + H2O

BH

+

+ OH

-..

Kb

= [BH+

][OH-

]

[B]


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The relative strength of an acid can bedetermined using: the magnitude of Ka (or pKa)

As Kaincreases, the strength of the acidincreases.

As pKadecreases, the strength of the acidincreases.

structural trends The strength of an acid, HX, depends on

the electronegativity of the atom containingthe acidic hydrogen (i.e. the electronegativityof X)

the stability of the conjugate base, X-


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Within the same period (row), acidityincreases as the electronegativity of elementX increases (i.e. left to right) electronegative elements can bear a negative

charge more easilyH-C < H-N < H-O < H-F

Within a group, the strength of an acidincreases moving down the group negative charge is more stable when spread

out over a larger region (i.e. larger ion)HCl is stronger than HF


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Acidity increases when the charge on theconjugate base can be delocalized over twoor more atoms through resonance.

Stronger acid

pKa= 4.74

Weaker acid

pKa= 15.9

CH3CH

2OH..

.. CH3CH2O..

....-

..

..CH3C

.. ..O

OH-..

..

..CH3C

.. ..O

O

..

..

..CH3C

.. ..-

O

O


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Hybridization: Acidity increases when more scharacter in the orbital. More s characterwill make more stable anion.Consider Alkanes, Alkenes, Alkynes

Acid: HCCH H2CCH2 H3CCH3

pKa: 25 44 50

Hybrid. sp sp2 sp3

% s: 50 33 25


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You must be able to predict the relativestrength of various acids:

pKa values indicate the following relative

acidities: strong inorganic acids > carboxylic acids> phenols > alcohols ~ water > terminalalkynes > alkanes

Be able to use structural trends.


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The strength of a base is inversely relatedto its conjugate acid. Strong acids form very weak conjugate bases.

Weak acids form stronger conjugate bases.

Substances with negligible acidity form verystrong conjugate bases.

HCl Cl-

CH3CO

2H CH

3CO

2

-

CH3OH CH

3O

-

NH3 NH

2

-

CH4 CH

3

-


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You should be able to: use the relative strength of various acids to

predict the relative strengths of their conjugatebases

use the strength of acids and bases to predictwhether an acid/base equilibrium favorsreactants or products.

Equilibrium favors the weaker acid (or theweaker base.


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Example:Does the following reaction favor thereactants or products?

CH3CH

2OH + CH

3NH

-

CH3CH

2O

-

+ CH3NH

2


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The following reaction does not look like aclassic acid-base reaction...neither reactantgains or loses an H+.

It is, however, a Lewis acid-base reaction.

The Lewis acid-base definition is the broadestdefinition of acids and bases.

CH3C

.. ..

....

..

..

.. ..

+ CH3O - CH

3C

....

-O

H

O

H

OCH3Lewis acid Lewis base


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Lewis acid: an electron pair acceptor an electrophile

electron lover

a substance that accepts a pair ofelectrons to form a new bond

CH3C

.. ..

....

..

..

.. ..

+ CH3O - CH

3C

....

-O

H

O

H

OCH3Lewis acid


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Lewis Base: an electron pair donor a nucleophile

nuclei lover

a substance with a pair of electronsthat can be donated to another nucleusto form a new bond

A B H + AB + H

Lewis

Base

Lewis

Acid


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Common acids (materials with acidic protons)used in organic chemistry: Inorganic acids:

HCl

HBr HI H2SO4 H3PO4

Carboxylic acids

Strong acids

R CO

O H


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Common acids (materials with acidicprotons) used or found in organicchemistry: Phenols

Alcohols

Water

Terminal Alkynes

O H

R O H

H O H

R C C H


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Common bases (or basic substances) usedor found in organic chemistry:

Hydroxide ion NaOH or KOH

Alkoxide ions Sodium methoxide

Sodium ethoxide

Potassium t-butoxide

CH3O

-

Na+

CH3CH2O

-

Na

+

CH3CO

-

K+

CH3

CH3


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Common bases used (or basic substances)used or found in organic chemistry:

Sodium hydride

Sodium amide

Amines or ammonia

Carbanions

NaH

NaNH2

NH3

R..-

C R'

R"

N


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Example: Complete the following acid-basereactions.

Ca(OH)2 + HCl

CH3CO

2H + NaOH

NaH + CH3CH2OH

CH3C CH + NaNH2


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Lewis Acids and Bases

Other Lewis Acids: BF3 ZnCl2 FeBr3

Have Available Acceptor Orbital

Other Lewis Bases: R-OH Br2

Have Lone Pair to Donate

Lewis Acid/Base Reactions Essentially Electrostatic

(Opposite Charges Attract)


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Other good examples involve metal ions.

Such bonds as the H2O ---> Co bond are

often called COORDINATE COVALENT

BONDSbecause both electrons aresupplied by one of the atoms of the bond.

HH

Co2+

BASEACID

OH

OH

Co2+

Lewis Acids & Bases


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The combination of metalions (Lewis acids) withLewis bases such as H2Oand NH3 ------>

COMPLEX IONSAll metal ions formcomplex ions with waterand are of the type

[M(H2O)x]n+

where x = 4and 6.

Lewis Acids & Bases

[Cu(NH3)4]2+


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Add NH3to light blue [Cu(H2O)4]2+

------> light blue Cu(OH)2and then deep

blue [Cu(NH3)4]2+

Lewis Acids & Bases


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Examples of Lewis Acid/Base Reactions

CH3C H + H

O

Cl CH3C H

O

H

+ Cl

Lewis

baseLewis

acid

CH3C H + CH3O

O

CH3C

O

OCH3

H

Lewis

acid

Lewis

base


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Type of Reaction in Organic Chemistry

Substitution:

Addition:

Elimination:

Rearrangement:

+ NaOH + NaClH3C Cl H3C OH

Br2

Br

Br

Br-HBr

F d l R i T


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Fundamental Reaction Typesbased on mechanism

Elementary Mechanisms:

Decomposition Pericyclic(Cycloaddition)

Recombination Rearrangement

Complex Mechanisms:

Insertion Addition

Elimination Substitution


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Decomposition Reaction

A B + C + D +...

ONO2

O2NO ONO2 CO2 + H2O + NO 2

Br+ Br


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Recombination Reaction

C

Cl + Cl

+ OH

A + B

Cl2. .

OH


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A

B

C

D

+

Pericyclic Reaction (Cycloaddition)

A-B + C-D

R t R ti


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BA

Rearrangement Reaction

N C O ONC

1

2

3

4

5

6

7

1

2

3

4

5

6

7


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:CH2

A

B

D

C

+

Insertion Reaction

A-B=C + D


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OHH2O Cl

B-H XB

alkenebase

H X+

Elimination Reaction

H Cl

+ +

+ + +

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