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Khairunnisah 084Sherli Hermanita 026Fahri Ihsan 007Dian Degusty 040

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y Reaction Mechanism is an overall description of howa reaction occurs.

y This basically describes in detail exactly what takes

place at each stage of a chemical transformationy ALL CHEMICAL REACTIONS INVOLVE BOND-

BREAKING AND BOND-MAKING.

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There are two of reaction pathway:

y Polar reactiony Free radical reaction

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Polar reactions involve a species with an even numberof electrons and thus have only electrons pairs in theirorbitals. This is where the electrons are taken by the

most electronegative atom in a bond.

In polar reaction have:y Electrophiley Nucleophile

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A molecule or ion that accepts a pair of electrons tomake a new covalent bond is called an electrophile(from the Greek for "electron loving").

y W ants to pair up with a negative charge.

Typical of electrophile

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A molecule or ion that donates a pair of electrons toform a new covalent bond is called a nucleophile(from the Greek for "nucleus loving").

W ants a to pair up with a positive charge.Type of nucleophile

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Electrons always flow from nucleophile toelectrophile:

Example:

In this reaction, chloride ion is the leaving group .

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y In this polar reaction, know as an electrophilic additionreaction the strong nucleophile the ethylene reacts withthe proton of hydrogen bromide (electrophile) to yieldbromoethane.

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R ule 1 Electrons move from a nucleophilic source to an electrophilicsink.

R ule 2 The nucleophilie can be either negatively charged or neutral.

R ule 3 The electrophile can be either positively charged or neutral.

R ule 4 The octet must be followed.

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Chemists propose two limiting mechanisms for nucleophilic displacementy a fundamental difference between them is the timing of bond breaking and

bond forming steps

At one extreme, the two processes take place simultaneously; designated S SNN22y S = substitutiony N = nucleophilicy 2 = bimolecular (two species are involved in the rate-determining step)y rate = k[haloalkane][nucleophile]

In the other limiting mechanism, bond breaking between carbon and the leaving

group is entirely completed before bond forming with the nucleophile begins.This mechanism is designated SSNN11 where S = substitution N = nucleophilic 1 = unimolecular (only one species is involved in the rate-determining step) rate = k[haloalkane]

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Nucleophilic substitution:

y -> reagent is nucleophily -> nucleophil replaces leaving groupy

-> competing reaction (elimination + rearrangements)

nucleophilicsubstitution

Nucleophile

++ C NuC XNu - X-

leavinggroup

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CH 3 -I

HO-

Nu -

RO-

HS-

RS-

I -

NH 3

HOH

CH 3 -SH

CH 3 -SR

CH 3 -OH

CH 3 -OR

H

CH 3 -O-H

CH 3 -NH 3+

CH 3 X CH 3 Nu X-

+An alcohol (after proton transfer)

An alkylammonium ion

An alkyl iodide

A sulfide (a thioether)

A thiol (a mercaptan)

An ether

An alcohol

Reaction: + +

Some nucleophilic substitution reactions

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y SN1 is illustrated by the solvolysis of tert-butyl bromidey Step 1: ionization of the C-X bond gives a carbocation intermediate

C

CH 3

CH 3H 3 C

+C

H 3 C

H 3 C

Br

H 3 C

slow, ratedetermining

A carbocation intermediate;carbon is trigonal p lanar

+ Br

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y Step 2: reaction of the carbocation (an electrophile) with methanol(a nucleophile) gives an oxonium ion

y Step 3: proton transfer completes the reaction

CH 3 O

H H 3 C

C

CH 3

CH 3

O CH 3

H

C

CH 3

CH 3

CH 3

O

H 3 C

HH 3 C

H 3 C

C

H 3 C

O

CH 3

H

f a t ++ ++

+++

+ f a tC

H 3 C

H 3 CO

H 3 C

O

CH 3

H

OHO

CH 3CH3

H

H

CH 3

H 3 C

CH 3 C

H 3 C

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An energy diagram for an S N1 reaction

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y both reactants are involved in the transition state of therate-determining step

y the nucleophile attacks the reactive center from the sideopposite the leaving group

C Br

H

HH

HO + C

H

H H

HO Br

H- H-

Tr siti st te witsi lt e s b bre i g

b f r i g

C

H

HH

H O + Br -

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An energy diagram for an S N 2 reactiony there is one transition state and no reactive intermediate

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y SN1 reactionsy governed by electronic factorselectronic factors , namely the relative stabilities

of carbocation intermediatesy relative rates: 3 > 2 > 1 > methyl

y SN2 reactionsy governed by stericsteric factorsfactors , namely the relative ease of

approach of the nucleophile to the site of reactiony relative rates: methyl > 1 > 2 > 3

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g oo d

po o

Br -, I -

H O -, C H 3 O-, RO -

CH

3S -

,RS -

H 2 O

CH 3 O H , RO H

CH 3 CO H , RCO H

O O

NH3 ,

RNH2 ,

R2

NH,

R3

N

CH 3 SH , RSH , R 2 S

Effec tiv e n ess N u cl e oph il e

m o d er a teCH 3 CO

-, RCO -O O

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y A radical (also called a free radical) is described as a neutralchemical species that contains an odd number of electronsand thus has a single unpaired electrons in its orbital.Radical reactions can involve:

y Bond forming and bond breakingy Substitutiony Insertiony Additiony

Eliminationy Rearrangementy Single electron transfer (SET)

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Type of Reactions

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y Addition Reactions are when two reactants addtogether to form a single product with no atoms leftover .

+ H Br

Br

HH

H

HH

BromoethaneEthylene

Example:

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y Elimination Reactions are when a single reactantbreaks down to form two products. This is theopposite of an addition reaction.

Example:

O HH

H

H

HH + H O H

Ethylene

Acid Catalyst

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y Substitution Reactions occurs when tworeactants exchange parts to give two newproducts.

Example:

CH

HH

H Cl Cl+ CH

ClH

H H Cl+Light

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y Rearrangement Reactions occurs when a singlereactant undergoes a reorganization of bonds andatoms to yield an isomeric product.

Example:

H

H3

CH2

C

H

H A ci Ca ta l st

CH 3

H

H

H 3 C

1-Bu t 2-Bu t

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PericyclicPericyclic reactionreaction: a reaction that takes place in asingle step, without intermediates, and involves acyclic redistribution of bonding electrons. There three

classes of pericyclic reactions:y 1. Cycloaddition: two molecules combine to form aring.

y 2. Electrocyclic: one molecule, an intramolecular

cyclization.y 3. Sigmatropic: an intramolecular rearrangement.

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y Ox idation is the loss of electrons or an increase inoxidation state by a molecule , atom , or ion.

y R eduction is the g ain of electrons or a decrease in

oxidation state by a molecule, atom, or ion.

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y Initiation The symmetrical breaking of a bondinto 2 free radicals.

C l C lC

lC

l

C l H-CH 3 C l H CH 3+ +

C l C l+ C l C l

Propagation The reaction of a free radical withnormal atoms to create new free radicals.

Termination The reaction of two free radical tocreate a normal bond again.

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