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By Third Group

1. DEVI ARIYANI

2. M. ARDIANSYAH

3. CITRA ARTI M.

4. TITIS ESWINDRO

5. RIRIS ANDRIANI

6. ISTIQOMAH UTAMININGSIH

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Elimination reactions involve the removal part of themolecule from a reactant. In this section we focus onpolar elimination reactions involving heterolytic bondbreaking. A fundamental example involves deprotonation

in conjunction with expulsion of a good leaving group,such as dehydrohalogenation. Elimination reactions canbeclassified according to the structural relationshipbetween the proton and the leaving group. The productsof eliminations are unstable divalent carbon speciescalled carbenes

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Elimination reactions involve the loss of elements from the starting material toform a new T bond in the product.

General Features of Elimination

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E1 Reaction

The E1 reaction competes with the SN1 reaction and

likewise goes through a carbocation intermediate

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Video1 : primary alkyle Halide

Video2: tertieryalkyle Halide

Animation

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Product depends on stability of the carbocation

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The rate of an E1 reaction increases as the number of R groups onthe carbon with the leaving group increases.

Increasing the Rate of an E1 Reaction

The strength of the base usually determines whether a reactionfollows the E1 or E2 mechanism. Strong bases likeOH andORfavor E2 reactions, whereas weaker bases like H2O and ROH favorE1 reactions.

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When elimination can occur in more than one direction,

the principal alkene is the one formed by loss of H from the

carbon having the fewest hydrogens, or forms the most substituted alkene

OHOH

CHCH33

CC CC

HH

CC

three protons on thisthree protons on this FF carboncarbon

two protons on thistwo protons on this FF carboncarbon

only one proton on thisonly one proton on this FF carboncarbon

HH

HH

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Example 1 : THE E1 MECHANISM FORALCOHOLS

Step 1:

An acid/base reaction.Protonation of the alcoholicoxygen tomake a better leaving group.This step is very fast andreversible. The lone pairs on

the oxygen make it a Lewisbase.

Step 2:

Cleavage of the C-O bondallows the loss of the good

leavinggroup, a neutral watermolecule, to give acarbocationintermediate. This is the ratedetermining step (bond

breaking isendothermic)

Step 3:

An acid/basereaction.Deprotonation bya base (a watermolecule) from aC atom adjacent

to the carbocationcenterleads to thecreation of theC=C

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Example 2 : THE E1 MECHANISM FORALKYL HALIDES

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Characteristics Of The E1 Mechanism

Characteristic Result

Mechanism Two Steps

Alkyl Halide More substituted halides react fastest

Rate: R3CX R

2CHX RCH

2X

Rate Equation V = k [RX]

First Order KineticsStereochemistry Trigonal planar carbocation intermediate

Base Favored by weak base such as H2O and ROH

Leaving group A better leaving group makes the reaction faster because the

bond to the leaving group is partially broken in the ratedetermining step

Solvent Polar protic solvents that solvate the ionic intermediates

(carbocation) are needed

Product ore substitute alkene favored (Zaitsev Rule)

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Sumber:Organic_Chemistry_4th_ed_-_Paula_Bruice(2) Hal 409

dan 410

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Digunakan Untuk MenjawabWhy Step pertama laju reaksinya lbh kecildrpd the second step

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