Post on 05-Apr-2020
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Type of ReactionType of Reaction
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SORACHAI SAELIM
Reactions and Their Mechanisms
1.Substitutions
2.Additions2.Additions
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3.Eliminations
4.Rearrangements
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The Use of Curved Arrows in Illustrating Reactions
C d h th fl f l t i -Curved arrows show the flow of electrons in a reaction-An arrow starts at a site of higher electron density (a An arrow starts at a site of higher electron density (a covalent bond or unshared electron pair) and points to a site of electron deficiency
BC
2 e 1 e2 e 1 e
A D
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Cleavage of Covalent Bonds
Homolysis
H l iHeterolysis
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Organic chemistry terms for Lewis acids and bases
(“electron-loving” reagents ): seek electrons
Electrophiles:
( electron loving reagents ): seek electrons to obtain a stable valence shell of electron-Are electron-deficient themselves
e.g. carbocations
Nucleophiles:
(“nucleus-loving” reagents): seek a proton or h i i l h d
Nucleophiles:
some other positively charged cente-Are electron-rich themselves e.g.
carbanionscarbanions
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M hMech.
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Heterolysis of Bonds to Carbons: Carbanions and Carbocations
Reaction can occur to give a carbocation or carbaniondepending on the nature of Zdepending on the nature of Z
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Carbocations have only 6 valence electrons and a positive chargeelectrons and a positive charge
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Carbanions have 8 valence electrons and a negative charge
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NucleophileNucleophile
Leaving Group
The nucleophile reacts at the electron deficient carbon
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Nucleophilic Substitution Reaction
-SN2 reactionS ti-SN1 reaction
Elimination Reactions
-The E2 Reaction-The E1 Reaction
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Nucleophilic Substitution Reaction: An SN2 Reaction
SN2 reaction: substitution, nucleophilic, 2nd order (bimolecular)
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A Mechanism for the SN2 Reaction
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The Stereochemistry of SN2 Reactions
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An SN1 Reaction
S 1 reaction: Substitution nucleophilic 1st order SN1 reaction: Substitution, nucleophilic, 1st order (unimolecular)
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Mech1. ….ชา
Mech…
2.
เร็ว
3
….เรว
3.
….เร็ว
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CarbocationsCarbocations
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Factors Affecting the Rate of SN1 and SN2 Reactions
SN2 Reactions
Steric hinderanceS e c de a ce
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S 1 tiSN1 reactions
Generally only tertiary halides undergo SN1 Generally only tertiary halides undergo SN1 reactions because only they can form relatively stabilized carbocations
CarbocationsCarbocations
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Th E R i-The E2 Reaction
Mech…
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Th E R iThe E1 Reaction
Th E i i h h S i d The E1 reaction competes with the SN1 reaction and likewise goes through a carbocation intermediate
1.
2.
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Substitution versus Elimination
SN1 versus E1N
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Substitution versus Elimination
SN2 versus E2N
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-Type of substrate
-Temperature
(Increasing temperature favors elimination over substitution)
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-Size of the Base/Nucleophile
-Large sterically hindered bases favor elimination because they cannot directly
approach the carbon closely enough to react in a substitutionapproach the carbon closely enough to react in a substitution
-Potassium tert-butoxide is an extremely bulky base and is routinely used to
favor E2 reaction
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SN1 SN2SN1 SN
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