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Type of ReactionType of Reaction

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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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