chemistry department ضيغملا نيسح .د organic chemistry 217 ......substitution nucleophilic...

© 2013 Pearson Education, Inc. Dr. Hussein Al-Mughaid

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Chemistry Department

Organic Chemistry 217

chapter 6

Organic halogen compounds:

Substitution and Elimination Reactions

كيمياء عضوية217ك

حسين المغيض. د

Part-1

© 2013 Pearson Education, Inc.

Alkyl halides (haloalkanes)

hydrocarbons that contain the C-X bond somewhere in the molecule X = F, Cl, Br, I

1. Electronegativities of the halides: F > Cl > Br > I

2. Bond lengths increase as the size of the halogen increases:

C—F < C—Cl < C—Br < C—I

Dr. Hussein Al-Mughaid-1

Primary, secondary, and tertiary halides


Other Classes of Alkyl Halides

1. Vinyl halides:

Halogen is bonded to sp2 carbon of alkene.

2. Aryl halides:

Halogen is bonded to sp2 carbon on benzene ring.


sp3 C next to C=C

3. Allylic halides:

4. Benzylic halides( later)

Allylic halide

Classify the following compounds as alkyl halide, vinyl halide, allylic halide, or aryl halide


Trends in Nucleophilicity

1. A negatively charged nucleophile is stronger than its neutral counterpart

2. Nucleophilicity decreases from left to right (periodic table ) decreasing


Trends in Nucleophilicity

3. Nucleophilicity increases down periodic table, as size and polarizability increase.

Increasing size


Big (bulky) nucleophile is Bad


Thinking question

The nucleophilic atom in the following two species is negatively charged oxygen. Circle the better nucleophile?

reason


Solvent System:

Aprotic polar Solvents:

dimethyl sufoxide (DMSO)

Protic and aprotic

do not have acidic protons and therefore cannot hydrogen bond.

protic polar solvents: have a hydrogen bonded to an oxygen or nitrogen (can hydrogen bond). O-H, N-H


How R—X React

Nucleophilic substitution

General reaction:

Example:

Leaving group

Leaving group

solvent

Solvent: protic or aprotic


How The Nu replaces X? (SN1 & SN2 mechanism)

substitution nucleophilic second order

The SN2 Reaction:

+

substitution nucleophilic first order The SN1 Reaction:

One step

two steps

carbocation

Step 1 Step 2


The SN2 Reaction

+ X _

Transition state

Bond breaking Bond forming general

E. diagram


Example


Factors that affect SN2 Reaction 1. Nature of R-X.

Reactivity Order: CH3-X > 1o > 2o >> 3o

Which one is more reactive?

Tertiary halides do not react via the SN2 mechanism,

due to steric hindrance.

Or

Give the product?


2. Nature of the leaving group X

Leaving Group Ability : I (best) > Br > Cl > F (worse)

Faster SN2 reaction?

3. Nature of the solvent aprotic?

Which solvent affords the faster reaction?

4. Nature of the nucleophile Strong(-vely charged)


The SN1 Reaction

General:

E. diagram • Forming the carbocation is an

endothermic step. (rate-determining step)

• Step 2 is fast with a low activation energy.

Step 1: Formation of the carbocation. Step 2: Attack of the nucleophile.


Overall reaction

mechanism: Step-1

Step-2

• If the nucleophile was neutral, a third step (deprotonation) will be needed.


Factors that affect SN1 Reaction 1. Nature of R-X.

Which one is more reactive? Or

Give the product?

Reactivity Order: 3o > 2o > 1o > CH3-X opposite to SN2.


2. Nature of the leaving group X

Leaving Group Ability : I (best) > Br > Cl > F (worse)

Faster SN1 reaction?

3. Nature of the solvent protic?

Which solvent affords the faster reaction?

4. Nature of the nucleophile weak(-neutral )


react via SN2 mechanism

react via SN1 mechanism


Secondary, Benzylic,

Allylic.

SN1 SN2

Strong Nu

aprotic solvent

weak Nu

protic solvent

Secondary, allylic and Benzylic halides

first C next to benzene ring

Benzylic carbon

Benzyl chloride

Benzylic (primary) carbocation (stabilized by resonance, try to draw them)

Benzylic (1o), allylic (1o), and secondary carbocations have the same stability (approximate)


faster rate of SN1

faster rate of SN2

A B

B A

(A) (B)

For you: Which of the two compounds shown below will undergo Solvolysis in aqueous ethanol more rapidly?

aqueous ethanol = CH3CH2OH in H2O


Vinyl Halides and Aryl Halides. Inert (unreactive)

Give the product?


Stereochemistry of SN1 and SN2

SN2 reactions will result in an inversion of configuration (Walden inversion).

+

R S


One More

Give the product?


Stereochemistry of SN1

The SN1 reaction produces mixtures of enantiomers. There is usually more inversion than retention of configuration.

Sp2, flat(2 faces) Step-1

Step-2

chiral achiral

(racemization)


Top (S)

Bottom (R)

-H+

+

Example:


2. Draw the structures of A and B?

Thinking question

A + B

1. Does the above reaction follow SN1 or SN2 mechanism?

3. How A is related to B?

A) Enatiomers B) Diastereomers


Summary of the SN1 and SN2 reactions


chemistry department ضيغملا نيسح .د organic chemistry 217 ......substitution nucleophilic...

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