12.5_Reactions_in_organic_compounds_1

8/7/2019 12.5_Reactions_in_organic_compounds_1

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OBJECTIVES:OBJECTIVES:

C H A PT E R 12 : I N T R O DUC T IO N TO O R GA N IC C H E M I S T RYC H A PT E R 12 : I N T R O DUC T IO N TO O R GA N IC C H E M I S T RY

12.5 REACTION IN ORGANIC COMPOUNDS12.5 REACTION IN ORGANIC COMPOUNDS

1.Explain covalent bond cleavage: homolytic cleavage to produce free radicals

Heterolytic cleavage to produce carbocation and

S K 0 2 7 C H E M I S T R YS K 0 2 7 C H E M I S T R Y

2. State the relative stabilities of 1o,2o,3o

Carbocations/Carboniums

Free radicals

CarbanionsExplain the inductive effect of alkyl group towards

the stability of carbocations and carbanions.


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OBJECTIVES:OBJECTIVES:



3. Define electrophile and nucleophile

i. types of electrophiles: Lewis acids, cations


ii. types of nucleophiles: Lewis base,anions and

electron rich sites in organic compounds


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All chemical reactions involved bond-breakingand bond-forming .

Reactions occur at the active site/functionalrou .

COVALENT BOND CLEAVAGE / FISSION


Reactive intermediate occurs in the course ofreaction

Types of covalent bond cleavage:

a. Homolytic cleavageb. Heterolytic cleavage


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HOMOLYTIC CLEAVAGE

..is a bond fission in which the two sharedelectrons of the covalent bond are split equallybetween the two atoms.

Occurs in a non- olar bond involvin two atoms


of similar electronegativity.

Free radicals (atoms/group of atoms containsan unpaired valence electron) are formed.

Cl Cl+

uv

Example:

Cl Cl


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HETEROLYTIC CLEAVAGE

.. Is the bond-breakage in which both electronsremain on one of the atoms.

Occurs in a polar bond involving two atoms ofdifferent electronegativities.

.

C + BrExample: C Br

C EC E+

Carbocation / carbonium

Carbanion


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CH3 CH2RCHR

R

C

R

R

R

<


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Relative Stabilities of intermediates

CH3 CH 2R C HR C

R

R

R

>>>- - - -

Methyl 1 2 3>>>

The alkyl group decreases the stability of a carbanion

The greater the number of alkyl group, the less stableis the carbanion


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Reagents and sites of organic reactions

..is an electron-deficient species which capable ofaccepting electron pairs.

it attacks electron-rich areas of organic molecules.

a) Electrophile

can be either neutral or positively charged.

Type Example

Cations NO2+, H3O

+

Lewis acid AlCl3, BF3Carbocation C+

Oxidising agents Br2, Cl2

Site of low e density

C = O

+ -

C OH

+ -

C X

+ -


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Reagents and sites of organic reactions

..is an electron-rich species which is capable ofdonatingelectron pairs.

it attacks low electron density areas of organic molecules.

b) Nucleophile

can be either neutral or negatively charged.

Type Example

Anions OH-, RO-, Cl-, CN-

Lewis base NH3, H2O, H2SCarbanion C-

Site of high e density C = C C C Benzenering


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12.5 REACTION IN ORGANIC COMPOUNDS12.5 REACTION IN ORGANIC COMPOUNDSExercises: 1. The bond dissociation energies for homolytic

cleavage are as below,

Bond Bond dissociation

energies (kJmol-1)

CH H 435

What is homolytic cleavage, write the free radicals that are

formed due to the homolytic cleavage and rank them

increasing order of stability.

CH3CH2 H 410

C6H5CH2 H 356

(CH3)2C H 397

(CH3)3C H 381


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12.5 REACTION IN ORGANIC COMPOUNDS12.5 REACTION IN ORGANIC COMPOUNDSExercises: 2. a) Classify the following species as free

radical, carbonium ion, carbanion,

electrophile or nucleophile.

i) (CH3)2CH-

ii) CH3CH2

iii) CH3O-

iv) NO2+

v) (CH3)3C+

b) Ethane reacts with bromine according to the equation

CH3CH3 + Br2

CH3CH2Br + HBri) Classify this reaction

ii) Write the structural formula of the organic speciesformed when CH3CH3 undergoes homolytic fission.

iii) Write the structural formulae of the species formedwhen CH3CH2Br undergoes heterolytic fission.


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12.5 REACTION IN ORGANIC COMPOUNDS12.5 REACTION IN ORGANIC COMPOUNDSExercises:

3. (a) Explain how free radicals are formed. Write an

equation for the bromine-bromine bond cleavagein the bromination of methane. State the type of

bond cleavage.

(b) Which of the following species is likely to be anelectrophile, and which a nucleophile ?

CH3OCH2CH3, AlCl3, CH3OH, NH3,

C6H5N2+, (CH3)CH2

+ ,OH- ,

CH3CH=CH2,

12.5_Reactions_in_organic_compounds_1

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