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Main classes of organic reactions Main organic reactions are:

(i) addition reaction, (ii) elimination reaction and (iii) substitution reaction.

All of these reactions may take place in either (i) polar mechanism, or through (ii) free radical mechanism.

Polar reactions may be electrophilic or nucleophilic.

Types of Steps in Reaction Mechanisms

• Bond formation or breakage can be symmetrical or unsymetrical

• Symmetrical- homolytic

• Unsymmetrical- heterolytic

• Not as common as polar reactions

• Radicals react to complete electron octet of valence shell

• A radical can break a bond in another molecule and abstract a partner with an electron, giving substitution in the original molecule

• A radical can add to an alkene to give a new radical, causing an addition reaction

Free Radical Reactions

• Three types of steps

• Initiation – homolytic formation of two reactive species with unpaired electrons

• Example – formation of Cl atoms form Cl2 and light

• Propagation – reaction with molecule to generate radical

• Example - reaction of chlorine atom with methane to give HCl and CH₃^.

• Termination – combination of two radicals to form a stable product:

CH3. + CH3.  CH3CH3

Steps in Radical Substitution

• Molecules can contain local unsymmetrical electron distributions due to differences in electronegativities

• This causes a partial negative charge on an atom and a compensating partial positive charge on an adjacent atom

• The more electronegative atom has the greater electron density

• Elements such as O, F, N, Cl more electronegative than carbon

Polar Reactions

• Rearrangement reactions – a molecule undergoes changes in the way its atoms are connected

Nucleophilic Substitution Reaction

Reaction

Reaction

Nucleophilic addition reactions:

Electrophilic addition reactions:

Addition reactions – two molecules combine

• Addition of Cl₂ and Br₂

carried out with either the pure reagents or in an inert solvent such as CH₂Cl₂

• addition of bromine or chlorine to a cycloalkene gives a trans-dihalocycloalkane

• addition occurs with anti stereoselectivity; halogen atoms add from the opposite face of the double bond

Br₂

CH₂Cl₂

Br Br

Br Br +

trans-1,2-Dibromocyclohexane (a racemic mixture)

Cyclohexene

+

CH₃CH=CHCH₃ Br₂

CH₂Cl₂ CH₃CH-CHCH₃ Br Br

2,3-Dibromobutane 2-Butene

+

Br₂

S S R R

racemic mixture CH₃

CH₃

H H

CH₃ CH₃

Br Br

H H

CH₃ CH₃

Br Br

H H

CH₃ CH₃ H

H CH₃

CH₃

H H

Br

Br

CH₃ CH₃

Br Br

H H

mesoS R Br₂

Racemic mixture

Addition of HX :

Carried out with pure reagents or in a polar solvent such as acetic acid Addition is regioselective

CH₃CH=CHCH₃ H Br CH₃CH-CHCH₃ H

Br

+ +





sec-Butyl cation (a 2° carbocation

intermediate) slow, rate

determining CH₃CH=CH₂ HBr CH₃CH-CH₂

Br H

CH₃CH-CH₂ H Br

1-Bromopropane (not observed) 2-Bromopropane

Propene

+ +

Br CH₃CHCH₂CH₃ CH₃CHCH₂CH₃ Br

sec-Butyl cation (an electrophile) +

Bromide ion (a nucleophile)

fast

2-Bromobutane

Markovnikov’s rule: in the addition of HX, H₂O, or ROH to an

alkene, H adds to the carbon of the double bond having the greater number of hydrogens

On similar reduction other hydrocarbons are produced.

CH₂= CH₂ +H₂ Ethane

CH₃ CH= CH₂ +H₂ Propane CH₃ CH= CH CH₃ +H₂ Butane

• Addition of H₂O

• addition of water is called hydration

• acid-catalyzed hydration of an alkene is regioselective; hydrogen adds preferentially to the less substituted carbon of the double bond

• HOH adds in accordance with Markovnikov’s rule

CH₃CH=CH₂ H₂O H₂SO₄

CH₃CH-CH₂ H OH

Propene 2-Propanol

+

CH₃C=CH₂ CH₃

H₂O H₂SO₄

HO CH₃

H CH₃C-CH₂

2-Methyl-2-propanol 2-Methylpropene

+

-Elimination: removal of atoms or groups of atoms from adjacent carbons to form a carbon-carbon double bond

– a type of -elimination called dehydrohalogenation (the elimination of HX)

• Saytzeff rule: the major product of a elimination is the more stable (the more highly substituted) alkene

Br CH₃CH₂O^-Na⁺ CH₃CH₂OH

2-Methyl-2-butene (major product) 2-Bromo-2-

methylbutane

2-Methyl-1- butene +

Br CH₃O^-Na⁺

CH₃OH +

1-Methyl- cyclopentene (major product) 1-Bromo-1-methyl-

cyclopentane

Methylene- cyclopentane

• Substitution reactions – parts from two molecules exchange

Free Radical Substitution Reaction

Electrophilic Substitution Reaction

Nucleophilic reactions: nucleophilic substitution (S_N)

Nucleophilic substitution: -> reagent is nucleophil -> nucleophil replaces leaving group

-> competing reaction (elimination + rearrangements)

nucleophilic substitution Nucleophile

+ ^{C X C Nu} +

Nu ^- X^-

leaving group

• in the following general reaction, substitution takes place on an sp3 hybridized (tetrahedral) carbon

tert-butyl bromide tert-butyl alcohol

21

Nucleophilic Substitution

• Some nucleophilic substitution reactions

CH₃-I HO ^-

Nu ^-

RO ^- HS ^- RS ^-

I ^-

NH₃ HOH

CH₃-SH CH₃-SR CH₃-OH CH₃-OR

H CH₃-O-H CH₃-NH₃⁺

CH₃X CH₃Nu X^-

+

An alcohol (after proton transfer) An alkylammonium ion

An alkyl iodide

A sulfide (a thioether) A thiol (a mercaptan)

An ether An alcohol

Reaction: + +

S

_N

1 reaction: unimolecular nucleophilic substitution

S_N1 is illustrated by the solvolysis of tert-butyl bromide

Step 1: ionization of the C-X bond gives a carbocation intermediate

Step 2: reaction of the carbocation (an electrophile) with aq. NaOH (a nucleophile) gives an alcohol

CH₃O

H H₃C C CH₃

CH₃

OCH₃ H

C CH₃

CHCH_{3 3} O

H₃C

H H₃C

H₃C C H₃C

O

CH₃

H

fast +

+ +

+

Step 2: reaction of the carbocation (an electrophile) with methanol (a nucleophile) gives an oxonium ion

Step 3: proton transfer completes the reaction

+ +

+ + fast

C H₃C

H₃C O H₃C

O

CH₃ H

O O H

CH₃ CH₃

H

H

CH₃ H₃C

H₃C C H₃C

(R)-Enantiomer Planar carbocation (achiral)

C H

Cl C₆H₅

Cl

C+

C₆H₅

H

Cl

CH₃OH -Cl^-

-H⁺ +

A racemic mixture Cl

C₆H₅ C₆H₅

C OCH₃ H

CH₃O C H

Cl

(R)-Enantiomer (S)-Enantiomer

24

An energy diagram for an S

_N

1 reaction

S

_N

1

S_N2 reaction: bimolecular nucleophilic substitution

C Br H

HH

HO + C

H

H H

HO Br

- -

Transition state with simultaneous bond breaking

and bond forming

C H

HH

HO + ^{Br -}

• both reactants are involved in the transition state of the rate-determining step

• the nucleophile attacks the reactive center from the side opposite the leaving group

• An energy diagram for an S_N 2 reaction

• there is one transition state and no reactive intermediate