Organic Reactions Dr. M. Abd-Elhakeem Faculty of Biotechnology Organic Chemistry Chapter 3.

OrganicReactions

Dr. M. Abd-Elhakeem

Faculty of Biotechnology

Organic Chemistry

Chapter 3

The majority of organic reactions take place at functional groups and are characteristic of that functional group.

the reactivity of the functional group is affected by stereo-electronic effects.

For example

a functional group may be surrounded by bulky groups which hinder the approach of a reagent and slow down the rate of reaction. This is referred to as steric shielding.

Electronic effects

can also influence the rate of a reaction. Neighboring groups can influence the reactivity of a functional group if they are electron-withdrawing or electron donating and influence the electronic density within the functional group.

Organic compounds burn exothermically to produce carbon dioxide and water if there is a plentiful supply of oxygen. This is known as complete combustion.

e.g. CH4 + 2O2 CO2 + 2H2O + Energy

1- Combustion

2 -Substitution Reaction

Substitution reactions a group or an atom in a particular chemical compound is replaced by another one.

There are three main types substitution reactions:

1- Electrophilic substitution:

A positive charged atom or group attack the electrons richest part of the compound

2- Nucleophilic substitution:

A negative charged atom or group attack the part of the lower electron density in the compound.

3- Free radical substitution

As a result of homogeneous break down of the attacking reagent a free radical is produced to attack a neutral molecule and replace one part of the molecule. The result is new molecule and new free radical

In the presence of ultraviolet light Alkanes react to form alkyl halide and hydrogen chloride.

CH4 + Cl2 CH3Cl + HCl

The mechanism for this reaction is known as free radical substitution.

A- Chlorination of Alkanes

CH4 + Cl CH3 + HCl

Cl2 Cl + Cl

CH3 + Cl2 CH3Cl + Cl

CH3ClCH3 + Cl

initiation step

two propagation steps

termination step

UV Light

CH3CH3CH3 + CH3minor termination step

Free radical substitution mechanism

Also get reverse of initiation step occurring as a termination step.

CH3Cl + Cl2 CH2Cl2 + HCl

Overall reaction equations

Conditions

CH2Cl2 + Cl2 CHCl3 + HCl

CHCl3 + Cl2 CCl4 + HCl

ultra-violet lightexcess chlorine

Further free radical substitutions

2. Electrophilic substitutions of Aromatic compounds

Benzene ring has six electrons circulate inside the ring thus benzene ring has a high electron density.

This resonance give a high chemical stability for benzene and cause the difficulty of double bond reaction

So Benzene generally give only substitution reaction except in addition of hydrogen

H

H

H

H

H

H

HNO3

H2SO4

Br2

FeBr3

H

H

H

H

H

H

N+

O

O

NO2

H

H

H

H

H

Br

H

H

H

H

HBr Br+ -

CH3CH2 Br

AlCl3

CH2CH3

H

H

H

H

H

H2SO4

SO3

H

H

H

H

H

(Friedl Craft)

+

H

H

H

H

H

H

S+

OH

O

O

Friedl Craft reaction is used to introduce any alkyl (R) or acyl (RCO) group into an aromatic ring in presence of AlCl3 as catalyst

Cl

AlCl3

An atom or group already attached to a benzene ring may direct an incoming electrophile to either the ortho-para positions or the meta position.

Directing groups

Atoms or groups that make the benzene molecule more reactive by increasing the ring's electron density are called activating groups. Activating groups serve as ortho-para directors when they are attached to a benzene ring.

An atom or group that makes the benzene molecule less reactive by removing electron density from the ring acts as a deactivating group. Deactivating groups direct incoming electrophiles to the meta position.

You can further classify activating and deactivating groups or atoms as strong, moderate, or weak in their directing influence. This table lists some typical activating and deactivating groups by the order of their strength.

NH2

OHORR

NO2

COOHCN

Ortho-Para director

Meta director

Halogen atoms show both activating and deactivating characteristics. Because

1- they have three pairs of unshared electrons, halogen atoms can supply electrons toward the ring.

2- due to their high electronegativity, halogen atoms also tend to remove electrons from the benzene ring. These conflicting properties make halogens a weak ortho-para director and also a ring deactivator.

NH2

HNO3

H2SO4

Br2

FeBr3

CH3CH2Br

AlCl3

SO3

H2SO4

o,p directing group

NO2

NH2

Br

NH2

NH2

CH3

SO3

NH2

+

+

+

+

NH2

NO2

NH2

BrNH2

CH3

NH2

SO3

NO2

HNO3

H2SO4

NO2

NO2

NO2

Br

Br2

FeBr3

CH3CH2Br

AlCl3

NO2

CH3

SO3

H2SO4

NO2

SO3

R m directing group

Substitution Reactions of Alkynes

CH CHNaNH2 / liq NH3

Ag(NH3)+/

-OH

Cu(NH3)+/

-OH

CH CNa

CH CAg + NH3

CH CCu + NH3

White ppt

Red ppt

Terminal alkynes show also substitution reactions. Where the terminal hydrogen is very easy to remove and replaced by a metal.

The last two reactions are used to differentiate between terminal alkynes and mid-alkynes or alkenes

C C CH3 CH3

Ag(NH3)+/

-OH

Cu(NH3)+/

-OH

NO Reaction

The easiness of removing the terminal hydrogen from alkyne molecule give some weak acidic properties of this molecule.

CH2 X

CH3

NaOR / alcohol

NaNH 2 / liq NH3

CH CNa

liq NH3

KCN

CH CH2CH3

CH2 OR

CH3

CH2 CN

CH3

CH2 NH2

CH3

X = Cl, Br, I, F

the halogen atom in the alkyl halide is readily removed by many reagents

The conventional method to replace a halogen is made by reaction with reagent contain alkali metal

THE END