New Way Chemistry for Hong Kong A-Level Book 3A1 Introduction to Organic Reactions 28.1Types of Reactive Species in Organic Chemistry 28.2Ways of Breaking

New Way Chemistry for Hong Kong A-Level Book 3A

1

Introduction to Organic Reactions

28.128.1 Types of Reactive Species in Organic Types of Reactive Species in Organic ChemistryChemistry

28.228.2 Ways of Breaking Covalent BondsWays of Breaking Covalent Bonds

28.328.3 Inductive and Resonance EffectsInductive and Resonance Effects

28.428.4 Types of Organic ReactionsTypes of Organic Reactions

Chapter 28Chapter 28


2

28.1 Types of Reactive Species in Organic Chemistry (SB p.82)

Free RadicalsFree Radicals

• Electrically neutral atoms or groups of atoms possessing an unpaired electron

• Highly reactive because of the unstable electronic configuration

e.g.


3


Electrophiles and NucleophilesElectrophiles and Nucleophiles

Electrophiles

• electron-deficient species that tend to accept electron(s)

• possess an empty orbital to receive the electron pair

• cations or free radicals seeking electron-rich centres

Nucleophiles

• electron-rich species that tend to seek an electron-deficient site for reaction

• possess lone pairs of electrons

• anions or molecules with lone pairs of electrons


4


Nature

Electrophile Nucleophile

Cation Free radical Anion

Molecule with lone

pair of electrons

Example

Br+, Cl+, NO2

+, R+, RCO+, SO

3H+

H•, Br•, Cl•, I•, R•, HO•, CH2

=CHCH2•,

Cl–, Br–, I–, RO–, CN–, OH–, RCOO–

H2O, ROH, ROR, NH3, RNH2, R2NH, R3N


5

Check Point 28-1 Check Point 28-1

Identify the following chemical species as electrophiles, nucleophiles, or one that could act as both an electrophile and a nucleophile.

(a) Cl– (d) CH3Cl

(b) C2H5+ (e) NO2

+

(c) NH3 (d) CH3OHAnswer

(a) Nucleophile

(b) Electrophile

(c) Nucleophile

(d) Electrophile and nucleophile

(e) Electrophile

(f) Electrophile and nucleophile



6

28.2 Ways of Breaking Covalent Bond (SB p.83)

HomolysisHomolysis

A curly arrow with half an arrow head ‘ ’ is used to indicate the movement of a single electron.


7


Energy must be supplied in, either in form of heat or

irradiation with light.

e.g. chlorine undergoes homolysis readily when heated, or

when irradiated with light of a wavelength that can be

absorbed by the molecule to form two chlorine radic

als


8


General equation of homolysis of a bond to carbon:

e.g.methane undergoes homolysis to form a methyl radical and a hydrogen radical


9


HeterolysisHeterolysis

• a curly arrow with a full arrow head ‘ ’ is used to indicate the movement of an electron pair.

• 2 charged fragments or ions formed


10


Heterolysis of a bond requires the bond to be polarized

The greater the difference in electronegativity between

the atoms, the greater is the polarization of the bonds.


11


The product of heterolysis of a bond to carbon depends on the electronegativity of the atom that is bonded to the carbon atom.


12


Which type of bond fission, homolysis or heterolysis, is most likely to occur in:

(a) a bond between identical atoms?

(b) a bond between atoms having widely different electronegativities?

(c) a bond between atoms having similar electronegativities?

Answer(a) Homolysis

(b) Heterolysis

(c) Homolysis

28.2 Ways of Breaking Covalent Bonds (SB p.85)


13

28.3 Inductive and Resonance Effects (SB p.86)

Inductive EffectInductive Effect

Due to the difference in electronegativity between two atoms linked up by bonds, the bonding electrons will displace towards the more electronegative atom. The atom exhibits a partial negative charge.

The electronic effect of a group that is transmitted by the polarization of electrons in bonds is called an inductive effect.


14


Inductive effect is represented by an arrow head in the middle of the covalent bond pointing in the direction of the displacement of electrons.

Electron-withdrawing group (X) exerts a negative inductive effect.

Electron-donating group (Y) exerts a positive inductive effect.


15


1. Groups which exert negative inductive effects (i.e. electron-withdrawing groups):

e.g.

–NO2 > –F > –COOH > –Cl > –Br > –I

2. Groups which exert positive inductive effects (i.e. electron-releasing groups):

e.g.

alkyl groups like –CH3, –C2H5, –C3H7


16


tert-butyl carbocation is the most stable because electron-donating groups exert positive inductive effects to reduce the positive charge on the carbon atom.

The greater the number of alkyl groups attached to the central carbon atom, the more stable is the carbocation.


17


Resonance EffectResonance Effect

Resonance effect is an electronic effect involving bond electrons or electrons present in unhybridized p orbitals.

The ion become more stable when the charge of the ion can be reduced or dispersed.

Resonance structures


18


The actual structure of carboxylate ion is the resonance hybrid of the resonance structures.

• The negative charge of the anion is dispersed

• This resonance stabilization is responsible for the high acidity of carboxylic acids


19


Another example:

Carbocation with the positively charged carbon atom directly bonded to a benzene ring

Its actual structure is represented by four resonance structures shown below:


20

Example 28-1Example 28-1The following equations represent the ionizations of two organic acids:

(a) Which conjugate base is more stable? Explain your answer.

Answer


Solution:

(a) Conjugate base 2 is more stable. The anion is stabilized by resonance effect and the negative charge of the anion is dispersed over two oxygen atoms. The two resonance structures of the anion are shown below:


21

Example 28-1 (cont’d)Example 28-1 (cont’d)(b) Which conjugate base is less stable? Explain your answer.

(c) Which is a stronger acid?

Answer


Solution:

(b) Conjugate base 1 is less stable because there is no resonance effect stabilizing the anion. Moreover, the positive inductive effect of the electron-releasing CH3CH2– group further destabilizes the anion.

(c) Acid 2 is a stronger acid than acid 1.


22


(a) Draw the two resonance structures for propanoate ion (CH3CH2COO–).

(b) State whether the following species exhibit positive or negative inductive effects.

(i) –I

(ii) –NO2

(iii)–COOH

(iv) –C2H5

Answer


(a)

(b) (i) Negative inductive effect

(ii) Negative inductive effect

(iii) Negative inductive effect

(iv) Positive inductive effect


23

28.4 Types of Organic Reactions (SB p.90)

Substitution ReactionsSubstitution Reactions

• An atom or a group of atoms of the reactant molecule is replaced by another atom or group of atoms

• Characteristic reactions of saturated compounds

e.g.

H2OCH3 – Cl + NaOH CH3 – OH + NaCl


24


Addition ReactionsAddition Reactions

• Two molecules react to give a single product

• Characteristic reactions of compounds with multiple bonds

e.g.


25


Elimination ReactionsElimination Reactions

• Atoms or groups of atoms are removed from two adjacent atoms of the reactant molecule

• Method for preparing compounds with multiple bonds

e.g.


26


Condensation ReactionsCondensation Reactions

• Two or more molecules join together, with a small molecule being removed

e.g.


27


Rearrangement ReactionsRearrangement Reactions

• A molecule undergoes reorganization of its constituent atoms or groups of atoms

e.g.


28

The END

Documents

New Way Chemistry for Hong Kong A-Level Book 3A1 Introduction to Organic Reactions 28.1Types of Reactive Species in Organic Chemistry 28.2Ways of Breaking