General Organic Chemistry (2)

Lecture 2

General Organic Chemistry (2)

1. Types of Organic Reactions
2. Isomerism

• Structural Isomerism

• Stereoisomerism

Types of Organic Reactions

There are four possible of alkyl substitution for carbon denoted 1⁰ (primary), 2⁰ (secondary), 3⁰ (tertiary) and 4⁰ (quaternary) e.g,

1.1Classification of organic reactions by type of chemical bonds braking

Homolysis

The movement of a single electron to each atoms with formation two free radicals.

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 radicals

General equation of homolysis of a bond to carbon:

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

Free Radicals

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

• Highly reactive because of the unstable electronic configuration

e.g.

tert -butyl carboradical is the most stable because electron-donating groups exert positive inductive effects and hyperconjugation effects to stabilize carbon atom.

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

Electron-accepting groups exert negative inductive effects to destabilize carboradical due to increase of electron deficit on the carbon atom.

If the carbon atom with the unpaired electron is related to the atom in a state of sp2-hybridization or with a heteroatom which has a lone pair of electrons, carboradical is more stable due to the delocalization of charge on the conjugating system (р-π-conjugation or р-р-conjugation)

Heterolysis

The movement of a pair of electrons to one atom. Two charged fragments or ions formed.

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.

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

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

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

Electron-accepting groups exert negative inductive effects to destabilize carbocation due to increase of electron deficit on the carbon atom.

If the carbon atom with the vacant orbital is related to the atom in a state of sp2-hybridization or with a heteroatom which has a lone pair of electrons, carbocation is more stable due to the delocalization of charge on the conjugating system (р-π-conjugation or р-р-conjugation)

tert -butyl carboanion is the least stable because electron-donating groups exert positive inductive effects to increase the negative charge on the carbon atom.

Unlike carbocation and carboradical, the greater the number of alkyl groups attached to the central carbon atom, the less stable is the carboanion.

Electron - accepting groups exert negative inductive effects to stabilize carboanion due to decrease of electron over on the carbon atom.

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

Nature	Electrophile	Nucleophile
Cation	Free radical	Anion	Molecule with lone pair of electrons
Example	Br+, Cl+, NO2+, R+, RCO+, SO3H+	H•, Br•, Cl•, I•, R•, HO•, CH2=CHCH2•,	Cl–, Br–, I–, RO–, CN–, OH–, RCOO–	H2O, ROH, ROR, NH3, RNH2, R2NH, R3N

Substitution 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.

H2O CH3 – Cl + NaOH ¾¾¾® CH3 – OH + NaCl

Addition Reactions

Two molecules react to give a single product. Characteristic reactions of compounds with multiple bonds.

e.g.

Elimination 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.

Condensation Reactions

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

e.g.

Rearrangement Reactions

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

e.g.

Isomerism

Structural Isomerism

 

 

Chain Isomerism

Chain isomers are isomers that have different carbon skeletons.
e.g.

Position Isomerism

Position isomers are isomers that have the same carbon skeleton and functional group. They differ only in the position of the functional group.
e.g.

Metamerism

Metamers are those isomers with the functional group interrupting the carbon skeleton at different positions.
e.g.

 

Tautomerism

Tautomers are those isomers with structures differing in arrangement of atoms. They are in dynamic equilibrium with each other.
e.g.

Functional Group Isomerism

Functional group isomers are isomers that have the same molecular formula but contain different functional groups.
e.g.

 

 

Stereoisomerism

Conformational Isomerism

Conformational isomers are stereoisomers that have different arrangements of their atoms in space due to free rotation about a covalent σ-bond.

e.g.

Conformational isomers interconvert into each other without breaking of chemical bonds. Unlike structural isomers, different conformers can’t usually be isolated, because theyinterconverttoo rapidly. This kind of isomers can be fined by physical and chemical methods only.

 

Geometrical Isomerism

Geometrical isomers are stereoisomers that have different arrangements of their atoms in space due to restricted rotation about a covalent bond.

Rotation of a carbon atom of a C = C bond through an angle of 90° causes the breaking of the p bond.

 

Geometrical isomers have different physical and chemical properties.

 

Physical properties	cis -But-2-ene	trans -But-2-ene
Melting point (°C)	–139	–106
Boiling point (°C)

trans -But-2-ene has higher melting point

Þ more regular and symmetrical structure

Þ molecules pack more compactly in crystal lattice

Þ difficult to break the lattice

Þ higher melting point

 

cis -But-2-ene has higher boiling point because it has net dipole moment

Þ molecules are held together by dipole-dipole interactions

Þ trans -isomer has no net dipole moment, their molecules are held by instantaneous dipole-induced dipole interactions

Þ dipole-dipole interactions are stronger

Þ cis -but-2-ene has higher boiling point

Another example:

cis -butenedioic acid and trans -butenedioic acid

Physical properties	cis -Butenedioic acid	trans -Butenedioic acid
Melting point (°C)
Solubility in water (gram of solution per 100 g of water at 25 °C)	78.8	0.7

trans -butenedioic acid has higher melting point
and form more extensive intermolecular hydrogen bonding.

While the cis -isomer forms intramolecular hydrogen bonding
Þ less extensive intermolecular hydrogen bonding
Þ molecules are less strongly held
Þ lower melting point

Although trans -butenedioic acid can form more extensive hydrogen bonds with water molecules, cis- butenedioic acid is more soluble in water than the trans -isomer because of the greater dipole moment.

cis - and trans -butenedioic acids have different chemical properties

Enantiomerism

• Enantiomerism occurs in those compounds whose molecules are chiral.

• A chiral molecule is one that is not superimposable with its mirror image.

• The chiral molecule and its mirror image are enantiomers.

 

sp ³-hybridized carbon atom with two or more identical groups attached is achiral and contains a plane of symmetry

sp ³- hybridized carbon atom with four different groups attached is chiral and do not contain a plane of symmetry

E.g. butan-2-ol

Plane-polarized Light

Light is an electromagnetic radiation.

Polarimeter is a device used for measuring the effect of optically active compounds on plane-polarized light.

If the tube of polarimeter is empty, or if an optically inactive substance is present, the axes of the plane-polarized light and the analyzer will be exactly parallel.

If the tube contains an optically active substance, the plane-polarized light will rotate as it passes through the tube.

If the analyzer is rotated in a clockwise direction, the rotation is said to be positive (+).

If the rotation is anticlockwise, the rotation is said to be negative (–).

Molecule whith one chiral carbon atom exists informs of two stereoisomers.

A compound with n chiral atoms can have a maximum of 2 ⁿ stereoisomers.

N = 2ⁿ, where N – number of optical isomers; n – number of chiral atoms

e.g. N = 2² = 4

 

 

Molecule having two chiral centers with identical substituent exists in three stereoisomeric forms. Because one isomer has a symmetry plane and is achiral.

Compounds that are achiral, yet contain stereogenic centers, are called meso compounds (meso forms).

e.g.

Tartaric acid exists in three stereoisomeric forms: two enantiomers and one meso form.

 

Racemic mixtures, or racemates, are 50:50 mixtures of (+) and (-) enantiomers. Racemic mixtures and individual diastereomers differ in their phisical properties, such as solubility, melting point, and boiling point.

Most reactions give chiral prodacts. If the reactant are optically inactive, the products are also optically inactive – either meso or racemic. If one ore both of the reactant is optically active, the product can also be optically active.

Questions and problems

1. Give the symbol for the first-order nucleophile addition reaction а) Е_S¹; b) N_A¹; c) A_N¹; d) A_E¹; e) S_N²; f) E; g) S_E¹.

2. What kind of reaction is the following one? а) rearrangement; b) condensation; c) addition; d) substitution;

3. Give the symbol for the following reaction а) Е_S¹; b) N_A¹; c) A_N¹; d) A_E¹; e) S_N²; f) E; g) S_E¹.

4. Dispose the carbocations as their stability grows: 1) , 2) , 3) . Explain. а) 1,2,3; b) 3,1,2; c) 2,3,1; d) 1,3,2; e) 2,1,3; f) 3,2,1.

5. What kind of isomerism is there in the following pair of compounds а) chain isomerism; b) position isomerism; c) metamerism; d) tautomerism; e) functional group isomerism; f) enantiomerism; g) geometrical isomerism; h) conformational isomerism.

6. What kind of formulas was used in the previous question? а) full structural; b) short structural; c) simplified structural; d) sterechemical; e) perspective; f) Newman formula; g) Fischer formula.

7. What kind of isomers has the same empirical formulas but different functional groups? а) chain isomerism; b) position isomerism; c) metamerism; d) tautomerism; e) conformational isomerism; f) enantiomerism; g) geometrical isomerism.

8. How many optical isomers has the following compound? In the following compound show chiral atoms as *. а) 2; b) 3; c) 4; d) 9; e) 8; f) compound has no optical isomers.

Lecture 2

General Organic Chemistry (2)

1. Types of Organic Reactions
2. Isomerism

• Structural Isomerism

• Stereoisomerism

Types of Organic Reactions

There are four possible of alkyl substitution for carbon denoted 1⁰ (primary), 2⁰ (secondary), 3⁰ (tertiary) and 4⁰ (quaternary) e.g,

1.1Classification of organic reactions by type of chemical bonds braking

Homolysis

The movement of a single electron to each atoms with formation two free radicals.

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 radicals

General equation of homolysis of a bond to carbon:

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

Free Radicals

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

• Highly reactive because of the unstable electronic configuration

e.g.

tert -butyl carboradical is the most stable because electron-donating groups exert positive inductive effects and hyperconjugation effects to stabilize carbon atom.

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

Electron-accepting groups exert negative inductive effects to destabilize carboradical due to increase of electron deficit on the carbon atom.

If the carbon atom with the unpaired electron is related to the atom in a state of sp2-hybridization or with a heteroatom which has a lone pair of electrons, carboradical is more stable due to the delocalization of charge on the conjugating system (р-π-conjugation or р-р-conjugation)

Heterolysis

The movement of a pair of electrons to one atom. Two charged fragments or ions formed.

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.

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

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

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

Electron-accepting groups exert negative inductive effects to destabilize carbocation due to increase of electron deficit on the carbon atom.

If the carbon atom with the vacant orbital is related to the atom in a state of sp2-hybridization or with a heteroatom which has a lone pair of electrons, carbocation is more stable due to the delocalization of charge on the conjugating system (р-π-conjugation or р-р-conjugation)

tert -butyl carboanion is the least stable because electron-donating groups exert positive inductive effects to increase the negative charge on the carbon atom.

Unlike carbocation and carboradical, the greater the number of alkyl groups attached to the central carbon atom, the less stable is the carboanion.

Electron - accepting groups exert negative inductive effects to stabilize carboanion due to decrease of electron over on the carbon atom.