2. Stereoisomerism

Formulas for imaging of stereo isomers

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.

C onformational isomers interconvert into each other without breaking of chemical bonds. Unlike structural isomers, different conformers can’t usually be isolated, because they interconvert too 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.

R otation 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)	4	1

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)	130	286
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.

sp³-hybridized carbon atom with two or more identical groups attached is achiral and contains a plane of symmetry

sp³-hybridized carbon atom with four different groups attached is chiral and do not contain a plane of symmetry

e.g. butan-2-ol

Properties of Enantiomers: Optical Activity

Enantiomers have identical physical properties

Physical properties	(+) Butan-2-ol	(–) Butan-2-ol
Boiling point (°C)	99.5	99.5
Density (g cm–3 at 20°C)	0.808	0.808
Refractive index (at 20°C)	1.397	1.397

One easily observable difference of a pair of enantiomers is their properties towards plane-polarized light. When a beam of plane-polarized light passes through a solution of an enantionmer, the plane of polarization rotates.

Two enantiomers that are mirror images of one another cause exactly the same extent of rotation, but in opposite directions, clockwise in one and anticlockwise in the other.

Enantiomers are also called optical isomers and said to be optically active.

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 in forms of two stereoisomers.

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

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