Isomerism

Ms. Anjelyn del Rosario

Chemistry 31

UNIVERSITY OF THE PHILIPPINES MANILAPadre Faura, Ermita, Manila

SS, 2009 – 2010

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Isomers

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Compounds that have the same molecular formula but are not identical

Isomerism

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• The two major classes of isomers are constitutional isomers and stereoisomers.

Constitutional/structural isomers have different IUPAC names, the same or different functional groups, different physical properties and different chemical properties.

Stereoisomers differ only in the way the atoms are oriented in space. They have identical IUPAC names (except for a prefix like cis or trans). They always have the same functional group(s).

• A particular three-dimensional arrangement is called a configuration. Stereoisomers differ in configuration.

The Two Major Classes of Isomers:

Isomerism

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Figure 5.3 A comparison of constitutional isomers and geometric stereoisomers

Isomerism

Structural Isomerism

• Structural isomers are molecules with the same chemical formulas but different molecular structures - different “connectivity”.

• They arise because of the many ways to create branched hydrocarbons.

• a.k.a. “Constitutional Isomers”

CH3

CH2

CH2

CH2

CH3

CH3

CH2

CH

CH3

CH3

n-pentane, C5H12

2-methylbutane, C5H12

Constitutional Isomers

The First 10 “Normal” AlkanesName Formula M.P. B.P. # Structural Isomers

• Methane CH4 -183 -162 1

• Ethane C2H6 -172 -89 1

• Propane C3H8 -187 -42 1

• Butane C4H10 -138 0 2

• Pentane C5H12 -130 36 3

• Hexane C6H14 -95 68 5

• Heptane C7H16 -91 98 9

• Octane C8H18 -57 126 18

• Nonane C9H20 -54 151 35

• Decane C10H22 -30 174 75

C1 - C4 are Gases at Room Temperature

C5 - C16 are Liquids at Room Temperature

1. Chain Isomers

have the same number of C and H atoms but different points of attachment

CH3

CH2

CH2

CH2

CH3

CH3

CH2

CH

CH3

CH3

n-pentane, C5H12

2-methylbutane, C5H12

Constitutional Isomers

2. Position Isomers

Differ in the positions of substituents or multiple bonds

Constitutional Isomers

Constitutional Isomers

3. Functional Isomers

Differ in their functional groups.

2. Stereochemistry

Stereochemistry

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the study of the spatial arrangement of atoms in molecules.

Stereochemistry

Example of Stereoisomers:

Stereoisomers differ only in the way the atoms are oriented in space. They have identical IUPAC names (except for a prefix like cis or trans). They always have the same functional group(s).

Conformational Isomers

I. Rotation about

Single Bonds

Conformational Isomers

• Conformer: a specific conformation

1. Rotation about single bonds

2. Amine inversion

• Molecules constantly rotate through all the possible conformations.

Conformation: any three-dimensional arrangement of atoms in a molecule that results from rotation about a single bond

Conformational Isomers: Rotation about Single Bonds

Example: Ethane

• Staggered conformation: a conformation about a carbon-carbon single bond in which the atoms or groups on one carbon are as far apart as possible from the atoms or groups on an adjacent carbon

H

H H

H H

H

Example: Ethane

Conformational Isomers: Rotation about Single Bonds

• Eclipsed conformation: a conformation about a carbon-carbon single bond in which the atoms or groups of atoms on one carbon are as close as possible to the atoms or groups of atoms on an adjacent carbon

H

H H

H

HH

Example: Ethane

Conformational Isomers: Rotation about Single Bonds

Conformational Isomers: Rotation about Single Bonds

• Torsional strain

– also called eclipsed interaction strain

– strain that arises when nonbonded atoms separated by three bonds are forced from a staggered conformation to an eclipsed conformation

– the torsional strain between eclipsed and staggered ethane is approximately 12.6 kJ (3.0 kcal)/mol

+12.6 kJ/mol

Conformational Isomers: Rotation about Single Bonds

• Dihedral angle (Q): the angle created by two intersecting planes

Conformational Isomers: Rotation about Single Bonds

Conformational Isomers: Rotation about Single Bonds

Example: Butane (C1-C2 bond)

Conformational Isomers: Rotation about Single Bonds

Example: Butane (C2-C3 bond)

(Anti is Greek for “opposite of ”; gauche is French for “left.”)

Conformational Isomers: Rotation about Single Bonds

Eclipsed Butane

– calculated energy difference between (a) the non-energy-minimized and (b) the energy-minimized eclipsed conformations is 5.6 kJ (0.86 kcal)/mol

Conformational Isomers: Rotation about Single Bonds

• anti conformation

– a conformation about a single bond in which the groups lie at a dihedral angle of 180°

CH3

H H

H H

CH3

Conformational Isomers: Rotation about Single Bonds

Angle strain the strain induced in a molecule when the bond angles are

different from the ideal tetrahedral bond angle of 109.5°.

Torsional strain the strain caused by repulsion between the bonding electrons of

one substituent and the bonding electrons of a nearby substituent.

Steric strain strain caused by atoms or groups of atoms approaching each

other too closely.

Conformational Isomers: Rotation about Single Bonds

Conformational Isomers: Rotation about Single Bonds

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• Large ring strain due to angle compression

• Very reactive, weak bonds

• Torsional strain because of eclipsed hydrogens

Conformational Isomers: Cycloalkanes

Cycloalkanes: Ring Strain

1. Cyclopropane

– puckering from planar cyclobutane reduces torsional strain but increases angle strain

– the conformation of minimum energy is a puckered “butterfly” conformation

– strain energy is about 110 kJ (26.3 kcal)/mol

Conformational Isomers: Cycloalkanes

Cycloalkanes: Ring Strain

2. Cyclobutane

– puckering from planar cyclopentane reduces torsional strain, but increases angle stain

– the conformation of minimum energy is a puckered “envelope” conformation

– strain energy is about 42 kJ (6.5 kcal)/mol

Conformational Isomers: Cycloalkanes

Cycloalkanes: Ring Strain

3. Cyclopentane

• Chair conformation: the most stable puckered conformation of a cyclohexane ring– all bond C-C-C bond angles are 110.9°

– all bonds on adjacent carbons are staggered

Conformational Isomers: Cycloalkanes

Cycloalkanes: Ring Strain

2. Cyclohexane

Axial and equatorial

Conformational Isomers: Cycloalkanes

Cycloalkanes: Ring Strain

2. Cyclohexane

ring flip

• Boat conformation: carbons 1 and 4 are bent toward each other– there are four sets of eclipsed C-H interactions and one flagpole interaction

– a boat conformation is less stable than a chair conformation by 27 kJ (6.5 kcal)/mol

Conformational Isomers: Cycloalkanes

Cycloalkanes: Ring Strain

2. Cyclohexane

• Twist-boat conformation– approximately 41.8 kJ (5.5 kcal)/mol less stable than a chair conformation

– approximately 6.3 kJ (1.5 kcal)/mol more stable than a boat conformation

Conformational Isomers: Cycloalkanes

Cycloalkanes: Ring Strain

2. Cyclohexane

• Half-chair conformation

Conformational Isomers: Cycloalkanes

Cycloalkanes: Ring Strain

2. Cyclohexane

Chapter 3 35=>

Conformational Energy

Conformational Isomers

II. Amine Inversion

• The lone-pair electrons on nitrogen allow an amine to turn “inside out” rapidly at room temperature.

• The lone pair is required for inversion: Quaternary ammonium ions—ions with four bonds to nitrogen and hence no lone pair—do not invert.

• amine inversion takes place through a transition state in which the sp3 nitrogen becomes an sp2 nitrogen.

Conformational Isomers

Amine Inversion

Configurational Isomers

I. Cis-Trans Isomers

Configurational Isomers

I. Cis-Trans Isomers Alkenes and cyclic alkanes

I. Cis-Trans Isomers Alkenes

Configurational Isomers

I. Cis-Trans Isomers

cis isomerthe isomer with the hydrogens on the same side of the double bond

trans isomerthe isomer with the hydrogens on opposite sides of the double bond

Configurational Isomers

I. Cis-Trans Isomers

Configurational Isomers

Exercise

Configurational Isomers

The E,Z System of Nomenclature

“If the hydrogens are on the same side of the double bond, it is the cis isomer; if they are on opposite sides of the double bond, it is the trans isomer.”

The Z isomer has the high-priority groups on the SAME side.

The E isomer has the high-priority groups on the OPPOSITE side.

Configurational Isomers

The E,Z System of Nomenclature

Rule 1. The relative priorities of the two groups depend on the atomic numbers of the atoms that are bonded directly to the sp2

carbon. The greater the atomic number, the higher is the priority.

Configurational Isomers

Rule 2. If the two substituents bonded to an carbon start with the same atom (there is a tie), you must move outward from the point of attachment and consider the atomic numbers of the atoms that are attached to the “tied” atoms.

The E,Z System of Nomenclature

Configurational Isomers

Rule 3. If an atom is doubly bonded to another atom, the priority system treats it as if it were singly bonded to two of those atoms. If an atom is triply bonded to another atom, the priority system treats it as if it were singly bonded to three of those atoms.

The E,Z System of Nomenclature

Configurational Isomers

Rule 4. In the case of isotopes (atoms with the same atomic number, but different mass numbers), the mass number is used to determine the relative priorities.

The E,Z System of Nomenclature

Configurational Isomers

Exercise:

Draw and label the E and Z isomers for each of the following compounds:

Configurational Isomers

I. Cis-Trans Isomers cyclic alkanes

The cis isomer has its substituents on the same side of the ring.

The trans isomer has its substituents on opposite sides of the ring.

Configurational Isomers

I. Cis-Trans Isomers cyclic alkanes

Configurational Isomers

Exercise:Determine whether each of the following compounds is a cis isomer or a trans isomer:

Configurational Isomers