B.Sc. Part II Semester- III Paper V: Organic Chemistry Unit 1: Stereochemistry [8] 1.1 Conformational isomerism – Introduction. 1.2 Representation of conformations of ethane by using Saw- Horse, Fischer (dotted line wedge) and Newmann’s projection formulae. 1.3 Conformations and conformational analysis of ethane and n-butane by Newmann’s Projection formula with the help of energy profile diagrams. 1.4 Cycloalkanes relative stability - Baeyer’s strain theory, Theory of strainless rings. 1.5 Conformations and stability of cyclohexane and monosubstituted cyclohexanes Cyclohexanol,bromocyclohexane and methyl cyclohexane. 1.6 Locking of conformation in t-butyl cyclohexane. 1.7 Stereoselective and stereospecific reactions: i) Stereochemistry of addition of halogens to alkenes: syn and anti addition. Example- Addition of bromine to 2-butene. (mechanism not expected) ii) Stereochemistry of elimination reaction: syn and anti elimination. Example-Dehydrohalogenation of 1-bromo-1,2diphenylpropane. (mechanism not expected)

Stereochemistry

Isomers are compounds with the same molecular formulae but that are structurally different in some way. It is important to be able to Stereochemistry/sdpatilnsr.blogspot.in/1

recognise isomers because they can have different chemical, physical properties and biological properties. Constitutional isomers differ in the order in which the atoms are connected so they can contain different functional groups and / or bonding patterns (e.g. branching) example: 1-propanol, 2-propanol and ethyl methyl ether (C3H8O) Stereoisomers have the same functional groups and connectivities, they differ only in the arrangement of atoms and bonds in space Conformational isomers (or conformers or rotational isomers or rotamers) are stereoisomers produced by rotation (twisting) about σ bonds, and are often rapidly interconverting at room temperature. example 1: butane : anti (left) and syn (center). Rotation about the C2-C3 σ bond is animated (right). Try rotating the model to look along the C-C to see the two extreme forms

Conformations of Ethane Introduction to Conformational Analysis Ethane, which is comprised of two methyl groups attached to each other, has properties very similar to those of methane. However, the complete 3-dimensional shape of ethane cannot be specified by the bond lengths and bond angles alone because ethane can internally rotate about its C-C bond. To understand why ethane has this extra degree of freedom, consider the cylindrically symmetric nature of $\sigma$ bonds. The $\sigma$ bond can maintain a full degree of overlap while its two ends rotate. Hence, the energetic barrier to rotation about sigma bonds is generally very low. Unlike pi bonds in alkenes, the C-C sigma bond does not hold the two methyl groups in fixed positions relative to one another. The different spatial arrangements formed by rotations about a single bond are called conformations or conformers.

Free rotation about the C-C sigma bond in ethane.

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Visualizing Conformations Several methods are used by organic chemists to help them visualize the conformations of molecules. One of these methods uses wedges to denote bonds that are extending out from the plane of the page toward the reader and dashes to indicate bonds that are going into the plane of the page away from the reader. This notation is frequently used to represent the tetrahedral geometry of sp3-hydridized carbons. A Newman projection can be used to specify the conformation of a particular bond with clarity and detail. A Newman projection represents the head-on look down the bond of interest. The circle in the Newman projection represents the atom in front of the bond, and the lines radiating from the center are the bonds of that atom. The bonds of the rear atom emerge from the sides of the circle.

How to draw a Newman projection.

Newman projections can be characterized by the angles formed between bonds on the front atom and bonds on the rear atom. Such angles are called dihedral angles. The full 3D shape of any molecule can be described by its bond lengths, bond angles, and dihedral angles. Conformations of Ethane While there are an infinite number of conformations about any sigma bond, in ethane two particular conformers are noteworthy and have special names. In the eclipsed conformation, the C-H bonds on the front and back carbons are aligned with each other with dihedral angles of 0 degrees. In the staggered conformation, the C-H bonds on the rear carbon lie between those on the front carbon with dihedral angles of 60 degrees.

The eclipsed and staggered conformations of ethane. Stereochemistry/sdpatilnsr.blogspot.in/3

Energetically, not all conformations are equally favored. The eclipsed conformation of ethane is less stable than the staggered conformation by 3 kcal/mol. The staggered conformation is the most stable of all possible conformations of ethane, since the angles between C-H bonds on the front and rear carbons are maximized at 60 degrees. In the eclipsed form, the electron densities on the C-H bonds are closer together than they are in the staggered form. When two C-H bonds are brought into a dihedral angle of zero degrees, their electron clouds experience repulsion, which raises the energy of the molecule. The eclipsed conformation of ethane has three such C-H eclipsing interactions, so we can infer that each eclipsed C-H "costs" roughly 1 kcal/mol.

Eclipsing interactions in ethane.

Steric Hindrance Eclipsing interactions are an example of a general phenomenon called steric hindrance, which occurs whenever bulky portions of a molecule repel other molecules or other parts of the same molecule. Because such hindrance causes resistance to rotation, it is also called torsional strain. The 3 kcal/mol needed to overcome this resistance is the torsional energy. Note that this figure is very small compared to the energy required to rotate around double bonds, which is 60 kcal/mol (the bond energy of a C-C pi bond). At room temperature, ethane molecules have enough energy to be in a constant state of rotation. Because of this rapid rotation, it is impossible to isolate any particular conformation in the way that cis- and trans- alkenes can be individually isolated. Although the term "conformational isomer" is sometimes used as a synonym for conformations, conformations of a molecule are not considered true isomers because of their rapid interconversion.

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Energy diagram for rotation about the C-C bond in ethan

Conformations of Butane The hydrocarbon butane has a larger and more complex set of conformations associated with its constitution than does ethane. Of particular interest and importance are the conformations produced by rotation about the central carbon-carbon bond. Among these we shall focus on two staggered conformers (A & C) and two eclipsed conformers (B & D), shown below in several stereo-representations. As in the case of ethane, the staggered conformers are more stable than the eclipsed conformers by 2.8 to 4.5 kcal/mol. Since the staggered conformers represent the chief components of a butane sample they have been given the identifying prefix designations anti for A and gauche for C.

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Four Conformers of Butane:

The following diagram illustrates the change in potential energy that occurs with rotation about the C2–C3 bond. The model on the right is shown in conformation D, and by clicking on any of the colored data points on the potential energy curve, it will change to the conformer corresponding to that point. The full rotation will be displayed by turning the animation on. This model may be manipulated by click-dragging the mouse for viewing from any perspective. Potential Energy Profile for Butane Conformers

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Summary of Conformational Stereoisomerism i. Most conformational inter-conversions in simple molecules occur rapidly at room temperature. Consequently, isolation of pure conformers is usually not possible. ii. Specific conformers require special nomenclature terms such as staggered, eclipsed, gauche and anti when they are designated. iii. Specific conformers may also be designated by dihedral angles. In the butane conformers shown above, the dihedral angles formed by the two methyl groups about the central double bond are: A 180º, B 120º, C 60º & D 0º. iv. Staggered conformations about carbon-carbon single bonds are more stable (have a lower potential energy) than the corresponding eclipsed conformations. The higher energy of eclipsed bonds is known as eclipsing strain. v. In butane the gauche-conformer is less stable than the anticonformer by about 0.9 kcal/mol. This is due to a crowding of the two methyl groups in the gauche structure, and is called steric strain or steric hindrance. vi. Butane conformers B and C have non-identical mirror image structures in which the clockwise dihedral angles are 300º & 240º respectively. These pairs are energetically the same, and have not been distinguished in the potential energy diagram shown here.

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