Geometry of Organic Compounds

H O C H C H H H H H CCH

sp 3 sp 2 sp Tetrahedral Planar Linear

Tetrahedron How to Draw a Tetrahedral Structure TbdTwo bonds on th thle plane of fth the paper, one t oward s you and one away f rom you.

In such structures, the bonds on the plane should be adjacent to each other. The following two structures are not so right.

Drawing a bond in between the bonds on the plane may give a wrong configuration The structures can also be drawn with one or no bonds on the plane of the paper Drawing Ethane - Projections

Replacing one H-atom in methane with a CH3 group gives ethane.

WdWedge st ruct ures Sawhorse Projection Newman Projection

Viewed from the front carbon to the back through the C -C bond

The front carbon is represented as a dot The back carbon is drawn as a circle and and bonds are drawn from the dot . bonds start from the circle. Dihedral Angle

The angle defined by X-C-C and C-C-Y plane is termed as dihedral angle.

Dihedral Angle Torsional angle is an alternative term used 0º C Y for dihedral angle. While dihedral angle is measured from 0-360º, torsional angle is negative positive X measured from 0-180º, as a positive value in the clockwise direction and a negative 180º C value in the anti-clockwise direction. Conformations Different spatial arrangements a molecule can adopt through rotation about sigma bonds. Based on the dihedral angle between two groups of interest, a molecular conformation can be defined using the following.

-30º 30º

syn

clinal anti

-150º 150º periplanar

anti-periplanar anti-clinal

syn-periplanar syn-clinal Conformations Staggered - A low energy conformation where the bonds on adjacent atoms bisect each other (60º dihedral angle), maximizing the separation.

Eclipsed - A high energy conformation where the bonds on adjacent atoms are aligned with each other (0º dihedral angle). a b

b a a b Gauche - Description given to two substitutents attached to adjacent atoms when their bonds are at 60º with respect to each other. a

60º b Strain in Organic Molecules

Ster ic ( non-bdditbonded interac ti)tions) – Des tabili za tion due to th e repul si on b et ween th e electron clouds of atoms or groups. Groups compete to occupy common space. Strain due to close contact of atoms separated by four or more bonds

Torsional – Strain resulting from eclipsing interactions between bonds on adjacent carbon atoms. Actual reasons are highly debated.

Angular – Destabilization due to distortion of a bond angle from its optimal value

Possible Reasons for Torsional Strain Ecldlipsed Staggered Conformations of Ethane

2.9 kcal/mol

0

The staggered conformation is more stable than the eclipsed conformation by 12 kJ/mol or 2.9 kcal/mol. There are infinite number of conformations in between both these conformers Conformations of Propane Conformations of Butane

Butane has steric and torsional strain in its eclipsed conformation Conformational Analysis of Butane Energy Corresponding to Interactions in Alkane Conformations

Tot al d est abili zati on = 19 kJ/ mol

Total destabilization = 16 kJ/mol Gauche Effect

The Gauche effect denotes the existence of a gauche conformer, which is more stable than the anti conformer. This effect is present in 1,2-difluoroethane (CH2FCH2F) for which the gauche conformation is more stable by 3.4 kJ/mol in the gas phase. Another example is 1,2-dime thoxye thane.

The increased stability of the gauche conformer is explained based on the possibility of hyperconjugation between C-F and C-H bonds. This is initiated by the very high electronegativity of fluorine. Effect of Intramolecular H-bonding ItIntramol ecul ar H-bdibonding btbetween two viiicinal groups confers an apprecibliable amount of stability (8-20 KJ/mol) to a conformer. For effective Intramolecular H-bonding to occur, the donor and the acceptor groups must be close to each other, which is possible only in the eclipsed or gauche conformation. The gauche conformations with a torsion angle of 60-70° between the interacting ggproups are ideally suited for intramolecular H-bonding.

In general, 2-substituted ethanols of the type, X-CH2-CH2-OH, where X = OH, NH2,F, Cl,Br,OCH3,NHCH3,N(CH3)2 etc. have the prefdferred gaucheconfiformations wihith OH and X forming intramolecular H-bonds. Usually the dihedral angle is slightly greater than 60°.