Geometry of Organic Compounds

H

CC

O

HHH

HC

H C C HH

sp3 sp2 sp

Tetrahedral Planar Linear

Tetrahedron

How to Draw a Tetrahedral StructureT b d th l f th t d d fTwo bonds on the plane of the paper, one towards you and one away from you.

In such structures, the bonds on the plane should be adjacent to each other. Thefollowing two structures are not so rightfollowing 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 paperThe 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.

W d t t Sawhorse ProjectionWedge structures Sawhorse Projection

Newman Projection

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

The front carbon is represented as a dot and bonds are drawn from the dot

The back carbon is drawn as a circle and bonds start from the circleand 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.

C Y

Dihedral AngleTorsional angle is an alternative term usedfor dihedral angle While dihedral angle is

0º

C

X

Y for dihedral angle. While dihedral angle ismeasured from 0-360º, torsional angle ismeasured from 0-180º, as a positive valuein the clockwise direction and a negative 180º

positivenegative

C value in the anti-clockwise direction.

ConformationsDifferent spatial arrangements a molecule can adopt through rotation about sigma bondsDifferent 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.

syn

-30º 30º

anti

clinal

-150º 150ºperiplanar

anti-periplanar anti-clinalanti periplanar anti clinal

syn-periplanar syn-clinal

ConformationsStaggered - A low energy conformation where the bonds on adjacent atoms bisect eachStaggered - 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

aab

b

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

a

b60º

Strain in Organic Molecules

St i ( b d d i t ti ) D t bili ti d t th l i b t thSteric (non-bonded interactions) – Destabilization due to the repulsion between the 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 valueAngular – Destabilization due to distortion of a bond angle from its optimal value

Possible Reasons for Torsional Strainl d dEclipsed  Staggered

Conformations of Ethane

2.9 kcal/mol

0

The staggered conformation is more stable than the eclipsed conformation by 12 kJ/molor 2.9 kcal/mol. There are infinite number of conformations in between both theseconformers

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

T t l d t bili ti 19 kJ/ l

Total destabilization = 16 kJ/mol

Total destabilization = 19 kJ/mol

Gauche Effect

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

l i 1 2 di th thexample is 1,2-dimethoxyethane.

The increased stability of the gauche conformer is explained based on the possibility ofhyperconjugation between C-F and C-H bonds. This is initiated by the very highelectronegativity of fluorineelectronegativity of fluorine.

Effect of Intramolecular H-bondingI t l l H b di b t t i i l f i bl tIntramolecular H-bonding between two vicinal groups confers an appreciable amountof stability (8-20 KJ/mol) to a conformer.For effective Intramolecular H-bonding to occur, the donor and the acceptor groupsmust be close to each other which is possible only in the eclipsed or gauchemust be close to each other, which is possible only in the eclipsed or gaucheconformation.The gauche conformations with a torsion angle of 60-70° between the interactinggroups are ideally suited for intramolecular H-bonding.g p y g

In general, 2-substituted ethanols of the type, X-CH2-CH2-OH, where X = OH, NH2, F,Cl OC C (C ) h h f d h f i i h OCl, Br, OCH3, NHCH3, N(CH3)2 etc. have the preferred gauche conformations with OHand X forming intramolecular H-bonds. Usually the dihedral angle is slightly greaterthan 60°.