Organic Chemistry By Dr. Mehnaz Kamal Assistant Professor, Pharmaceutical Chemistry Prince Sattam Bin Abdulaziz University

1- What is Conformational Isomers. 2-How they could be Classified? 3-. What is Eclipsed & Staggered conformation 4-What are Newman " and sawhorse projection. 5-Conformation of cycloalkane.

Conformations Different internal arrangements of the atoms in a molecule that differ by rotation(s) about one or more of the single bonds. Conformations are NOT ISOMERS they are different arrangements of the SAME MOLECULE. The connection pattern of the atoms does not change. Most Atoms Attached By A Single Bond Are Free To Rotate Around The Bond So The Two Ends Of The Bond Can Rotate Relative To Each Other The Carbon And Three Hydrogen Atoms On The Right End Of The Bond Can Spin Relative To The Carbon And Other Three Hydrogens On The Left End Of The Bond fix rotate

In the Newman projection the molecule is viewed along the axis of a rotatable bond. Newman projections Conformations  The conformation of a molecule containing two tetrahedral atoms linked together can be represented as a "Newman " or as sawhorse projections. John von Neumann

Conformations Sawhorse Projections are very similar to Newman Projections, but are used more often because the carbon-carbon bond that is compressed in a Newman Projection is fully drawn out in a Sawhorse Projection. *Sawhorse Projections

Carbon arrangement

Conformations of Lower Alkanes

 Interconversion of conformers proceed by an infinite number of intermediate conformers, however, only a few of them have energy minima. Staggered conformation: a conformation about a carbon-carbon single bond where the atoms on one carbon are as far apart from atoms on the adjacent carbon Eclipsed conformation: a conformation about a carbon-carbon single bond where atoms on one carbon are as close as possible to the atoms on the adjacent carbo n

H HHH CCCC :::.. Longer Distance Repulsions Are Less For Staggered (D > D) d D Electron Pair Repulsions Determine The Energy Difference Shorter Distance Larger Repulsions For Eclipsed ECLIPSEDSTAGGERED a Torsional Strain The force that opposes the rotation of one part of a molecule about a bond while the other part of the molecule is held fixed The torsional strain between eclipsed and staggered ethane is approximately 2.9 kcal/mol

Each eclipsed conformation now consists of two eclipsed C-H bonds and one C-H eclipsed with a CH 3 bond. We can infer that the latter eclipsing interaction has an energetic "cost" of 3.3 kcal/mol. Conformations of Higher Alkanes Bear in mind that these energies do not measure electronic repulsion in any absolute sense, but merely provide figures relative to the more stable staggered state.

In the most stable conformation, the two methyl groups lie as far apart from each other as possible with a dihedral angle of 180 degrees. This particular staggered conformation is called anti. The other staggered conformation has a Me-Me dihedral angle of 60 degrees and is called gauche The gauche form is less stable than the anti form by 0.9 kcal/mol due to steric hindrance between the two methyl groups.

Conformations Of Butane (Newman) CH 3 HH H H (0 o ) SYN (0 o ) CH 3 H H H H (60 o ) GAUCHE (60 o ) CH 3 H H H H ECLIPSED () ECLIPSED (120 o ) CH 3 H H HH (180 o ) ANTI (180 o ) CH 3 H HH H (240 o ) ECLIPSED (240 o ) CH 3 H HH H (300 o ) GAUCHE (300 o ) (C2-C3 bond)

SYNANTI GAUCHE Conformations Of Butane Sawhorse Projections GAUCHE ECLIPSED

ExampleExample

2- Geometric isomerism in cyclic compounds The most illustrative example is cyclohexane Rotation of group ( atoms ) around sigma bonds, result in different conformations ( conformer ) I-Geometric isomerism…cont 2-Stereoisomerism…cont.

If cyclohexane were planar all of the hydrogens would be eclipsed, resulting in torsional strain. Planar Cyclohexane Cyclohexane is not planar... (doesn’t exist) There would also be angle strain - a hexagon has 120 o internal angles.

Chair Boat Ball-and-stick model In chemistry, the ball-and-stick model is a molecular model of a chemical substance which is to display both the three-dimensional position of the atoms and the bonds between them.chemistrymolecular modelchemical substancethree-dimensionalatomsbonds

Stereochemistry 2- Geometric isomerism in cyclic compounds Axial hydrogen ( Ha ) : H parallel to axis (in vertical plane) Equatorial hydrogen ( He ) : H in plane (in horizontal plane) upon flipping and reflipping between conformers, axial becomes equatorial and equatorial becomes axial

Mono-substituted cyclic compounds, do not exhibit cis or trans geometrical ……… cis or trans

2- Geometric isomerism in cyclic compounds 1,2-disubstituted : same as 1,4-disubstituted Cis both down = e,a both up = a,e Trans up and down = a,a down and up = e,e In summary : a,e e,a a,a e,e a,a e,e 1,4-disubistuted cyclohexane as 1,2

Stereochemistry 2- Geometric isomerism in cyclic compounds 1,3-disubstituted : both down = e,e both up = a,a up and down = a,e down and up = e,a

Label each of the following disubstituted rings as cis or trans and as a,a ; e,e ; or a,e Trans e,e Cis a,e

ENERGYENERGYENERGYENERGY boat chair The Chair Conformation Has Lower Energy Than The Boat 29 kJ/mol 0 kJ/mol

In a chair conformation, six H are equatorial and six are axial For cyclohexane, there are two equivalent chair conformations –all C-H bonds equatorial in one chair are axial in the other, and vice versa

Cis-trans isomerism trans-1,4-dimethylcyclohexane –the diequatorial-methyl chair conformation is more stable by approximately 2 x (-1.74) = kcal/mol cis-1,4-dimethylcyclohexane