Stereochemistry - Conformational Flashcards
ySTEREOCHEMISTRY
- branch of chemistry that take into account the ____ ____ ____ of _____ in ____
- concerned with the _____ structure of compounds, specifically the _____ geometry of CARBON with _____ attached groups
- 2 types
- relative spatial arrangement
- atoms
- molecules
- 3D
- tetrahedral
- 4
- Conformational
- Configurational
CONFORMATIONAL vs CONFIGURATIONAL
differ in spatial arrangement due to:
CONFORMATIONAL
- rotation around single bonds
CONFIGURATIONAL
- requires bond breaking and and reformation
What are the POSSIBLE individual isolations in CONFORMATIONAL STEREOCHEMISTRY?
+ reason
NONE
- the molecules INTERCONVERT too RAPIDLY that individual isolations are impossible
3 KINDS of strains present in STEREOCHEMISTRY
+ where found
+ occurs due to what reason?
- Angle strain
- found ONLY in rings
- due to expansion/compression of bond angles - Torsional strain
- found in any alkanes, alkenes, alkynes (eclipsing formations)
- due to repulsion between electron clouds in eclipsing bonds - Steric strain
- found in any alkanes, alkenes, alkynes (eclipsing and gauche formations)
- due to repulsion between atoms/groups of atoms that are close to each other
Other term for STERIC STRAIN
Van der Waal’s strain
2 representations of CONFORMATIONAL STEREOCHEMISTRY
+ their 2 conformations
+ WHICH of the 2 conformations have lower energy and more unstable
- Sawhorse Projection
- Newman Projection
a. Staggered (low PE; more stable)
b. Eclipsed (high PE: less stable)
PALATANDAAN in drawing the 2 conformations in CONFORMATIONAL STEREOCHEMISTRY
STAGGERED
- 60 degrees yung bonds LAGI in both newman and sawhorse
ECLIPSED
- 0 degrees yung bonds LAGI in both newman and sawhorse
+ consistent yung itsura nya regardless of what alkane it is
CONFORMATIONAL STEREOCHEMISTRY
To rotate on a single bond, _____ _____ ____ are overcome
small energy barriers
The most stable conformation in alkanes
zig-zag shape
CONFORMATIONAL STEREOCHEMISTRY
- draw the energy diagram per angle conformations for the ff:
- ethane
- propane
- draw the 6 butane (newman) conformations
+ which is the most stable
+ difference in energy from MOST STABLE CONFORMATION
*and draw its example staggered conformation
- ethane (additional 12 kJ/mol from staggered to eclipsed)
- propane (additional 14 kJ/mol from staggered to eclipsed)
- most stable/anti-confirmation: dihedral angle (angle between the 2 methyl grps) is 180 degrees*
- least stable: eclipsing bonds (higher than ^* by 19 kJ/mol in energy)
- gauche conformation: steric staggered bonds ((higher than ^* by 3.8 kJ/mol in energy)
Determine the strain energy in kJ/mol + strain components for the ff interactions:
- H - H eclipsing interaction
- H - CH3 eclipsing interaction
- CH3 - CH3 eclipsing interaction
- CH3 - CH3 gauche interaction
- 4.0 kJ/mol (torsional)
- 6.0 kJ/mol (torsional & steric)
- 11.0 kJ/mol (torsional & steric)
- 3.8 kJ/mol (steric)
Why does H-H eclipsing interaction only contain torsional strain?
no steric strain bc hydrogen atoms are very small
steric strain only applies to bulky groups getting closed
BAEYER STRAIN THEORY
states that
- what ring sizes are too strained to exist?
- sp3 carbons have bond angles of _____
- ___ ____ or ____ ____ bond angles are ____ assuming ____ ring polygon angles
however… what was proven?
- other than 5 or 6 membered rings (so butane and propane)
- 109.5 degrees
- too large or too small
- impossible
- flat
however, since cycloalkanes in real life are not “planar” (flat all throughout) at all (contains bends), they still exist yet are unstable
5 membered-rings and above relieve angle strain through?
puckered conformations
3 IMPORTANT points of the BAEYER STRAIN THEORY in terms of:
- ideal shape to minimize strains
- cause for greater strains in a ring
- very cause of angle strain + other term for this strain
- tetrahedral
- angle value becoming farther from 109.5 = greater strain
- cyclic compounds forces CARBON ATOMS into FIXED ANGLES that may deviate from 109.5, causing BAEYER (angle) STRAIN
CYCLOPROPANE
- ____ bonds
- 2 types of strains + level of intensity + due to what?
- structure
+ draw newman
- banana bonds
- HIGH angle and HIGH torsional strain due to C - H eclipsing bonds
- very planar
CYCLOBUTANE
- 2 types of strains + level of intensity
- structure
+ draw newman
- BALANCE between angle and torsional strain
- slight bending
CYCLOPENTANE
- 2 conformation types
- 1;2 types of strains + level of intensity + due to what? + how
+ structure of second conformation type
+ draw newman
- Planar (envelope)
- high torsional strain
- no angle strain - Non-Planar / Puckered (butterfly)
- BALANCE between angle and torsional strain due to puckering
- slight bending which reduces TS but still not deviating from 109.5 thus limited changes to angle strain
CYCLOHEXANE
- 2 conformation types (both are ______)
- which of the 2 is more stable?
- difference in energy value between the 2
- 2 types of bonds + how many in a cyclohexane + projection + sequence
+ draw newman for each conformation
- non-planar
1. Chair - more stable
2. Boat - 23 kJ/mol higher in energy than chair
a. 6 axial bonds (alternate upwards/downwards per carbon atom)
b. 6 equatorial (alternate upwards/downwards diagonally per carbon atom)
*basically each carbon atom has 1 axial and 1 carbon
Are axial and equatorial bonds only present in CYCLOHEXANES?
no
If the axial bond of a carbon atom is projected upwards, its partner equatorial bond will be projected _____ side-ways
downards
WHAT do you call the interconversion from 1 chair conformation to a ____ conformation
RING FLIPPING
- 1 chair conformation to 1 chair conformation
*boat conformation is IN THE PROCESS
In RING FLIPPING, what are the 2 types of bonds that are also interconverted?
AXIAL -> EQUATORIAL
EQUATORIAL -> AXIAL
- take note that despite the change in type of bonds, their projections is the same (upward -> upward diagonally)
- ALTHOUGH remember that the specific carbon atom that you are observing will change upon perspective
ENERGY BARRIER in ring flipping is?
45 kJ/mol
