CHEM111 - molecular geometry

sp3 hybridised shape

tetrahedral

sp2 hybridised shape

trigonal planar

sp hybridised shape

linear

isomers

molecules with the same molecular formula but different structure

three types of constitutional isomers

chain, positional, functional group

stereoisomers

same order of attachment of atoms but a different spatial arrangement

chiral

molecules that are non-superimposable on their mirror image

achiral

molecules that are identical to their mirror image

most common form of chiral molecule

tetrahedral carbon bonded to four different groups

why are enantiomers difficult to isolate

they have identical physical and chemical properties

how are enantiomers labelled

right or left handed

R/L handed notation for chiral centres

R or S

R/L notation for metal ligand complexes

delta or lambda

R/L notation for helical molecules

P or M

how can enantiomers be identified

through rotation of the plane of polarised light

how is rotation of plane of polarised light to the left indicated 

(-)

how is rotation of plane of polarised light to the right indicated 

(+)

rotation of plane of polarised light in racemic mixture

no rotation

racemic mixture

1:1 mixture of two enantiomers

diastereomers

molecules that are non-superimposable non mirror images

notation for distinguishing diastereomers

E and Z

E notation means

two highest priority groups on opposite sides of the double bond

Z notation means

two highest priority groups on the same side of the double bond

how to determine the number of possible stereoisomers, for a compound with n chiral centres

2^n

Fischer projection

a 2D representation of the configuration of chiral molecules

meso isomer

achiral, contains at least two stereocentres and has internal symmetry so it is optically inactive

fractional recrystallisation

mixing cation-enantiomers with a chiral anion to get diastereomeric salts which can be separated by crystallisation

chiral chromatography

natural separation of diastereomers through a column containing starch

kinetic resolution

using an enzyme that prefers the reaction of one diastereomer over another

asymmetric catalysis

reaction so that only one enantiomer is produced

conformation

arrangements of the molecule with the same molecular formula but there’s free rotation around each bond in the molecule

preferred conformers are

lower in energy

why is changing conformation relatively lower energy than changing configuration

changing configuration involves the breaking and reformation of bonds

different conformations of a molecule might have different…

behaviours and properties

staggered

eclipsed

why is staggered the preferred conformer

it minimises steric clash and electron repulsion

how is the staggered form stabilised

interactions between filled and unfilled orbitals

what notation/drawing is used for cycloalkanes like cyclohexane

chair and boat

axial

groups above and below the plane of the ring

equatorial

groups sticking out from equator plane of the ring

what conformation is every C-C bond in cyclohexane

staggered

staggered cyclohexane

eclipsed cyclohexane

why is free rotation not possible in the ring system

restricted rotation about the C-C bonds

what is the most stable conformation for monosubstituted systems

where the substituent is in the equatorial position because axial would be destabilised by other interactions

what is the steric preference for larger substituent groups

have them further away from each other

what happens to the chair forms in solution

they exist in equilibrium with each other

disubstituted cyclohexanes can result in

diastereomers

both cis conformers and both trans conformers have the same

energy

what conformation is preferred for disubstituted cyclohexanes with two different substituted groups

where the larger group is equatorial