Week 4 (Molecules-molecular orbitals. Symmetry and isomerism) Flashcards
Problems with the Lewis model
(paramagnetism)
The Lewis model for covalent bonding was a brilliant but
flawed model.
e.g. it could not account for the magnetism of O2
Paramagnetism, weak attraction of a substance to an external magnetic field, caused by presence of a unpaired electrons in the substance.
In the Lewis model all electrons are paired, yet O2 is magnetic, a property that implies the presence of unpaired
electrons.
A more sophisticated model is therefore needed to explain these properties.
.
Molecular orbital theory
We combine atomic orbitals (AOs) of elements in a molecule to create
molecular orbitals (MOs)
Then fill, Molecular Orbital diagram with electrons like we would fill an Atomic Orbital
diagram using Hund’s first rule and the Pauli exclusion principle
Nuclei of given molecule placed at equilibrium distance and electrons added.
Linear Combinations of Atomic Orbitals (LCAO)
(wavefunctions)
Wave-particle duality-
particles, e.g electrons, can be treated as waves. For electrons these called wavefunctions.
Waves can be added together, provide constructive
interference (bigger wave), destructive interference (smaller wave).
By combining the Atomic Orbital
wavefunctions of two or more interacting
atoms we can make Molecular Orbitals
either constructively or destructively
Linear Combinations of Atomic Orbitals (LCAO)
The simplest approximation to MOs is to consider them to result from…
simple linear combinations of AOs.
Form MO by adding (constructive interference) or subtracting the appropriate atomic orbitals.
Isomerism Definition:
What are the types
existence of molecules have same
no. of same kinds of atoms (and hence the same formula) but differ in chemical and physical properties
- structural isomers (constitutional isomers)
- stereoisomers
Structural isomers defintion
(3 types)
Structural isomers have same molecular formula, but different bonding arrangement of atoms.
- Chain isomerism
- Functional group isomerism
- Position isomerism
Structural isomers
- Chain isomerism:
occurs when chains are branched.
The longer the
chain, more branching is possible and therefore more possible
structural isomers.
Structural isomers
functional group isomerism:
same molecular mass, but contain different functional groups due to
isomerism.
Structural isomers
Position isomerism:
In this case, the alkyl (carbon) chain stays the
same, with the functional group(s) moving round the skeleton.
- NOT same as functional group isomerism, as functional groups remain constant
Stereoisomers definition:
what the 2 types
molecules with the same molecular formula and sequence of bonded atoms.
Differ in the three-dimensional orientations of their
atoms in space
Chiral and Achiral molecules
Achiral molecules,
superimposable on their
mirror image
Chiral molecules, non
super imposable on their
mirror image (like hands)
Chirality
molecules that have a “handedness” i.e. nonsuperimposable by any rotation or movement (termed “translation” in
chemistry).
The “handedness” comes from the fact that you cannot rotate or move a left hand onto a right hand.
This means the hands are “chiral” (from the Greek word for hand)
A Racemic mixture
a 50/50 mix of two enantiomers
Each chiral centre will be either R (r for right) or S (Latin for left is sinister). To determine
whether an enantiomer is R or S there is a system called…
the Cahn-Ingold-Prelog rules.
Alternatively the enantiomers can be labelled (+) or (-). This is determined by how they…
polarise light.
Why are enantiomers are often referred to as “optical isomers”
Light from lamp can be polarised using a filter.
If the polarised light passes through an optically active molecule it can be rotated.
Enantiomers
will rotate the light in different directions.
If there is a racemic mixture of enantiomers (50/50) there will be no angle of rotation observed.
Chiral centres (2):
Chiral centres are tetrahedral atoms (usually carbons) that have four different substituents
Chiral centres can
be denoted with a
star
Diastereoisomers
Diastereoisomers are stereoisomers that are not mirror images of each
other.
Cis isomers - “cis” means the nonhydrogen groups on same side
of hexagon of carbons
Trans isomers - “trans” means, “ “ on other sides
of hexagon of carbons
Diastereoisomers
This can also happen across…
alkenes as they do not rotate.
(not just typically sterosiosmers)
Can diastereoisomers be both chiral and achiral?
Yes
Achiral molecules are
superimposable on their
mirror image
Chiral molecules are non
super imposable on their
mirror image (like hands)
Organic compounds
Defintion:
a large class of chemical compounds in which
one or more atoms carbon, covalently linked to atoms of other elements,
most commonly hydrogen, oxygen, or nitrogen.
Carbons can “catenate” or form covalent bonds with other carbons to from chains and rings of
carbons.
Why is it being able to draw stable carbon compounds efficiently and accurately is therefore very important?
Counting only molecules that are under 30 Carbon atoms, there are many
combinations of atoms that are possible to create stable compounds.
It has been calculated that there are (10^63) stable compounds are possible.
There aren’t enough carbon atoms in the universe to make them all.
What varies across stable organic compounds
very different properties.
What are all the different ways that carbon can bond for amino acids (8)
2 carbon chain
3 carbon chain
9 carbons
chain
rings
straight chain
branched chain
ring and chain
All have similar properties:
all soluble in water, all both acidic and basic
(amphoteric), all be joined with other amino
acids to form proteins
This is because the chemistry of organic
molecules depends much less on the number or
the arrangement of carbon or hydrogen atoms
than on the other types of atoms (O,N, S, P, Si…)
in the molecule.
We call parts of molecules containing small
collections of these other atoms (O,N, S, P, Si…)
functional groups, simply because they are groups of atoms that determine the way the molecule works.
All amino acids contain two functional groups: an
amino (NH2or NH) group and a carboxylic acid (CO2H)
group (some contain other functional groups
as well)
