week 4 - Molecular orbitals, symmetry, isomerism

what is the limitation of the Lewis model?

• does not accurately show the presence of unpaired electrons

what is molecular orbital theory?

• combining atomic orbitals of elements in a molecule to create molecular orbitals

• the nuclei of a given molecule are placed at an equilibrium distance and electrons are added

What are the two different ways of making molecular orbitals?

Constructive interference and destructive interference

which type of molecular orbital promotes bond formation (bonding orbital)?

constructive interference

which type of molecular orbital avoids bond formation (antibonding orbital)?

destructive interference

what is the result of adding two waves providing constructive interference?

a bigger wave

what is the result of adding two waves providing destructive interference?

• a smaller wave

• includes node

what is the equation for bond order?

½ (no. of electrons in bonding orbitals - no. of electrons in antibonding orbitals)

are bonding or antibonding orbitals lower in energy than the original atomic orbitals?

bonding orbitals

what is a diatomic molecule?

any chemical compound that is made up of only 2 types of atoms

which axis is taken as the internuclear axis?

z

which 2 types of p-orbitals overlap sideways, which types of bonds do they form and what do their shapes look like?

• px and py orbitals

• pi and pi* bonds

• buns of a burger (constructive) and 4 clouds (destructive)

which type of p orbital overlaps head-on (axial), which types of bonds do they form and what does its shape look like?

• px orbital

• sigma and sigma* bonds

• cylindrical shape with large oval in the middle (constructive) and 2 bowling pins (destructive)

what does the mixing of atomic orbitals result in?

• new hybridised orbitals of equal energy (degenerate) that participate in bonding

what factors can influence the types of combinations that will result in bonding?

• symmetry

• relative energies of orbitals

• not all combinations of atomic orbitals will result in bonding (symmetry disallowed)

what is a heterodiatomic/heteronuclear diatomic molecule?

a molecule that consists of 2 non-identical atoms

what is a homodiatomic molecule?

a molecule that consists of 2 identical atoms

what happens when the energy separation of atomic orbitals increases and what does this result in?

• efficiency of overlap in the molecular orbital decreases

• a polar molecule

what is meant by a polarised bond?

the electrons have a greater probability of being located nearer one of the atoms

describe the molecular orbital energy level diagram for the homodiatomic molecules formed via the period 2 elements Li2 to N2

• 2s and 2p orbitals mix

• this is because the difference in energy of these two orbitals is small

describe the molecular orbital energy level diagram for the homodiatomic molecules formed via the period 2 elements O2 and F2

• 2s and 2p orbitals do not mix

• this is because the difference in energy of these two orbitals is large so they do not overlap

what is a requirement of an atom for it to participate in bonding?

• must have an unpaired electron

• if not, then electron promotion occurs where an electron moves up to a high energy orbital

which are some of the molecules that are exceptions to the octet rule?

• BeH2 / BeCl2 (4 valence electrons)

• BH3 (6 valence electrons)

what are organic compounds?

a large class of chemical compounds in which one or more atoms of carbon are covalently linked to atoms of other elements, most commonly hydrogen, oxygen or nitrogen

what do carbons do when forming organic compounds?

they catenate/form covalent bonds with other carbons to form chains and rings of carbon

what are the shapes in which carbons can bond?

• straight chain

• branched chain

• rings

• ring and chain

what are the properties of amino acids?

• all soluble in water

• both acidic and basic (amphoteric)

• joins together to form proteins

what largely determines the properties of organic compounds?

• the TYPE of atoms (e.g. O, N, S, P, Si) (known as functional groups) that are connected to the organic compounds other than carbon and hydrogen

• depends less on the number/arrangement of carob/hydrogen atoms

what are the functional groups in amino acid?

• amino (NH2 or NH)

• carboxylic acid (CO2H)

• some contain other functional groups as well

describe the role of hydrocarbon framework and functional groups

• is made up of chains and rings of carbon atoms and it acts as a support for the functional groups

• determines the way the molecule works both chemically and biologically. can be represented as R

in 3D notation what do these bonds represent:

• normal straight line

• dashed line

• wedged line

• in the same plane as the page/screen

• going into the page/screen

• is coming out of the page/screen

what does the hybridisation in organic compounds represent?

• the number of bonds between the carbons (whether it is single/double/triple etc)

• e.g. sp3 hybridisation represents 4 electron domains and single bond between carbon atoms like alkane

• sp2 - alkene

• sp - alkyne

what is isomerism?

the existence of molecules that have the same molecular formula but differ in chemical and physical properties

the 2 main categories are:

• constitutional (structural)

• stereo (spatial)

even though they have the same molecular formula, properties such as boiling/melting point and reactivity can significantly differ due to their different structures

what is structural isomerism?

same molecular formula but a different bonding arrangement (connectivity) of the atoms

what are the types of structural isomers?

• chain

• functional group

• position

what is chain isomerism?

• occurs when chains are branched

• longer chain = more branching possible = more possible structural isomers

what is functional group isomerism?

• same molecular mass but contain different functional groups

what is position isomerism?

• the carbon chain remains the same but the position of the functional group changes

• NOTE: the functional group itself remains the same, it only changes its location in the molecule

what is stereoisomerism?

same molecular formula, same bonding arrangement (connectivity) of atoms but different 3D orientations of the atoms in space

what are the types of stereoisomers?

• Diastereomers

  • geometrical (cis/trans) isomers

  • conformers —> rotamers

  • enantiomers

what is the difference between enantiomers and diastereomers?

enantiomers:

• non-superimposable

• mirror images

• all chiral carbons inverted

• all show optical activity

diastereomers:

• non-superimposable

• not mirror images

• some chiral carbons inverted

• most show optical activity, not all