OChem topic 1 a+b - molecular representation, structure and bonding

- shapes of molecules, isomers, functional groups, naming, structure and bonding, hybridisation

sp³ hybridised carbon

• tetrahedral

• a carbon atom that is bonded to 4 separate atoms is always sp³ with a 109.5˚ bond angle - explained using VSEPR theory

• always drawn with two bonds in the plane and one dashed and one wedged

racemate

• an equal mixture of two enantiomers

phenyl group

• a semi-saturated version of cyclohexane

• often abbreviated to Ph when not key to the reaction

• often drawn as one of the resonance forms but can actually be considered a hybrid of two equivalent contributing resonance structures - Kekule

ⁱPr

isopropyl group

isomers

• compounds that containing the same atoms ie. same molecular formula but are bonded together in different ways

• their properties may still be very different

constitutional isomers

• isomers that have different connectivity eg. propanol and isopropanol

stereoisomers

• where the connectivity is the same but atoms have different orientations in space

what is the stereochemical relationship between…?

either:

1. enantiomers

2. diastereoisomers

what is the oxidation level of a carbon based on?

the number of bonds the central C atom has to heteroatoms (not carbon or hydrogen)

what are the names of the 5 oxidation levels?

4. carbon dioxide

3. carboxylic acid

2. aldehyde

1. alcohol

0. alkane

steps to naming a compound

1. identify the principle functional group - usually the suffix (alkenes and alkynes are only ever suffixes)

2. identify the longest continuous chain that contains the principal functional group

3. assign numbers - distinguishes between isomers

• ortho?

• meta?

• para?

• 1,2- disubstituted = o

• 1,3- disubstituted = m

• 1,4- disubstituted = p

how to determine the priority of substituents in more complex molecules

• priority follows oxidation level for basic functional groups - eg. carboxylic acid>ketone>alcohol

• oxygen is assigned higher priority than other functional groups eg. alcohol>amine

what are the bonds/bonding in methane?

• filed MOs formed from the overlap of hybrid AOs

• the tetrahedral geometry in methane is energetically beneficial as by making 4 new sigma bonds carbon ends up with a filled shell of 8 valence electrons - leads to the most effective orbital overlap and so the greatest energy gain on bonding

• each molecular orbital arises form a combination of H 1s AO with a mixture of the 2s and 2p carbon AOs

• H 1s gets equal overlap with each 2p - there is also equal antibonding/destructive overlap

hybridisation

• a carbon atom doesn’t undergo any physical process of rearranging between its original AOs and hybrid AOs - just a model to visualise the geometry

• hybrid orbitals formed are degenerate

sp³ hybrid AOs

• each has 25% s character and 75% p character

• each has two lobes with one larger due to constructive and destructive overlap

• each has a planar node through the nucleus

• 4 hybrid AOs are equivalent in energy and direction and point to the 4 corners of a tetrahedron

sp² hybrid AOs

• creates three equivalent sp2 orbitals + one p orbital left unchanged

• trigonal planar geometry bc the directionality of the constituent orbital is maintained 2s, 2px, 2py - all in plane so not entering the 3rd dimension

• maximising the distance between the orbitals means the remaining 2pz orbital is perpendicular to the plane

• 33% s character, 67% p character

sp hybrid AOs

• creates two equivalent sp hybridised orbitals

• each hybrid orbital has 50% s and 50% p character making the carbon atom linear - direction is along the axis of the associated p orbital

• the two remaining p orbitals are perpendicular to the plane of the sp orbital sand each other

which is lower in energy; sp² or sp³?

• sp2 hybridised orbitals are lower in energy as they have a greater s character

• this means there will be more efficient overlap (in terms f energy and shape) between an sp2 and an s

what hybridisation with BH₃ and ⁺CH₃ adopt? (they’re isoelectronic)

sp2

isolobal

• when molecular fragments possess the same number of frontier MOs with similar shape and symmetry at approximately the same energy with the same number of electrons available for bonding eg. boranes and carbocations - both sp2 with empty p

• ie. we know carbocations react with Nu- through interactions w empty p orbital so we can correctly predict boranes will react by the same interaction