Organic Chem + Isomerism

Hybridisation in Carbon

Electrons in valence shell must be excited or promoted from 2s to 2p orbitals so as to get 4 unpaired electron

How to tell if is stronger bond?

Hybrid orbitals with higher % of s character overlap to form a stronger bond

sp3 bonding (diagram)

Shape: Tetrahedral about C

Bonds: C—H head on overlap with 1s orbital of H to form sigma bond

sp2 bonding (diagram)

Structure: Trigonal planar about C

Bonds:

C=C: Head-on overlap between sp2 and sp2 to form sigma bond, sideway overlap between 2p and 2p to form π bond

C-H: Head-on overlap between sp2 of C and 1s of H to form sigma bond

sp bonding (diagram)

C≡C: Head-on overlap between sp and sp to form sigma bond, 2 sideway overlaps between 2p and 2p to form 2π bonds

C-H: Head-on overlap between sp2 of C and 1s of H to form sigma bond

Conditions for delocalisation

Continuous overlap of p orbitals over at least 3 adjacent atoms, allowing for delocalisation of π electrons

How to see how many delocalised electrons

1. Count how many negative charge

2. Count how many double bonds

Benzene

• Bond order of C=C in benzene is 1.5

• 6 C atoms in benzene are sp² hybridised, hence each C has an unhybridised p orbital containing one electron

• 6 unhybridised p orbitals in benzene overlap continuously, allowing for delocalisation of 6 π electrons

Nucleophiles

• Electron rich (π bonds or lone pair of electrons), can donate electrons

• Can be neutral molecules with lone pair of electron (H2O, NH3)

  • Negatively charged ions (OH-, CN-)

Electrophiles

• Electron deficient, can accept an electron pair

• Can be atoms with partial positive charge (Br in Br2, H in HBr)

• Positively charged ions (CH3+, Br+)

• Atom with empty orbital in valence shell (Al in AlCl3)

Homolytic fission

• When 2 bonding atoms are of relatively similar electronegativities, 2 shared electrons in covalent bond can be split equally between 2 atoms

  • Forms radical, contains unpaired electron and is extremely reactive

Heterolytic fission

• 2 atoms of different electronegativities, shared electrons are split unequally

• More electronegative atom acquires both bonding electrons forming anion, less electronegative atom forms cation

Bond formation

1. Two radicals collide to form stable product

2. Electron rich region donates an electron pair to an electron deficient region

Substitution

Atom is substituted for another atom

Addition

2 species react to form a single product without any elimination of atom

Elimination

Atoms are removed to form a product with a double bond

Hydrolysis

Molecule is split by providing water

Condensation

2 molecules react to form a bigger molecule with the elimination of small molecule

Oxidation

O added

Reduction

H added

Structural isomerism

1. Chain isomers: straight vs branched

2. Positional isomers: different location of group on hydrocarbon chain

   1. Different physical properties, similar chemical properties

3. Functional group isomers

   1. different physical and chemical properties

Cis-trans isomerism

2 different groups on each of the C atoms with restricted rotation, conversion of cis isomer to trans isomer requires π bond to be broken

Cis VS Trans properties

Chiral centre

C is attached to 4 different atoms (use Mercedes logo to find from structural formula)

Enantiomers

• Non-superimposable mirror images of each other

• Rotate plane of plane-polarised light by equal angles in opposite directions

• Have similar chemical properties

• Have similar physical properties

• Different biological properties

Racemic mixture

• Solution that contains equal concentrations of enantiomers

• Equal and opposite rotation of plane polarised light results in net zero enantiomer activity

  • Hence mixture is optically inactive

Mesocompounds

• Contains 2 chiral centres, but has a plan of symmetry

• Optically inactive as effect on plane polarised light by each half of the molecule is equal in magnitude and opposite in directions, hence cancel each other out