M4S1: Basic concepts and hydrocarbons

Aromatic compounds

Contain benzene ring

Aliphatic compunds

Contain carbon & hydrogen joined together in straight chain

Alicyclic compound

Compound contains (non-aromatic) ring

Structural isomers

• Chain isomer

• Positional isomer

Chain isomer

• Different arrangements of carbon skeleton

• Similar chemical properties

• Different physical properties

Positional isomer

• Functional group attached to different carbon atom

• Different physical properties & chemical properties

Shape of alkane molecule

• Tetrahedral - 109.5 degrees

• Each carbon equally repel, each carbon 4 bonds

Sigma bond

Orbitals positioned so that they overlap to form new, shared orbital lying directly between bonded atoms

Herterolytic fission

Bond break unevenly with one bonded atom receiving both electrons

Homolytic fission

Bond break evenly, each atom receive one electron from bonding pair, radical formed

Synthesis of haloalkanes from alkane steps

• Initiation

• Propagation

• Termination

Initiation

• Radical produced, photodissociation

• Cl₂ (UV)» 2Cl(radical)

Propagation

• Radical used up and create chain reaction

• First Cl(radical) attack methane molecule:

  • Cl(radical) + CH₄ » CH₃(radical) + HCl

• New methyl radical attack another Cl₂

  • CH₃(radical) + Cl₂ » CH₃Cl + Cl(radical)

• New Cl(radical) can attack CH₄ molecule, so on until Cl₂ or CH₄ used up

Termination

• Two radicals join together = stable molecules

Pi bond

• Two p orbitals overlap sideways above and below

• Weaker than sigma bonds as electron density spread out above & below nuclei

Alkene bond angle

Trigonal planar - 120 degrees

Sterioisomers

Same structural formula but different arrangement

E - isomer

Higher priority groups on opposite sided

Z- isomer

Higher priority group on same side

CPI rules

Highest priority group on each of C=C carbons is the one with the atom with highest atomic number directly bonded to carbon atom (if priority groups are different)

Cis - isomer

Two identical groups on same side of C=C bond

Trans - isomer

Two identical groups on opposite sides of C=C bond

Electrophilic addition reaction

1. C=C repel electrons in X-Y bond, polarises X-Y bond

2. Two electrons from C=C bond attack delta positive X atom = bond between carbon and X

3. X-Y bond broken, Y takes electrons = negative ion

4. C=C bond break, carbon 1 took electrons = carbon 2 positively charged

5. Y^- acts as nucleophile, attack positively charged carbocation intermediate, donating lone pair electrons & forming new bond with carbon 2

6. H₂C=CH₂ + X-Y » CH₂XCH₂Y

Produce alkane from alkene

• React with hydrogen gas

• Ni catalyst

• 150 degrees Celsius

Alkene reacting with halogens

• Form dihaloalkanes

• Electrophilic addition reaction

Test for unsaturation

• Use pipette add 2cm³ orange bromine water to test tube

• Use another pipette to add about 2cm³ unknown hydrocarbon sample

• Stopper pipette & shake

• Alkene = orange » colourless

Synthesis alcohols

• Alkenes hydrated by steam

• 300 degrees Celsius

• 60 - 70 atm

• Solid phosphoric (V) acid - H₃PO₄ catalyst

Synthesis of haloalkanes from alkene

Alkenes undergo electrophilic addition reaction with hydrogen halides to form haloalkanes

Markownikoff’s rule

Major product from addition of hydrogen halide (HX) to an unsymmetrical alkene is the one where hydrogen adds to carbon with most hydrogen attaches

Why longer chain = more induced dipole - dipole interactions

Larger molecular surface area so more surface contact between molecules and there are more electrons to interact