alkanes

• what are alkanes?


  → are saturated ,, straight chain hydrocarbons with single C-C bonds only

  • the general formula is CnH2n+2

  ◌ 1 carbon ; METHane - CH4

  ◌ 2 carbon ETHane - C2H6

  ◌ 3 carbon ; PROPane - C3H8

  ◌ 4 carbon ; BUTane - C4H10

  ◌ 5 carbon ; PENTane - C5H12

  ◌ 6 carbon ; HEXane - C6H14

  ◌ 7 carbon ; HEPTane - C7H16

  ◌ 8 carbon ; OCTane - C8H18

  ◌ 9 carbon ; NONane - C9H20

  ◌ 10 carbon ; DECane - C10H22
• bonding in alkanes

  → C-C and C-H bonds in alkanes are all single covalent bonds

  you can use 'orbital overlap' model to explain how bonds are formed;

  • covalent bonds can form between 2 atoms if theyre arranged so that the outer atomic orbitals overlap
  • orbitals then combine to form a shared orbital that forms a bond between atoms
  • how well orbitals overlap determines how strong the bond is

  ◌ in alkanes, atomic orbitals on carbon and H2 atoms are positioned so they overlap to form a new, shared orbital lying DIRECTLY BETWEEN BONDED atoms

  • this type of COVALENT bond is called a SIGMA (σ-) BOND

  
• the shape of alkanes


  → each carbon is surronded by FOUR electron pairs in for sigma (σ-) bonds

  • the σ-bonds act as axes around which the atoms can rotate freely

  ◌ remember ; repulsion between these electron pairs results in a 3D tetrahderal arrangement around each carbon atom with a bond angle of 109.5°

  
• boiling point of alkanes

  → within the fractions of crude oil ,, there are many different alkane molecules

  • the forces holding them together are INDUCED DIPOLE-DIPOLE FORCES (London forces) ,, and to boil ,, these forces have to be overcome
• effect of chain length

  → within the alkanes ,, as the chain length increases ,, the boiling point increases - this is because ;

  • there is a GREATER NUMBER OF ELECTRONS
  • the INDUCED DIPOLE-DIPOLE forces between the molecules become STRONGER
  • so ,, MORE energy is required to break these forced - meaning a HIGHER boiling point
  • in longer chained molecules ,, there are also more points of contact between the molecules ;

  ◌ this leads to MORE INDUCED DIPOLE-DIPOLE FORCES

  ◌ it takes MORE energy and therefore a HIGHER temperature to break the INDUCED DIPOLES

  
• effect of branching


  → isomers of the alkanes have the same relative molecular mass and molecular formula

  ◌ a BRANCHED ISOMER has a LOWER boiling point than an UNBRANCHED ISOMER

  • in a BRANCHED alkane ,, there are FEWER point of contact between the molecules ,, leading to FEWER INDUCED DIPOLE-DIPOLE FORCES between the molecules
  • BRANCHED molecules can't get as close to each other as branched molecules

  
• chemical reactions of the alkanes ;

  • reactivity of alkanes


  → alkanes do not readily react with most reagents

  ◌ reasons for the lack of reavtivity include ;

  • C-C and C-H σ- bonds are strong
  • C-C bonds are non-polar
  • C-H bonds are considered to be non-polar (due to similar electronegativities
• chemical reactions of the alkanes ;

  → Combustion of alkanes ;

  • alkanes are useful as fuels as they combust in a plentiful supply of oxygen to release heat ,, also forms carbon dioxide and water

  → Complete combustion ;

  • ALKANE (g) + O2 (g) → CO2 + H2O (l)

  → Incomplete combustion ;

  • ALKANE (g) + O2 (g) → CO (g) + H2O (l)
  • ALKANE (g) + O2 (g) → C (s) + H2O (l)
• chemical reactions of the alkanes ;

  → substitution reactions of alkanes ;

  • alkanes take part in substitution reactions - a hydrogen atom in the alkane is replaced by a halogen atom

  → bond fission ;

  • breaking of a covalent bond
  • a single bond is shared - a shared pair electrons between two atoms
  • it can break in two ways ; HETEROLYTIC or HOMOLYTIC fission
• chemical reactions of the alkanes ;

  • bond fission

  → Heterolytic fission ;

  • bond breaks unevenly with one of bonded atoms receiving both electrons from bonded pair

  general equation for heterolytic fission ;

  • XY → X+ + Y-

  → Homolytic fission ;

  • bond breaks evenly and each bonding atom receives one electron from the bonded pair
  • 2 electronically uncharged RADICALS are formed - these are particles that have an unpaired electron

  general equation for homolytic fission ;

  • XY → X● + Y●

  because of the unpaired electron ,, these radicals are very reactive
• halogenation of alkanes ; 1


  → alkanes react with halogens in the presence of UV radiation or at a temperature of about 300°C

  ◌ the overall equation for this reaction is ;

  • CH4 (g) + Cl2 (g) → CH3Cl + HCl

  methane chloromethane

  → reaction is called FREE RADICAL SUBSTITUTION and takes place in a series of steps (reaction mechanism)
• halogenation of alkanes ; 2

  ◌ RADICAL SUBSTITATION is a type of substitution reaction in which a radical replaces a different atom/a group of atoms

  • covalent bonds are broken by HOMOLYTIC fission to form RADICALS with an unpaired electron
  • a hydrogen atom in the alkane is substituted by a halogen atom

  → for the other halogens ;

  • bromine will react in a similar way
  • iodine isn't reactive enough and barely reacts
  • fluorine is too reactive and would likely cause an explosion
• mechanism for chlorination of alkanes ; 1

  ◌ INITIATION ;

  → first step in a radical substitution in which free radicals are generated by UV radiation

  • in this stage ,, reaction is started when the Cl-Cl bond in chlorine molecules broken by homolytic fission forming two chlorine RADICALS

  UV light provides the energy for the fission ;

  Cl - Cl → Cl● + Cl●
• mechanism for chlorination of alkanes ; 2

  ◌ PROPAGATION ;

  → this is the 2 repeated steps in a radical substituion that built up products in a chain reaction

  • propagation steps could continue until all the reactants have been used up

  the propagation stage has two steps ;

  STEP 1 - CH4 + Cl● → ●CH3 + HCl

  STEP 2 - ●CH3 + Cl2 → CH3Cl + Cl●

  
  

  ◌ TERMINATION ;

  → this stage removes radicals ,, stopping reaction

  • 2 radicals combine to form a mol

  there are many possible termination steps ,, due to large no of radicals in reacting mixture ;

  Cl● + Cl● → Cl2

  CH3● + CH3● → C2H6

  CH3● + Cl● → CH3Cl
• limitations of radical substitution

  → radical substitution gives us a way to make haloalkanes - however it has problems which limits its use in synthesis

  • radical substituion reactions lead to the formation of a mixture of products

  ◌ E.G. - in the termination step ,, chlorine ,, thane and chloromethane are produced
• substitution at different positions in the carbon chain

  → for methane - all four hydrogen atoms are bonded to the same carbon atom ,, so only one monochloro compound is possible CH3Cl

  ◌ similarly this is true for ethane when one substitution takes place ,, CH3CH2Cl

  → if the carbon chain is longer ,, we'll get a mixture of monosubstituted isomers by substitution at different positions in the carbon chain

  • E.G. - pentane could form three monosubstituted isomers

  ◌ with furthur substitution ,, there are even more possibilities - even for ethane two different isomers are shown ;