Chapter 12

• What are alkanes?


  They're saturated hydrocarbons containing single C-C and C-H bonds as σ-bonds that can freely rotate around it?
• What are σ-bonds?


  They are sigma bonds - they're the strongest type of covalent bond and are formed by the overlapping of orbitals directly between bonding atoms.
• What shape do alkanes take around each carbon atom?


  They adopt a tetrahedral shape and therefore have a bond angle of 109.5°. This is bc each C atom is bonded to 3 H atoms and another C angle so there are 4 bonding pairs of electrons.
• How does the boiling point of alkanes change with carbon-chain length?


  The boiling point increases as there is a greater surface area so more there is more surface contact between the molecules. Since London force act between molecules that are in close surface contact the molecules will be greater so more energy is required to overcome them.
• How does branching affect the boiling point within alkanes


  With more branches there is a decrease in surface area of contact which results in weaker London forces and hence a lower boiling point. The branches get in the way and prevent branched molecules from getting closer to the straight-chain = less intermolecular forces.
• How reactive are alkanes and why?


  Alkanes are not very reactive. Their C-C and C-H bonds are σ-bonds which are strong - require large amounts of energy to brea so a high bond enthalpy. Their C-C bonds are non-polar and the C-H bonds are not polar since their electronegativity values are similar.
• What is complete combustion?


  In surplus oxygen supply, alkanes will burn completely to produce Co2 and water.
• What is incomplete combustion?


  In a limited oxygen supply, there is not enough oxygen for the fuel to completely burn. As a result CO is produced which is poisonous when inhaled or just C particulates (Soot).
• What is free radical substitution?


  When a species with an unpaired electron is used in the replacement of an atom within a hydrocarbon to create a haloalkane.
• Free radical substitution


  What happens in the initiation stage?Cl2 -> 2Cl•

  UV light provides enough vibration for the bond to break; which leads to homolytic fission when a bond breaks and one e- ends up in each atom (Shown by curly arrows). At this point the reaction is at equilibrium.

  
• Free radical substitution


  What happens in the propagation stage?Propagation is the continuation of the reaction.

  1st: The e- shift towards the left within the e- cloud due to the like -ve charge to create a polar molecule.

  The product in the second propagation is the product from the 1st radical.

  Propagation can continue til all the reacts have been used up and is terminated when two radicals collide.

  
• Free radical substitution


  What happens in the termination stage?Two radicals collide to form a molecule with all e- pairs. When they collide both radicals react and are removed from the reaction mixture.

  
• What is the limitation of free radical substitution? 1


  At the propagation stage further substitution can occur. For example in methane we form chloromethane but this can be then be further substituted until all hydrogens are replaced with CL.
• What is the limitation of free radical substitution? 2


  Also, with longer carbon chains a mixture of monosubstituted isomers will be created. Which will create different properties hence why it's not ideal to use it to synthesis one organic compound.
• What are alkanes?


  - Saturated hydrocarbons (CnH2n+2)

  - Main components of natural gas and crude oil

  - Used as fuels
• What is the bonding in alkanes?

  - Each carbon atom joined to four others by single covalent bonds (4 sigma bonds)

  - Sigma bond
• What is a sigma bond?

  - Type of covalent bond

  - Result of the overlap of two orbitals, one from each bonding atom
• What is the shape of alkanes?

  - Tetrahedral

  - 109.5 degrees - bond angle
• How does sigma bonds affect the shape of alkanes?


  - Acts as axes around which the atoms can rotate freely

  - So, shape's not rigid
• How is crude oil separated into different alkanes?

  - Fractional distillation

  - Separates into fractions in distillation tower

  - Each alkane has different boiling points

  - Uses heat to evaporate alkanes into different fractions

  - Bottom (long hydrocarbons), top (short)
• How does chain length affect the boiling point of alkanes?

  - London forces act between molecules on close surface contact

  - Chain length increases --> greater surface area of contact

  - Greater London forces (more energy required)

  - Increases bp
• What is the effect of branching on boiling points of alkanes?

  - Branched isomers have lower boiling points

  - Fewer surface points of contact

  - Branches get in the way --> prevents branched molecules getting closer

  - Decreases London forces --> lowers bp
• Describe the reactivity of alkanes

  Lack of reactivity:

  - C-C an C-H sigma bonds are strong

  - C-C non-polar

  - C-H considered non-polar (en is really similar)
• Describe the combustion of alkanes

  - Alkanes react with plentiful supply of O2 to produce CO2 and water

  - Gives out heat
• Why are alkanes used as fuels?

  - Readily available

  - Easy to transport

  - Burn in plentiful supply of oxygen without releasing toxic products
• What is the equation that can be used for balancing complete combustion of any alkane?

  CxHy + (x + y/4)O2 --> xCO2 + y/2 H20
• What happens when oxygen is limited during combustion of alkanes?

  - Incomplete combustion

  - H atoms in alkanes are always oxidised to water

  - Combustion of carbon incomplete

  - May produce CO or C (soot)
• Give equations to compare the different degrees of combustion

  - C7H16 + 11O2 --> 7CO2 + 8H2O

  - C7H16 + 7 1/2 O2 --> 7CO + 8H2O

  - C7H16 + 4O2 --> 7C + 8H2O
• Describe the reactions of alkanes with halogens?

  - Alkanes react with halogens in presence of sunlight (UV) radiation provides energy for reaction

  - Substitution reaction
• Give an example of an alkane reacting with a halogen

  CH4(g) + Br2(l) --> CH3Br(g) + HBr(g)
• What is a reaction mechanism?

  Series of steps that represent a chemical reaction that show how electrons are thought to move during reaction
• Describe the mechanism for the bromination of methane

  - Radical substitution

  - Three stages : Initiation, propagation, termination
• What is a radical?

  A very reactive species with an unpaired electron
• Describe the initiation (1st) stage of the mechanism of the bromination of methane

  - Covalent bond in bromine broken by homolytic fission

  - Each Br atom takes one electron from pair --> two Br radicals

  - Br-Br --> Br· + ·Br
• Describe the propagation (2nd) stage of the mechanism of the bromination of methane

  - Chain reaction (products of 1st acts as reactants in 2nd)

  - 1st - Br· (radical) reacts with C-H bond in methane forming methyl radical (·CH3) and HBr

  - 2nd - Each methyl radical reacts with another Br molecule forming CH3Br with Br·
• Show the propagation steps in the mechanism of the bromination of methane

  1st - CH4 + Br· --> ·CH3 + HBr

  2nd - ·CH3 + Br2 --> CH3Br + Br·

  
  

  - New bromine radical reacts with another CH4 molecule --> cycle
• Describe the termination (3rd) stage of bromination of methane


  - Two radicals collide forming molecule with all electrons paired

  - Both radicals removed from reactant mixture stopping reaction
• What are the limitations of radical substitution in organic synthesis?


  - Further substitution

  - Substitution at different positions in a carbon chain
• What is further substitution?

  - After CH3Br is formed, another bromine radical can collide with a bromomethane molecule, substituting a further H atom to form dibromomethane, CH2Br2.

  - Can continue until all H atoms have been substituted.

  - The result is a mixture of compounds.
• List what different alkanes are used for

  - C1 - C4 - gas, used in domestic fuel

  - C5 - C9 — petrol

  - C10 - C16 — kerosene

  - C17 - C20 — diesel