Analía Bellizzi – Chemistry Classes

Ronald Reagan High School

AS ALKANES

Alkanes


Alkanes are hydrocarbons with single bonds between 2 consecutive C atoms

Physical Properties of Alkanes

  • They have the General Formula: CnH2n+2 with all C–C bonds. Example: C2H6 – Ethane.
  • They can exist as LINEAR, BRANCHED or forming rings: CYCLOALKANES.
  • They are non-polar .
  • They have only weak intermolecular forces. (Van der Vaals only), that makes alkanes water insoluble.
  • They have low melting and boiling points; The values increase with the size (number of carbons in the molecule).
  • Linear alkanes have higher melting and boiling points than branched alkanes since the surface of contact increase and the intermolecular forces are more frequent.
    • Examples: hexane: boiling point = 69oC
    • 2,2 dimethylbutane: boiling point = 49oC.

Examples:

hexane: boiling point = 69oC

2,2 dimethylbutane: boiling point = 49oC.

  • At  room temperature:
    • the first 4 alkanes (1 to 4 C in the molecule)  are gases
    • The ones with 5 to 17 C in the molecule  are liquids
    • Alkanes with more than 17 C are solids
  • Also, linear Alkanes with even number of carbons have higher melting and boiling points that the ones that have odd number of carbon atoms in their molecule.

Structure of the –C-C– bond

  • The C- C bond each carbon atom is sp3 hybridized with 109o bond angles and tetrahedral structure.
  • These orbitals then form a sigma (σ) bond with each hydrogen or carbon. These bonds can rotate.
 
All three pictures above correspond to the molecule of Hexane (C6H14)
  • Most alkanes can exist as a unique chain (linear), branched, or forming rings (cycloalkanes)  Note that cycloalkanes do not have the same general formula of alkanes.
cycloalkanes

Chemical properties

The C-C and C-H bonds in alkanes are very strong since they are non-polar covalent compounds, therefore, alkanes are relatively inert in regards to most chemical reagents.They can react under certain conditions in the following reactions:

    1. Combustion (Oxidation – Burning)
    2. Catalytic or thermal cracking, also called PYROLYSIS (Breaking the molecule in smaller parts using catalysts or heat).
    3. Substitution reactions (One or more of the hydrogen atoms in the molecule is replaced

A) Combustion

1) Complete combustion

Alkanes burn in with enough amount of oxygen to produce CO2 and H2O.
Gases burn much more easily than liquid and solid alkanes. A gaseous mixture of a small alkane and oxygen are extremely explosive.

 2C2H6(g) + 7O2(g) →   4CO2(g) + 6H2O(g)

These reactions are used commercially when fuels such as natural gas, petrol and oil are burnt in air.

2) Incomplete combustion

Incomplete combustion (where there isn’t enough oxygen present) can lead to the formation of carbon and/or carbon monoxide. Carbon monoxide is produced as a colorless poisonous gas.

2CH4(g) + 2O2 (g) → 2CO (g) +4H2(g)

Carbon monoxide form an irreversible compound with hemoglobin – making it unable to carry oxygen. If you breath in enough carbon monoxide you will die not because you cannot breath but because the oxygen cannot be transported inside your blood.

B) Catalytic or thermal cracking

Big molecules of Alkanes are heavy, they do not flow or burn easy. Our industry needs smaller and more reactive materials. We can use themal cracking to break these large molecules into smaller ones. The sample is vaporized and passed over a hot catalyst. Bonds inside the molecule are broken and double bonds are formed since there is a lack of hydrogens saturate the new compounds. It is a random process so the double bonds can produce a mixture of compounds. Since one molecule is broken in two, cracking is an example of a thermal decomposition reaction.

cracking alkane

The reaction above shows the cracking reaction of an alkane (Octane) to obtain hexane and ethene (notice the double bond in ethene)

The animation below shows the reaction in an interactive way. 

source: http://www.bbc.co.uk/schools/gcsebitesize/science/aqa_pre_2011/oils/polymersrev1.shtml
 
The following video shows the cracking of alkanes and how to recognize the alkenes obtained
 
source: http://www.youtube.com/watch?v=ZYyKUePdC2Y

 These reactions are used commercially when fuels such as natural gas, petrol and oil are burnt in air.

C) Substitution reactions (Chlorine or Bromine)These reactions of alkanes with all halogens are very similar, the rate of reaction is the only difference. We will concentrate in one example but remember that once you know one of them, you know them all.

A mixture of chlorine and methane:

                      a) Does not react if kept in the dark at room temp.
                      b) Does not react if kept in the dark at 300oC
                      c) Reacts at room temperature if exposed to sunlight or U.V. light.
                      d) Explodes if exposed to bright sunlight or sparked.

So, energy is required to initiate the reaction.Four products are formed from this reactionchlorination

 

 

 

 

These products are explained in terms of a step by step reaction:

 

      • CH4(g)        + Cl2(g)      CH3Cl(g) + HCl(g)
      • CH3Cl(g)     + Cl2(g)    CH2Cl2(g) + HCl(g)
      • CH2Cl2(g)   + Cl2(g)      CHCl3(g) + HCl(g)
      • CHCl3(g)     + Cl2(g)        CCl4(g) + HCl(g)

Each of these reaction is an example of a substitution reaction (a H atom is substituted by a Cl atom)


Mechanism: FREE RADICAL SUBSTITUTION

power point presentation about MECHANISMS

Chlorination of Methane (You will be asked to draw and explain the path of electrons in each reaction)
The reaction occurs ONLY in the presence of a free radical.
free radical is an atom with an unpaired electron that once belonged to a bond.
Halogens form free radicals in presence of UV light. This type of fission is called HOMOLYTIC. So, Chlorine molecules will split HOMOLITICALLY to form CHLORINE RADICALS. 
This formation of the free radicals will trigger the chain reaction for the chlorination of methane.
There are three steps in the reaction:I
  1. Initiation: the free radical form
  2. Propagation: the free radical reacts with more molecules forming more free radicals
  3. Termination: two free radicals get together forming new molecules
 
a) Initiation:
 
The free radical is formed (the reaction shows Chlorine but the mechanism works the same with Bromine
NOTICE THE HALF ARROWS THAT SHOW THAT ONLY ONE ELECTRON IS TRANSFERRED. 
b) Propagation: 
Radicals are very reactive. They will react with another Chlorine molecules (we are not interested in these)  or methane molecules, forming more free radicals (these are the important).The methyl radical produced by this reaction initiates further propagation steps (see below)

c) Termination:
This process occur when two radicals collide together and react to form a molecule with no radical production. We know that we have two radicals in the reaction: chlorine radical (Cl•) and methyl radical (CH3•)
Bromination of methane occurs by the same mechanism. Just replace Bromine by Chlorine in the formulas. Since they have similar properties, they react in the same way.