1. Organic REACTIONS: ALkanes
Chapter 10.2:
Structure, bonding and chemical reactions involving functional group inter-conversions are key strands in organic chemistry
Chapter 20.1:
Key organic reaction types include nucleophilic substitution, electrophilic addition, electrophilic substitution, and redox reactions. Reaction mechanisms vary and help in understanding the different types of reactions taking place.
Organic REACTIONS: ALkanes

2. Alkanes
Saturated hydrocarbons where carbons in the chain are singly bonded to one another
Ex: Methane Ethane Propane Butane Pentane
Reactivity: relatively low
Carbon-hydrogen bond relatively strong (relatively high bond energy)
Only slightly polar (electronegativity difference of 0.4)
Two RXN types are important:
1:Combustion: rapid, exothermic oxidation of combustible materials
2: Substitution: two main types
FRCR: free radical chain reaction
Nucleophilic (SN1, SN2)

3. Alkanes: Combustion
Combustion: rapid, exothermic oxidation of combustible materials.
Most common alkane RXN
Requires:
oxidizer (oxygen)
fuel source (alkane)
source of ignition (required to reach activation energy)

4. Alkanes: Combustion
Complete combustion of hydrocarbons produces CO2 and H2O
All carbon converts to CO2 and all Hydrogen converts to H2O.
When balancing:
 # of C in the alkane = # CO2 molecules produced
 # of H in the alkane = 2 X H2O molecules produced
In most situations, combustion of hydrocarbons is incomplete because of insufficient oxygen.
Products of incomplete combustion are responsible for a large amount of urban pollution:
 carbon monoxide (CO)
 carbon (soot)

5. Alkanes: Combustion
Burning other hydrocarbons (unsaturated) is very similar
Alkanes
Alkene
Alkynes
Arenes
The more unsaturation (higher C:H ratio)  the higher the smokiness due to unburned carbon
CO2 and H2O are greenhouse gases = absorb radiation and increase heat average world temp
CO toxin as binds irreversibly to hemoglobin in blood
C (soot) causes respiratory distress and contributes to smog and global dimming

6. Alkanes-Substitution RXNs: FRCR
Free radical chain reaction:
alkane RXT with halogen = halogenoalkanes
One Hydrogen (H) in the alkane is replaced by a halogen (X)
  reaction of ethane with chlorine:
CH3-CH3(g) Cl2(g)  CH3-CH2-Cl(g) H-Cl(g)
Ethane chlorine chloroethane hydrogen chloride

7. Alkanes-Substitution RXNs: FRCR
Reaction usually brought about by exposure to UV light or high temps (provides energy of activation)
Chloroethane can RXT with more Cl2  1,2-dichloroethane and 1,1-dichloroethane
High amounts of Cl2  eventually convert to hexachloroethane (substitute all H with Cl)

8. Alkanes-Substitution RXNs: FRCR
Free radical: any molecule or atom with a single unpaired electron
   highly reactive
 Reaction proceeds in 3 distinct phases.
RXN of CH4 with Cl2 example:
Initiation: free radicals are produced
Propagation: products are formed and radicals are reformed
Termination: radicals are used up

9. Alkanes-Substitution RXNs: FRCR
1. Initiation phase:
Source of E (often UV light) can break covalent bond between the 2 Cl atoms
Releasing unpaired Cl atoms (free radicals)
Photochemical homolytic fission: each atom results in one e- (“equal splitting”).
Therefore, heterolytic fission is unequal splitting  both electrons result with one atom
Large reduction in stability for Cl when this happens

10. Alkanes-Substitution RXNs: FRCR
2. Propagation:
Unstable Cl• readily forms new covalent bond with whatever is present
Here, H atom from CH4
Cl radical pulls H atom (including its e- which is currently shared with carbon atom) off of CH4
This forms HCl and free radical, •CH3
CH3• will then pull a Cl atom off a Cl2 molecule, reforming a chlorine radical. Continues in a chain reaction.

11. Alkanes-Substitution RXNs: FRCR
3. Termination occurs when all of the radicals are consumed.
Cl• radicals can combine with each other to form a molecule of Cl2
OR they can combine with a CH3• to form CH3Cl
OR 2 methyl radicals can combine to form ethane
Since ethane found to be produced during the halogenations of methane
Mechanism for this reaction is indeed the one illustrated in the diagram.. So we know it’s good!

12. Alkanes-Substitution RXNs: FRCR
If bromine were used instead of chlorine
Dark brown color provides simple visual method to monitor the progress of the reaction
As the brown colored bromine is consumed, the color would gradually fade
Note: reaction is not
observed in the dark
There is no source of
energy to create the
necessary radicals

13. 20.1: Alkanes-Substitution RXNs: Nucleophilic
Nucleophilic substitution of halogenoalkanes:
Nucleophile is e- rich and attack areas of e- deficiency
Nucleophile can be anything with a lone pair of electrons, but common examples are:
Hydroxide ion:OH-
Ammonia: NH3
Cyanide ion: CN-
Electrophile is e- deficient and accepts e- pairs from a nucleophile
Electrophile can be anything e- deficient
Common examples are:
Hydride ion: H+
Bromide ion: Br+
Nitrate ion: NO2+

14. 20.1: Alkanes-Substitution RXNs: Nucleophilic
Nucleophilic substitution of halogenoalkanes:
Polar C-X bond means C atom is e- deficient=electrophile
It can be attacked by a nucleophile such as OH-
General reaction:

15. Alkanes-Substitution RXNs: Nucleophilic
Nucleophilic substitution can occur by two distinct “mechanisms”
Mechanism: a step-wise model of how a reaction occurs
Rate-determining step: In a chemical reaction with more than one step (and many of them do)
The slowest step determines the overall rate of reaction
Balanced equation implies that a reaction occurs in only one step – this is often not the case!

16. Alkanes-Substitution RXNs: Nucleophilic
Molecularity: # of molecules involved in rate-determining step
Unimolecular: one molecule is involved
Bimolecular: two are involved
Termolecular: Three involved, and so on
Termolecular steps and above are quite rare because the probability of three particles colliding simultaneously is very low
Nucleophilic substitution can occur by two distinct “mechanisms”
SN2
SN1

17. Primary Halogenoalkane Subst: SN2
SN2 type mechanisms
Substitution, Nucleophilic, 2 (bimolecular).
Nucleophilic substitution reaction that has two molecules in the rate-determining step.

18. Primary Halogenoalkane Subst: SN2
Nucleophile attacking electrophile C on the opposite side of leaving group results in an inversion of the atoms around the carbon (stereospecific)

19. Tertiary Halogenoalkane Subst: SN1
SN1 type mechanisms:
Stands for Substitution, Nucleophilic, 1 (unimolecular)
Nucleophilic substitution reaction that has one molecule in the rate-determining step
Ex: a haloalkane undergoes slow, heterolytic fission to produce a carbocation intermediate and a halide ion
“X” is any halogen
Carbocation means a positively charged carbon ion

20. Tertiary Halogenoalkane Subst: SN1
Step 1:
Relatively slow due to the energy input required to break the carbon-halogen bond.
Curved arrow that starts on C and moves to the halogen (X) indicates that electrons move from carbon to the halogen.

21. Tertiary Halogenoalkane Subst: SN1
Step 2:
Lone pair electrons on OH- is attracted to this + carbocation, and form a coordinate bond (dative)
2nd step is much quicker so the 1st step is rate-determining.
1 molecule is involved in the rate-determining step = unimolecular,
Therefore  SN1 mechanism

22. Comparing SN1 and SN2
SN1 or an SN2 mechanism depends on the nature of the haloalkane.
1° Primary haloalkanes tend to undergo SN2 substitution
Easy for the nucleophile (OH- in ex) to access the carbon to attack it  No large carbon atoms in its way
3° Tertiary haloalkanes tend to undergo SN1 substitution
Difficult for the nucleophile to access the carbon while the surrounding carbons “shield it”
2° Secondary halogenoalkanes, both mechanisms can occur

23. Speed of a nucleophilic substitution reaction
Effect of the mechanism:
SN1 occur faster than SN2
In general: tertiary > secondary > primary.
Influence of the leaving group:
Polarity of C–X bond
C—F is most polar C—I is less polar
Would expect that C—F would be faster to leave
Strength of C–X bond
Stronger bonds take longer to break
C– I > C– Br > C– Cl > C–F
Strength predominate for rate of RXN (over polarity)

24. Speed of a nucleophilic substitution reaction
SN2
Prefers solvents that are polar and aprotic (no H-bonds in the solvent=no proton or H+)
They tend to solvate the Na+ (kind of like dissolve) and leaves the nucleophile bare and more reactive
Good solvents are: propanone and ethanenitrile
SN1
Carbocation intermediate is planar, so nucleophile can attack from any position (not stereospecific and can result in racemic mixture)
Prefers polar and protic solvents to help stabilize the carbocation intermediate
Good solvents are: water, alcohols, carboxylic acids

25. Summary of Alkane Nucleophilic Substitution RXNS