Reactions of Organic Compounds
How an organic compound will react depends on the group they belong to.
- Alkanes are very stable (C-C & C-H bonds strong, not reactive).
- Those compounds with a functional group tend to be more reactive.
1. Combustion: all organic compounds can undergo combustion reactions in the presence of oxygen.
Complete combustion: in the presence of excess O2, reaction can go to completion.
- Products are CO2 & H2O
Ex. C3H8(g) + 5O2(g) ---> 3CO2(g) + 4H2O(g)
Incomplete combustion: when O2 is limited and hydrocarbon and reaction cannot go to completion.
- Products are CO & H2O (CO is toxic!)
- May be some C & CO2 as well
Ex. 2C3H8(g) + 7O2 (g) ----> 6CO(g) + 4H2O(g)
2. Substitution Reaction:
- alkanes and arenes can undergo substitution reactions.
- one reactant takes the place of an H on the alkane/benzene ring
- Usually in reaction with a halogen
- UV light needed to split the bond between halogen molecules (homolytic fission). This creates free radicals (atoms with an unpaired electron - very reactive!!)
- Produces a haloalkane
Example: CH4(g) + Cl2(g) ------> CH3Cl(g) + HCl(g)
Mechanism:
3. Addition Reaction
- Alkenes and alkynes (reaction happens at site of double or triple bond)
- When reacting with halogen gas, halogen halide or with water
- π bond (weaker of the bonds) is broken during reaction. Gives two new bonding positions, allowing the halogen atoms (or H/OH) to ‘add’ on.
Ex1. ethene + bromine gas
Ex2. propene + water (hydration reaction: adding water)
* By using a nickel catalyst and heat we can make an alkene undergo an addition reaction with H2. This breaks double bonds and forms alkanes, which have higher boiling/melting points. Called hydrogenation, because you are adding hydrogens.
- This is done with unsaturated oils to make margarine.
* Using this reaction, there is a test to distinguish between alkenes and alkanes. Bromine water (brown in colour) is added to the unknown. If it is an alkene, it will undergo an addition reaction, and become colourless as the haloalkane is formed. If the unknown is an alkane, no reaction will occur, colour will remain.
4. Esterification
Alcohol + carboxylic acid à ester
Dehydration synthesis: a molecule of water is produced.
5. Polymerization of Alkenes
- the creation of long chains of repeating alkene units.
- a type of addition reaction that breaks the double bonds and joins the alkenes together.
- Chains of repeating alkene units are called polymers. A single alkene unit is called a monomer.
- Often uses a free radical to act as a catalyst to jumpstart the process and create a chain reaction.
- Most plastics are polymers of alkenes. Ex. Polyethene (polythane):
Ex.
6. Oxidation of Alcohols
- selective oxidation that targets only the carbon bonded to the –OH and leaves the rest of the hydrocarbon alone. (unlike combustion, which is complete oxidation)
- oxidizing agents are used. Usually KMnO4 or K2Cr2O7.(these change colour as oxidation occurs, so you can see colour change from brown to green)
- reaction depends on whether alcohol is primary, secondary or tertiary
Primary alcohols
- two-step reaction. First step products an aldehyde, the second a carboxylic acid.
Ex:
Secondary alcohols:
- react to produce a ketone
Ex:
Tertiary alcohols do not readily oxidize under similar conditions (no colour change).
7. Substitution of haloalkanes
- Same idea as substitution of alkanes, but different mechanism, because alkanes are non-polar, and haloalkanes have a polar bond.
Nucleophile: a reactant that is electron-rich (negative) and is attracted to the electron-poor (positive) parts of a molecule.
Electrophile: a reactant that is electron-poor and is attracted to electron-rich parts of a molecule.
- In the bond of carbon to a halogen, the halogen atom exerts a stronger pull on the shared electrons (higher electronegativity). The halogen gains a partial negative charge (δ-), and therefore the carbon a partial positive charge (δ+).
- The electron deficient carbon is what causes these reactions. (SN reactions = substitution nucleophilic)
- The carbon is attracted to electron-rich reactants. During reaction, the carbon halogen bond breaks (heterolytic fission). The halogen leaves (leaving group) and the negatively charged reactant bonds.
- Mechanism depends on whether the haloalkane is primary, secondary or tertiary.
Primary haloalkanes: SN2 reaction
- Halogen bonded to a carbon with two hydrogens. H’s are small, leaving the C wide open.
- This mechanism has an unstable transition state, where the C is bonded to the halogen and the nucleophile reactant.
Tertiary haloalkanes SN1 reaction
- C bonded to halogen less available, due to large C atoms around it (called steric hindrance).
- Because of this, the haloalkane must ionize first, with the halogen leaving the carbon. The intermediate is a carbocation. The nucleophile is then free to bond.
Secondary Haloalkanes
- mechanism is a mixture of SN1 & SN2
Questions
Read pages 383-396.
Questions: p.385 #9.10
p. 389 #11, 12
p. 392 #13, 14
p. 396 # 15, 16
Further reading: pages 396-399 #17,18,19