Organic Chemistry - WordPress.com · •Can react with non-polar HALOGENS ... chlorine, bromine or...
Transcript of Organic Chemistry - WordPress.com · •Can react with non-polar HALOGENS ... chlorine, bromine or...
Organic ChemistryREACTIONS
Grade 12 Physical Science
Mrs KL Faling
SUBSTITUTION REACTIONS
• This is a reaction where an atom or group of
atoms is replaced by another atom or group of
atoms
• Substitution only occurs in saturated compounds
(single bonds)
• Substitution reactions offer require heating and/or
a catalyst
Example:
One of the H’s from methane is replaced
by a chlorine. The H and the other chlorine then form HCℓ
TYPE 1:Alkanes + HALOGENS (HALOGENATION)
• Alkanes are generally non-reactive NON-
POLAR nature
• Can react with non-polar HALOGENS
• Occurs in presence of sunlight, UV light or heat
EXAMPLE:
Practice
•Ethane + Bromine
•Butane + Iodine
•Methane + Fluorine
•Hexane + Chlorine
NOTE: The bromine test for saturation
• Bromine is used to test what homologous series an
unknown hydrocarbon belongs to:
• If Bromine is added to an alkane (saturated) a non-
spontaneous substitution reaction occurs and the
solution remains brown.
• (If the hydrocarbon is unsaturated a spontaneous
addition reaction occurs and the water goes clear)
SUBSTITUTION IN ALCOHOLS
• Alcohols react with hydrogen halides to
produce halo-alkanes and water
• (OPPOSITE TO THE HYDROLYSIS of halo-
alkanes)
• The reaction is thus reversible
NOTE:
• Reactions only occur with hydrogen and
chlorine, bromine or iodine
• HF is not reactive
Primary and secondary alcohols
• React slowly
• React best at high temperature
• An acid like H2SO4 should be added as a catalyst
PRACTICE
• Methanol + HBr
• Ethanol + HI
• Propan-1-ol + HF
• Butan-2-ol + HCℓ
Tertiary alcohols
• Occurs easily and fastest
• The tertiary alcohol is converted to a tertiary
halo-alkane
• Occurs at room temperature
• Only works with hydrogen bromide and
hydrogen chloride.
Practice
SUBSTITUTION IN HALO-ALKANES with BASES
• Halo-alkanes react with an aqueous
solution of a DILUTED metal hydroxide to
form an alcohol.Concentrated
hydroxide
solutions will
cause a different
type of reaction
EXAMPLE
Chloro methane + KOH
2 Bromo-2,3 dimethyl butane +
NaOH
ADDITION REACTIONS
• Unsaturated compounds can undergo
addition
• Double bonds become single bonds and
triple bonds become double bonds and
an extra “something” is added.
4 types of addition reactions
1. Hydrohalogenation
2. Halogenation
3. Hydration
4. Hydrogenation
1. HYDROHALOGENATION
• Addition of a hydrogen halide to an alkene
Conditions and process
• HCℓ, HBr or HI must be added to an alkene
(NOT HF)
• No water must be present
• During the reaction the double bond breaks and the
hydrogen attaches to one side and the halogen to the
other side. The hydrogen will always attach to the most
“primary” carbon of the two.
Practice:
• Propene + HBr
• 2 methyl-1-butene + HCℓ
• 3 ethyl-2-pentene + HI
• 3,3,4 trimethyl-1-pentene + HBr
2. HALOGENATION
• The addition of a halogen
• No catalyst is necessary
• Only with chlorine and bromine (DIATOMIC
MOLECULES!!!!!) One gets added each side of the
“broken” double bond.
• Iodine is to slow and fluorine is to volatile
Example
Practice
• Propene + Chlorine gas
• 1-butene + Bromine gas
• 2,3 dimethyl-2-hexene + Cℓ2
• 4,5,6 triethyl-2-octene + Br2
3. Hydration
• Addition of excess water to an alkene to produce
an alcohol.
• A strong acid is required as a catalyst generally
H3PO4 + H2SO4
EASY EXAMPLE:
Markovnikov’s rule for asymmetrical alkenes
• When the H2O splits during the addition reaction
the single hydrogen atom will go to the carbon
with the most hydrogens and the hydroxyl
functional group (O-H) will go to become tertiary
over secondary over primary.
• (worked the same way in hydrohalogenation)
Example
Practice
• 2-butane + H2O
• 1-butane + H2O
• 2 methyl-2-pentene + H2O
• 3,4 diethyl-2-heptane + H2O
4. Hydrogenation
• The addition of hydrogen gas in a hydrogen
atmosphere (NO OXYGEN) to an unsaturated
hydrocarbon which then results in a more saturated
hydrocarbon.
• The alkene must be dissolved in a non-polar solvent
like hexane or benzene
• Needs a catalyst: Pt, Ni or Pd
• This is sometimes called catalytic hydrogenation and
is used in making margarine.
Example
Practice
• propene + hydrogen
• 2 methyl-propene + hydrogen
• 3 methyl-but-1-ene + hydrogen
APPLICATIONS OF ADDITION REACTIONS
• Hydrogenation of plant oils – plant oils are
unsaturated and due to multiple bonds are in
liquid form.
• Hydrogen is added in the presence of nickel as a
catalyst and the plant oil hardens and produces
margarine.
ELIMINATION
1. Dehydrohalogenation
2. Dehydration (of alcohols)
3. Thermal cracking and catalytic
cracking
Dehydrohalogenation
A hydrogen halide is eliminated from a
haloalkane to for an alkene.
The reaction must be strongly heated
during reflux – the vapours must condense
and return to the reaction vessel during
heating.
What you need for dehydrohalogenation?
• A strong base dissolved in pure ethanol
• This is known as warm ethanolic NaOH (or
KOH)
• The product is an alkene, water and a
halide salt.
Example
Practice
• 1 Bromo-butane + NaOH
• 2 Chloro-pentane + KOH
• 1 Bromo-propane + KOH
• 3 Chloro-hexane + NaOH
DEHYDRATION (of alcohols)
• H2O is removed from an alcohol.
• It is a catalysed acid reaction.
• Concentrated sulfuric acid is used as a
dehydrating agent. (OR phosphoric acid)
• The product is an alkene and water
Example
ASYMMETRICAL alcohols
• We can focus on the main
product which is that the “H” will
come off the more secondary
carbon.
• AKA The one with the least H’s ->
This will form the main product (by-
products can also be formed)
Practice
1. 2 methyl-1-propanol
2. 3,3 dimethyl-2-hexanol
3. 3 ethyl-2-pentanol
Cracking
• Hydrocarbons can be cracked to make smaller
more useful molecules.
• Thermal cracking occurs at a high temperature
and pressure.
• Catalytic cracking occurs at a lower temperature
but requires a catalyst.
EXAMPLE – The product of thermal cracking is multiple short chains – at least ONE must be an alkene
Complete Ex 9 Page 122