Download 10.2 Functional group chemistry Hydrocarbons

Hydrocarbons
Essential idea
 Structure, bonding and chemical reactions involving
functional group inter-conversions are key strands in
organic chemistry.
ALKANES
 General Formula: CnH2n+2
 Alkanes are saturated hydrocarbons.
 Hydrocarbons refer to compounds that contain
hydrogen and carbon only.
 Saturated means they contain all single carboncarbon bonds.
LO-1
Alkanes have low reactivity and undergo
free-radical substitution reactions.
 Alkanes have low reactivity because alkanes have
 Saturated nature
 Non-polar character
ALKANES HAVE LOW CHEMICAL
REACTIVITY
 Alkanes contain only C-C and C-H bonds.
 Both of these are strong bonds with bond
enthalpies of 348 kJ/mol and 412 kJ/mol,
respectively.
 These molecules will only react in the presence of
a source of energy strong enough to break these
bonds.
 Most alkanes are stable under most conditions
and can be stored, transported and compressed
safely.
 The C-C and C-H bonds are non-polar so they are
not susceptible to attack by most reactants.
Application and skills
 Alkanes:
 Writing equations for the complete and incomplete
combustion of hydrocarbons.
 Explanation of the reaction of methane and ethane
with halogens in terms of a free-radical substitution
mechanism involving photochemical homolytic
fission.
Combustion
 Complete combustion
 Incomplete combustion
 2CH4(g) + 3O2(g)
2CO(g) + 4H2O(l)
 Carbon monoxide is poisonous.
Why is Carbon Monoxide Poisonous?
 Carbon monoxide acts as a poison by combining with
hemoglobin in the blood. Hemoglobin normally reacts
with oxygen from the air and transports the oxygen to
the parts of the body which need it. Carbon monoxide is
much more reactive with hemoglobin than oxygen is.
It combines to form a stable compound with hemoglobin,
preventing the transport of oxygen around the body. The
person dies by suffocating from the inside (nasty!).
Halogenations
Reaction of methane with chlorine
Reaction of ethane with chlorine
Homolytic Fission and Free radical substitution
Learning outcomes
Understand
 Alkenes are more
reactive than
alkanes and
undergo addition
reactions.
 Application and Skills
 Write equations for the
reactions of alkenes with
hydrogen and halogens.
 Write equations for the
reactions of symmetrical
alkenes with hydrogen halides
and water.
 Outline the addition
polymerization of alkenes.
Reactions of Alkenes
 Hydrocarbons that have –C=C- undergo addition reactions.
 Addition of hydrogen ( alkene to Alkane)
 Reaction conditions
 Catalyst : Nickel( Pt or Pd)
 Temperature : 573 K or 300 oC
12
Reactions of Alkenes
 Addition of halogen to alkene
Reaction of Alkenes
 Reaction of Alkenes and Hydrogen halides
Reaction of Alkenes
 Reaction of ethene and water gives ethanol
Test to differentiate between
Alkane and Alkene
 Alkenes decolorize bromine water
Animation
Video
POLYMERIZATION
 Alkenes readily undergo addition reactions by
breaking their double bond.
 They can be joined together to produce long
chains known as polymers.
 The alkene used in the reaction is known as
the monomer and its chemical nature will
determine the properties of the polymer.
H H
C=C has the repeating unit
H H
H H
-C-CH Hn
Polymerization
ECONOMIC IMPACT
 Alkenes are used as starting materials in the
manufacture of many industrially important
chemicals.
 Polymers are a major product of the organic chemical
industry.
 Most of our most common and useful plastics are
polymers of alkenes.
 Ethene polymerizes to form polyethene also known as
polythene. (structure shown previously)
 It has excellent insulating properties.
 It played an essential role in the development of radar
during World War II.
 It is commonly used in household containers, carrier
bags, water tanks and piping.
 Propene polymerizes to form
polypropene often called polypropylene.
 It is used in the manufacture of clothing.
 More specifically the production of thermal
wear.
H H
C=C has the repeating unit
H CH3
H H
-C-C
H CH3 n
 Polychloroethene is known as PVC, poly
vinyl chloride.
 It is one of the world’s most important
plastics.
 It is used in construction materials,
packaging, and electrical cable sheathing.
 Its synthesis produces dioxins which are
toxic and are linked to a variety of cancers.
H H
C=C has the repeating unit
H Cl
H H
-C-C H Cl n
 Polytetrafluoroethene is known as PTFE
 It is often marketed as Teflon.
F F
C=C has the repeating unit
F F
F F
-C-C F F n
Alkene Reaction Summary
Alcohols
Learning outcomes
 Understand:
 Alcohols undergo esterification (or condensation)
reactions with acids and some undergo oxidation
reactions
 Apply their knowledge to:
 Write equations for the complete combustion of
alcohols.
 Write equations for the oxidation reactions of primary
and secondary alcohol (using either acidified
potassium dichromate(VI) or potassium
manganate(VII) as the oxidizing agent).
Learning outcomes
 Apply knowledge to:
 Explain the importance of distillation and reflux in the
isolation of the aldehyde and carboxylic acid products
when primary alcohols are oxidized.
 Write the equation for the condensation reaction of an
alcohol with a carboxylic acid, in the presence of a
catalyst (e.g. concentrated sulfuric acid) to form an
ester.
ALCOHOLS
 General Formula: CnH2n+1OH
 Alcohols contain the –OH functional group.
 The –OH group is polar and increases the solubility in
water and the volatility of alcohols when compared to
alkanes of similar molar mass.
COMBUSTION OF ALCOHOLS
 Alcohols react with oxygen to produce CO2 and
H2O.
 These reactions produce significant amounts of
energy.
 The amount of energy released increases as we go
up a homologous series because more CO2 is
produced.
 As in the case with hydrocarbons, in the case of
limited oxygen, CO will be produced instead of
CO2.
2CH3OH + 3O2 → 2CO2 + 4H2O
Esterification
An alcohol reacts with an alkanoic acid to form an ester and
water.
Oxidation
Alcohols are oxidized to alkanals (aldehydes)
or alkanones (ketones
Primary Alcohols are Oxidized to Alkanals
The most common oxidizing agents are KMnO4 in basic solution, K2Cr2O7
in acidic solution, or oxygen from the air. The mechanisms of such
reactions are usually complex
Anima
tion
Aldehy
de
Anima
tion
Carbox
ylic
acid
Secondary Alcohols are oxidized to
Alkanones
Tertiary Alcohols are not
easily oxidized
Evidence for oxidation of alcohols
Video
Learning outcomes
 Understand:
 Halogenoalkanes are more reactive than alkanes.
 Halogenoalkanes can undergo nucleophilic
substitution reactions.
 Nucleophiles are electron-rich species containing a
non-bonding pair of electrons that is donated to an
electron-deficient carbon
 Apply their knowledge to:
 Write equations for the substitution reactions of
halogenoalkanes with aqueous sodium hydroxide.
HALOGENOALKANES
 General Formula: CnH2n+1X (X=Halogen)
 Halogenoalkanes contain an atom of fluorine,
chlorine, bromine or iodine bonded to the carbon
skeleton of the molecule.
 Due to the presence of the halogen, they possess a
polar bond unlike the non-polar alkanes.
Boiling points of some isomers
Draw a conclusion about relationship
between branching and boiling points
of halogenoalkanes
Reactivity of halogenoalkane
Reactivity of halogenoalkanes
 Bond strength Vs Bond Polarity
Bond strength is the winner
Nucleophiles
 A nucleophile is a species (an ion or a molecule) which
is strongly attracted to a region of positive charge in
something else.
 Nucleophiles are either fully negative ions, or else have
a strongly - charge somewhere on a molecule.
Common nucleophiles are hydroxide ions, cyanide
ions, water and ammonia.
 The nucleophiles are attracted to the electron deficient
carbon in the halogenoalkane which leads to a reaction
where substitution of the halogen occurs.
 This reaction is called Substitution Nucleophilic and has
the shorthand notation of SN.
 During this reaction, the carbon-halogen bond breaks and the
halogen is released as a halide ion.
 When both of the shared electrons go to one product, it is known
as heterolytic fission.
 The halogen that becomes detached is sometimes referred to as
the leaving group.
C2H5Br + OH- → C2H5OH + Br-
Nucleophilic substitution reactions.
 Halogenoalkanes undergo nucleophilic substitution
reaction
 Simples way to write
1OH
as Nucleophile
 The halogenoalkane is usually refluxed with
aqueous sodium hydroxide, NaOH(aq), but some RX
molecules are reactive enough to hydrolyse when just
mixed with water
Learning outcomes
 Understanding
 Benzene does not readily undergo addition reactions
but does undergo electrophilic substitution reactions.
Reactivity of benzene
 Benzene does not readily undergo addition reaction
due to delocalized bonding
 Benzene undergo electrophilic substitution reaction
Benzene and electrophiles
 Because of the delocalised electrons exposed above
and below the plane of the rest of the molecule,
benzene is obviously going to be highly attractive to
electrophiles - species which seek after electron rich
areas in other molecules
 The electrophile will either be a positive ion, or the
slightly positive end of a polar molecule.
 Benzene undergo substitution reaction because it
needs high energy to break delocalized electron cloud.
Examples of substitution reactions
 Nitration
 Chlorination