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Chemical properties of the alkanes
Explain why the old, non-systematic name for the alkanes was the paraffins:
Free radicalsAlmost all alkane reactions involve the formation of free radicals what are they:
Combustion When alkanes are heated in a plentiful supply of air, combustion occurs Alkanes are energetically unstable with respect to water and carbon dioxide They only burn when they are in the gaseous state
Explain what happens when a candle burns:
Sketch and energy profile for the combustion of propane:
Cracking Cracking happens when alkanes are heated in the absence of air
The products of the cracking of long-chain hydrocarbons are shorter chain molecules Ethane is cracked industrially to produce ethene
What are the conditions for catalytic cracking and what are the benefits of using a catalyst?
Write an equation for the cracking of dodecane to form one mole of ethene and an alkane product:
Breaking bonds
Homolytic fission Homolytic fission involves the equal sharing out of electrons in a
bond Each atom receives one electron from the bond when it splits This leaves the atoms with one unpaired electron The atoms are uncharged These ‘free radicals’ are very reactive
Why are free radicals so reactive?
In what type of situation is homolytic fission most likely to occur?
Draw a dot-cross and a curly arrow diagram for homolytic fission of chlorine (NB the curly arrow is wrong in your textbook):
Heterolytic fission Heterolytic fission involves the unequal sharing of the electrons in
a bond Both electrons go to one atom
This results in the formation of two charged particles
In what type of situation is heterolytic fission usually seen?
Draw dot-cross and curly arrow diagrams for heterolytic fission:
Reactions with chlorine Alkanes only react with chlorine when a mixture of the two is exposed to sunlight or
ultraviolet light The light provides the energy required to break the very strong bonds This is an example of a substitution reaction There are three steps to the chain-reaction:
Initiation:The Cl-Cl bond is easier to break than C-H
Propagation:There are several possible propagation steps
Termination:There are also several possible termination steps
Explain the difference in products produced when: methane reacts with a limited supply of chlorine and with a plentiful supply of chlorine:
Chemical and physical properties of alkanes
Chemical properties Alkanes contain atoms of 2 elements: ____________ and
__________
Alkanes are _____________ hydrocarbons
This means that they contain no carbon-carbon ___________ bonds
Alkanes are very _________________ because they contain very __________ bonds
They do burn, and in sufficient oxygen produce _________ and __________ ___________.
Physical properties Alkanes are ________________ in water
In straight-chain alkanes boiling temperature _______________ with chain length; however, this
trend is not followed by branched structural ________________.
ActivityNumber these alkanes in order of increasing boiling point:
Propane Methane Octane Dotriacontane Heptane
1
ActivityLabel the following pairs of structural isomers with highest boiling point (H) and lowest boiling point
(L):
Butane 2-methylpropane
Hexane 2,2-dimethylbutane
2,2,4-trimethylpentane Octane
ActivityBriefly explain the choices you have made in the above activities:
Structural Isomerism
“Isomers are compounds with the same molecular formula but different structural formula”
Structural isomers have the same molecular _____________ but different
molecular ______________. This means that they have the same number of
_________ of the same ___________, but that these atoms are arranged
differently.
Exercise:
Determine displayed, skeletal, structural, molecular and empirical formulae for all structural isomers of the 1st five compounds in the alkane homologous series (methane-pentane), you can use the Molymod kits to help you do this.
The alkenes
Comparison of bonding in ethane and ethene
Ethane The carbon-carbon single bond ( bond) runs through the central axis of the molecule The two ends of the ethane molecule are free to rotate relative to each other
Ethene The ethene molecule is flat Ethene contains a carbon-carbon bond and a carbon-carbon double bond ( bond) The bond lies above and below the central axis The bond prevents rotation around the central axis
The carbon-carbon bond is less than twice as strong as the carbon-carbon bond, why?
What is and electrophile and why do alkanes readily react with them?
Naming alkenes We use the IUPAC naming system to name alkenes The suffix used is –ene We must use a number to indicate the position of the double bond
Complete the table of alkenes:
ethene
but-1-ene but-2-ene
2-methylpent-2-ene
2,4-dimethylhex-2-ene
Alkenes – Chemical reactions
Introduction Alkenes are far more reactive than alkanes They readily react with electrophiles They undergo addition reactions Addition reactions involve heterolytic fission of the
double bond
Electrophiles Species containing and atom with a whole or partial ________________ charge
They are attracted to areas of high _____________ density
They form a ______________ bond using a pair of electrons from the bond
The most common electrophile is ________________
Addition reactions1. Reaction with hydrogen
Alkenes do not react with hydrogen under normal conditions…
Conditions:
2. Reaction with acidified potassium manganate(VII) This reaction involves addition across the double bond and oxidation
The products are alkane diols
Can be used as a test for _______________ as alkanes do not react
Conditions:
Observations:
3. Reactions with halogens Alkenes react halogens to form halogenoalkanes Reactions happen at room temperature and unlike the reaction of alkanes with halogens do
not involve ______ _____________
Reactions become __________ vigorous down the halogen group (F > Cl > Br > I)
4. Reaction with bromine water Bromine water is an _______________ solution of bromine
Addition takes place across the double bond and the bromine water becomes
_______________
There are two different products _____________________ and _____________________
2-bromoethanol is formed because _____ ions from water also take part in the reaction
5. Reactions with hydrogen halides Hydrogen halides are compounds containing ________________ and a halogen
e.g.
Hydrogen halides react with alkenes readily at room temperature
In the reactions hydrogen halides with higher alkenes (n > 2) there are two possible products
Both possible products are formed but one is favoured, in this case ___________________
The major product can be predicted using Markovnikov’s rule
Markovnikov’s rule:
“When HX adds across an asymmetric double bond, the major
product is the molecule in which hydrogen adds to the carbon
atom in the double bond with the greater number of hydrogen atoms already attached to it”
Reaction mechanisms – “Curly Arrows”
Key words
Electrophile –
Nucleophile –
Carbocation –
Curly arrow –
Reaction mechanisms Mechanisms are diagrams that represent how organic chemistry reactions happen in terms of
the movement of electrons
A single-headed curly arrow shows…
A double-headed curly arrow shows…
Mechanism for the reaction of ethene with hydrogen bromide
Mechanism for the reaction of propene with hydrogen bromideRemember Markovnikov? There are two possible products:
1.
2.
Both mechanisms involve carbocations (careful there’s a mistake in your book!):
The carbocation in reaction ____ is more stable
This is because the __________ groups donate electrons to stabilise the positive charge
As a result the major product is ___________________
Mechanism for the addition of bromine to ethene
The bromine-bromine bond is partially polarised by the electron rich ___ bond
The first stage of the reaction is ________________ attack
This is followed by _______________ attack on the carbocation by Br-
Polymerisation – The most important reaction of the alkenes
Key wordsPolymer –
Monomer –
Synthesis of polymers Polymers are widely made in addition reactions
Addition occurs across a double or triple bond
Common polymers1. Poly(ethene) (polythene)
Formed from the polymerisation of ____________________
Comes in 2 forms __________ or ____________
Conditions of polymerisation reaction determine which form is produced:
1.1 Low-density polythene (LDPE)
Properties Conditions of polymerisation reaction
Diagram of structure
Uses
1.2 High-density polythene (HDPE)
Properties Conditions of polymerisation reaction
Diagram of structure
Uses
2. Poly(propene)
Properties Conditions of polymerisation reaction
Monomer
Polymer
3. Poly(chloroethene) (PVC)
Properties Conditions of polymerisation reaction
Monomer
Polymer
4. Poly(tetrafluoroethene) (PTFE / “Teflon”)
Properties Monomer
Polymer
Properties of polymers The physical (e.g. _________________ , __________________ and ________________) and
chemical properties of polymers depend on their molecular structure:
1. Average length of polymer chain Tensile strength
Melting temperature
2. Branching of the chain Tensile strength
Melting temperature
Density
3. Intermolecular forces between chains Tensile strength
Melting temperature
4. Cross-linking between chains Rigidity
Hardness
Flexibility
Melting temperature