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Organic Chemistry
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Menu Lessons 1-11: Lesson 1 – Homologous Series Lesson 2 – Isomers Lesson 3 – Meet the Families Lesson 4 – Alkanes Lesson 5 – Alkenes Lesson 6 – Alcohols Lesson 7 – Halogenoalkanes Lesson 8 – Reaction Pathways Lesson 9 – HL – Meet the Families (again) Lesson 10 – HL – SN1 and SN2 Revisited Lesson 11 – HL – More Nucleophiles
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Menu Lessons 12-18: Lesson 12 – HL – Elimination Reactions Lesson 13 – HL – Condensation Reactions Lesson 14 – HL – Condensation Polymerisation Lesson 15 – HL – Geometric Isomerism Lesson 16 – HL – Optical Isomerism Lesson 17 – HL – More Reaction Pathways
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Lesson 1
Homologous Series
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Overview Copy this onto a double-page spread. You
should add to it as a regular review throughout the unit.
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Assessment
This unit will be assessed by:
A test at the end of the topic (75%)… An internal assessment (25%)
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Lesson 1: Homologous Series
Objectives:
Reflect on previous knowledge of organic chemistry
Understand the term ‘homologous series’
Conduct the fractional distillation of crude oil
Understand and use the variety of different types of formula used in organic chemistry
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Organic Chemistry
Organic chemistry is the chemistry of carbon containing compounds.
From the very simple: methane
To the very complex: Haem B
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Homologous Series A homologous
series is a family of compounds that differs only by the length of its hydrocarbon chain
Members share: General formula Chemical properties
Three such series are the: Alkanes Alkenes Alcohols
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Homologous Series and Boiling Points
What do you think will be the trend in melting/boiling points as you go down a homologous series?
Why?
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Formulas
Draw the compound with the formula C4H8O
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What did you get?
Clearly a molecular formula is not enough!
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Types of Formula Empirical Formula C4H8O C4H8O
Molecular Formula C4H8O C4H8O
Full Structural Formula Aka displayed formula
Condensed Structural Formula Note the ‘=‘ used for the C=C double bond
Skeletal formula Not required but v. useful Used in data booklet for complicated structures Do not use in exam answers!
CH2=CHCH2CH2OH CH2=C(CH3)CH2OH
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Thinking About Formulas
Produce a table to summarise each of the formulas. Include columns for: What they show Pros Cons How you make them
Draw full structural, condensed structural and skeletal formulas for at least 5 of the C4H8O compounds (not the cyclic ones)
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Key Points
Organic chemistry is the chemistry of carbon containing compounds
A homologous series is a family of organic compounds differing only by the length of their carbon chains
The melting and boiling point increases as you go down a homologous series
Displayed formulas show the unambiguous arrangement of atoms in a compound
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Lesson 2
Isomers
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Lesson 2: Isomers
Objectives:
Describe the term structural isomer
Draw a name the non-cyclic alkanes
Draw and name the straight-chain alkenes
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Isomers Compounds with the same
molecular formula but different structural formula
The 20 different C4H8O compounds from last lesson are isomers of each other
These are all structural isomers Same number of each atom,
but bonded in a different order
You would have even more if you included geometric and optical isomers
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Structural Isomers of the Alkanes The (non-cyclic) alkanes have the general
formula CnH2n+2
Draw full and condensed structural formulas for every isomer of every one of the alkanes up to n = 6 If you finish early, draw each as a skeletal formula
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Did you get them all?
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And skeletally
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Naming Straight-chain alkanes Suffix:
Tells us the functional group of the molecule
For alkanes it is ‘-ane’
Prefix: Tells us the length of the
longest carbon chain: 1 carbon: meth- 2 carbons: eth- 3 carbons: prop- 4 carbons: but- 5 carbons: pent- 6 carbons: hex-
Example 1: ethane
Example 2: butane:
Task: write in the names of the 4 straight chain alkanes next to your diagrams from last slide
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Naming branched-chain alkanes Start by naming the longest chain
Add extras to say the size of a branch, its position and how many of that branch
Branch Size: 1 carbon: methyl- 2 carbons: ethyl- 3 carbons: propyl-
Position: Number the carbons in the longest chain Choose numbers to minimise the total
numbers used
Number of same branches One branch – nothing Two branches – di- Three branches – tri- Four branches – tetra-
Example 1: 2-methylpropane
Example 2: 2,3-dimethylbutane
Task: name the remaining alkanes
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The straight-chain alkenes Alkenes are the same as alkanes but have one C=C
double bond.
The suffix for the alkene homologous series is ‘-ene’
Task: draw full structural and skeletal formulas for each of the straight-chain alkenes up to C6 and name them. Do the branched ones as well if you have time
Hint: you need to state the position of the double bond, but only if there is the possibility of multiple isomers: i.e. ‘but-2-ene’ or ‘hex-1-ene’ but only ‘ethene’ not ‘eth-1-ene’
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Did you get them?
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Key Points
Structural isomers have the same number of each atom but they are connected differently
When naming compounds The longest carbon chain forms the prefix The functional group tells you the suffix
Sometimes numbers need to be used to tell you where this functional group is
Side chains and other groups are named according to what they are, how many there are and their position
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Lesson 3
Meet the Families
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Lesson 3: Meet the Families
Objectives:
Meet and learn to recognise the 7 functional groups required for the SL course
Produce a mind-map summarising each of the homologous series
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Functional Groups Table (landscape) You need to research and produce a
mind-map summarising the following functional groups: Alkane Alkene Alcohol Aldehyde Ketone Carboxylic acid Halide/Halogenoalkane
Your table should have four columns including: Name of functional group General structural formula (use ‘R’ to
signify a carbon chain) Rules for naming them (including the
position where relevant) A named example Relative volatility Relative solubility in water
For alcohols and halides you should include a branch to explain the difference between 1o, 2o and 3o
You should also have a branch called ‘Other Functional Groups’ that just allows you to recognise the groups:
Amine Ester Benzene
If HL you should leave space for four more functional groups
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Building Organic Compounds
Use molecular models to make any of the compounds mentioned in your mind-map: Draw it (structural and skeletal) Name it Give it to a friend and challenge them to do the
same
Only go up to 6 carbons
Only include branched-chains for the alkanes
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Key Points
There are 7 functional groups we need to know in detail and 3 extra we need to be able to recognise
We will look at each in detail over the rest of the unit
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Lesson 4
Alkanes
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The following is a computer-generated representation of the molecule, methyl 2-hydroxy benzoate, better known as oil of wintergreen.
a) Deduce the empirical formula of methyl 2-hydroxy benzoate and draw the full structural formula, including any multiple bonds that may be present…The computer-generated representation shown does not distinguish between single and multiple bonds.
b) Name all the functional groups present in the molecule.
H
H
H
H
H
H
H H
CC
C
C
C
C
C
C
O
OO
Reviewing Your Notes
You should spend 60 seconds reviewing
your notes from last lesson before
attempting this.
Your notes and mind-map must be ready for me to inspect.
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Lesson 4: Alkanes
Objectives:
Explain the stability of the alkanes
Observe the combustion of alkanes
Describe the free-radical substitution reactions of alkanes and its mechanism
Observe the free-radical substitution of hexane
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Combustion of Alkanes
The alkanes really don’t do much Combustion is of one of two notable reactions (this is why we use them for
fuels)
Complete combustion: alkane + oxygen carbon dioxide + water
Incomplete combustion: Alkane + oxygen carbon + carbon monoxide + carbon dioxide + water The amounts of C, CO and CO2 will vary depending on conditions
Task: Observe the combustion of the gas from the gas taps (propane/butane mix) and of a small amount hexane (in spirit burners). Hold the end of a clean boiling tube just over the flame for 15 seconds, this will collect soot from the flame. Record all observations clearly and try to account for them Include balanced equations to describe the (complete) combustion
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Why so boring stable?
There are at least two reasons why alkanes are so unreactive
Task: Think back to your knowledge of molecular structure, and look at the tables of bond-enthalpies in the data booklet to see if you can work out why.
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Halogenation Alkanes will undergo halogenation if reacted
with a halide in the presence of u.v. light.
For example:
C2H6(g) + Cl2(g) CH3CH2Cl(g) + HCl(g)
ethane chloroethane
This reaction is an example of free radical substitution
u.v.
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Radicals
Radicals are species with unpaired electrons They are crazy reactive
Halogens form radicals when hit by uv light of the right frequency:
Cl2 2 Cl•
The dot after the Cl represents the unpaired electron and tells us we have a radical
This process is called homolytic fission – the bond breaks equally with one electron going to each chlorine
Task: draw Lewis structures for the Cl2 molecule and each of the Cl• radicals
u.v.
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Reaction Mechanism: Free Radical Substitution
Cl2 2 Cl•
Cl• + C2H6 C2H5• + HCl
C2H5• + Cl2 C2H5Cl + Cl•
Cl• + Cl• Cl2 Cl• + C2H5• C2H5Cl
C2H5• + C2H5• C4H10
Initiation Radicals formed by homolytic
fission
Propagation These steps feed each other the
radicals needed to continue
Termination Any two radicals can combine to
terminate the reaction Concentration of radicals is low so
this is a rare event A single radical can cause thousands of cycles of the propagation stage before
it reaches termination This same mechanism applies to all of the halogens The alkane can be substituted multiple times, until every H has been replaced
u.v.
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Key Points
Alkanes are unreactive
They release a lot of energy on combustion, and are easy to handle which makes them good fuels
Undergo free radical substitution to form halogenoalkanes and a hydrogen halide in the presence of UV light
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Lesson 5
Alkenes
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Lesson 5: Alkenes
Objectives:
Describe the main addition reactions of the alkenes
Extract an alkene from a citrus fruit
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Reactivity of Alkenes
Alkenes are considerably more reactive than alkanes and are a major industrial feedstock
The reactivity is due to the double bond: The double bond contains 4 electrons This is a significant amount of charge
which: Makes it attractive to electrophiles Enables it to polarise approaching molecules
Most reactions of alkenes are addition reactions where two molecules come together to make one new one
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Alkenes and hydrogen
Alkene + hydrogen alkane
Reaction conditions: Hot Ni catalyst
This is an addition reaction, in which the hydrogen adds across the double bond
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Alkenes and hydrogen halides
Alkene + hydrogen halide halogenoalkane
Reaction conditions: This reaction occurs very readily and needs no
special conditions
This is an addition reaction, in which the hydrogen halide adds across the double bond
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Alkenes and halogens
Alkene + halogen dihalogenoalkane
Reaction conditions: This reaction occurs very readily and needs no special
conditions
If the halogen used is an aqueous solution of bromine (bromine water), the orange-brown colour of bromine solution is decolourised. This is the standard test for alkenes.
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Alkenes and water
Alkene + water alcohol
Reaction conditions: Water must be steam Phosphoric or sulphuric acid catalyst
This is the process used to make industrial ethanol Fermentation from sugar would be far too expensive!
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Polymerisation Under the right conditions, alkene molecules will add to each
other creating a polymer
In this case, 1-bromo-2-fluoroethene polymerises to form poly-1-bromo-2-fluroethene
Conditions: Vary from alkene to alkene but often include high pressure,
temperature and a catalyst
The carbons in the C=C double bonds form the carbon chain, everything else hangs off this chain
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Drawing polymers Draw three-monomer lengths of the polymers
formed by:
Propene
Styrene
Pent-2-ene
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Key Points Alkenes undergo addition reactions with:
Hydrogen Hydrogen halides Halogens Water (steam)
Alkenes undergo addition polymerisation
Alkenes are very economically important due to the range of products they can make
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Lesson 6
Alcohols
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Lesson 6: Alcohols
Objectives:
Explain the relative ease of combustion of the alcohols
Describe the oxidation reactions of the alcohols
Investigate the oxidation reactions of the alcohols
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Alcohols as Fuels Alcohols combust more readily than equivalent
alkanes but release less energy since they are already partially oxidised
Alcohol + oxygen carbon dioxide + water
Alcohols are used as fuels: As a fuel for cars – either pure or blended with petrol Methanol as fuel for competitive motorsports including
dragsters and monster trucks
Much fuel ethanol is fermented from crops…crops that could otherwise be eaten, forcing up food prices. Is this ok?
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Oxidation of alcohols The most important reactions of the alcohols are
their oxidations
A range of compounds will oxidise them so the oxidiser is often represented as [O]
One oxidising agent you need to know is potassium dichromate, K2Cr2O7. When using this, orange Cr (VI) is reduced to green Cr (III)
More on what this means in the oxidation and reduction unit
See next slide for details
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Oxidation reaction scheme
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Key Points
Alcohols are highly combustible
Primary alcohols oxidise to form aldehydes, which oxidise to form carboxylic acids
Secondary alcohols oxidise to form ketones
Tertiary do not oxidise due to the 3 strong C-C bonds surrounding the –OH carbon
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Lesson 7
Halogenoalkanes
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Lesson 7: Halogenoalkanes
Objectives:
Describe the substitution reactions of halogenoalkanes with a strong base
Understand the SN1 and SN2 mechanisms for nucleophilic substitution
Produce an animation showing the two different mechanisms
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Nucleophilic Substitution
One of the most important reactions undergone by halogenoalkanes is nucleophilic substitution
A nucleophile is a ‘nucleus-loving’ species that is attracted to positive charges. Nucleophiles have either full negative charges or delta-
negative charges Water and hydroxide are both nucleophiles
In this case we can also call the reaction ‘hydrolysis’
The carbon in the carbon-halogen bond has a + charge due to the greater electronegativity of the halogen This makes it susceptible to attack by nucleophiles
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Halogenoalkanes and strong bases A substitution reaction takes place, where the halogen atom is displaced by the
hydroxide ion
halogenoalkane + sodium hydroxide alcohol + sodium chloride
Conditions: Aqueous base Gently warmed (can at room temperature, but may be quite slow)
This is a nuclephilic substitution. The C attached to the halogen is + due to the high electronegativity of the halogen The OH- ion (our nucleophile) is attracted to the + carbon
A nucleophile is a species with a negative charge or a lone pair that is attracted to positive/delta-positive atoms
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SN1 – Unimolecular nucleophilic substitution – animation here
Unimolecular because only one molecule is involved in the rate determining step
The rate determining step involves the spontaneous breaking of the carbon-halogen bond and is a heterolytic fission, forming a halide ion and a carbocation intermediate The stability of the carbocation intermediate is a key factor in SN1
The attack by the nucleophile (OH-) is very fast, but does need the carbocation to be formed first
The rate is only dependent on the concentration of the halogenoalkane: Rate = k[halogenoalkane]
Note: the curly arrows show the movement of pairs of electrons
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SN2 – Bimolecular nucleophilic substitution – animation here
Bimolecular because two molecules are involved in the rate determining step
In the rate determining step, the nucleophile (OH-) attacks at the same time as the carbon-halogen bond breaks.
The reaction passes through a negative transition state where the carbon has a ‘half-bond’ to both the –OH and the –Br with an overall negative charge
The rate is dependent on both the concentration of the halogenoalkane and the nucleophile Rate = k[halogenoalkane][nucleophile]
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SN1 or SN2?
1o halogenoalkanes predominantly undergo SN2
2o halogenoalkanes undergo a mix of SN1 and SN2
3o halogenoalkanes predominantly undergo SN1
You do not need to know why at SL, but will find out more at HL
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Refresh
Halogenoalkanes undergo substitution with strong bases to form alcohols
The reaction has two possible mechanisms: SN1: the C-X bond breaks and then the nucleophile attacks SN2: the nucleophile attacks at the same time as the C-X
bond breaks
The mechanism depends on the halogenoalkane: 1o - SN2 2o - SN1 and SN2 3o - SN1