Main Menu 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
Main Menu Organic Chemistry Organic chemistry is the chemistry of carbon containing compounds. From the very simple: methane To the very complex: Haem B
Main Menu 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
Main Menu 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?
Main Menu Formulas Draw the compound with the formula C 4 H 8 O
Main Menu What did you get? Clearly a molecular formula is not enough!
Main Menu Types of Formula Empirical Formula C 4 H 8 O C 4 H 8 O Molecular Formula C 4 H 8 O C 4 H 8 O 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! CH 2 =CHCH 2 CH 2 OH CH 2 =C(CH 3 )CH 2 OH
Main Menu 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 C 4 H 8 O compounds (not the cyclic ones)
Main Menu 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
Main Menu Lesson 2: Isomers Objectives: Describe the term structural isomer Draw a name the non-cyclic alkanes Draw and name the straight-chain alkenes
Main Menu Isomers Compounds with the same molecular formula but different structural formula The 20 different C 4 H 8 O 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
Main Menu Structural Isomers of the Alkanes The (non-cyclic) alkanes have the general formula C n H 2n+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
Main Menu 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
Main Menu 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
Main Menu 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’
Main Menu 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
Main Menu 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
Main Menu 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 1 o, 2 o and 3 o 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
Main Menu 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
Main Menu 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
Main Menu Refresh 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 C C C C C C C C O O O 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.
Main Menu 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
Main Menu 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 CO 2 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
Main Menu 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.
Main Menu Halogenation Alkanes will undergo halogenation if reacted with a halide in the presence of u.v. light. For example: C 2 H 6 (g) + Cl 2 (g) CH 3 CH 2 Cl(g) + HCl(g) ethane chloroethane This reaction is an example of free radical substitution u.v.
Main Menu Radicals Radicals are species with unpaired electrons They are crazy reactive Halogens form radicals when hit by uv light of the right frequency: Cl 2 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 Cl 2 molecule and each of the Cl radicals u.v.
Main Menu Reaction Mechanism: Free Radical Substitution Cl 2 2 Cl Cl + C 2 H 6 C 2 H 5 + HCl C 2 H 5 + Cl 2 C 2 H 5 Cl + Cl Cl + Cl Cl 2 Cl + C 2 H 5 C 2 H 5 Cl C 2 H 5 + C 2 H 5 C 4 H 10 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.
Main Menu 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
Main Menu Lesson 5: Alkenes Objectives: Describe the main addition reactions of the alkenes Extract an alkene from a citrus fruit
Main Menu 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
Main Menu 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
Main Menu 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
Main Menu 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.
Main Menu 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!
Main Menu 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
Main Menu Drawing polymers Draw three-monomer lengths of the polymers formed by: Propene Styrene Pent-2-ene
Main Menu 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
Main Menu 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
Main Menu 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?
Main Menu 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, K 2 Cr 2 O 7. 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
Main Menu 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
Main Menu Lesson 7: Halogenoalkanes Objectives: Describe the substitution reactions of halogenoalkanes with a strong base Understand the S N 1 and S N 2 mechanisms for nucleophilic substitution Produce an animation showing the two different mechanisms
Main Menu 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
Main Menu 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
Main Menu S N 1 – Unimolecular nucleophilic substitution – animation hereanimation 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 S N 1 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
Main Menu S N 2 – Bimolecular nucleophilic substitution – animation hereanimation 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]
Main Menu S N 1 or S N 2? 1 o halogenoalkanes predominantly undergo S N 2 2 o halogenoalkanes undergo a mix of S N 1 and S N 2 3 o halogenoalkanes predominantly undergo S N 1 You do not need to know why at SL, but will find out more at HL
Main Menu Refresh Halogenoalkanes undergo substitution with strong bases to form alcohols The reaction has two possible mechanisms: S N 1: the C-X bond breaks and then the nucleophile attacks S N 2: the nucleophile attacks at the same time as the C-X bond breaks The mechanism depends on the halogenoalkane: 1 o - S N 2 2 o - S N 1 and S N 2 3 o - S N 1