Module 4: Core Organic Chemistry Flashcards
What are organic compounds?
Compounds containing carbon
Why is carbon special?
Each carbon atom can form 4 covalent bonds, and can bond to other carbon atoms to form long chains
What is a hydrocarbon?
What are the two types?
A compound containing only hydrogen and carbon.
Can be saturated (single bonds only) or unsaturated (contains at least one carbon-to-carbon double bond)
What is a homologous series?
A family of compounds with similar chemical properties where successive members differ by CH2
What is a functional group?
The part of the organic molecule mostly responsible for chemical properties
What are the different types of hydrocarbons? Define them
Saturated/unsaturated
- Aliphatic - carbon atoms joined in straight or branched chains or non-aromatic rings
- Alicyclic - carbon atoms joined in ring structures
- Aromatic - some/all carbon atoms in a benzene ring
What are the homologous series of aliphatic hydrocarbons?
- Alkanes - only have single carbon-to-carbon bonds
- Alkenes - have at least 1 double carbon-to-carbon bond
- Alkynes - have at least 1 triple carbon-to-carbon bond
Give the main rules of organic nomenclature
- The stem of the name indicates the number of carbons in the longest continuous chain
- Prefixes/suffixes indicate functional groups (side chains are alkyl groups). These are labelled with the smallest possible numbers giving their position of the chain
Give some examples of functional groups
- Nitrile (-CN)
- Amine (-NH2)
- Acyl chloride (-COCl)
- Ester (-COOC-)
- Ketone (-CH2C(O)CH2-)
- Aldehyde (-COH)
Give the general formulae of:
- Ketones
- Carboxylic acids
Ketones - CnH2nO
Carboxylic acids - CnH2nO2
Give the general formulae of:
- Alcohols
- Alkenes
- Alkanes
Alcohols - CnH(2n+1)OH
Alkenes - CnH2n
Alkanes - CnH(2n+2)
What do displayed formulae show?
Relative positioning of atoms and the bonds between them
What do skeletal formulae show?
Uses the smallest amount of details to show atom’s positions
How do skeletal formulae work?
- A line represents a single bond
- Each corner is a carbon atom
- All hydrogens are removed, any functional groups are not
What are structural isomers?
Compounds with the same molecular formulae but different structural formulae.
Often occurs through branches/positioning
What are functional isomers?
Structural isomers with the same molecular formulae but different functional groups
Name and define the two types of bond fission
- Homolytic fission - each atom takes 1 of the shared electrons. Forms 2 radicals
- Heterolytic fission - one of the bonded atoms takes both of the electrons. Forms 2 oppositely charged ions
What is a radical?
A species with an unpaired electron
What do reaction mechanisms show?
Show how reactions occur. Their curly arrows indicate the movement of a pair of electrons. An arrow with half a head indicates the movement of one electron
What are the different types of reaction?
- Addition - 2 reactants join to form one product
- Substitution - an atom/group of atoms is replaced by a different atom/group of atoms
- Elimination - involves the removal of a small molecule from a larger one. 1 reactant forms 2 products
What is the general formula of alkanes?
C(n)H(2n+2)
What are the main uses of alkanes?
Mostly used as fuels as they are the main components of crude oil
Describe the bonding in alkanes
Saturated hydrocarbons - only have single bonds.
Their bonds are sigma bonds - each carbon atom has four
Describe sigma bonds
The end-to-end overlap of two orbitals, 1 from each bonding atom.
The sigma bond has two electrons shared between bonding atoms, positioned on a line directly between them.
Sigma bonds can rotate freely
Describe the shape of alkanes
Each carbon atom is surrounded by 4 others in a tetrahedral arrangement, with a bond angle of 109.5.
Shapes aren’t rigid as sigma bonds act like axes around which atoms can rotate freely
How is crude oil separated and why?
Fractional distillation separates crude oil into fractions.
Possible due to variations in boiling points between components
Explain the effect on chain length of the boiling point of alkanes
As chain length increases, boiling point increases.
London forces act between molecules in close surface contact.
Increased chain length = larger surface area = more surface contact = greater London forces = more energy needed to overcome them
Explain the effect of branching on the boiling points of alkanes
As branching increases, boiling points decrease.
More branches = fewer surface points of contact between molecules = fewer London forces = less energy needed to overcome them.
Branches = molecules are less compacted together = weaker London forces = less energy needed to overcome them
Explain the reactivity of alkanes
Not very reactive because:
- carbon-carbon and carbon-hydrogen bonds are strong
- carbon-carbon bonds are non-polar
- carbon and hydrogen have similar electronegativities so carbon-hydrogen bonds are considered non-polar
What different reactions can alkanes take part in?
- Complete combustion
- Incomplete combustion
- Halogenation
Describe complete combustion in alkanes
Used as fuels because:
- Readily available
- Easy to transport
- Burn in oxygen without releasing toxic products
alkane + oxygen –> carbon dioxide + water
Describe the incomplete combustion of alkanes
In insufficient oxygen supplies, alkanes can form toxic carbon monoxide and sometimes soot (solid carbon).
alkane + oxygen –> carbon monoxide/carbon + water
Describe the halogenation of alkane
Under UV light, alkanes undergo a substitution with halogens, forming a haloalkane and hydrogen halide. This occurs by radical substitution
What are the three steps of radical substitution?
- Initiation
- Propagation
- Termination
Describe the initiation step of radical substitution of methane with bromine
Homolytic fission under UV.
Br2 –> 2Br*
Describe the propagation step of radical substitution of methane with bromine
A chain reaction
CH4 + Br* –> *CH3 + HBr
CH3 + Br2 –> CH3Br + Br
Describe the termination step of radical substitution of methane with bromine
Br* + Br* –> Br2
*CH3 + *CH3 –> C2H6
CH3 + Br –> CH3Br
What are the limitations of radical substitution in organic synthesis?
- Further substitution - can occur until all hydrogen atoms are substituted
- Isomers - will produce a mixture of monosubstituted isomers by substitution at different points on the carbon chain. Time consuming to separate isomers
Describe the general structure of alkenes
Unsaturated hydrocarbons (have at least 1 C=C).
Aliphatic alkenes with 1 double bond have the general formula C(n)H(2n)
Describe the double bond in alkenes
Each carbon atom in the double bond has 3 out of its 4 electrons used in 3 sigma bonds, including 1 to the other carbon in the double bond.
The last electron in a p-orbital for each carbon atom forms a pi bond.
Describe a pi bond
The sideways overlap of 2 p-orbitals, one from each carbon atom of the double bond,
Locks the atoms in position and prevents them from rotating
What is the shape around the double bond of an alkene?
Trigonal planar (angle: 120), as the 3 regions of electron density around each carbon atom repel as far apart as possible
What are stereoisomers?
Have the same molecular formula but different structural formulae (arrangement of atoms in space).
What are the two types of stereoisomer?
- E/Z isomerism (occurs in compounds with a C=C)
- Optical isomerism (occurs in many types of compounds)
Why do E/Z isomers exist?
Rotation around the double bond is restricted, so groups attached to each atom are fixed relative to each other
What conditions must be met for a compound to show E/Z isomerism?
- Must have a C=C bond
- Must have different groups attached to each carbon atom on either side of the double bond
What is cis-trans isomerism?
A special case of E/Z isomerism where 1 of the attached groups on each side of the double bond are the same.
Cis isomers have the same groups on the same side of the double bond (Z)
Trans isomers have the same groups on opposite sides of the double bond (E)
How do you assign E/Z isomers (Cahn-Ingold-Prelog nomenclature)?
- Z isomers have the highest priority groups on the same side of the double bond
- E isomers have the highest priority groups placed diagonally across the bond
To assign priority:
- Highest atomic number of the atom directly attached to C = highest priority
- If they are the same, the group with the higher atomic number at the first point of difference = highest priority
Explain the reactivity of alkenes
Much more reactive than alkanes due to the pi bond.
Because the pi bond is concentrated above and below the plane of the sigma bond (on the outside), pi electrons are more exposed so the bond breaks easily
What are the different types of reactions alkenes can undergo?
Addition reactions:
- Hydrogenation
- Halogenation
- Addition with hydrogen halides
- Hydration
- Polymerisation
Describe the hydrogenation of alkenes
hydrogen + alkene –> alkane
Uses a nickel catalyst
Describe the halogenation of alkenes
halogen + alkene –> (di)haloalkane
Room temperature
Adding bromine water tests for a C=C bond - a positive result turns from orange to colourless
Describe the addition of alkenes with hydrogen halides
alkene + hydrogen halide –> haloalkane
Room temperature
Forms unsymmetrical molecules, so 2 structural isomers
Describe the hydration of alkenes
alkene + steam –> alcohol
Acid catalyst, needs to be steam not water
Forms unsymmetrical molecules, so 2 structural isomers
What is electrophilic addition?
Why does it occur?
The reaction mechanism for addition reactions of alkenes. Involves the breaking of the bond in a small molecule by heterolytic fission
The high electron density of the pi bond in an alkene attracts electrophiles
What is an electrophile?
An atom/group of atoms that is attracted to an electron-rich centre and accepts an electron pair
Describe how you would draw the mechanism of electrophilic addition
First diagram:
- Partial charges on the small molecule
- Curly arrow going from the bond of the small molecule of the partial negative atom
- Curly arrow going from the alkene double bond to the partial positive atom
Second diagram:
- The partial positive atom now joined to a carbon
- Single bond between 2 carbons
- Lone pair shown on the negative ion with negative charge shown
- Curly arrow going from lone pair to (shown positive) carbocation
Last diagram:
- Product shown
Describe how to assign the minor and major products of electrophilic addition of alkenes (Markownikoff’s rule)
- The major product is the one with the halogen joined to the carbon attached to the most carbons because:
- Carbocations can be primary (attached to 1C), secondary (attached to 2Cs) or tertiary (attached to 3Cs)
- Tertiary carbocations are the most stable because alkyl groups push electrons to the carbocation so the +ve charge is more spread out
- In addition reactions, the hydrogen tends to attach to the least stable carbocation
What is a polymer?
How are they named?
Large molecules formed from many repeat units of smaller molecules called monomers
Named after the monomer with the prefix ‘poly’
How do alkenes form polymers?
Addition polymerisation - forms long chains. Usually carried out at high temperate and pressure, using catalysts.
Give some common polymers and their uses
- PVC (poly(vinylchloride) = poly(chloroethene): mostly used in pipes, films and sheeting
- Poly(propene): used to make children’s toys
- Ploy(tetrafluoroethene): used in non-stick pans
Why are polymers used so commonly?
What are the environmental concerns?
Cheap, convenient and readily available
Their unreactivity makes many non-biodegradable and so hard to dispose of
What are we doing to reduce the environmental effects of polymers?
- Recycling
- PVC recycling
- Burning for fuel
- Feedstock recycling
- Bioplastics
- Biodegradable polymers
- Photodegradable polymers
Describe how recycling and PVC recycling is reducing the environmental effects of polymers
Recycling: conserves finite fossil fuels, decreases waste in landfill, polymers must be sorted by type before being made into new products
PVC recycling: disposal + recycling is hazardous due to high chlorine content. Dumping in landfill is unsustainable + burning releases toxic chemicals, so now we use solvents to dissolve the polymer and reuse the PVC
Describe how burning for fuel and feedstock recycling is reducing the environmental effects of polymers
Burning for fuel: waste polymers incinerated to produce heat, generating steam to drive a turbine, producing electricity
Feedstock recycling: reclaims monomers/gases/oils from waste polymers to produce new products. Can handle unsorted + unwashed polymers
Describe how the use of bioplastics and biodegradable polymers is reducing the environmental effects of polymers
Bioplastics: produced from starch/cellulose/plant oils and proteins = a renewable and sustainable alternative to oil-based polymers. Also protects oil reserves
Biodegradable polymers: broken down by microorgansims into biological compounds, Usually bioplastics or contain additives. Compostable polymers degrade fully
Describe how the use of photodegradable polymers is reducing the environmental effect of polymers
Oil-based and contain bonds weakened by absorbing light to start degradation. Sometimes use light-absorbing additives
What is the general formula of alcohols?
C(n)H(2n+1)OH
Describe the uses of some alcohols
Methanol is used as a fuel and chemical feedstock (starting material of producing many compounds)
Ethanol is used in alcoholic drinks, fuel and solvents
Describe the properties of alcohols
Less volatile, have higher melting points and greater water solubility than the corresponding alkanes.
The differences get smaller as carbon chain length increases.
Explain the properties of alcohols compared to alkanes
- Alkanes are non-polar (electronegativity of C + H are similar so they have non-polar bonds), so the intermolecular forces between them are weak London forces
- Alcohols are polar (polar O-H bond), so can also form hydrogen bonds between O-H groups = higher boiling points
- Alcohols form hydrogen bonds with water = more soluble
- As hydrocarbon chain length increases, influence of the -OH group decreases so solubility decreases
Describe how we can classify alcohols
- Primary - the -OH group is attached to a C atom that is attached to only one alkyl group (with methanol as an exception)
- Secondary - the -OH group is attached to a C atom that is attached to 2 alkyl groups
- Tertiary - the -OH group is attached to a C atom that is attached to 3 alkyl groups
Name the different reactions that alcohols undergo
- Combustion
- Oxidation
- Dehydration
- Substitution with hydrogen halides
Describe the combustion of alcohols
Burn completely in a plentiful oxygen supply forming carbon dioxide and water.
An exothermic reaction - increasing chain length = increasing quantity of heat per mole released
Describe the general oxidation of alcohols
Uses an oxidising agent [0] (usually acidified potassium dichromate - K2Cr2O7 with H2SO4).
Turns from orange to green as dichromate ions are turned into chromium ions.
Tertiary alcohols can’t oxidise
Describe the oxidation reactions of primary alcohols
- Distil an alcohol with acidified potassium dichromate –> aldehydes
- Reflux the alcohol/aldehyde with acidified potassium dichromate –> carboxylic acids
Describe the oxidation reaction of secondary alcohols
Reflux the alcohol with acidified potassium dichromate –> ketones
Describe the dehydration reaction of alcohols
A water molecule is removed from the alcohol, forming an alkene
Reflux with a (phosphoric) acid catalyst
Describe the substitution with hydrogen halides of alcohols
alcohol + hydrogen halide –> haloalkane + water
Heated under reflux with sulfuric acid and a sodium halide - the hydrogen halide is formed in situ, e.g
NaBr + H2SO4 –> NaHSO4 + HBr
overall: (alcohol) + NaBr + H2SO4 –> (haloalkane) + NaHSO4 + H2O
What is a haloalkane and how would you name it?
Organic substances that contain a halogen.
To name a haloalkane with multiple halogen molecules, list the halogens along with their carbon number in alphabetical order
How do haloalkanes react?
The carbon-halogen bond is polar (halogen more electronegative) so the electron pair is held closer to the halogen, giving the carbon atom a slight positive charge.
Nucleophiles are attracted to the electron deficient carbon atom, where they donate an electron pair to form a new covalent bond
What is a nucleophile and give some examples of one
A species that donates a lone pair of electrons.
e.g OH-, H2O, NH3
What is the reaction mechanism for a
haloalkane being replaced by a nucleophile?
Nucleophilic substitution
What is hydrolysis?
Water or an aqueous hydroxide solution breaks a bond forming two products (the molecule has split)
Describe how you would draw the mechanism of nucleophilic substitution with a single negative ion
First diagram:
Draw the partial charges on the halogen (-) -carbon (+) bond. An arrow goes from the carbon-halogen bond to the halogen. An arrow goes from the lone pair on the ion to the positive carbon
Second diagram:
The original ion has now replaced the halogen in the bond. Halogen is drawn with the full negative charge
Describe how you would draw the mechanism of nucleophilic substitution with water or ammonia
First diagram:
Haloalkane drawn with partial charges on the halogen(-) - carbon(+) bond. Water drawn with partial charges on all atoms. Arrow goes from halogen-carbon bond to halogen. Arrow goes from lone pair on water/ammonia to positive carbon.
Second diagram:
Ammonia / water has replaced the halogen in the bond. Oxygen / ammonia has a full positive charge and the halogen has a lone pair and full negative charge. Arrow goes from an OH/NH bond to the O/N. Arrow goes from the lone pair to the H.
Third diagram:
OH or NH2 has replaced the halogen in the bond, forming H(halogen)
How does the rate of hydrolysis of haloalkanes change down group 7 and why?
Depends on the carbon-halogen bond strength.
C-X bond enthalpies decrease down the group, so it becomes easier to break the bond, so rate increases.
Fluoroalkanes are unreactive as it takes too much energy to break the bond.
How would you test how the rate of hydrolysis of haloalkanes changes down group 7?
- Add 1-chlorobutane, 1-bromobutane and 1-iodobutane to aqueous silver nitrate in three test tubes
- Time how long it takes for a precipitate to form.
- Ethanol solvent lets water and the haloalkane mix so there is only one layer
How does the position of the halogen atom in a haloalkane change its reactivity and why?
Tertiary haloalkanes react more quickly than primary haloalkanes due to the increased stability of the carbocation
What are organohalogen compounds and what are they used for?
Molecules with at least one halogen atom joined to a carbon chain.
Often used as solvents, polymers, refrigerants and pesticides.
What is the ozone layer and what is it useful for?
The outer edge of the stratosphere.
Ozone absorbs biologically damaging UV-B radiation, protecting from genetic damage and skin cancer.
What is the usual state of ozone in the stratosphere?
Why is it changing?
Ozone is continually broken down and re-formed by UV in the stratosphere.
First, UV breaks oxygen into oxygen radicals:
O2 –> 2O
Then a steady state is set up where the rate of breakdown = the rate of reformation.
O2 + O <–> O3
Human activity (particularly CFC use) has upset the equilibrium
What are CFCs and why are they especially problematic for the ozone layer?
CFCs are chlorofluorocarbons.
Used to be the most common aerosol along with HCFCs.
They are very stable from strong carbon-halogen bonds so they have a long residence time in the atmosphere.
How do CFCs break down in the stratosphere?
UV provides the energy for the homolytic fission of the C-Cl bond, called photodissociation.
CF2Cl2 –> CF2Cl* + Cl*
Describe how CFCs lead to the breakdown of the ozone layer
The Cl* produced from the breakdown of CFCs is very reactive, and breaks ozone down into O2.
Propagation 1:
Cl* + O3 –> ClO* + O2
Propagation 2:
ClO* + O –> Cl* + O2
Overall:
O3 + O –> 2O2
This is a chain reaction
How are nitric oxide radicals formed? Give the equations
NO* radicals are formed through lightning strikes and aircraft travel.
Describe how nitric oxide radicals can break down ozone in the ozone layer
Propagation 1:
NO* + O3 –> NO2* + O2
Propagation 2:
NO2* + O –> NO* + O2
Overall:
O3 + O –> 2O2
The radicals act as a catalyst
What is heating via reflux used for?
Used to heat reactions that would happen too slowly at room temp without boiling off the solvent, reactants or products.
How would you heat a reaction via reflux safely?
The heat source could be a bunsen burner, a water bath for reactions below 100 degrees or a heating mantle for flammable liquids.
Anti-bumping granules are added for smooth boiling.
What is distillation used for and how should the apparatus be set up?
Used to purify the products of a reaction, as the reaction may not complete or could produce by-products.
Joints between the apparatus should be greased so it comes apart easily at the end.
Describe how you would separate an aqueous and organic layer
- Identify which layer is which, then pour the mixture into a separating funnel.
- Close the funnel and invert several times to mix.
- Put a conical flask under the funnel, remove the stopper and open the tap until all the lower layer has drained.
- Collect the other layer in a separate conical flask
Describe how you would remove acid impurities from an organic product
- Add aqueous sodium carbonate to the separating funnel.
- Slowly open the tap, holding the stoppered funnel upside down to release any gas pressure
- Remove the sodium carbonate layer and wash the organic layer with water. Separate again
Describe how you would dry an organic product
Add some anhydrous salt (drying agent, e.g calcium chloride, calcium sulfate, magnesium sulfate) to the mixture until it appears to be a fine powder, then decant the liquid
How would you ensure there is only one organic liquid in your experimental product?
- Clean and dry the original distillation apparatus
- Redistil the solution, set up to only collect the product with the boiling point of the target compound
- This removes any organic liquids with similar boiling points to the target
What is mass spectrometry used for?
Mass spectra help find the molecular mass of an organic compound and information about its structure
How does mass spectrometry work?
- In the spectrometer, the compound loses an electron, becoming a cation (molecular ion).
- The mass spectrometer detects the mass/charge (m/z) ratio, giving the molecular mass.
- On a mass spectrum, the molecular ion (M+) peak is at the highest m/z value, showing molecular mass.
- There will also be a small M+1 peak at one m/z higher due to heavier carbon isotopes
What is fragmentation (in terms of mass spectrometry)?
In the spectrometer, some molecular ions break down into fragments, causing the other peaks on the mass spectrum.
Mass spectra can identify molecules because different structures fragment differently.
How does infrared radiation affect bonds in molecules?
- Bonds possess energy and vibrate around a central point
- Increasing temperature increases vibrations, so absorbing IR radiation either makes the bond stretch/compress or to change bond angle
- Intensity of this depends on the mass of atoms (heavier atoms vibrate slower) and bond strength (stronger bonds vibrate faster)
- Bonds can only absorb radiation with the same frequency as the bond’s natural frequency, measured using wavenumber
What is infrared spectroscopy used for and how?
Used to identify pollutants, and is found in breathalysers.
It does this by helping to identify functional groups.
The IR spectrum plots transmittance against wavenumber, with each peak representing a type of bond.
Below 1500cm^-1 is the fingerprint region - unique peaks that can identify the molecule
