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Organic Chemistry
IUPAC
● International Union of Practical and Applied Chemistry ● Decides on naming conventions for substances. IUPAC naming rules: ● Identify the longest unbroken carbon chain. ● Identify any shorter branches attached to the main chain, such as methyl, ethyl and propyl (CH 3 , C 2 H 5 , C 3 H 7 ). ● Number the carbon atoms where the branch is attached by starting at the end that gives the lowest numbers.
Organic chemistry involves the study of compounds containing carbon. Features of organic compounds: ● Can form strong covalent bonds with each other. ● They can be single, double and triple bonds. ● Can be arranged as straight chains, branched chains or rings. ● Other (groups of) atoms can be placed on the carbon atoms in different positions.
Homologous series: ● A series of compounds similar in structure, in which each member differs from the next by a common repeating unit - e.g., CH 2. Properties: ● Share a general formula. ● Same functional group. ● Similar chemical properties. ● Gradual changes in physical properties.
Types of formula: ● Molecular formula Shows actual number of each type of atom present. E.g., ethanol is C 2 H 6 O. ● (Condensed) structural formula Specifies exactly which atoms are bonded together. E.g., ethanol is CH 3 CH 2 OH ● Displayed formula Shows the bonds between the atoms in a molecule and their relative placing (stick diagram). ● Skeletal formula A shorthand used to show the formulae of larger molecules. Each line is a single covalent bond between carbon and another element, normally carbon. All other elements, except H, are represented by chemical symbols. ● Bare ends of sticks assumed to be CH 3.
Functional groups and isomerism: ● Organic compounds are grouped according to similar structures and chemical properties owing to a common functional group (the reactive part of the molecule) - e.g., -OH in alcohols. ● Branched alkanes contain alkyl groups which are attached to carbons in the main parent chains.
- e.g., methyl CH 3 , ethyl C 2 H 5 , propyl C 3 H 7
- Given the symbol “R” in general formulae.
Structural/Positional isomerism: ● Molecules that share the same molecular formula, but have different structural formulae. ● Structural isomers with different arrangements of the carbon skeleton are called chain isomers (may have different chemical and physical properties). ● Molecules are only isometric if changing one isomer to another involves breaking bonds. ● Can arise from placing a functional group in a different position (e.g., pent-1-ene Vs pent-2-ene), or if they have different functional groups (e.g., C 6 H 12 can be hexane or cyclohexane). Geometric isomerism: ● Sigma bonds are stronger than pi bonds. This is due to orbitals in sigma bonds overlapping more than orbitals in pi bonds. ● Geometric isomers occur in molecules where there is a double bond, where rotation is restricted around it. This locks the positions of groups around each carbon in the double bond pair. ● Z(usammen): Larger groups on the same side (above/below) of the molecule. E(ntdecken): Larger groups on opposite sides (above + below) of the molecule. ● Requirements for geometric isomerism:
- Restricted rotation (C=C double bond)
- Two different groups on the left carbon.
- Two different groups on the right carbon. ● CIP rules:
- Rank the atoms attached directly to each carbon atoms in the double bond based on the largest atomic number.
- If the first are identical, rank the 2nd, 3rd, 4th etc until a difference is found.
- Identify this as the largest group connected to this carbon.
Halo(gen)alkanes: ● Alkanes where one or more of the hydrogen atoms are replaced by halogen atoms. ● Fluoro, chloro, bromo, iodo are used as prefixes to the names of molecules. ● Position in chain indicated. ● Listed in alphabetical order. ● The carbon with the greatest net mass of atoms which are connected to it takes priority (lower number). ● e.g., 4-bromo-4-chloro-3-fluoropentane
Fractional distillation:
Fuel gases 25 oC C 1 - C 4
Gasoline 40 C 4 - C 12
Naphtha 110 C 7 - C 14
Kerosene 200 C 11 - C 15
Diesel 300 C 15 - C 19
Residue 350 >C 20 ● Crude oil (aka petroleum) is a mixture of hundreds of different hydrocarbons, most of which are alkanes.
● When fossil fuels are burned, sulfur compounds also burn to form sulfur oxides: S + O 2 → SO 2 ● Sulfur dioxide is an acidic gas which dissolves in rainwater, reacting with water and oxygen to form very dilute sulfuric acid (SO 2 + H 2 O + 0.5O 2 → H 2 SO 4 ), causing acid rain. ● SO 2 is also a harmful gas, as it is a lung irritant which can increase respiratory problems. ● Acid rain can:
- Cause plant damage.
- Kill certain life forms.
- Increase weathering corrosion rates of stone and limestone structures.
Nitrogen oxides, NOx: ● NO is formed in car engines, which reacts with oxygen to become NO 2. Can form acid rain in the same way as SO 2 , but with nitric acid instead. ● N 2 + O 2 → 2NO 2NO + O 2 → 2NO 2 4NO 2 + O 2 + H 2 O → 4HNO 3 ● Involved in the chemistry of photochemical smog.
- Produces chemicals: harmful to respiration; irritating to eyes and lungs; causing of headaches and fatigue.
Carbon soot, C: ● Fine black powder dust. A product of incomplete combustion. ● Harmful when absorbed into fine tissue of linings of the nose and lungs. ● Causes coughing, sore throats, sneezing and respiratory issues. ● Coarse solid particles (10 microns in diameter) can cause more irritation and even lung damage. ● They are carriers of ‘polycyclic aromatic hydrocarbons (PAHs), which are carcinogenic molecules of formula CxHy (where x > 6 and y > 6).
- Unburned hydrocarbons, which reduce air quality.
- These fine particles of soot are called particulates - believed to cause a phenomenon called ‘global dimming’ (scattering of sunlight).
Flue gas desulfurization: ● The processing of fuels to remove sulfur compounds, preventing SO 2 formation. ● Costly and still fallible. Acid gas scrubbers: ● Alkaline slurry of Ca(OH) 2 and H 2 O is sprayed into the flue gases from the power station, In a neutralisation reaction, harmful calcium sulfite is formed, which is readily oxidised to form calcium sulfate - a useful commercial product which is crystallised out. It’s used to plaster board. ● Process: Ca(OH) 2 + SO 2 → CaSO 3 + H 2 O CaSO 3 + 0.5O 2 → CaSO 4 ● Dry scrubbing: CaCO 3 + SO 2 → CaSO 3 + CO 2 CaSO 3 + 0.5O 2 → CaSO 4
Catalytic converters: ● Use of metals like platinum, rhodium and palladium on the catalyst bed. ● Catalyst beds designed to maximise the surface area over which car exhaust gases pass. To do this, it has a honeycomb structure. ● Pt/Rh: 2NO 2 → N 2 + 2O 2
● Pt/Pa: 2CO + O 2 → 2CO 2 ● Pt/Pa: C 7 H 16 + 11O 2 → 7CO 2 + 8H 2 O
Alkanes lack functional groups, so their chemistry is limited to combustion and substitution reactions. Radicals: ● The only species reactive enough to overcome the high Ea required to break C-C and C-H bonds. ● Free radicals are (groups of) atoms with a single unpaired electron, produced from the homolytic fission of a bond. Homolytic fission: ● The split of a molecule into atoms, each containing the same number off unpaired electrons from a covalent bond. ● A : B → .A + .B ● Single headed curved arrow (→ but curved and without one of the lines at the tip) = One electron passed. ● Double headed curved arrow (→ but curved) = Two electrons passed.
Free radical substitution: ● A 3 stage chain reaction, with each stage having one or more reactions, in which the hydrogen of an alkane is substituted with a halogen atom, resulting in the formation of a haloalkane. ● The more halogen, the more the alkanes are substituted (e.g., from CH 4 to CHCl 3 or CCl 4 ). ● The less halogen, the less they are substituted (e.g., from CH 4 to just CH 3 Cl). Initiation: ● A diatomic halogen molecule, with the help of UV radiation, gets split into two free radicals. ● e.g., Cl 2 → 2.Cl Propagation: ● Removes a hydrogen to form HR and a free radical of the alkane. ● The alkane free radical reacts with another diatomic halogen molecule to form CnH2n-mRm and another halogen radical to restart from the first propagation step or proceed to termination. ● e.g., CH 4 + .Cl → .CH 3 + HCl .CH 3 + Cl 2 → CH 3 Cl + .Cl
Termination: ● Radicals react to end the chain reaction. ● e.g., 2.Cl → Cl 2 2 .CH 3 → C 2 H 6 .CH 3 + .Cl → CH 3 Cl
There are 3 types of UV that enter the Earth: UVA (tans), UVB and UVC (both harmful). Ozone intercepts UVB and UVC, which is good, because both are very harmful to organisms. ● UVB can cause: melanoma and non-melanoma skin cancer; eye disorders and cataracts; suppression of the immune system; shrinking of plants’ leaves; reduced yield of crops; changing of the chemical composition of plants, reducing quality and nutritional value; etc. ● More surface UV can: increase concentration of hydrogen peroxide; increase O 3 on the surface, causing respiratory issues. Ozone: ● The Earth’s stratospheric ozone layer makes the planet habitable by absorbing harmful solar UV radiation before it reaches the planet’s surface. ● Ozone is produced continually in the upper stratosphere where UV radiation dissociates O 2 to 2 .O.
Nucleophilic substitution with water = hydrolysis of haloalkanes: ● Heat under reflux with water. ● Normal substitution as above for step 1. ● For step 2, draw an arrow from the bond between the oxygen and one hydrogen atom. ● Step 3 shows an alkane with OH in the place of one of its hydrogens, with the second product being HR (where R is the halogen).
Nucleophilic substitution with cyanide: ● Heat under reflux with an ethanolic solution of NaCN or KCN. ● Halogen replaced with -CN. Very useful for increasing the size of the carbon chain. ● The nitrile can be hydrolysed to form a carboxylic acid by heating under reflux with a strong acid such as HCl, producing the carboxylic acid and ammonium.
Nucleophilic substitution with ammonia: ● Heat in a sealed tube with excess ammonia dissolved in ethanol. ● Poor yield, and a mixture of products can be made. Secondary and tertiary amines can also form, as the produced amine is also a nucleophile. ● The yield can be maximised by using excess ammonia to ensure the main product is the primary amine.
R-NH 2 Primary amine
R-NH Secondary amine
R-N Tertiary amine
R-N+^ Quaternary ammonium salt
Primary haloalkane (Process: SN2 due to 2 reactants in step 1):
- Lone pair in nucleophile attracted to δ+^ charge on carbon.
- The shared pair of electrons between the carbon and halogen is repelled towards the halogen.
- Movement continues until the halogen is expelled as a negatively charged ion, and the nucleophile takes its place. ● This happens on the side away from the halogen, as the halogens δ-^ charge naturally repels the nucleophile. Tertiary haloalkane (Process: SN1 due to 1 reactant in step 1): ● Cannot attack from the side away from the halogen, as that side is cluttered with alkyl groups. ● Two stage process:
- A small proportion of the haloalkane ionises, giving a carbonation and a halide ion.
- The nucleophile instantly reacts with the carbocation to form the completed molecule. ● This is best for tertiary haloalkanes, as the carbocations have more alkyl groups surrounding them to spread the positive charge. ● In primary and secondary haloalkanes, carbocations are too unstable due to having fewer alkyl groups. Secondary haloalkane: ● Can use either SN2 or SN1, but SN2 is preferred, as carbocations formed for SN1 are still quite unstable.
The rate of nucleophilic substitution depends on the haloalkane that is present. ● C-Cl strongest, then C-Br, then C-I. ● The strength of C-F bonds makes fluoroalkanes unreactive and of little use in organic chemistry.
In elimination reactions, the OH-^ ion acts as a base, removing a proton from the C atom neighbouring a C-Br (etc) bond. The result is production of an alkene.
- The OH-^ anion removes a proton (H+) from a beta carbon.
- The pair of electrons from the C-H bond moves to form a double bond between the two carbon atoms.
- This causes the electron pair in C-Br to be repelled onto the Br until the bond breaks, releasing a Br-^ ion.
Nucleophilic substitution Electron pair donors
● Aqueous OH-^ ions in reflux solvent form more alcohol, as the hydroxide ion acts like a nucleophile. ● Requires a dilute, aqueous base at moderate temperatures in water. ● Preferred in primary haloalkanes.
Elimination Proton (H+) acceptors
● The ethanol in reflux solvent forms more alkene, as the hydroxide acts as a base. ● Requires a strong, concentrated base at high temperatures in ethanol. ● Preferred in tertiary haloalkanes
In the case of unsymmetrical haloalkanes, there are different possible alkene products depending on the hydrogen removed due to there being more than one beta carbon.
Steric effect: ● Nucleophiles must be able to get close to a carbon atom to attract it. ● Bulky groups will hinder the approach of a nucleophile, but will have little effect on a base attacking an unhindered proton.
Electrophilic addition: ● An electrophile is a species that is attracted to electron-rich regions in other atoms/ions. ● They either carry a partial or full negative charge. ● The double bond in alkenes is an area of high electron density, so will attract electrophiles such as HBr or H 2 SO 4. In an addition reaction, the pi bond breaks, with the pair of electrons then being used to combine other atoms/groups onto the alkene. This creates a saturated compound. The high electron density in a C=C double bond induces a dipole in Br 2 , Cl 2 , etc, which enables the molecule to split and the bromine to substitute a hydrogen.
Polymers: ● A molecule made up of smaller molecules called monomers. ● Addition polymers typically result when the pi electrons in substituted alkene monomers form C-C sigma bonds between the molecules. ● Poly(ethene) is the most simple polymer.
Atactic PP ● CH 3 groups are randomly oriented. ● Lack of regularity makes it impossible for molecules to closely pack. ● Waste product from production of isotactic PP.
● Road paint, roofing felt, sealants, adhesives.
Syndiotactic PP ● Every alternate CH 3 is oriented the same way. ● Chains pack closely, but not as much as in isotactic PP. Thus, it is softer.
● Plastic film, medical tubing, medical bags, medical pouches.
Poly(chloroethene) ● Mainly atactic chains (random Cl orientation). ● Mainly amorphous, with only small areas of crystallinity. ● Pure, so hard and rigid. ● Pd-pd IMFs due to C-Cl being polar. ● Plasticisers added to PVC to reduce the effectiveness of these pd-pd interactions. Makes it more flexible.
● Guttering, plastic windows, electrical cable insulation, sheet material (for flooring), footwear, clothing, etc.
● Production: Polymerising chloroethene.
Poly(tetrafluoroethene) ● Mainly straight chains, so chains tend to pack well, making PTFE fairly crystalline. ● Large mp (327oC). ● Very resistant to chemical attack.
● Non-stick coating, coating of vessels (in food and chemical industries).
● Production: Polymerising tetrafluoroethene.
Alcohols can have positional isomers - i.e., the -OH group can be on different carbons. This can cause different classes of alcohol, of the same logic as haloalkanes: ● Primary alcohol (e.g., propan-1-ol) ● Secondary alcohol (e.g., propan-2-ol) ● Tertiary alcohol (e.g., 2-methylpropan-2-ol) When there are multiple -OH groups in a molecule, when naming it, you MUST keep the “e” in the alkane/alkene parent name. ● E.g., propan e -1,2,3-triol - NOT propan-1,2,3-triol
Alcohols can be oxidised using oxidising agents such as acidified potassium dichromate (VI), which contains Cr 2 O 7 2-^ ions. A colour change from orange → green occurs when the oxidising agent is reduced to Cr3+. ● Primary and secondary alcohols can be oxidised. ● Tertiary alcohols won’t be oxidised, as the oxidation of the -OH group of an alcohol involves the removal of a hydrogen from a) the carbon attached to the -OH group and b) the -OH group. Since the carbon attached to the -OH group doesn’t have any hydrogens attached to it, it won’t oxidise.
Primary alcohol → Aldehyde (-al) → Carboxylic acid (-oic acid) Secondary alcohol → Ketone (-one)
To favour aldehyde production with a 1o^ alcohol: ● React at rtp. ● … or heat, then distill the product to prevent further oxidation. Aldehydes have a lower bp than alcohol due to having no hydrogen bonds. To favour carboxylic acid production with a 1o^ alcohol: ● Heat under reflux. To test for a carboxylic acid, add sodium hydrogen carbonate. Observe for fizzing and effervescence.
Test for aldehyde Method
Tollen’s reagent 1. Add 5 drops of sodium hydroxide solution to 2cm^3 of silver nitrate solution in a test tube.
- Add enough dilute ammonia solution to dissolve the brown precipitate completely.
- Add 10 drops of the unknown substance to the tube and gently warm in a beaker of hot water.
- A positive test shows a silver mirror in the tube.
Fehling’s solution 1. Add 10cm^3 of Fehling’s solution to a test tube, then add 10 drops of the unknown substance to the tube.
- Gently warm in a beaker of hot water.
- A positive test shows the production of a brick-red precipitate.
Ethanol can be dehydrated to ethene by heating with excess concentrated H 2 SO 4 at temperatures of around 170oC. This is an elimination reaction, as water is removed. ● This process allows ethanol to be the starting point for the manufacture of many plastics, as an alternative to oil. This is because ethanol is not finite.
The use of bioethanol as a fuel is considered by some to be carbon neutral. However, you need to factor in emissions in transport, harvesting, milling, grinding and distilling.
