Organic Chemistry - Basics
Functional Group - Atoms that give the compound it's characteristics
Homologous Series - Same functional group different length hydrocarbon chain, examples; Alcohols, Alkanes, Alkenes, Halogenoalkanes, Aldehydes, Ketones, Carboxylic acid, Esters.
Saturated hydrocarbons - compounds that contain only carbon and hydrogen and have single covalent bonds, general formula: CnH2n+2
Structural Isomers: same molecular formula but different structural formula
Chain isomers: skeletal formulae are different branched in different ways
Positional isomers: functional group attached to a different carbon atom
Stereoisomers/EZ isomerism: same molecular formula but different arrangement in space. E=trans / Z=cis
Fractional distillation: separation by boiling point.
Fractionating column, larger hydrocarbon chains run to residue at the bottom as their boiling points are too high to be vapourised. These end up as fuels and petrochemicals. Kerosene - Aeroplane fuel.
Cracking (Ceramic) : breaking long chain alkanes into smaller alkanes
Catalytic cracking cuts costs producing large amounts of branched hydrocarbons useful for making petrol
Isomerisation(Platinum with inert aluminium oxide): changing straight-chain alkanes to branched-chain alkanes
Reforming(Platinum): changes straight-chain alkanes to cyclic alkanes
Oxidising alkanes by combustion:
CH4(g)+2O2(g) → CO2(g)+2H2O(g) - Complete Combustion
Smaller molecules are more volatile so they burn more easily, but larger molecules are more exothermic as more bonds broken. Alkanes are good fuels; propane for central heating/cooking, butane for camping gas.
CH4(g)+1.5O2(g) → CO(g)+2H2O(g) - Incomplete Combustion
In a lack of oxygen carbon monoxide is formed instead of carbon dioxide. CO is extremely poisonous, they bind with the haemoglobin in the red blood cells better than oxygen. This will lead to oxygen deprivation thus headaches and nausea.
Percentage Yield: Actual over Theoretical x100
Atom economy: Mr of useful product/sum Mr of all products
Heterolytic bond fission: breaking covalent bond to form 2 different substances
Homolytic bond fission: breaking covalent bond to form 2 free radicals
Electrophilic addition of alkenes (Nickel Catalyst / 150℃)
C2H4 + H2 → C2H6 -
Margerine(S) is made by hydrogenating vegetable oil (L)
C2H4 + Br2 → C2H4Br2 -
This reaction can be used to test the presence of double bonds. Adding orange bromine water will decolourise the solution. Mechanism: double bond repels the electrons in Br2, polarising it (δ+ and δ-). Heterolytic fission of Br occurs, one of them (Br+) attaching itself to the C atom. Br- then attaches to the carbocation formed
Adding hydrogen halides can cause 2 different products to be formed
Free Radical Substituition
Halogenoalkanes from Halogens and Alkanes
CH4 + Cl2 → CH3Cl + HCl
1. INITIATION - UV light forms Cl radicals - Cl2 → 2Cl・
2. PROPAGATION - Cl・+ CH4 → CH3・ + HCl
Repeated until no Cl2/CH4 CH3・+ Cl2 → CH3Cl + Cl・
e.g. CH3・+CH3・→ C2H6
Problems: Yield of CH3Cl isn't high as a mixture of products formed.
4. Further Substituition CH3CL → CH2Cl2
CH2Cl2 → CHCl3
CHCl3 → CCl4
Alkenes - Unsaturated hydrocarbons, has one or more double bonds CnH2n. Double bonds are reactive, they can open to join with other atoms. A double bond is made up of: σ - sigma bond - 2 s-orbitals overlap in a straight line - there's high electron density between them, which attracts electrophiles π - pi bond - 2 p-orbitals overlap. There are two parts to it - one above and one below the molecule, as the orbitals are dumb-bell shaped.Double bonds cannot rotate due to this, causing cis/trans isomerism.
Addition polymerisation: double bonds in alkenes open up and join together to form long chain polymers ethene becomes poly(ethene)
Poly(chloroethene), also known as PVC, has a wide range of uses such as insulation on wires and water pipes
Poly(tetrafluoroethene), also known as PTFE, is useful on frying pans as they have non-stick properties
Hydroxyl Group -OH - Primary alcohols have 1 alkyl group attached to the OH carbon. Secondary alcohols have 2 alkyl groups, tertiary have 3. Small alcohols are miscible with water, as hydrogen bonds form between them (hydroxyl group of the alcohol is polar and so are the water molecules). Polar nature decreases as the size of molecule increases. Uses of ethanol: alcoholic drinks, methylated spirits (industrial solvent with purple dye, making it undrinkable), bioethanol
How they are made
1. Hydration of ethene (Hot Phosphoric Acid / 300℃ / 60atm
C2H4(g) + H2O(g) ⇔ C2H5OH(g)
Dehydration reverse reaction: phosphoric acid catalyst, 170℃
2. Fermentation of glucose (30-40℃ / Anaerobic Conditions / Yeist)
C6H12O6(aq) → 2C2H5OH(aq) + 2CO2(g) -
This is an exothermic reaction. Yiest dies when over 15% ethanol. Fractional distillation increases concentration of ethanol.
Esterification (Conc Sulfuric Acid)
alcohol + carboxylic acid ⇔ ester + water
e.g. ethanol + ethanoic acid ⇔ ethylmethanoate + water
Stable due to strong halogen-carbon bonds, volatile, non-flammable and non-toxic – CFCs found in aerosol cans, fridges, and air conditioning, until we found out that they were destructing the ozone layer. Alternatives: HCFCs (hydrochlorofluorocarbons), HFCs (hydrofluorocarbons) – these are temporary alternatives to CFCs. Less/no chlorine means less ozone depletion. They are eventually broken down in the atmosphere. However, these are much worse than CO2 molecules in terms of enhancing of the greenhouse effect. Nowadays aerosols have pump spray systems and ammonia is used in fridges, and CO2 is used to make foamed polymers instead of CFCs.
Oxidation of Alcohols
Oxidation of alcohol (Acidified Potassium Dichromate)
Combustion oxidises alcohols - they can be distilled/refluxed using an oxidising agent which turns from orange to green
PRIMARY ALCOHOLS distilled with oxidising agent producing ALDEHYDEs
ethanol + [O] → ethanal + water C2H5OH + [O] → CH3COH + H2O
To prevent further oxidation (forming carboxylic acid), removal of the aldehyde out of the oxidising solution is needed as soon as it is formed.
CARBOXYLIC ACID - REFLUX (continual heating) so that it is vigorously oxidised Vapourised compounds are condensed back into the reaction mixture with a vertical condenser.
ethanal + [O] → ethanoic acid CH3COH + [O] → CH3COOH
SECONDARY ALCOHOLS are oxidised to KETONES when refluxed with the oxidising agent.
propan-2-ol + [O] → propanone + water
CH3CH(OH)CH3 + [O] → CH3C=OCH3 + H2O
Ketones cannot be oxidised further by refluxing. Also, TERTIARY ALCOHOLS are not easily oxidised with oxidising agents (the only way is to burn).
Infrared spectroscopy – identifying molecules - Infrared radiation is passed through chemical sample. Different covalent bonds absorb different frequencies of this radiation. The graph shows ‘peaks’ at specific frequencies to show which bond is present.
Uses: breathalysing a driver will show the amount of ethanol vapour in their breath. CO absorbs a certain frequency of infrared radiation so it can be passed through sample air to identify CO gas.
Hydrolysis of Halogenoalkanes
Hydrolysis of halogenoalkanes - Nucleophilic Substitution
Halogens are more electronegative than carbon, thus carbon-halogen is polar. This means that C (δ+) easily attracts the nucleophiles (electron pair donator) e.g. OH- Bromomethane can be hydrolysed to ethanol:
C2H5Br + OH- → C2H5OH + Br-
Bond enthalpy decreases down the halogen group so iodoalkanes are hydrolysed the fastest (weaker bonds can be broken more easily).
OH- comes from warm aqueous potassium hydroxide (KOH) or sodium hydroxide (NaOH) and this is done under reflux.
Mechanism: OH- is attracted to the δ+ in C of bromomethane. Heterolytic fission of C-Br – electron pair is taken by Br. OH- attaches to the carbocation as Br- falls off.
C2H5Br + H2O → C2H5OH + H3O+ + Br-
Mixing halogenoalkanes with water forms alcohol and aqueous halide (here, H2O is the nucleophile). You can compare the reaction rates of different halogenoalkanes by adding silver nitrate (AgNO3) solution: Ag+ + Br- → AgBr (silver halide) Iodoalkane - White precipitate Bromoalkane - Cream precipitate Chloroalkane - Yellow precipitate
Mass spectrometry - Uses: Probes to Mars have carried mass spectrometers to identify molecules on the planet. Level of pollutants in the air can be studied.
Mass spectrum gives mass/charge on the x-axis and the abundance on the y-axis. Molecules are bombarded with electrons to produce + ions of different fragments, and the bar represents molecular mass of the fragment.
Useful in differentiating between molecules that have the same molecular formula such as propanone and propanal, however propanal will have a C2H5+ fragment.
Electrophilic Addition - Alkene + HBr ----------------> Halogenoalkane
Nucleophilic Substitution - Halogenoalkane -------> Alcohol
Free Radical Substitution - Alkane + Chlorine -----> Chloroalkane
Elimination - Alcohol + H2SO4 -------------------------> Alkene
Oxidation- Primary Alcohol + K2Cr2O7 + H2SO4 --->Aldehyde or Carboxylic Acid
Hydration(300'C / 60 atm / Hot Phosphoric Acid): Ethene + Steam --> Ethanol
Halogenation OR Electrophilic Addition: Ethene + Bromine -->1,2-dibromoethane
Addition of a hydrogen Halide: Ethene + Hydrogen Bromide --> 1-bromoethane
Hydrogenation(150*C / Nickel Catalyst): Ethene + Hydrogen --> Ethane
Formation of an alcohol by hydrolysis of a halogenoalkane: Nucleophilic substitution / Reflux
1-chloropropane --> propan-1-ol
(need to know mechanism ^)
Combustion of an alcohol: CnH2n+1+(1.5n)O2 --> nCO2 + (n+1)H2O
Hydration of ethene(300*C / 60atm / Phosphoric Acid):
Ethene + Steam --> Ethanol
Fermentation(yeast / 37*C): Glucose --> Ethanol + Carbon dioxide
Oxidation of alcohols(H2SO4 / K2CR2O7) : Primary alcohol: propan-1-ol
Distillation: Propan-1-ol + [O] --> Propanal (aldehyde)
Reflux: Propan-1-ol + 2[O] --> Propanoic acid (carboxylic acid)
Secondary alcohol: butan-2-ol
Butan-2-ol + [O] --> Butanone (ketone)
Tertiary alcohols do not oxidise
Dehydration of an alcohol(Reflux / H2SO4): Ethanol --> Ethene + water