The Kekule structure of benzene is wrong because:
- Bond length between carbon atoms are all constant and it's somewhere between the length of a c-c bond and a c=c bond (c-c longer than c=c)
- Enthalpy change of hydrogenation is less than expected => the structure is more stable than initially expected
- Does not undergo typical alkene reactions => less reactive
These traits arise due to benzene's structure which is due to multiple p-orbitals overlapping forming a delocalised pi system of electrons (in this case, it contains 6 electrons as it has 6 p-orbitals).
- Low reactivity is due to the spreading out of the electrons which decreases the electron density of the bonds and therefore reduces it's reactivity with electrophiles as they are less strongly attracted.
- The lower enthalpy change of hydrogenation of benzene compared with the kekule structure is due to the greater stability of the delocalised pi system compared to the localised pi bonds (which are more electron dense), the greater the stability of the pi-eletrons, the lower the energy they have.
Reactions with arenes occur via electrophilic substitution, this maintains the pi-electron system maintaining the stability of the molecule. Compared to alkene reactions, arenes react much more slowly and have a higher activation energy hence catalysts are quite often needed.
When naming molecules containing a benzene ring that isn't the main functional group, the prefix 'PHENYL' is applied to indicate the prescence of the ring, normal naming conventions follow.
- NITRATION: Nitration occurs when an arene is reacted with: - concentrated H2SO4 and HNO3 (the prescence of H2SO4 is needed to produce nitronium ion: NO2+, this acts as the electrophile in the reaction) 2H2SO4 + HNO3 --> NO2+ + 2HSO4- + H30+ - at a 50-60 degrees c temperature The nitro-group can be substituted into both the 1 and 3 groups
- HALOGENATION: Conditions: - in the dark - room temperature - halogen carrier catalyst (AlBr3/FeBr3) or Iron which then forms the halogen carrier in situ. Observations: - HBr fumes are steamy and acidic - Orange Br2 is decolourised Comparison with reaction of Br2 with cyclohexene: - Benzene reacts less easily as it is less electron dense due to the pi-system of electrons as opposed to localised pi-bonds therefore this polarises the bromine molecule less which is why a catalyst is needed, it polarises the bromine so that it can then be substituted into the benzene ring. - Higher E_a with benzene due to lesser polarising effect of benzene as opposed to cyclohexene.
Phenol is Benzene with an -OH group, it is also a set of compounds containing a phenol group.
- Not very soluble in water despite the fact it can form hydrogen bonds, the large non-polar hydrocarbon part of the molecule is hydrophobic.
- When disolved in water, it will partially dissociate acting as a weak acid forming the PHENOXIDE ion and H+ ions.
- Phenol is more acidic than water an ethanol because of the delocalisation of the oxygen atom in the -OH group this allows the hydrogen atom to dissociate more easily.
- Phenol is more reactive than benzene due to the oxygen in the -OH group, the oxygen has a p-orbital that overlaps with the delocalised pi-system of electrons of benzene, the lone pair from the p-orbital becomes delocalised into the pi-system of electrons of benzene increasing the electron density.
- Phenol is very soluble in a solution of NaOH as it forms the phenoxide ion which is ionic and therefore very soluble. C6H5OH + NaOH --> C6H5-Na+ + H2O
- Phenol reacts with group 1 metals producing hydrogen gas and the phenoxide ion. C6H5OH + Na --> 1/2H2 + C6H5-Na+
- Phenol can react with bromine where it is substituted into the 2/4/6 positions producing HBr in the process
- Conditions: in aqueous solution C6H5OH + 3Br2 --> C6H5OHBr3 + 3HBr
- Forms a white precipitate of 2,4,6-tribromophenol
- Phenols are used in paints so that they harden in time, this is due to a reaction between phenols and aldehydes
- Used to make plastics, like bakelite, bis-phenol-A is another phenol based plastic, it sparked controversy due to it's possible toxicity and use in food packaging
- Antiseptics such as TCP (Trichlorophenol) are phenol based. Phenol was initially used neat as a antiseptic but it proved to be corrosive.
Carbonyl compopunds are characterised by having a C=O group, however carboxylic acids are excluded.
- Primary Alcohol + [O] --> Aldehyde + Water
- Secondary Alcohol + [O] --> Keytone + Water
- [O], the oxidising agent is often pottasium dichromate which is ORANGE then upon oxidising a compound becomes GREEN due to the Cr3+ ion.
- PERMANENT DIPOLE, which means permanent dipole interactions are present and hence the boiling point of these compounds is greater than that of their cousin alkanes
- 2,4-dinitrophenylhydrazine forms a orange precipitate upon reacting with a carbonyl compounds, the product of this can be purified by recrystallisation and then the melting point measured to identify the particular compound
- REDUCTION can be acheived by reaction with SODIUM TETRAHYDRIDOBORATE: NaBH4, forming a secondary alcohol from a ketone and a primary alcohol from an aldehyde. This process is a form of NUCLEOPHILIC ADDITION as the C atom in the C=O group is paritally positively charged, it is attracted to negatively charged molecules, NaBH4 produces hydride ions: H-, these form a bond with the carbonyl compound producing an intermediate of the from R-CH2O- which is then forms R-CH2OH when it reacts with a water molecule.
- Tollens reagent can be used to distinguish aldehydes from ketones, it is a mixture of SILVER NITRATE in AQUEOUS AMMONIA, it acts as an oxidising agent and the aldehyde is oxidised to a carboxylic acid thereby reducing Ag+ ions to Ag which then forms a silver mirror, obviously a ketone can't do this as it cannot be oxidised further.
- Because carbonyl groups are polar where the carbon takes on a delta + charge, they are unreactive with electrophiles however they are reactive with nucleophiles. Sodium Hydridoborate
- (D) Aliphatic - Non-aromatic compounds
Boiling points of carboxylic acids are greater than that of alcohols as the C=O group makes the O-H bond even more polar, because the molecule has 2 oxygen groups it can form 2 hydrogen bonds with adjacent carboxylic acids forming a DIMER. This means that the surface area of the 2 molecules is greater than what it would be if they weren't bonded => Van de Waal forces are greater. Short chain carboxylic acids are very soluble in water due to their ability to form hydrogen bonds, as chain length increases they become progressively more insoluble due to long non-polar hydrocarbon tails.
Carboxylic acids react like any of acid does, here are the general acid reactions:
- Acid + Base --> Salt + Water
- Acid + Carbonate --> Salt, CO2 + H20
- Acid + Metal --> Salt, H2
- Acid + Ammonia --> Ammonium salt (contains a ammonium ion, NH4+)
Carboxylic acids can be prepared either from an aldehyde or a ketone:
- Aldehyde + [O] --> Carboxylic acid
- Primary Alcohol + 2[O] --> Carboxylic acid + Water
Esters are formed from a condensation reaction between a carboxylic acid and an alcohol, their names are derived in the following way:
- The prefix is determined by the alcohol, for example ethanol gives ethyl
- The carboxylic acid determines the name of the actual ester group, for example propanoic acid gives propanoate
- Combine the 2 names and add any other groups onto the name as a prefix, in the example: ethyl propanoate
- The ester group is -COOC- in displayed formula
Esters smell sweet and are often used in cosmetics and flavourings.
- Carboxylic acid + Alcohol --> Ester + Water (Conditions: Concentrated sulphuric acid catalyst and heat, reflux is usually employed as well)
Acid anyhydrides are similar to esters they have the form R-OCOCO-R where the 2 oxygens on the outside are bonded via 2 covalent bonds to the carbon. They are named by taking the name of the carboxylic acid used to form then, remove 'acid' replacing it with 'anhydride' for example Ethanoic acid would produce Ethanoic anhydride.
- Acid Anhydride + Alcohol --> Ester + Carboxylic acid (conditions: gentle heat)
Esters can be broken down by the addition of water, this is known as HYDROLYSIS. It will occur spontaneously but very slowly so an aqueous acid or aqueous base is added to speed up the rate of reaction.
Base-catalysed hydrolysis of an ester: (Conditions: Reflux with aqueous NaOH)
- Methyl Propanoate + OH- --> Methanol + Propanoate ion This is a good reaction as it is irreversible therefore has a very high yield, the propanoate ion can be converted to propanoic acid with the addition of an aqueous acid.
Acid-catalysed hydrolysis of an ester: (Conditions: Reflux with aqueous HCl)
- Methyl Propanoate + H2O --> Methanol + Propanoic acid
Triglycerides are simply a type of ester, they are formed from glycerol which is Propan-1,2,3-triol, each -OH group forms an ester link with a carboxylic acid, forming a triglyceride. Fatty acids refer to carboxylic acids that react with glycerol to form triglycerides.
- A fatty acid is said to be unsaturated if it has 1 double bond, it is said to be polyunsaturated if it has more than 1 double bond
- A fatty acid is said to be saturated if it has no double bonds
For carboxylic acids: trans,cis,20,2(3,4) is read as following:
- the first number in the sequence, in this case: 20, is the number of carbons.
- the second number tells you how many double bonds are present and the first words indicate whether they are cis/trans
- finally, the numbers in brackets indicate where the double bonds are present
trans,cis,18,2(2,6) would be named in IUPAC nomenclature as: trans,cis-2,6-octadecadienoic acid (dien implies two double bonds)
- Saturated fats than non-saturated have higher boiling points due to C=C double bonds making fatty acids less flexible and therefore less able to pack closely together hence they are usually liquids.
Biodiesel can be formed from vegetable oils, it is carbon neutral as plants take in CO2 when they live and then it is released during combustion as biodiesel, this helps combat climate change
Amines are derivatives of ammonia, the hydrogens in ammonia have been replaced, one at a time:
- 1 H replaced: Primary amine, 2 Hs replaced: Secondary amine, 3 Hs replaced: Tertiary amine.
- They smell fishy
- Higher boiling points that similar alkanes due their ability to form hydrogen bonds with adjacent (amine) molecules.
- Soluble in water due to ability to form hydrogen bonds.
- Act as organic bases due to the N atom which has a lone pair of electrons which can be donated.
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Preparation of aliphatic amines: Ethanolic ammonia + Halogenoalkane --> Amine + HX (where X is the halogen) (very wrong, acids and bases can't exist in the same solution at room temp, the amine would act as a base forming the salt of the amine and HX).
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Preparation of aromatic amines: Nitrobenzene heated with tin and conc. HCl reduces nitrobenzene forming an aromatic amine and water.o
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Azodyes:
R-NH2, the R group gives the prefix of the name and NH2 => suffix = amine
- For Secondary amines, the longer group is given precedence and the other group forms the prefix, where N-(prefix name) is used, N denotes that the other group is attached to the nitrogen
- For Tertairy amines, the above naming scheme is also employed.
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Amines are weak bases (as ammonia is a weak base), ammonia has a lone pair of electrons located on the N atom and can therefore accept protons. Amine + Acid --> Salt (water is not produced as an addition proton is needed to form the ammonium ion)
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Preparation of aliphatic amines: Aliphatic amines can be produced by warming halogenoalkanes with excess ammonia, using ethanol as a solvent. Alkane + Ammonia --> Amine (Mechanism: Nucleophilic substitution)
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Preparation of aromatic amines: Aromatic amines can be produced by reducing nitroarenes using tin and conc. hydrochloric acid Nitroarene + n[H] --> Aromatic amine + H2O
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Production of Azo dyes Azo dyes are produced in 2 steps:
- HCl and NaNO3 are used together to form HNO2 which reacts with Phenylamine producing the benzenediazonium ion and water, this must be carried out under 10 degrees C.
- The benzenediazonium ion is then added to Phenol dissolved in NaOH, The benzenediazonium ion reacts with the Phenol in a coupling reaction forming a -N=N- bond, this is called the Diazo group.
Amino acids are combinations of amines and carboxylic acids. They have the general formula H2N-CHR-COOH where R is a side group dependent upon the amino acid, for glycine it's simply H.
- At a specific pH, the isoelectric point, the amino acid will form a zwitterion where the carboxylic acid will have dissociated and the H+ ion been accepted by the NH2 group forming NH3+, when the pH is increased above the isoelectric point, the zwitterion acts as an acid donating a proton retaining the NH2 group. When the pH is decreased below the isoelectric point, the zwitterion acts as an alkali accepting protons forming an NH3+ group.
Polymers are composed of monomers which are smallest unit of a polymer.
Peptides and proteins are composed of amino acids, 2-50 amino acid length chains are called polypeptides, chains of length greater than 50 amino acids are called proteins. Both are formed from a condensation reaction between amino acid by the elimination of a water molecule. Polypeptides and proteins can be broken down into their constituent units by hydrolysis where water is added breaking the peptide bond between amino acids.
Polyesters are formed by the condensation reaction between monomers where water or another small molecule is eliminated as the monomers join. Terylene is a polymerisation between ethane-1,2-diol and benzene-1,4-dicarboxylic acid. Terylene is used as a fibre for making clothing, rope, safety belts etc.
Polylactic acid is obtained by the polymerisation of 2-hydroxypropanoic acid (lactic acid), it is used for waste sacks, disposable eating utensils and medical applications, it's main advantage is that it is biodegradable.
Polyamides are polymers linked together by amide links (peptide links) -CONH-. Nylon-6,6 is a polyamide where
(D) Chromatography - An analytical technique that separates components in a mixture between a mobile phase and stationary phase
- The mobile phase in chromatography can either be liquid or a gas, this separates the mixture by relative solubility
- The stationary phase can be a solid (such as in TLC and gas chromatography) or liquid (in gas chromatography) Chromatography works on the basis that different components are adsorbed to the solid phase by differing amounts therefore as the solvent pushes on, the components move on at different paces.
TLC - Thin layer chromatography, A stationary phase is a thin layer of adsorbent that sits on a solid support called the TLC plate, frequently SiO2, or Al2O3 are used as solid phases. A liquid solvent then acts as the mobile phase which moves up the TLC plate. As a sample is added to the plate and the plate sat in solvent, the components start to separate at different speeds due to their different adsorptions. The Rf value is the proportion of distance moved by component to distance moved by solvent, obviously it is always less than 1 as the distance moved by the solvent can't be more than that of the solvent.
Limitations:
- Compounds often have similar Rf values
- Unknown compounds have no reference of Rf values
- Picking a solvent which separates all the components of the mixture may be difficult
GC, Gas chromatography is an analytical method use to separate volatile components in a mixture. The stationary phase is a liquid or solid coated on the inside of a tube called the chromatography column, different columns are used depending on what components are being separated out. The mobile phase is, believe it or not, a gas! The equivalent of Rf factors in TLC are retention times in GC, a retention time is the time taken for a component to pass from the inlet to the column detector which is then compared to data tables.
Limitations:
- Thousands of chemicals may share the same retention times, detector response and peak shape
- Not all substances will be detected or separated
- Unknown compounds have no reference times and are therefore meaningless
GC can be combined with mass spectrometry to provide a far more powerful analytical tool in identifying compounds. First components are separated using GC and then Separated components are analysed using MS. Mass spectra can be analysed and compared with spectral databases for identification of components. GC-MS is used in forensics (e.g. drugs), environmental analysis (e.g. monitoring organic pollutants and presence of pesticides in foods), airport security (e.g. detecting explosives) and space probes (e.g. to analyse materials found)
NMR, Nuclear Magnetic Resonance works on the principle that molecules will interact with low-energy radio frequency radiation, a strong magnetic field is applied using an electromagnet and low-energy radio-frequency radiation is applied. Protons and neutrons (nucleons) have a property called spin, nucleons with opposite spins pair up canceling each other out, in nuclei with an odd number of nucleons there is an overall spin which will mean the nucleus will interact with magnetic field, these nuclei can line up with magnetic fields either in the same direction or oppositely depending upon which way the residual spin is orientated. When a nucleus is in it's low-energy spin state, it can be promoted to it's upper-energy spin state providing the change in energy is exactly equal to it's energy gap, this is called excitation, nuclei in excited states drop back down emitting a radio frequency photon in the process of relaxation. NMR employs the technique of repeatedly exciting and relaxing nuclei, this is called resonance.
The magnetic field felt by a nucleus is dependent upon 2 factors:
- The applied strong magnetic field
- The weaker magnetic field generated from the electrons surrounding the nucleus
The effect of the latter point is called nuclear shielding, atoms in different environments (where they are in relation to neighbouring atoms) means that the energy gap will change.
- Tetramethylsilane, or TMS, is used in every NMR run to calibrate the machine, it produces a small peak which is defined as delta (chemical shift) = 0.
- Deuteriated solvents such as CDCl3, are used as solvents as NMR analysis takes place in solution.
In proton NMR, D2O is sometimes employed as a solvent, it substitutes protons in N-H and O-H groups forming N-D and O-D groups which give no peak as Deuterium is composed of a proton and a neutron => the opposite spins of the nucleons leaves no residual magnetic field and hence deuterium doesn't interact with the magnetic field applied.
- NMR is the same technology as MRI, it was renamed as people found 'Nuclear' in NMR intimidating... MRI scans are used to image the body internally.
- n+1 rule: when there are n hydrogens attached to a carbon, the splitting pattern shows n+1 peaks.
There are 2 types of isomerism:
- Stereoisomerism - same structural formula different 3D arragement • Optical Isomerism - 4 different groups attached to a carbon center: CHIRAL CARBONS • E-Z Isomerism, different groups attached to c=c, Z isomer is where the same groups line up on top or on the bottom, E isomer is where they are on opposing sides
- Structural isomerism - same molecular formula different structural formula.
In nature proteins and other structures are produced many of which are CHIRAL, practically always only 1 isomer is present, this is due to the fact that enzymes are chiral themselves and therefore can produce only a single isomer. When these molecules are replicated in a laboratory, both isomers are produced. However, recently there has been a development called CHEMICAL CHIRL SYNTHESIS whereby a single isomer can be produced by REAGENTS FIXED TO POLYMER RESINS
Many pharceutical drugs are chiral in nature, often only 1 isomer is pharmacologically active and the other may be harmful, this leads to a problem which can be solved in multiple ways:
- The other optical isomer can be tested to see whether is has any harmful side effects, if it does, it needs to be seperated, if not then the dosage of the drug can be increased so that enough of the pharmacologically active isomer is present
- CHEMICAL CHIRAL SYNTHESIS can be employed to only produce 1 of the isomers.
- Use of natural chiral molecules as starting materials (e.g. L-AMINO ACIDS and SUGARS)
Enzymes or bacteria can be used to produce the drub which promote STEREOSELECTIVITY (obvoiusly only works for stereoisomers and not chiral compounds).
Seperating isomers is EXPENSIVE however it REDUCES POSSIBLE SIDE EFFECTS and IMPROVES PHARMACOLOGICAL ACTIVITY