6.1 Light and Electrons
- 2 light models- wave theory and particle theory
- Speed of light = wavelength x frequency
- Particle theory- light as a stream of packet of energy called photons. Energy of photon= frequency x Planck constant
- Atoms become excited by absorbing energy. When excited atoms lose energy and return to their ground state energy is emitted as electromagnetic radiation. The light can be split up into an emission spectrum
- An emission spectrum consists of a series of coloured lines on a black background
- An absorption spectrum shows black lines on a continuous spectrum background
- Lines get closer together when frequency increases, and the lines in both spectra of an element are in the same places
- When an atom is excited, electrons jump into higher energy levels (absorption). Later they drop back down (emission)
- An electron can only possess definite quantities of energy (quanta)
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6.8 Ultraviolet and Visible Spectroscopy
- A spectrometer measures the quantity of light absorbed by a solution at each wavelength. The result is an absorption spectrum.
- In infrared and visible spectroscopy, the light is split into two beams, one passes through the sample solution and the other passes through pure solvent. The light in the emerging beams is compared to give the absorption spectrum
- The three following features of a spectrum are used to interpret it
- 1. The wavelength of radiation absorbed
- 2. The intensity of absorption
- 3. The shape of the absorption band
- A reflectance spectrum can be used to identify pigments in paintings. Light is shone onto the paint surface, and the composition of reflected light is examined
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6.9 Chemistry of Colour
- Coloured substances absorb radiation in visible part of the spectrum. Energy absorbed causes changes in electronic energy, and electrons are promoted from the ground state to an excited state
- The electrons which are excited are the outermost ones, involved in bonding or in lone pairs
- The energy to excite an electron is the excitation energy.
- Coloured organic compounds often contain unsaturated groups (eg. C=C, C=O or N=N), which are usually part of an extended delocalised system called the chromophore.
- Electrons in double bonds are more spread out and have a lower excitation energy, meaning the compounds absorb in the visible region
- Functional groups are often attached to chromophores to enhance or modify the colours of the molecule
- Many dye molecules have different colours in acidic and alkaline conditions, so can be used as acid-base indicators.
- Coloured inorganic compounds usually contain transition metals. Orbitals are split into different energy levels, are the size of the energy gap usually corresponds to light in the visible part of the spectrum
- Electron transfer- when absorption of visible light causes the transfer of an electron from the ground state of one atom to the excited state of another adjacent atom
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- In gas-liquid chromatography, the mobile phase is an inert carrier gas, and the stationary phase is a small amount of high bp liquid held on solid support
- Each component in a sample has a different affinity with the mobile phase compared to the stationary phase. Each distributes itself to different extents between the two phases, so will emerge from the column of solid at different times.
- The most volatile compounds emerge first
- A detector monitors the compounds coming out of the column, and a recorder plots the chromatogram.
- The time that a compound is held on a column under given conditions is the retention time, and is characteristic of a compound
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- The bond angle in benzene is 120°, and all carbon-carbon bonds are the same length- shorter than C-C, but longer than C=C
- Each C atom has 4 electrons for bonding, 3 are used to form C-C bonds with 2 other C atoms and a H. The remaining electrons are spread across the structure through electron delocalisation.
- Additional evidence for electron delocalisation comes from electron density maps.
- The Kekulé structures are benzene structures with three single bonds and three double bonds. They were proposed in 1865, but have been disproven
- The cyclic delocalisation of electrons in benzene makes it more stable that suggested by kekulé structure. Benzene maintains it stability by only undergoing reactions which preserve the ring
- Arenes are hydrocarbons which contain rings stabilised by electron delocalisation. They are sometimes called aromatic hydrocarbons.
- A benzene ring where a hydrogen atom has been substituted by another group is a phenyl group.
- Benzene rings can be joined together to give fused ring systems
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12.4 Reactions of Arenes
- The high electron density means that benzene reacts with electrophiles, in substitution reactions (which preserve the stable delocalised ring)
- Bromination- bromine reacts with benzene in the presence of an iron or iron (III) bromide catalyst. Br+ is the electrophile
- Nitration- benzene reacts with a nitrating mixture of conc nitric acid and conc sulphuric acid. If the temperature is under 55°C, the result is nitrobenzene. The electrophile is NO2+. At higher temperatures, more NO2 groups are added
- Sulphonation- benzene is heated under reflux with conc sulphuric acid. The electrophile is SO3
- Chlorination- Benzene is reacted with chlorine with an aluminium chloride catalyst. Must be done under anhydrous conditions as the catalyst reacts violently with water
- Freidel-Crafts reactions involve the substitution of halogen containing compounds onto the benzene ring
- Alkylation- benzene is warmed with chloromethane and aluminium chloride, to produce methyl benzene
- Acylation- benzene is treated with an acyl chloride (or acid anhydride) and aluminium chloride
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13.6 Oils and Fats
- Most oils and fats are esters of propane-1,2,3-triol (glycerol) and long-chain carboxylic acids. Three carboxylic acid molecules bind to one molecule of glycerol to form a triester.
- The carboxylic acids are sometimes called fatty acids. The alkyl groups can be saturated (no double bonds) or unsaturated (one or more double bonds
- Oils and fats can be split up by hydrolysis, usually done by heating with NaOH
- Saturated triglyceride molecules can pack closer together, as the chains are straight. The attractive forces between chains will be strong, so more energy is needed to separate them, leading to a high melting point. This is why most saturated fats are solids
- The double bonds in unsaturated molecules cause the chains to kink. They cannot pack closely together, so attractive forces are weaker. Less energy is needed to separate the chains, so the melting points are lower and unsaturated fats are liquids
- Oils can be converted to fats via hydrogenation. The addition of hydrogen breaks the double bonds in unsaturated fats.
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13.10 Azo Compounds
- Contain the N=N group. Compounds where the R groups on either side are arene groups are more stable than if they were alkyl groups
- Azo compounds are formed through a coupling reaction between a diazonium salt and a coupling agent.
- Diazonium salts have a triple bonded N2 group attached to an arene ring. They are very unstable as they tend to lose the triple bonded N2 group to N2 (g) if the temperature rises above 5°C.
- They are prepared by adding ice cold sodium nitrate to a solution of arylamine in dilute acid, at a temperature of less than 5°C. This is diazotisation.
- The diazonium salt then reacts with a coupling agent (such as phenol or phenylamine) to form an azo dye
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