C HEMISTRY AS R EVISION Chains, Energy and Resources: Module 2

K EY W ORDS Volatility; the ease that a liquid turns into a gas. Increases as boiling point decreases Reflux; the continual boiling and condensing of a reaction mixture to ensure that the reaction takes place without the contents of the flask boiling dry Esterification; the reaction of an alcohol with a carboxylic acid to produce an aster and water Dehydration; an elimination reaction in which water is removed from a saturated molecule to make an unsaturated molecule Hydrolysis; a reaction with water or aqueous hydroxide ions that breaks a chemical compound into two compounds Nucleophillic Substitution; a type of substitution reaction in which a nucleophile is attracted to an electron deficient centre or atom, where it donates a pair of electrons to form a new covalent bond Limiting Reagent; the substance in a chemical reaction that runs out first Molecular Ion; the positive ion formed in mass spectrometry when a molecule loses an ion Fragmentation; the process in mass spectrometry that causes a positive ion to split into piece, one of which is a positive fragment ion Radical; a species with an unpaired electron Electrophile; an electron pair acceptor Nucleophile; an electron pair donor

P RODUCTION OF E THANOL Hydration of ethene Ethene (g) + Water (g)  Ethanol (l) Catalyst: phosphoric acid Temperature: 300°C Reaction is reversible, 5% conversion rate. Unreacted gasses are passed through again. Overall 95% conversion achieved. Fermentation Carbohydrate (aq)  Ethanol (aq) + Carbon Dioxide (g) Catalyst (as enzyme): yeast (zymase) Temperature: 20°C to 37°C Only around 14% conversion rate. Any higher at the zymase enzyme starts to become denatured Must be carried out in conditions free of oxygen to prevent undesirable products forming and affecting the flavour

U SES OF A LCOHOLS Alcoholic drinks Solvent in methylated spirits For removing paint and ink stains from clothing and as fuel in spirit burners Petrol additive to improve combustion as it increases the octane rating Feedstock in the production of organic chemicals

A LCOHOLIC S OLUBILITY AND L OW V OLATILITY OF A LCOHOLS Relatively high melting and boiling points of alcohols are because of their hydrogen bonds. This decreases their volatility Alcohols dissolve in water because hydrogen bonds form between the polar OH groups of the alcohol and water molecules Solubility decreases as chain length increases A large part of the alcohol molecule is made up on a non polar hydrocarbon chain The hydrocarbon change does not form hydrogen bonds with water molecules

P RIMARY, S ECONDARY AND T ERTIARY A LCOHOLS 1° alcohol -OH group is attached to a carbon atom with no alkyl groups or bonded to one alkyl group 2° alcohol -OH group is attached to a carbon atom bonded with two alkyl groups 3° alcohol -OH group is attached to a carbon atom bonded to three alkyl groups

C OMBUSTION OF A LCOHOLS In a plentiful supply of oxygen, alcohols burn completely to form Carbon Dioxide and water. C 2 H 5 OH (l) + 3O 2 (g)  2CO 2 (g) +3H 2 O (l)

O XIDATION OF A LCOHOLS Degree of Alcohol Oxidising Agent[O]2[O]Colour Change 1°K 2 Cr 2 O 7 / H 2 SO 4 (Distil) Aldehyde + H 2 O (Reflux) Carboxylic Acid + H 2 O [O] Orange  Green 2[O] Orange  Green 2°K 2 Cr 2 O 7 / H 2 SO 4 KetoneResistant to further oxidation [O] Orange  Green 2[O] No colour change; stays orange 3°Resistant to oxidation No colour change; stays orange

E STERIFICATION OF A LCOHOLS Formed when an alcohol is warmed with a carboxylic acid in the presence of Sulphuric Acid. Water is also formed. Used as adhesives and solvents in the chemical industry. Used as flavours and fragrances in food and perfumes

D EHYDRATION OF A LCOHOLS Alcohol is dehydrated to form an alkene in the presence of Sulphuric Acid Alcohol is heated under reflux in the presence of the catalyst

H YDROLYSIS OF H ALOGENOALKANES AS A S UBSTITUTION REACTION Halogen atoms more electronegative than carbon atoms. Bonded electron pair attracted more the halogen atom than the carbon, resulting in a polar bond, or dipole. The electronegativity of halogens decreases down the group, resulting in a decrease in polarity of the carbon halogen bond from fluorine to iodine. The electron deficient carbon atom in halogenoalkanes attracts nucleophiles The nucleophile replaces the halogen atom in the halogenoalkane, forming a compounding containing a different functional group When halogenoalkanes react with an aqueous solution of hot hydroxide ions, a nucleophillic substitution occurs, the product of this is an alcohol.

N UCLEOPHILLIC SUBSTITUTION IN THE H YDROLYSIS OF P RIMARY H ALOGENOALKANES – T HE M ECHANISM Hydroxide ion has a lone pair of electrons. These are attracted and donated to the electron deficient carbon atom in the halogenoalkane. This is a nucleophillic attack. Must be heated. The donation of the electron pair leads to the formation of a new covalent bond between the oxygen atom of the OH ion and the carbon atom. The carbon halogen bond breaks by heterolytic fission, both electrons from the bond move to the halogen, forming a halide ion. Also known as substitution by rear side attack…

R ATE OF H YDROLYSIS OF P RIMARY H ALOGENOALKANES Halogenoalkane is heated with aqueous silver nitrate, with ethanol added Water behaves as the nucleophile Ethanol acts as the common solvent, ensuring that the halogenoalkane and aqueous silver nitrate mix together and react As the reaction takes place, halide ions form, forming a ppt of silver halide Cl and Br = white ppt, I = yellow ppt Bond enthalpy is more important that bond polarity The rate of reaction increases from the fluoroalkanes to the iodoalkanes as the carbon halogen bond enthalpy weakens

U SES OF C HLOROETHENE AND T ETREFLUOROETHENE TO PRODUCE PLASTICS PVC AND PTFE Chloroethene can be manufactured to produce PVC (Poly vinyl chloride) Coating of pans and metal surfaces. Nail polish Tetrafluoroethene can be manufactured to produce PTFE (poly Tetrafluoroethene) Drainpipes, plastic window frames, sports equipment, children’s toys and packaging

CFC S : R EASONS FOR D EVELOPMENT AND I MPACT Used as refrigerants Unreactive, non toxic, non flammable gas Break down in the presence of UV to produce Chlorine free radicals. These radicals catalyse the breakdown of ozone Ozone blocks much of the UV entering Earth’s atmosphere. The degradation of the ozone layer means more UV and an increased incidence of skin cancers.

G REEN C HEMISTRY : A LTERNATIVES TO CFC S Hydrofluoroalkanes, HFC, alternatives to CFC. Non flammable and non toxic Still deplete the ozone layer, but at one tenth the rate of CFC Only a temporary solution until something better can be found CO 2 used as an alternative as the blowing agent for expanded polymers

I NFRARED S PECTROSCOPY IR causes covalent bonds to vibrate Modern breathalysers measure ethanol in breath by using infrared spectroscopy Identifies functional groups

M ASS S PECTROSCOPY