CHEMISTRY NOTES Homologous series- a group of organic compounds containing the same functional group in which each successive member in the unit increases by the unit -CH2 Characteristics of a homologous series: 1. All members of a particular series have the same functional group - COOH for carboxylic acids 2. The members of a particular series increase by the unit -CH2 3. All members of a series have similar chemical properties because they have the same functional group 4. Each member in the particular series have physical characteristics that chnage in a regular way, for eg as the number of carbons in the straight chain alkane increases, the BP increases. Reactions of alkanes: 1. Cracking - Catalysts are Aluminium or silicon oxide temperature w catalyst - 400 - 500 degrees celsius temperature w/o catalyst - 700 - 900 deg celsius 2. Free Radical Substitution Free radical: a species or fragment of species that has an unpaired electron Stages: Initiation, Propagation, Termination Substitution takes place in UV light 3. Combustion Reactions of alkenes 1. Oxidation Using hot KMNO4 - alkenes become aldehydes, ketones, carboxylic acids / CO2 Using cold KMNO4 - alkenes become diols 2. Hydrogenation - to form alkanes Nickel - catalyst 140 deg celsius 3. Reaction with steam - catalyst: phosphoric acid 4. Addition of hydrogen halides: Electrophilic addition Electrophile - a species or fragment of species that has a partially positive or positive charge and attacks electron rich areas of molecules eg H+ 5. Reaction with hot H2SO4 to from alchols Under reflux conditions, the acid reforms Reactions of alcohols 1. Oxidation Primary alcohols oxidised to aldehydes and further to carboxylic acids Secondary alcohols oxidised to ketones 2. Reduction with carboxylic acids (Esterification) To form esters 3. Reduction of alcohols To form alkenes 4. Iodoform test alkali environment includes secondary alcohols which have a methyl group on the same carbon as the OH group. The iodine replaces all the hydrogens on the methyl group forming triiodomethane Reactions of Halogenalkanes Primary halogenoalkanes - halogenoalkanes which contain a halogen bonded to a carbon that is bonded to one other carbon Secondary halogenoalkanes - halogenoalkanes which contain a halogen bonded to a carbon that is bonded to two other carbons Tertiary halogenoalkanes - halogenalkanes which contain a halogen bonded to a carbon that is bonded to three other carbons 1. Primary halogenoalkanes undergo SN2 reaction- Substitution nucleophilic 2nd order reaction Both the primary halogenoalkane and the nucleophile are involved in the rate determining step which is the only step. 2. Tertiary halogenolalkanes undergo SN1 reaction Only the halogenoalkane is involved is involved in the rate determining step Tertiary alcohols are formed Reactions of Carbonyl groups Testing for carbonyl groups 1. Brady's reagent - yellow to orange if carbonyl group is present 2. Tollen's reagent - Silver mirror test. Is a mexture of silver complex ions in excess ammonia. If aldehydes are present, a silver mirror is formed because the silver complex ions are reduced 3. Fehling's Reagent - substance is warmed with complex (ii) ions. Aldehydes are oxidised to carboxylic acids. Fehling's changes from blue to orange/brick red Oxidation - KMNO4 or K2CRO7 Reduction of carbonyl groups - using lithium aluminium hydride Aldehydes reduced to primary alcohols Ketones reduced to secondary alcohols AROMATICS Benzne reactions: Chlorination of Benzene Reagents : Catalyst : Ferric chloride / Iron (iii) chloride, Chlorine molecule Nitration of Benzene Reagents : Concentrated nitric and sulphuric acid (catalyst). Nitronium ion is the electrophile. Methylbenzene/Toulene reactions: Methyl groups are 2,4 directing. Have a positive inductive effect. The carbon in methyl group is positive. The electrons are pushed the ring of delocalised pi electrons Nitration of Toulene: Reagents: Sulphuric acid (catalyst) and Nitric acid Products : toulene + nitril group, water and sulphuric acid Bromination of Toulene Reagents: toulene + Br2 ---> Toulene + Br + HBr Nitrobenzene reactions The nitro groups have a negative inductive effect. The nitro groups pull electrons to itself away from the ring of delocalised pi electrons in the benzene. The odd bonding between nitrogen and oxygen causes the negative inductive effect. Nitrogen should have 3 bonds but it has 4 bonds while oxygen has an unpaired electron which causes a negative charge. The double bond on the oxygen alternates between the two oxygen atoms. In order for the nitro group to be stable, the electrons are shared between the two oxygens. So as a result of this, conjugative effect, electrons are being pulled from the benzene ring to stabilise that portion of the group, making the benzene ring less reactive. The electrons are being pulled from the ring because nitrogen is more electronegative than carbon and facilitates the pulling away of electrons. The nitro group is 3 directing Reduction of nitrobenzene Reagents: Concentrated HCL and tin products : aniline/phenylamine/benzeneamine + water (involved in diazotisation reaction with nitrous acid) Nitration of benzene Acid mixture - conc nitric and sulphuric acid Phenols very reactive - Halogens : ring extends, oxygen included, oxygen donates electrons to the ring - Sodium : forming sodium phenoxide + water -Magnesium and Aluminium : magnesium/aluminium phenoxide + hydrogen gas - acid halides eg. ethanoyl chloride : ester + HCL - Diazotisation and coupling reactions involving phenol