2.3.3 Testing for Halide Ions

• Dissolve halide ions solution in nitric acid and then add drops of silver nitrate.
• The nitric acid is to prevent any false positive results from carbonate ions precipitating out with silver ions
• Ag⁺ (aq) + X^- (aq) → AgX (s)
• AgX would be a precipitate: silver chloride is white, silver bromide is cream, silver iodide is yellow.
• Add dilute then concentrated ammonia after as the colours are too similar.
  • If the precipitate dissolves in dilute ammonia: chloride
  • If the precipitate does not dissolve in dilute, but does in concentrated ammonia: bromide
  • If the precipitate does not dissolve in either: iodide

3.1.12 Acids and bases

• Acid: proton donor
• Base: proton acceptor
• pH = –log₁₀[H+ ]
• Water is slightly dissociated.
• Ionic product of water: K_w = [H⁺ ][OH^–]
• Disocciation constant: pK_a = –log₁₀(K_a) because K_a can have a large range.
• K_a is larger for stronger acids as they disocciate completely in water.
• A buffer solution maintains an approximately constant pH, despite dilution or addition of small amounts of acid or base.
• E.g. if an acid dissociates, we have HX --> H⁺ + X^-. If we add a base, it reacts with the H⁺ ions, so [X^-] goes up and [H⁺] goes down by equal amounts. K_a and [HX] do not change.
• Acidic buffer solutions contain a weak acid and the salt of that weak acid.
• Basic buffer solutions contain a weak base and the salt of that weak base.

3.3.3 Halogenoalkanes

• Haloalkanes contain polar bonds as halogens are more electronegative than carbon.
• Nucleophiles like positive. :NH₃, ^-:OH, CN:^-
• Nucleophilic substitution: haloalkanes form alcohols or amines.
• Nucleophile attacks the δ⁺ carbon and the electrons are transferred to the halogen.
• The greater the Mr of the halogen in the polar bond, the easier it breaks e.g. CI has a weaker bond than CF.
• Nucleophilic substitution can only happen for 1^o and 2^o haloalkanes.

• Haloalkanes undergo elimination reactions at high temperatures under alcoholic conditions to form alkenes.
• The nucleophile accepts a proton so a hydrogen atom is removed from the haloalkane to form water.

• Elimination only occurs with 2^o and 3^o haloalkanes.
• Different conditions will result in different reactions
• NaOH (hot, in ethanol): an elimination reaction occurs to form an alkene
• NaOH (warm, aqueous): a nucleophilic substitution reaction occurs, and an alcohol is formed
• Ozone and CFCs absorb UV light

3.3.4 Alkenes

• Alkenes are unsaturated hydrocarbons with a double covalent bond, a centre of high electron density.
• Alkenes undergo electrophillic addition about the double bond.
  • Double bond is broken. Carbocation (3 bonds) is formed. This has a positive charge. Tertiary = more stable = more likely to form.
• Electrophiles: electron acceptors. They are attracted to areas of high electron density e.g. the double bond.
• Common electrophiles: HBr, Br2, H2SO4.
• Products: alkyl hydrogensulfates or haloalkanes.

• Addition polymers: formed from alkenes. The double bond of the monomer breaks to form a repeat unit and many of these join together.
• Addition polymers are unreactive.
• Conditions: high pressure and temperature for branched chains with weak intermolecular forces, and vice versa.
• Uses: unreactive and non-biodegradable so plastics e.g. PVC.

3.3.5 Alcohols

• Alcohols are produced by hydration of alkenes with an acid catalyst.
• Ethanol is produced industrially by fermentation of glucose.
• The conditions for this process. Ethanol produced industrially by fermentation is separated by fractional distillation and can then be used as a biofuel.
• Alcohols can be primary, secondary and tertiary. This is the number of alkyl groups the closest carbon is connected to.
• Primary alcohols can be oxidised to aldehydes which can be further oxidised to carboxylic acids.
• Secondary alcohols can be oxidised to ketones.
• Tertiary alcohols are not easily oxidised.
• Oxidising agent: acidified potassium dichromate(VI) and heat.
• Further oxidation: under reflux conditions.
• Alcohols can form alkenes from elimination reactions with acid catalysts.

• These alkenes can produce addition polymers without using monomers from crude oil.

3,3.8 Aldehydes and Ketones

• Aldehydes are readily oxidised to carboxylic acids.
• Chemical tests to distinguish between aldehydes and ketones
  • Fehling’s solution:
  • Tollens’ reagent:
• Aldehydes can be reduced to primary alcohols, and ketones to secondary alcohols, using NaBH4 in aqueous solution. These reduction reactions are examples of nucleophilic addition.
• The nucleophilic addition reactions of carbonyl compounds with KCN, followed by dilute acid, to produce hydroxynitriles.

• The reducing agent: NaBH₄ provides the nucleophile H^-.

• Aldehydes and unsymmetrical ketones form mixtures of enantiomers when they react with KCN followed by dilute acid. The hazards of using KCN.

3.3.9 Carboxylic acids and Esters

• Carboxylic acids: COOH
• Carboxylic acids: weak acids, form COO^- + H⁺.
• Esters: formed by reacting carboxylic acid + alcohol, in the presence of an acid catalyst under reflux. Water is a by-product.
• Esters: smell nice. Used as perfume, food flavouring.
• Vegetable oils: esters of glycerol (alcohol).
• Biodiesel: esters formed from vegetable oils and methanol with catalyst.
• Esters can be hydrolysed to form alcohols and carboxylic acids/ salts of carboxylic acids under acidic or alkaline conditions.
• If acidic conditions: alcobol and carboxyic acid.
• If alkaline conditions: alcobol and salt e.g. COO^-Na⁺.
• Soap: Hydrolyise vegetable oils and animal fats in alkaline conditions.

3.3.10 Aromatic Chemistry

• Benzene is an aromatic compound containing six carbons and six hydrogens and a ring of delocalised electrons.
• Each bond has an intermediate length between a single and double bond.
• Benzene has a high melting point because of the delocalised outer electron from the p-orbital, but a low boiling point as they are non-polar (can't be dissolved in water).
• Benzene had an even lower enthalpy change (-208 kJ/mol) than predicted (-360 kJ/mol).
• Aromatic compounds undergo electrophillic substitution because of high electron density

• This monosubstitution takes place at 55 ^oC.

3.3.11 Amines

• Amines are formed when a H in ammonia is replaced with an R group.
• Primary amine: two H. Secondary amine: one H. Tertiary amine: no H.
• To form an amine: nucleophilic substitution of a haloalkane with ammonia. Use excess ammonia to form a primary amine.

• To form a secondary amine, nucleophilic substitution of a primary amine with haloalkane.
• Another way to form amines: reduction of nitrile.

• Conditions of reduction: LiAlH₄ and dilute acid OR hydrogen and nickel catalyst.
• Amines are weak bases. More electrons available, more basic.
• Amines undergo nucleophilic substitution as they are nucleophiles.
• quaternary ammonium saltsPosts

3.3.16 Chromatography

• Chromatography is used to separate mixtures.
• TLC (thin layer) is used to analyse small samples e.g. dyes.
• Stationary phase: thin metal sheet coated in alumina (Al₂O₃) or silica (SiO₂)
• Different Rf values: each substance has different attraction to stationary and mobile phases.
• To identify the substances, use UV light.
• Sometimes we use two solvents because some of the substances do not separate/dissolve with the first/either solvent.
• Less polar = greater Rf value.

7.7.1 Amino Acids

• Amino acids have two functional groups (-NH₂ and -COOH).
• 2-aminocarboxylic acids are naturally occuring amuino acods.

• General structural formula: RCH(NH₂)COOH (there are 20 of these).
• Amino acids are amphoteric (acidic and basic).
  • -COOH groups: act as acids
  • -NH₂ groups: act as bases
  • -OH and -CONH₂ groups: not able to act as acids or bases
• Amino acids will react with acids and bases.
• Zwitterions are formed when amino acids act within themselves (intramolecularly).
• A zwitterion is an ion with both a positive (-NH₃⁺) and a negative (-COO^-).
• Zwitterions have strong intermolecular forces because of the charges.
• Amino acids are therefore soluble crystalline solids.
• A solution of amino acids in water will exist as zwitterions with both acidic and basic properties, and act as buffer solutions.
• If an acid is added: -COO^- forms -COOH (zwitterion becomes +ve).
• If a base is added: NH₃⁺ forms the -NH₂ group (zwitterion becomes -ve).
• The isoelectric point is a point when neither the negatively charged or positively charged ions dominate and the amino acid exists as a neutral zwitterion.

7.7.2 Proteins

• Dipeptides: the -NH₂ group of one amino acid reacts with the -COOH group of another amino acid in a condensation reaction.
• It has the NH₂ and the COOH groups at the ends, and a peptide bond (the H-N-C=O) in the middle.
• Polypeptide: formed when many amino acids join together to form a long chain of molecules.
• Protein
• Primary: many peptide covalent bonds
• Secondary: weakly negative N and O forming hydrogen bonds in a α-helix or β-pleated sheet
• Tertiary: between the R-groups
  • weak hydrophobic
  • disulphide bonds
  • hydrogen bonds
  • ionic bonds
• Hydrogen bonding occurs between the C=O groups and N-H groups
• Ionic attractions occur between the side chains of amino acids, for example between -COO^- and -NH₃⁺
• Cysteine is an amino acid that has a -CH₂SH side group; two molecules can react together to make a sulfur-sulfur bridge between the two molecules
• Hydrolysis: the reversal of condensation. Break the peptide link and add H, H and O to form COOH and NH₂ again.
• Hydrolysis conditions: chemically or using enzymes
  • Reagent: concentrated hydrochloric acid. Boiled for hours, slow reaction.
  • Enzyme: room temperature.
• You can identify the amino acid products using TLC.
• Amino acids can be located on a chromatogram using developing agents e.g. ninhydrin or ultraviolet light and identified by their R_f values.