module 4.2

• what is the general formula to alcohols?

  C n H 2n+1 OH
• primary alcohols - the -OH group is attached to a carbon atom that is bonded to only one alkyl group (or no alkyl groups)
• secondary alcohol - the -OH group is attached to a carbon atom that is bonded to two alkyl groups
• tertiary alcohols - the -OH group is attached to a carbon atom that is bonded to 3 alkyl groups
• the bond between oxygen and hydrogen in the hydroxyl group of alcohols is polar

  this is due to oxygen being more electronegative than hydrogen, leading to an unequal sharing of electrons

  as a result, the hydrogen atom acquires a partial positive charge (δ+), while the oxygen atom gains a partial negative charge (δ-)
• the partially positively charged hydrogen atom (δ+) can form intermolecular hydrogen bonds with the lone pairs of electrons oon oxygen atoms found in other molecules, including water molecules or other alcohol molecules
• solubility

  alcohols with small carbon chains (methanol to propanol) dissolve in water because the hydroxyl group forms strong hydrogen bonds with water molecules
• solubility

  larger alcohols have increased non-polar hydrocarbon regions that interfere with hydroxyl-water hydrogen bonding. so the solubility of alcohols in water decreases as their molecular size increases
• boiling points of alcohols increase as the chain length increases - as molecules get longer, there are more surface area contacts and so stronger induced dipole-dipole intermolecular forces

  so more energy needed to overcome these attractive forces and therefore higher boiling point
• boiling point

  the ability of alcohol molecules to hydrogen bond with each other leads to higher boiliing points in comparison to alkanes of similar molecular size

  this is because the hydrogen bonds between alcohol molecules are the strongest type of intermolecular force of attraction so more energy is needed to overcome this force
• what does it mean for a substance that is volatile?

  it evaporates easily at room temperature and pressure.
• volatility increases as the boiling point decreases. alcohols have hydrogen bonds and this makes them less volatile than a corresponding alkane
• alcohols combust completely when burned in an excess of oxygen, breaking all C-C and C-H bonds. this results in the production of carbon dioxide and water.
• complete of combustion of ethanol

  C2H5OH (l) + 3O2 (g) -> 2CO2 (g) + 3H2O (g)
• oxidation of alcohols can be performed using an acidified potassium dichromate (VI) solution (K2Cr2O7), which changes colour from orange to green as the reaction proceeds
• this colour change is due to the reduction of dichromate (VI) ions (Cr2O7 2-) to chromium (III) ions (Cr 3+)
• to isolate the aldehyde, gently heat the alcohol in a distillation apparatus with a limited amount of potassium dichromate (VI) and distil the aldehyde as it forms to prevent further oxidation. to obtain the carboxylic acid, heat the alcohol with an excess of dichromate (VI) under reflux conditions
• secondary alcohols, such as propan-2-ol, can be converted into ketones by refluxing with acidified dichromate (VI)

  this process does not allow for further oxidation of the ketone

  
• tertiary alcohols do not oxidise under these conditions -> no reaction -> stays orange colour
• halide substitution

  halide ions like Cl-, Br- and I- will react with alcohols

  the halide takes the place of, or substitutes, the alcohol group to form a haloalkane
• general equation for halide substitution

  ROH + HX -> RX + H2O
• although the halide is the species directly involved in the reaction, the reagent would be a hydrogen halide like HCl

  for substitution with bromide, a salt like sodium bromide is usually used, which reacts with the sulfuric acid to make HBr in situ
• an acid catalyst such as concentrated sulfuric acid is added and the mixture is warmed to increase the rate of reaction. however, when iodide is reacted then phosphoric acid is used. this is because sulfuric acid oxidises iodide ions to iodine - so the yield of the desired iodoalkane is very low
• dehydration of alcohols, facilitated by concentrated sulfuric acid, results in the formation of alkenes through the elimination of water

  alcohol -> alkene + water
• dehydration happens when an -OH group from one carbon atom and an H atom on an adjacent carbon atom lost to form a water molecule. next a π-bond forms between the two adjacent carbon atoms
• on the laboratory scale, the alcohol is heated under reflux with concentrated sulfuric acid (H2SO4) or concentrated phosphoric acid (H3PO4) for about 40 minutes. the concentrated strong acid is a catalyst in the reaction

  
• haloalkanes are saturated organic compounds that contain carbon atoms and at least one halogen atom
• the oxidation of primary alcohols can proceed in two stages, initially forming an aldehyde and subsequently a carboxylic acid

  
• nucleophiles: electron pair donors
• nucleophilic substitution: a chemical reaction in which an atom or group of atoms is exchanged for a nucelophile
• nucleophilic substitution

  • a nucleophile (NUC) approaches the haloalkane, which has a partially positive carbon atom (δ+)
  • the nucleophile donates its lone pair of electrons to the δ+ carbon, forming a new covalent bond
  • the original bond between the δ+ carbon and the halogen breaks heterolytically as the halogen atom takes both the shared electrons
  • the halogen departs as a halide ion (X-), being replaced by the nucleophile

  
• hydrolysis - chemical reaction in which water is a reactant. there are alkali hydrolysis reactions where -OH- is the reacting species
• refluxing is a technique used to heat reactions involving volatile, flammable organic compounds
• • anti-bumping granules are added to the flask to prevent violent boiling
  • the condenser has cold water flowing through it, which condenses evaporated organic vapours back into liquid, returning them to the reaction vessel
  • this process allows the organic substances to be heated to their boiling point, speeding up the reaction, without losing material
  • heating is done electrically to avoid open flames that could ignite flammable vapours

  
• distillation is used to separate mixtures based on differences in boiling point
• • mixture is heated gently in a distillation apparatus
  • components evaporate in order of their increasing boiling points
  • if the boiling point of the desired product is known, collection can commence when the thermometer shows this temperature
  • for some reactions, the product has a lower boiling point than reactants. as the products forms, it evaporates from the reaction mixture and condenses separately, preventing further reaction