Organic 5: Alcohols

ROH CnH2n+1OH

The bond angle of an OH group is 105 degrees Because the 109.5 tetrahedral angle is reduced because of two lone pairs Lone pairs repel more than bonding pairs

Shorter chain alcohols are soluble because the OH group can hydrogen bond to water molecules Longer chain alcohols are insoluble because they have a larger non-polar hydrocarbon region, so cannot form hydrogen bonds

From crude oil Fermentation

Ethene is produced when crude oil fractions are cracked Ethene can then by hydrated (in electrophilic addition reaction with water) to form ethanol

Glucose is broken down using enzymes in yeast in anaerobic respiration C6H12O6(aq) —> 2C2H5OH(aq) + 2CO2(g)

Enzymes can be denatured if temperatures are too high But reaction will be too slow in low temperatures So a compromise temperature of around 35 degrees is used

To prevent oxidation of ethanol to ethanoic acid

Ethanol must be distilled by fractional distillation after fermentation At boiling temperature 78 degrees Celsius As aqueous solution produced by fermentation is impure

Dehydration

Acid catalysed elimination Reagent: Excess conc H2SO4 or H3PO4 Conditions: Warm, under reflux 1) Lone pair of electrons from oxygen are attracted to H+ from acid 2) Electrons in C-H bond next to the carbon attached to the OH group move to the C-C bond, and form a double C=C bond 3) Electron pair in C-O bond move to the H2O+ group, which is removed to give ethene, H+ and water

Advantages - Faster reaction - Purer product - Continuous process Disadvantages - High technology equipment needed - Ethene is non-renewable resource - High energy costs for producing high pressures

Advantages - Sugar is a renewable resource - Production uses low level technology Disadvantages - Batch process, so is slow - Not continous - Ethanol is not pure, so needs purifying by fractional distillation

The carbon dioxide released is balanced by the carbon dioxide absorbed by the plant during photosynthesis 2 molecules are released in fermentation 4 molecules are released during combustion 6 molecules are absorbed during photosynthesis

Because it comes from plants like sugar cane, which can be replanted

Fuel e.g. in picnic stoves

Primary alcohol + [O] –> aldehyde + H2O Primary alcohol + 2[O] –> carboxylic acid + H2O

1) Using acidified potassium dichromate (K2Cr2O7 + H2SO4) 2) Orange dichromate (VI) ions are reduced to green chromium(III) ions 3) Heat and then distill off product to prevent further oxidation into carboxylic acid

1) Using acidified potassium dichromate (K2Cr2O7 + H2SO4) 2) Mixture is refluxed, so vapour condenses back into the flask, so all alcohol will be oxidised into acid

1) Add [Ag(NH3)2]+ (Tollen’s Reagent) to sample 2) Heat gently 3) Aldehydes will be oxidised into carboxylic acids 4) The silver ions in [Ag(NH3)2]+ will be reduced to silver atoms, forming a silver mirror 5) No visible change for ketones

CH3CHO + 2Ag+ + H2O —> CH3COOH + 2Ag + 2H+