C15 Flashcards
ethanol
- derived from the fermentation of sugars in fruits etc
- a nervous system depressant
- interferes with transmission of nerve impulses
- in larger amounts, leads to loss of balance, poor coordination, impaired vision and inability to judge speed
- excessive long term use can be fatal
- ethanol is absorbed through the walls of the stomach and small intestine into blood stream
—-> some is eliminated unchanged in urine and breath- rest = broken down by liver
general formula of alcohols
-OH functional group attached to hydrocarbon chain
- relatively reactive
- CnH2n+1OH
how to name alcohols
suffix -ol
prefix -hydroxy used if other functional groups = present
shape of alcohol molecules
- O atom has 2 bonding pairs and 2 lone pairs
C-O-H angle = around 105 degrees
—–> 109.5 degree angle of tetrahedron = pushed down by presence of l.p - these l.p repel each other more than the e- pairs in a covalent bond
classification of alcohols
primary: 1 R group attached to C w/ -OH
methanol, w/ no R groups attached to C, = primary
secondary: 2 R groups attached to C w/-OH
tertiary: 3 R groups attached to C w/-OH
physical properties of alcohols
-OH group means that hydrogen bonding occurs between the molecules
- this is why they have higher melting and boiling points than alkanes of similar Mr
- the -OH group of alcohols can hydrogen bond to water molecules, but the non-polar hydrocarbon chain cannot
- this means that the alcohols with short hydrocarbon chains are soluble in water because the hydrogen bonding predominates
- in longer chain alcohols the non-polar hydrocarbon chain dominates and the alcohols become insoluble in water
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long chain alcohol
- hydrogen bonding still present
- also VdW forces between long carbon chain
- the long chain = non-polar so does not mix with water
- the whole molecule becomes insoluble in water, as the VdW force predominates over hydrogen bonds
test for primary/ secondary alcohol
- orange acidified potassium dichromate, acidified with H2SO4
- turns green
Cr +6 oxidised –> Cr 3+
uses and production of ethanol
- used as an intermediate in the manufacture of other organic chemicals
- used as solvent in cosmetics
- used in the manufacture of drugs, detergents
- made industrially by reacting ethene (made from cracking crude oil) with steam, using a catalyst of phosphoric acid
- also made from sugars by fermentation
making ethanol from crude oil
ethene = produced when crude oil fractions = cracked
- ethene = hydrated, water is added across the double bond
(see notes for mechanism)
making ethanol by fermentation
- carbohydrates from plants (sugar cane/ sugar beet) = broken down into sugars and then converted into ethanol by the action of enzymes from yeast
- key step = breakdown of sugar in anaerobic respiration
conditions:
- enzymes from yeast
- anaerobic
- 35 degrees C
- fermentation
- the rate of this chemical reaction = affected by temperature
- slow at low temp but enzymes are made ineffective if temp = too high
- a compromise temp of around 35 degrees
- air = kept out of the fermentation vessels to prevent oxidation of ethanol to ethanoic acid
- once the fermenting solution contains around 15% ethanol, the enzymes are unable to function and fermentation stops
—–> batch production - ethanol may then be distilled by fractional distillation, as its bp = 78, lower than water
——> so requires separation
dehydration of ethanol –> ethene
using acid catalyst - excess, hot, H2SO4 or passing vapour over heated aluminum oxide
carbon neutral
no net CO2 emmissions into atmosphere
biofuel
fuel produced from renewable biological resources
e.g. ethanol made from fermentation of sugars
combustion of alcohols
burn completely to CO2 + water if there is sufficient O2
alcohols = excellent fuels- they are highly flammable and release a lot of energy when burned
—> producing flames with low sootiness
elimination reactions of alcohols
dehydration
- small molecule leaves parent molecule
- in the case of alcohols, small molecule = always water
- water = made from -OH group and a hydrogen atom from the C next to the OH group
- so elimination reactions of alcohols = always dehydrations
alcohols can be dehydrated with excess hot concentrated sulfuric acid or by passing their vapours over heated aluminium oxide
see mechanism in notes
oxidation of alcohols
combustion = usually complete oxidation
alcohols can be oxidised gently and in stages
- primary alcohols can be oxidised to aldehydes, RCHO
—> aldehydes can be further oxidised to carboxylic acids, RCOOH - secondary alcohols are oxidised to ketones, R2CO
- tertiary alcohols are not easily oxidised
- this is because oxidation would need a C-C bond to break, rather than a C-H bond (which occurs when an aldehyde = oxidised)
ketones are not oxidised further for the same reason
——> THERE ARE NO H ATTACHED TO THE C, ONLY ALKYL GROUPS
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THE EXPERIMENTAL DETAILS
- a solution of acidified potassium dichromate/ potassion permanganate, acidified with dilute sulfuric acid, is often used to oxidise alcohols –> aldehydes/ ketones
—-> it is the OXIDISING AGENT - in the reaction, the orange dichromate (VI) ions are reduced to green chromium (III) ions
or Mn 8+ —> Mn 2- (purple - colourless)
—> this is also a test for primary/ secondary alcohols
to oxidise a primary alcohol to an aldehyde
e.g. ethanol –> ethanal
- dilute acid and less potassium (IV) dichromate than is needed for complete oxidation to carboxylic acid = used
- the mixture = heated gently, but with the reciever cooled in ice to reduce evaporation of the product
- ethanal (bp 21) vapourises as soon as it is formed, and distills off
- this stops it being oxidised further to ethanoic acid
- unreated ethanol remains in the flask
- the notation [O] is often used to represent O2 from the oxidising agent- must still balance
to oxidise ethanol to ethanoic acid
- concentrated sulfuric acid and excess potassium dichromate (VI) is used for complete reaction
- the mixture is refuxed
—> vapour condenses and drips back into the reaction flask - whilst the reaction mixture = refluxing, any ethanol or ethanal vapour will condense and drip back into the flask until eventually it is all oxidised into acid
- after refulxing for around 20 mins, distill off the ethanoic acid along with any wayer
- TWICE AS MUCH OXIDISING AGENT IS USED IN THIS REACTION COMPATED WITH THE OXIDATION OF ETHANAL
even using the [O] notation for oxidation, the equation must still balance
oxidising a secondary alcohol to a ketone
- secondary alcohols are oxidised to ketones by acidified dichromate
aldehydes and ketones
both have the C=O group
—> CARBONYL GROUP
- in aldehydes, at END of hydrocarbon chain
RCHO
- al suffix
- in ketones, in BODY of chain
RCOR’ - one sufix
test for aldehydes/ ketones
- have similar physical properties
- 2 tests can tell them apart
- both involve gentle oxidation
aldehydes = oxidised to carboxylic acids
RCO + [O] –> RCOOH
– this is the second stage of oxidation of a primary alcohol
ketones are NOT CHANGED by gentle oxidation
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tollens silver mirror test
- tollens reagent = gentle oxidising agent
- it is a solution of silver nitrate in aqueous ammonia
- it oxidises aldehydes but has no effect on ketones
- it contains colourless silver (I) complex ion, containing Ag+ which are reduced to metallic silver Ag as aldehyde = oxidised
- on warming an aldehyde with tollens, a deppsit of metallic silver = formed
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Fehling’s test
- fehling’s reagent = gentle oxidising agent
- contains blue copper (II) complex ions which will oxidise aldehydes but not ketones
- during the oxidation, the blue solution gradualy changes to a brick red ppt of copper (i) oxide
Cu(2+) +e- —> Cu+
- on warming an aldehyde with blue Fehling’s splution, a brick red ppt gradually forms
look at method for distillation of a product
