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3.3.4-5 Alkenes and Alcohols (Addition Polymers (Low Density Polythene…
3.3.4-5 Alkenes and Alcohols
Structure, Bonding and Reactivity
Alkenes are unsaturated hydrocarbons.
Bonding in alkenes involves a double covalent bond, a centre of high electron density; the double bond cannot rotate
Physical properties of alkenes are similar to those of alkanes
Very reactive due to double bond which is an area of high electron density to attracts electrophiles
Test for alkenes:
add reddish/brown bromine solution to the sample; the bromine will add across the double bond in an alkene, forming a compound which is colourless
if no alkene present, then bromine water will remain reddish/brown
Addition Polymers
Addition polymers are formed from alkenes and substituted alkenes; are unreactive
Monomers: the repeating unit of addition polymers.
unreactive due to strong non polar C-H and C-C bonds
polychloroethene (PVC)
uses: drainpipes (without plasticiser), aprons (with plasticiser), vinyl records
can be modified with plasticisers; small molecules between polymer chains forcing them apart and allowing them to slide across each other
Low Density Polythene (LDP)
ethene polymerised at high temperature and high pressure via free radical mechanism
branching; decreased intermolecular forces; flexible and stretches well
uses: packaging, cable insulation, sheeting
High Density Polythene (HDP)
slightly higher than room temperature and pressure; uses Zieglar-Natta catalyst
decreased branching; increased intermolecular forces; more rigid and chains pack together
uses: milk bottles, buckets, bottles
Production of alcohols
hydration of alkenes
water is added across the double bond
conditions: phosphoric acid catalyst, steam
fermentation of glucose
conditions: 35°C , anaerobic, yeast
C₆H₁₂0₆ --> 2C₂H₅OH + 2CO₂
separated by fractional distillation
uses: alcoholic drinks, solvent, fuel
biofuel: a fuel derived immediately from living matter
biofuel advantages: renewable; alternative energy source from crude oil; may be carbon neutral; reduce greenhouse gas emissions
biofuel disadvantages: expensive to produce; space and time and energy used to produce crops can be better spent on feeding people; not carbon-neutral (carbon needed to transport crops and fuel, and when processing crops)
carbon neutral-fuel: carbon dioxide released when burnt is balanced by carbon dioxide absorbed during photosynthesis
Reactions of Alcohols
Combustion
alcohols burn completely in oxygen to give carbon dioxide and water
incomplete combustion produces carbon monoxide (or carbon), and water
Elimination (Dehydration)
excess hot concentrated sulfuric acid (or phosphoric (V) acid), or passing vapours over heated aluminium oxide
small molecule is lost; water
alkene is formed; Alkenes produced by this method can be used to produce addition polymers without using monomers derived from crude oil.
high temperature; 600K
Oxidation
primary alcohols
oxidised to aldehydes
functional group RCHO
further oxidised to carboxylic acid
functional group RCOOH
dilute sulfuric acid; less potassium dichromate (VI); when aldehyde is formed it is quickly distilled off to prevent further oxidation
hot concentrated sulfuric acid, excess potassium dichromate (VI; reacted under reflux (ethanol or ethanal vapour condenses back into the reaction flask)
secondary alcohols
oxidised to ketones
functional group RCOR
Acidified potassium dichromate(VI)
tertiary alcohols
not easily oxidised
have to break stronger C-C bond, rather than C-H bond
Tests:
Tollens reagent (silver mirror)
silver nitrate in aqueous ammonia
oxidation of aldehydes, not ketones
deposit of metallic silver is formed on inside of test tub when warming, as colourless silver (I) ions are reduced to metallic silver, if aldehyde is present
Fehling's test
contains blue copper (II) ions
oxidation of aldehydes, not ketones
solution will gradually change from blue to brick red, when warmed, if aldehyde is present
Carboxylic Acid:
react with metal carbonate, and collect gas; bubble gas through limewater
if limewater goes cloudy, gas is carbon dioxide, so substance is a carboxylic acid
Sammer Sheikh