Topic 6 Flashcards
Test for a C=C
Bromine water decolourises (electrophillic addition)
Structural isomers
Compounds with the same molecular formula, but different structural formulae (chain, position, functional group isomerism)
Stereoisomers
Compounds with the same molecular and structural formulae, but a different spatial arrangement of atoms (geometric and optical isomerism)
Process of fractional distillation
Crude oil heated and enters distillation column as a vapour.
Column is hotter at the bottom and cooler at the top.
The vapour cools as it rises through a series of bubble caps.
Different fractions condense at different heights in the column depending on their boiling temperature.
The liquid of each fraction is piped away (gas leaves at top).
Fractional distillation fractions
Refinery gas Petrol Naptha Kerosene Diesel oil Lubricating oil Fuel oil Bitumen
Cracking
Use of heat/catalysts to break down long chain hydrocarbons into shorter chain alkanes, alkenes and hydrogen.
Increases supply of shorter fractions (economically important)
Increases branching in chains
Alkenes are an important feedstock for chemicals
Reforming
Process that converts straight hydrocarbon chains into branched and cyclic hydrocarbons, by heating with a catalyst (eg. platinum).
More efficient combustion
Smoother burning, less knocking
Pre-ignition less likely
Isomerisation: straight to branched
Another type: straight to cyclic alkane to arene
Pollutants
Carbon monoxide
Lead
Sulfur dioxide
Oxides of nitrogen
Sigma bonds
Covalent bonds formed when electron orbitals overlap end on (axially)
Pi bonds
Covalent bonds formed when electron orbitals overlap side on
Free radical
A chemical species with an unpaired electron
Homolytic fission
A bond breaks to form two free radicals
Heterolytic fission
A bond breaks to form two oppositely charged ions
Electrophile
A chemical species which is attracted to an area of high electron density and accepts a pair of electrons to form a covalent bond.
Nucleophile
A chemical species which is attracted to an area of (partial) positive charge and donates a pair of electrons to form a covalent bond.
Free radical substitution
Alkane + halogen
Initiation: Molecule -> 2 free radicals
Propagation Molecule + free radical -> molecule + free radical
(note free radical changes each step and homolytic fission)
Termination: Free radical + free radical
Ends because a stable species is formed
Electrophillic addition
Alkene + halogen
Thermal stability of carbocations
Stability increases as the number of alkyl groups on the positively charged carbon atom increases (primary, secondary, tertiary).
Alkyl groups contain a greater electron density that H atoms, so they are electron releasing and electrons from the alkyl group are attracted towards the positively charged carbon atom. This reduces its positive charge and has a stabilising effect.
Alkene + hydrogen
Name: Hydrogenation
Product: Alkane
Conditions: heat, nickel catalyst
Alkene + halogen
Name: Halogenation
Product: dihalogenoalkanes
Alkene + water
Name: Hydration
Product: Alcohol
Conditions: heat, phosphoric acid catalyst
Alkene + hydrogen halide
Product: Halogenoalkane
Oxidation of alkenes
Product: Doil
Conditions: Potassium manganate (VII) (oxidising agent), water
Colour change from purple to colourless
Primary halogenoalkane
1 R group attached to the R carbon atom bonded to the halogen
Testing for reactivity of halogenoalkanes
Ethanol used as solvent
Add silver nitrate solution
Use water bath (50 degrees) to control temp
Time how long precipitate takes to form
Trend in reactivity of halogenoalkanes
Slowest Chloro- primary Bromo- secondary Iodo- tertiary Fastest
Alkyl groups are electron releasing
C-X bond weakest in tertiary compounds
Weaker the bond, faster the hydrolysis
Nucleophillic substitution reactions
Halogenoalkanes + nucleophile
Primary alcohol
There is 1 alkyl group attached to the carbon atom which carries the OH group.
Chlorination of primary and secondary alcohols
Reagent: Phosphorus (V) chloride (PCl5)
Conditions: Room temp
Extra products: Phosphorus chloride (POCl3) and HCl
Chlorination of tertiary alcohols
Reagent: Conc. HCl
Conditions: Shake at room temp
Extra products: Water
Bromination of alcohols
Reagent: Hydrobromic acid
-made from KBr and 50% conc. H2SO4
Extra products: Water
Iodination of alcohols
Reagent: phosphorus (III) iodide- PI3
-made from red phosphorus and iodine
Conditions: Heat under reflux
Extra products: H3PO3
Elimination/dehydration of alcohols
Reagent: Phosphoric (V) acid- catalyst
Products: Alkene and water
Oxidation of primary alcohols
Oxidising agent: Acidified potassium dichromate (VI) solution
Distill with excess alcohol- aldehyde and water
Heat under reflux with excess oxidising agent- carboxylic acid and water
Oxidation of secondary alcohols
Oxidising agent: Acidified potassium dichromate (VI) solution
Heat under reflux
Produces ketone and water
Test for oxidation
Potassium dichromate (VI) solution (reduced) Orange -> green
Test for aldehyde
Fehling’s solution or Benedict’s solution (reduced)
Warm
Blue -> red-brown
Test for carboxylic acid
Add magnesium or calcium carbonate
Effervescence
Why does incomplete combustion occur?
There is a lack of oxygen.
What happens to the products of free radical substitution when there is excess halogen?
Further substitution reactions
Halogenoalkane and potassium hydroxide
Conditions: heat under reflux
Products: alcohol, potassium halide
Reaction: nucleophilic substitution
Halogenoalkane and potassium cyanide
Conditions: heat under reflux
Products: nitrile, potassium halide
Reaction: nucleophilic substitution
Halogenoalkane and ammonia
Conditions: heat in a sealed tube at high pressure
Products: primary amine, hydrogen halide
Reaction: nucleophilic substitution
Halogenoalkane and water
Conditions: warm
Products: alcohol, hydrogen halide
Reaction: nucleophilic substitution
Halogenoalkane and ethanolic potassium hydroxide
Conditions: heat, ethanol used as a solvent
Products: alkene, water, potassium halide
Reaction: elimination (OH- acts as a base, not nucleophile)
Note: reacts with H atom on the C atom next to the C atom bonded to the halogen!
Practical notes for chlorination of a tertiary alcohol
Unwanted substances in products: alcohol, HCl, water
Two layers- upper organic layer wanted
Ensure alcohol in lower layer- anhydrous calcium chloride
Remove lower layer- separating funnel
Remove HCl- sodium hydrogen carbonate, remove lower layer
Remove water- anhydrous sodium/magnesium sulphate
Distil and collect 50-52 degrees fraction
Test product: ethanol, NaOH, warm, nitric acid and silver nitrate = white ppt
How does burning fuels lead to the corrosion of buildings?
SO2 and NO2 form when fossil fuels are burned
React with water forming sulfuric acid and nitric acid
Acid rain
Reacts with limestone forming soluble compounds
Which type of covalent bond is stronger?
Sigma bonds are stronger than pi bonds
Good overlap of S orbitals in sigma bonds
Poor overlap of P orbitals in pi bonds
Because P orbitals are parallel
What happens to pi and sigma bonds in addition reactions?
Sigma bonds remain
Pi bonds break because they are weaker
Because orbital overlap is lateral, so overlap is poor
Carbocation intermediate forms
Why does the initiation step for free radical substitution involve breaking Cl-Cl bond rather than C-H bond?
The Cl-Cl bond is weaker than a C-H bond
Why do halogenoalkanes react with NaOH faster than with water?
OH- is a stronger nucleophile than water
Higher concentration of OH- ions in NaOH than water
