Unit 3: Section 3 - Alkenes and Alcohols
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What is an alkene?
Unsaturated hydrocarbons with a C=C double bond
What is the general formula of an alkene?
CnH2n
Why is there no rotation about the C=C double bond?
Due to the π orbital - electron density above and below the single bond, which holds the carbon atoms in place
Are alkenes more or less reactive than alkanes? Why?
More reactive, due to high electron density of double bond and the fact the pi-bond is slightly easier to break
What intermolecular forces of attraction do alkenes have?
Only Van der Waals due to non-polar bonds
Are alkenes soluble in water? Why?
No, non-polar bonds (van der Waals’ < hydrogen bonding)
Name and describe the 3 kinds of isomers alkenes can have
• Chain isomers (branched chains) • Position isomers (C=C on different carbon atoms) • Geometric E-Z isomers (Z is when 2 highest atomic number chains are on the same side of the double bond; E is when they’re on opposite sides)
What is the equation for the complete combustion of pent-2-ene?
CH3CH=CHCH2CH3 + 71/2O2 -> 5CO2 + 5H2O
What is an electrophile?
Electron deficient atoms/ions which accept a pair of electrons
What is the most stable type of carbocation intermediate? Why?
Alkyl groups have a posivite inductive effect, so the most stable carbocation is the one bonded to the most other carbon atoms
Major products will be formed from which kinds of carbocations?
Tertiary (or the most stable available)
What conditions are needed for the electrophilic addition of H2O to an alkene? What is the product?
Acid catalyst, usually phosphoric acid Alcohol
What conditions are needed for the electrophilic addition of a hydrogen halide to an alkene?
Room temperature
What conditions are needed for the electrophilic addition of a halogen molecule to an alkene?
Room temperature and organic solvent e.g. ethanol
How does a molecule with a non-polar bond react as if it is an electrophile?
C=C double bond with a high electron density induces a temporary dipole in the halogen molecule -> δ+ atom attracted to double bond
How would you turn the product from sulfuric acid and ethene into an alcohol and how does that show that sulfuric acid catalyses the addition of water to an alkene?
Add water H2SO4 reforms, showing it catlayses the hydration of alkenes
What is an addition polymer?
Many monomers bonded together via rearrangement of bonds without the loss of any atom or molecule
What are monomers? What form do they usually take?
Molecules which combine to form a polymer Usually have a C=C bond which breaks to leave a repeating pattern
Give 3 uses of poly(chloroethene) / PVC?
Drainpipes Vinyl Aprons
Give 2 examples of plasticisers
Esters Phthalates
What are plasticisers?
Small molecules that get between polymer chains to force them apart and allow them to slide over one another
How do the physical properties of PVC change due to a plasticiser? What applications does this lead to?
PVC with a plasticiser become flexible, used for aprons Without a plasticiser, PVC is rigid, used for drainpipes
Why do things containing mainly C-C and C-H bonds not decompose easily?
Bonds are non-polar so are not attacked by enzymes
Why is a lack of biodegradability in compounds with C-C or C-H bonds a problem?
Disposal is very problematic