Intro, Alkanes, Haloalkanes Flashcards
Relative reactivity of halogens in halogenoalkanes test and result
- Dissolve in ethanol
- add dilute nitric acid and silver nitrate
- Add aqueous sodium/potassium hydroxide.
- The time for a precipitate to form is recorded -
Iodoalkanes - Yellow, Bromoalkanes - Cream, Chloroalkanes - White
- Iodoalkanes - fastest rate of reaction and the rate slows as you ascend up the group.
- Proves bond strength is more important than polarity for haloalkanes
Trends in bond polarity of halogenoalkanes
- size of halogen atoms increases as you go down the group.
↳ Distance between electron pair and nuclei increases - Strength of C-halogen bond decreases down group
↳ less attraction due to increased distance - you would expect c-f bond to be most reactive due to this but bond enthalpy override this
Trend in Bond Enthalpy of Halogenoalkanes
- size of the halogen increases down group.
- bond enthalpy decreases down the halogen group.
↳ positive nucleus of the halogen is further away from the (-)pair of electrons as you go down group.
attraction weaker = weaker covalent bond.
Ozone depletion equation & cause
- chlorofluorocarbons (CFCs) result in ozone depletion
- Chlorine catalyses ozone depletion via free radical substitution
1. Cl· + O₃ –> ClO· + O₂
ClO· + O₃ –> 2O₂ + Cl·
ClO· + O –> 2O₂ + Cl·
overall ozone depletion equation: O + O₃ –> O₂
problems with ozone depletion: causes whole in ozone, allows for UV to enter atmosphere which can cause cancer.
What is a saturated molecule?
- Compounds which contain only single bonds.
- They have the maximum number of hydrogens for a given no. of carbons
- e.g. cycloalkanes (still saturated even though CnH2n)
Types of Cracking
Thermal Cracking -
- High temperatures 500-900.c
- High pressure, up to 700kPa
- Produces mainly ALKENES due to thermal decomposition
Catalytic Cracking -
- High Temp - 450.C, Low pressure 1-2atms
- zeolite catalyst ( makes process cheaper and faster)
- produces AROMATIC HYDROCARBONS,branched & short-chain alkanes.
Free Radical Substitution
Initiation - bond in diatomic halogen molecules is broken by homolytic fission under UV light
Cl₂ –> 2Cl· <– forms 2 free radicals
Propagation - Chain reaction. Free radicals attack alkane molecules generating more free radicals.
CH₄ + Cl· –> ·CH₃+ HCl
·CH₃ + Cl₂ –> CH₃Cl + Cl·
Termination - free radicals react to produce a stable molecule
·CH₃ + ·CH₃ –> C₂H₆
Methods of removing combustion waste products
Catalytic Converters
- Inside exhaust pipes
- catalysing reaction between monoxide and nitrous oxide/hydrocarbons
- CO₂ N₂ H₂O are produced
Flue Gas Desulfurisation
- remove SO₂ from flue gases
SO₂ + CaCO₃ –> CaSO₃ + CO₂
Melting & Boiling point of Alkanes
- The stronger the Vander Waals forces the higher the MP & BP
- The strength of Vander Waals Forces is dependent on size of alkane molecule –> longer chain = larger surface area for Vander Waals interactions
- The strength of the Vander Waals forces is dependent on shape of alkane molecules
↳ Straight chain stack closer together than branched so greater area to interact with other molecules for Vander Waals interactions
Complete vs Incomplete combustion
Complete Combustion:
C₄H₁₀ + 6.5O₂ —> 4CO₂ + 5H₂O
-exothermic
INCOMPLETE COMBUSTION:
C₄H₁₀(g) + 4.5O₂(g) —> 4CO₂(g) + 5H₂O(g)
C₄H₁₀(g) + O₂(g) —> 4C(g) + 5H₂O(g)
Combustion Waste Products
- CO - toxic by binding to haemoglobin
- Soot - causes respiratory problems & global dimming
- Smog - Unburnt hydrocarbons passed to atmosphere.
↳ reacts with nitrogen oxides and ozone forming
photo-chemical smog. - Nitrogen oxides - formed by high pressure and temperatures
- SO₂ - present inn carbon fuels
- Acid rain -formulated by NO(n)s and SO₂ w/water
↳ harms aquatic life, infrastructure & crops
E-Z Isomerism naming priority
For single atoms, a higher atomic mass gives a molecule higher priority
- For groups of atoms, look at the atom directly bonded to the carbons in the double bond.
(If same look down alkyl group)
What is a Nucleophile?
An atom or compound with a negtive or partial negative charge which is able to form a covalent bond by donating a lone pair of electrons
mechanisms for haloalkane reacted with cyanide
- potassium/sodium cyanide
- reflux and in ethanol
- extends carbon chain
mechanism for halogenoalkane reacted with excess ammonia
- in ethanol
- in excess amonia
- forms amines & ammonium salt
mechanism for halogenoalkane reacted with Na/KOH
- alcohol forms
- aqueous alkalis e.g NaOH, KOH
- nucleophilic
mechanism for elimination using a haloalkane
- in ethanol
- under reflux
- produces an alkene
