Alcanes Flashcards
What is general formula for alkanes? How about for alkyl R grps and saturated alkane rings?
alkane: CnH2n+2
alkyl R grp: CnH2n+1
alkane ring: CnH2n
Give possible explanation why alkanes hv low mp, bp
- alkanes hv simple molecular structure
- low amt energy needed to overcome weak id-id attract n
=> low mp,bp
Why is there diff in bp in branched chain alkanes and corresponding straight chain isomers (of same molecular formula)?
- extent of surface area of contact: greatest for straight chain and smallest for most branched chain
- extent of distort n of e- cloud: highest for straight chain, lowest for most branched chain
- extent of intermolecular id-id attract n: greatest for straight chain, smallest for most branched chain
=> energy required to overcome imf of attract n: greatest for straight chain, least for most branched chain
Explain solubility of alkanes in non-polar and polar solvents
- Soluble in non-polar eg benzene
bcos energy released fr form of id-id attract n btw alkane molecules & solvent molecules enough to overcome id-id attract n btw alkane molecules & that btw solvent molecules - Insoluble in polar eg water
bcos energy released fr form of id-id btw alkane molecules & solvent molecules inadequate to overcome id-id attract n btw alkane molecules & pd-pd/H bond (for solvent eg water) btw solvent molecules
Why are alkanes non-polar?
C & H atoms hv almost similar electronegativity
Why r alkanes generally unreactive?
- saturated & non-polar
- no region of low or high e- density, so unreactive towards polar reagents
- Strong C-C or C-H bonds (require large amt energy to break)
What 2 main reactions do alkanes undergo?
- Combustion
- Free radical substitution (FRS)
What is the general eqn for complete combustion of alkane? So what are general pdt?
What is a possible eqn of incomplete combustion of methane? So what r the pdt?
CxHy + (x+0.25y)O2 –> xCO2 + 0.5yH2O
(CO2 & H2O r pdt)
possible eqn:
CH4 + 1.5 O2 –> CO + 2H2O
(CO2, H2O AND CO r pdt)
What are free radicals?
highly reactive species (atom or grp of atoms) w an unpaired e- formed by homolytic fission
Define homolytic fission
breaking of covalent bond btw 2 atoms so each atom retain oni 1 of shared e-, causing radical form n
Define heterolytic fission
breaking of covalent bond btw 2 atoms so 1 atom retain both shared e-, causing form n of ions
What does a full and half arrow represent?
full arrow: transfer of 2e- (pair of e-)
half arrow: transfer of one e-
Describe FRS mechanism using rxn of propane w limited Br to form bromo-1-propane
Name: Free radical sub n
Initiation:
homolytic fission (2 half arrow) of Br-Br under UV light –> 2Br ·
Propagation: (radical + molecule -> another radical)
1) H-CH2CH2CH3 + Br · –> ·CH2CH2CH3 + HBr
2) ·CH2CH2CH3 + Br2 –> Br-CH2CH2CH3 + Br ·
Termination: (3 eqn! A+A, B+B & A+B)
2Br · –> Br2
·CH2CH2CH3 + Br · –> CH3CH2CH2Br
·CH2CH2CH3 + ·CH2CH2CH3 –> CH3CH2CH2CH2CH2CH3
(in other situations where C radical is in the middle, u write formula sth like this:
CH3CHCH3
|
CH3CHCH3
)
Assuming equal probability of H being substituted, how to determine ratio of mono sub pdt formed?
- Draw out structure of alkane for easy visualisation
- determine H atoms that gives specific pdt and group them
eg sub for Ha gives 1-chloropropane, sub for Hb gives 2-chloropropane - take no of Ha & Hb atoms and put them into a ratio to get the final ratio
If eg Cl is limited vs excess, how does it affect product of FRS?
- Limited: major pdt is monosub
- Excess: disub, trisub, tetrasub oso obtained
Name the main pollutants that motorcar engines emit. How do they form and what are their harmful effects?
i. CO
- fr incomplete fuel combust n
- cause CO poisoning
ii. Unburnt hydrocarbons
- fr incomplete fuel combust n
- bcome photochemical smog in strong sunlight, cause lung damage
iii. Lead compounds
- eg (TO REMEMBER) Lead bromide vapour
- fr add n of tetraethyllead (IV) to petrol
- lead poisoning, accumulat n cause brain damage
iv. Oxides of nitrogen
- eg NO, NO2; referred as NOx
- form acid. rain, corrode buildings, destroy crop, marine life & cause respiratory problem in human
v. SO2
- form acid rain, corrode buildings, destroy crops, marine life
What does catalytic converter do? What condition does it work?
- speeds up conversion of harmful pollutants eg CO, NOx, unburnt hydrocarbons into harmless pdt eg H2O, CO2, N2, in motor vehicle exhaust system
- car must b run on unleaded petrol (bcos lead poisons/deactivates catalyst)
Give 3 rxn occurring in a catalytic converter. Name catalyst for each
- 2NO + 2CO –> 2CO2 + N2 (Rh as catalyst)
- CxHy + (x+0.25y)O2 –> xCO2 + 0.5yH2O (Pd/Pt as catalyst)
- 2CO + O2 –> 2CO2 (Pd/Pt as catalyst)
Name greenhouse gases. What are the consequences of enhanced greenhouse effect?
- H2O, CO2, CH4, N2O, O3, CFCs
Consequences: - more drought, flooding
- less snow, ice
- more extreme weather incidences
- rising sea lvl
stronger gh effect warm ocean, partially melt glaciers, other ice, increasing sea lvl. Ocean water oso expand if warmed, contribute further to rise in sea lvl
What is the problem with petroleum use? How to deal with it?
- chemical feedstock (raw material to produce many sorts of pdt eg plastic, fuel), natural source of hydrocarbon
BUT - non-renewable resource bcos natural petroleum deposits r finite; once exhausted, no way to restore them
THUS - must use alternative, renewable sources to preserve resources as well as minimise harmful pollut n (save environ)
eg solar, wind pwr, bio-fuels - must oso recycle
How is recycling important?
preserves Earth’s natural resources (eg petroleum), protect environ by reducing:
- pollut n caused by dispose waste, plastic incinerat n
eg harmful chemicals, ghg gases
- need for raw materials; rainforests can b preserved
eg deforestation, cause habitat destruct n, global warming
- amt energy needed in making pdt
eg recycle aluminium less energy need than extract fr ore
Explain stability of radical
Case 1: if there is adjacent benzene ring:
- lone (unpaired) electron is delocalised into benzene ring, stabilising radical
Case 2: number of electron donating group (not adjacent bond to benzene):
- the more electron donating alkyl group, the more stable (to better disperse +ve charge of radical)
