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La Chimie H2 > Alcanes > Flashcards

Alcanes Flashcards

1
Q

What is general formula for alkanes? How about for alkyl R grps and saturated alkane rings?

A

alkane: CnH2n+2

alkyl R grp: CnH2n+1

alkane ring: CnH2n

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2
Q

Give possible explanation why alkanes hv low mp, bp

A
  • alkanes hv simple molecular structure
  • low amt energy needed to overcome weak id-id attract n
    => low mp,bp
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3
Q

Why is there diff in bp in branched chain alkanes and corresponding straight chain isomers (of same molecular formula)?

A
  • 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

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4
Q

Explain solubility of alkanes in non-polar and polar solvents

A
  • 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
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5
Q

Why are alkanes non-polar?

A

C & H atoms hv almost similar electronegativity

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6
Q

Why r alkanes generally unreactive?

A
  1. saturated & non-polar
  2. no region of low or high e- density, so unreactive towards polar reagents
  3. Strong C-C or C-H bonds (require large amt energy to break)
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7
Q

What 2 main reactions do alkanes undergo?

A
  1. Combustion
  2. Free radical substitution (FRS)
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8
Q

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?

A

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)

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9
Q

What are free radicals?

A

highly reactive species (atom or grp of atoms) w an unpaired e- formed by homolytic fission

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10
Q

Define homolytic fission

A

breaking of covalent bond btw 2 atoms so each atom retain oni 1 of shared e-, causing radical form n

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11
Q

Define heterolytic fission

A

breaking of covalent bond btw 2 atoms so 1 atom retain both shared e-, causing form n of ions

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12
Q

What does a full and half arrow represent?

A

full arrow: transfer of 2e- (pair of e-)

half arrow: transfer of one e-

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13
Q

Describe FRS mechanism using rxn of propane w limited Br to form bromo-1-propane

A

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
)

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14
Q

Assuming equal probability of H being substituted, how to determine ratio of mono sub pdt formed?

A
  • 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
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15
Q

If eg Cl is limited vs excess, how does it affect product of FRS?

A
  • Limited: major pdt is monosub
  • Excess: disub, trisub, tetrasub oso obtained
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16
Q

Name the main pollutants that motorcar engines emit. How do they form and what are their harmful effects?

A

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

17
Q

What does catalytic converter do? What condition does it work?

A
  • 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)
18
Q

Give 3 rxn occurring in a catalytic converter. Name catalyst for each

A
  1. 2NO + 2CO –> 2CO2 + N2 (Rh as catalyst)
  2. CxHy + (x+0.25y)O2 –> xCO2 + 0.5yH2O (Pd/Pt as catalyst)
  3. 2CO + O2 –> 2CO2 (Pd/Pt as catalyst)
19
Q

Name greenhouse gases. What are the consequences of enhanced greenhouse effect?

A
  • 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
20
Q

What is the problem with petroleum use? How to deal with it?

A
  • 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
21
Q

How is recycling important?

A

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

22
Q

Explain stability of radical

A

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)

La Chimie H2 (25 decks)

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