Alkanes Flashcards

1
Q

What are alkanes?

A

Alkanes are saturated hydrocarbons that contain carbon atoms which are sp3 hybridised and are tetrahedrally bonded to either H atom or other C atoms

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2
Q

What is the general formula of alkanes?

A

CnH2n+2

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3
Q

What is the general formula of cycloalkanes?

A

CnH2n

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4
Q

What happens to the melting and boiling point of alkanes as the number of C atom increases and why?

A

Melting and boiling point increases as alkanes have simple covalent structures and as the number of C atoms increases, the number of electrons in the molecule increases and more energy is required to overcome the stronger id-id interactions between molecules

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5
Q

Are alkanes more or less dense than water and why?

A

Less dense, the packing of alkane molecules is less as compared to water molecules due to weaker id-id interactions between alkane molecules compared to the stronger hydrogen bonds between water molecules

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6
Q

What happens to the density of alkanes as the number of C atoms increases and why?

A

Density increases as when the number of carbon atom increases, molar mass increases, density increases as the increase in molar mass outweighs the increase in volume

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7
Q

Are alkanes soluble in water and why?

A

All alkanes are insoluble in water
- Alkanes cannot form effective interactions with water molecules
- During the dissolution of alkanes in water, the energy needed to overcome the stronger hydrogen bonds between water molecules is not compensated by the energy released when weaker id-id are formed between alkane and water molecules

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8
Q

Are alkanes generally reactive or unreactive and why?

A

Alkanes are generally unreactive towards polar and ionic reagents
- alkanes are non-polar thus unreactive towards polar reagents
- C-C and C-H bonds are very strong, a large amount of energy is required to overcome these strong bonds in order for reaction to occur

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9
Q

What are free radicals?

A

Species that posses at least one unpaired electron

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10
Q

What are the reagents and conditions for free radical substitution?

A

Condition: UV light
Reagents:
- for a large proportion of mono-substituted products, a large excess of hydrocarbon used
- for a large proportion of poly-substituted products, a large excess of halogen used

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11
Q

What is the reactivity of halogens in free radical substitutions (most reactive to least reactive)?

A

F2 > Cl2 > Br2 > I2
- F2 gives explosive reaction even in absence of UV light
- Cl or Br reaction proceeds rapidly only in UV light
- I2 no reaction under normal lab conditions

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12
Q

What is the stability of hydrocarbon radicals (most stable to least stable) and why?

A

tertiary radical > secondary radical > primary radical > methyl radical
- alkyl substituents which are electron-donating help stabilize the radical

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13
Q

What other factors can stabilize hydrocarbon radicals?

A
  • Radicals can be stabilised by delocalised pi electron systems like benzene ring
    -partially filled p orbital can participate in resonance with adjacent p orbitals, leading to greater stability
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14
Q

What factors destabilize hydrocarbon radicals?

A
  • the greater the s character of the orbital, the half-filled orbital containing the free radical is held more closely to the nucleus, electron affinity of orbital increases, destabilizing the radical (alkyl > alkenyl / aryl > alkynyl)
  • Higher electronegativity, greater electron affinity of the half-filled orbital, more unstable
  • Smaller atom, electron-deficient orbital spread out over a smaller volume, decrease in stability
  • destabilized by electron-withdrawing groups
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15
Q

What is cracking?

A

Cracking is a process where bigger hydrocarbon molecules are broken down into smaller molecules

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16
Q

What are the two types of cracking?

A
  • Thermal cracking: very high temperature and pressure
  • Catalytic cracking: In presence of catalyst (Al2O3 or SiO2), high temperature and pressure
17
Q

What are the pollutants in car exhaust and where do they come from?

A
  • CO and unburnt hydrocarbons (from incomplete combustion)
  • Oxides of nitrogen, NO and NO2 (from reaction of N2 and O2 due to high temperatures in the combustion engine)
18
Q

What contributes to the formation of photochemical smog?

A

CO, unburnt hydrocarbons and oxides of nitrogen

19
Q

What pollutants contribute to the formation of acid rain?

A

nitrogen oxides and SO2

20
Q

What do catalytic convertors do?

A

Convert harmful gas in exhaust into less harmful gas via the action of catalysts like platinum, rhodium and palladium

21
Q

What reaction do Pt/Pd catalyse?

A
  1. CO + 1/2O2 –> CO2
  2. C + O2 –> CO2
  3. CxHy + (x + y/4) O2 –> xCO2 + y/2 H2O
22
Q

What reaction do Rh catalyse?

A

2NO + 2CO –> 2CO2 + N2

23
Q

What poisons the catalyst in the catalytic converter?

A

Lead and Maganese (Pb and Mn)

24
Q

What gases cause the greenhouse effect?

A

Carbon dioxide, methane, N2O, halogenoalkanes, ozone, water vapour

25
Q

What is the enhanced greenhouse effect?

A

Greater retention of thermal energy in the earth’s atmosphere, leading to rise in global temperatures

26
Q

What does ozone do in the stratosphere and troposphere do respectively?

A

Stratosphere: useful role by absorbing harmful UV radiation from the sun
Troposphere: Harmful role as an air pollutant and a greenhouse gas

27
Q

What reactions can alkanes undergo?

A
  1. Combustion
  2. Free radical substitution
  3. Cracking