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Chemistry Sem 2 > Alkanes > Flashcards

Alkanes Flashcards

1
Q

Alkanes

A
  • Saturated hydrocarbons → only single bonds
  • Open-chain alkanes (acyclic/non-cyclic)
  • Cycloalkanes (alicyclic/ring)
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2
Q

Open-chain alkanes

A
  • CnH2n+2

- Each C atom sp³ hybridised → tetrahedral → 109.5°

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3
Q

Cycloalkanes

A
  • CnH2n

- Functional group isomerism with alkenes

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4
Q

Classification of C atoms

A
  • Primary (1°), secondary (2°), tertiary (3°), quaternary (4°)
  • Depends on no. of C atoms bonded to it
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5
Q

Classification of H atoms

A
  • Primary (1°), secondary (2°), tertiary (3°)
  • Depends on type of C atom bonded to it
  • 0° → methane
  • 4° → C cannot expand octet → cannot exist
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6
Q

Isomerism (2)

A
  1. Constitutional/structural isomerism

2. Stereoisomerism

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7
Q

Constitutional isomerism

A
  • Same molecular formula, diff structural formula
  • Branching of hydrocarbon chains
  • Chain isomers/position isomers
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8
Q

Stereoisomerism (2)

A
  1. Enantiomerism

2. Cis-trans isomerism

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9
Q

Enantiomerism

A
  • Chiral carbon centre
  • No plane of symmetry
  • Non-superimposable mirror images of each other
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10
Q

Cis-trans isomerism

A
  • Some cycloalkanes
  • Ring structure restricts free rotation of bonds
  • 2 diff on at least 2 C atoms
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11
Q

Physical properties

A
  • ΔEN between C & H negligible → non-polar

- Id-id interactions

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12
Q

Boiling and melting points

A
  • Increase with increasing no. of C atoms
  • No. e⁻ per molecule ↑ → size of e⁻ cloud ↑ → ease of polarisation of e⁻ cloud ↑ → strength of id-id ↑ → overcome at increasingly high T
  • Decrease with increasing degree of branching
  • Surface area available for intermolecular interactions ↓
  • Weaker id-id → overcome at lower T
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13
Q

Solubility

A
  • Soluble in non-polar solvents

- Insoluble in water and highly polar solvents

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14
Q

Density

A
  • Increases with increasing molecular size
  • Level off about 0.8 gcm⁻³
  • Less dense than water
  • Separatory funnel
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15
Q

Preparation of alkanes

A
  1. Reduction of alkenes

2. Reduction of other unsaturated hydrocarbons

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16
Q

Reduction of alkenes

A
  • Addition reaction, catalytic hydrogenation
  • Reagent and conditions: H₂(g), Ni, heat OR H₂(g), Pt/Pd
  • Finely divided platinum, palladium or nickel
  • Nickel least active → requires elevated T
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17
Q

Reduction of other unsaturated hydrocarbons

A
  • Alkynes: Pd + heat

- Benzene: Raney Nickel (Ni+Al) + 150°

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18
Q

Reactions of Alkanes (2)

A
  • Unaffected by polar reagents
  • Undergo reactions with non-polar reagents (e.g. Cl₂ and O₂)
  1. Combustion
  2. Halogenation (Free radical substitution)
19
Q

General lack of reactivity (2)

A
  1. Non-polar: no region of high e⁻ density to attract electrophilic reagents, no e⁻-deficient sites to attract nucleophilic reagents
  2. Relatively strong C-C and C-H bonds which do not break under normal conditions
20
Q

Combustion

A
  1. Complete: produce CO₂ and H₂O

2. Incomplete: also produce CO and C

21
Q

Halogenation

A
  • Substitution of ≥ 1 H atoms by halogen atoms
  • Energy input → heat/UV light → initiate
  • Liquid or gaseous phase
  • Aqueous condition → H₂O molecules react with any radical present
22
Q

Chlorination

A
  • Decolourisation of greenish-yellow Cl₂

- Formation of white HCl fumes that turn damp blue litmus paper red

23
Q

Bromination

A
  • Decolourisation of reddish-brown Br₂

- Formation of white HBr fumes that turn damp blue litmus paper red

24
Q

Fluorination

A

Dangerously exothermic

25
Q

Iodination

A

Does not occur as overall reaction is endothermic → thermodynamically unfavourable

26
Q

Free radical

A
  • Atom/group of atoms that has an unpaired e⁻ → •
  • Highly reactive
  • Intermediates
  • Electrically neutral
27
Q

Stability of alkyl radicals

A
  • Depends on number of alkyl groups attached to C atom with unpaired e⁻
  • C with unpaired e⁻ is e⁻-deficient
  • Alkyl groups are e⁻ donating
  • More alkyl groups → more stable
28
Q

Homolytic fission

A

Bond breaks in such a way that each of the atoms involved in forming the bond acquires one of the 2 bonding electrons, forming free radicals

29
Q

Free-radical substitution (3)

A

Chain reaction:

  1. Initiation → bond breaking
  2. Propagation → bond breaking & forming
  3. Termination → bond forming
30
Q

Initiation

A
  • Under UV light/heat

- Cl₂ supplied with energy to split into free radicals via homolytic fission of Cl-Cl bond → Cl•

31
Q

Propagation

A

(a) Hydrogen abstraction → highly reactive Cl• collides with methane molecule and abstracts 1 H atom from it → form HCl + methyl radical

CH₄ + Cl• → HCl + •CH₃

(b) Halogen abstraction → methyl radical reacts with Cl₂ → forms chloromethane and chlorine free radical

•CH₃ + Cl₂ → CH₃Cl + Cl•

Then (a), (b), (a), (b)

32
Q

Termination

A

Cl• + Cl• → Cl₂

  • CH₃ + Cl• → CH₃Cl (desired product)
  • CH₃ + •CH₃ → CH₃CH₃ (trace quantity)
33
Q

Major product

A
  • CH₄ + Cl₂
  • Limited: CH₃Cl major product
  • Excess: CCl₄ major product
34
Q

2nd substitution (Propagation steps)

A

(a) CH₄ + Cl• → HCl + •CH₃
(b) •CH₃ + Cl₂ → CH₃Cl + Cl•

Then (a), (b), (a), (b)

Propagation steps (a) and (b) give rise to many molecules of CH₃Cl

CH₃Cl can undergo further chlorination via propagation steps (c) and (d) to form CH₂Cl₂

(c) CH₄ + Cl• → HCl + •CH₃
(d) •CH₃ + Cl₂ → CH₃Cl + Cl•

Then (c), (d), (c), (d)

35
Q

Theoretical probability factor of the possible products formed

A
  • Assuming all H atoms have same reactivity
  • Label different types of H atoms on the compound
  • Count
36
Q

Experimental ratio of different products formed

A

Look at the stability of the alkyl formed

37
Q

Natural occurrences of alkanes

A
  • Natural gas and crude oil/petroleum
  • Fractional distillation
  • Cracking, reforming, alkylation
38
Q

Pollution problems

A
  1. Pollutants from internal combustion engine
  2. Environmental consequences of CO, NOx and unburnt hydrocarbons
  3. Use of catalytic converters
  4. Other pollutants form hydrocarbon fuels
39
Q

Pollutants from internal combustion engine

A
  • CO, NOx, unburnt hydrocarbons
  • Incomplete combustion
  • Reaction time too short
40
Q

Environmental consequences of CO, NOx and unburnt hydrocarbons

A
  • CO → combines with haemoglobin → renders it ineffective in O₂ transport
  • Unburnt hydrocarbons → photochemical smog → yellowish-white haze → irritates respiratory tract
  • NOx → acid rain → renders farmlands unsuitable for cultivation
41
Q

Use of catalytic converters

A
  • Remove the 3 main pollutants

- Converted into CO₂, N₂, and water vapour

42
Q

Other pollutants form hydrocarbon fuels

A
  • CO₂ → greenhouse effect → global warming
  • SO₂ → S as an impurity → acid rain → flue-gas desulfurisation
  • Lead compounds → poisonous → brain damage
43
Q

Importance of petroleum

A
  • Energy supply
  • Basic raw materials
  • Fossil fuels → finite → non-renewable
  • Must conserve and recycle
44
Q

Recycling

A
  • Conserve natural resources

- Protects environment

Chemistry Sem 2 (5 decks)

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