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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
Iodination
Does not occur as overall reaction is endothermic → thermodynamically unfavourable
26
Free radical
- Atom/group of atoms that has an unpaired e⁻ → • - Highly reactive - Intermediates - Electrically neutral
27
Stability of alkyl radicals
- 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
Homolytic fission
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
Free-radical substitution (3)
Chain reaction: 1. Initiation → bond breaking 2. Propagation → bond breaking & forming 3. Termination → bond forming
30
Initiation
- Under UV light/heat | - Cl₂ supplied with energy to split into free radicals via homolytic fission of Cl-Cl bond → Cl•
31
Propagation
(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
Termination
Cl• + Cl• → Cl₂ * CH₃ + Cl• → CH₃Cl (desired product) * CH₃ + •CH₃ → CH₃CH₃ (trace quantity)
33
Major product
- CH₄ + Cl₂ - Limited: CH₃Cl major product - Excess: CCl₄ major product
34
2nd substitution (Propagation steps)
(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
Theoretical probability factor of the possible products formed
- Assuming all H atoms have same reactivity - Label different types of H atoms on the compound - Count
36
Experimental ratio of different products formed
Look at the stability of the alkyl formed
37
Natural occurrences of alkanes
- Natural gas and crude oil/petroleum - Fractional distillation - Cracking, reforming, alkylation
38
Pollution problems
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
Pollutants from internal combustion engine
- CO, NOx, unburnt hydrocarbons - Incomplete combustion - Reaction time too short
40
Environmental consequences of CO, NOx and unburnt hydrocarbons
- 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
Use of catalytic converters
- Remove the 3 main pollutants | - Converted into CO₂, N₂, and water vapour
42
Other pollutants form hydrocarbon fuels
- CO₂ → greenhouse effect → global warming - SO₂ → S as an impurity → acid rain → flue-gas desulfurisation - Lead compounds → poisonous → brain damage
43
Importance of petroleum
- Energy supply - Basic raw materials - Fossil fuels → finite → non-renewable - Must conserve and recycle
44
Recycling
- Conserve natural resources | - Protects environment

Chemistry Sem 2 (5 decks)

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