Alkanes Flashcards
Alkanes
- Saturated hydrocarbons → only single bonds
- Open-chain alkanes (acyclic/non-cyclic)
- Cycloalkanes (alicyclic/ring)
Open-chain alkanes
- CnH2n+2
- Each C atom sp³ hybridised → tetrahedral → 109.5°
Cycloalkanes
- CnH2n
- Functional group isomerism with alkenes
Classification of C atoms
- Primary (1°), secondary (2°), tertiary (3°), quaternary (4°)
- Depends on no. of C atoms bonded to it
Classification of H atoms
- Primary (1°), secondary (2°), tertiary (3°)
- Depends on type of C atom bonded to it
- 0° → methane
- 4° → C cannot expand octet → cannot exist
Isomerism (2)
- Constitutional/structural isomerism
2. Stereoisomerism
Constitutional isomerism
- Same molecular formula, diff structural formula
- Branching of hydrocarbon chains
- Chain isomers/position isomers
Stereoisomerism (2)
- Enantiomerism
2. Cis-trans isomerism
Enantiomerism
- Chiral carbon centre
- No plane of symmetry
- Non-superimposable mirror images of each other
Cis-trans isomerism
- Some cycloalkanes
- Ring structure restricts free rotation of bonds
- 2 diff on at least 2 C atoms
Physical properties
- ΔEN between C & H negligible → non-polar
- Id-id interactions
Boiling and melting points
- 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
Solubility
- Soluble in non-polar solvents
- Insoluble in water and highly polar solvents
Density
- Increases with increasing molecular size
- Level off about 0.8 gcm⁻³
- Less dense than water
- Separatory funnel
Preparation of alkanes
- Reduction of alkenes
2. Reduction of other unsaturated hydrocarbons
Reduction of alkenes
- 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
Reduction of other unsaturated hydrocarbons
- Alkynes: Pd + heat
- Benzene: Raney Nickel (Ni+Al) + 150°
Reactions of Alkanes (2)
- Unaffected by polar reagents
- Undergo reactions with non-polar reagents (e.g. Cl₂ and O₂)
- Combustion
- Halogenation (Free radical substitution)
General lack of reactivity (2)
- Non-polar: no region of high e⁻ density to attract electrophilic reagents, no e⁻-deficient sites to attract nucleophilic reagents
- Relatively strong C-C and C-H bonds which do not break under normal conditions
Combustion
- Complete: produce CO₂ and H₂O
2. Incomplete: also produce CO and C
Halogenation
- 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
Chlorination
- Decolourisation of greenish-yellow Cl₂
- Formation of white HCl fumes that turn damp blue litmus paper red
Bromination
- Decolourisation of reddish-brown Br₂
- Formation of white HBr fumes that turn damp blue litmus paper red
Fluorination
Dangerously exothermic
Iodination
Does not occur as overall reaction is endothermic → thermodynamically unfavourable
Free radical
- Atom/group of atoms that has an unpaired e⁻ → •
- Highly reactive
- Intermediates
- Electrically neutral
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
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
Free-radical substitution (3)
Chain reaction:
- Initiation → bond breaking
- Propagation → bond breaking & forming
- Termination → bond forming
Initiation
- Under UV light/heat
- Cl₂ supplied with energy to split into free radicals via homolytic fission of Cl-Cl bond → Cl•
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)
Termination
Cl• + Cl• → Cl₂
- CH₃ + Cl• → CH₃Cl (desired product)
- CH₃ + •CH₃ → CH₃CH₃ (trace quantity)
Major product
- CH₄ + Cl₂
- Limited: CH₃Cl major product
- Excess: CCl₄ major product
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)
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
Experimental ratio of different products formed
Look at the stability of the alkyl formed
Natural occurrences of alkanes
- Natural gas and crude oil/petroleum
- Fractional distillation
- Cracking, reforming, alkylation
Pollution problems
- Pollutants from internal combustion engine
- Environmental consequences of CO, NOx and unburnt hydrocarbons
- Use of catalytic converters
- Other pollutants form hydrocarbon fuels
Pollutants from internal combustion engine
- CO, NOx, unburnt hydrocarbons
- Incomplete combustion
- Reaction time too short
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
Use of catalytic converters
- Remove the 3 main pollutants
- Converted into CO₂, N₂, and water vapour
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
Importance of petroleum
- Energy supply
- Basic raw materials
- Fossil fuels → finite → non-renewable
- Must conserve and recycle
Recycling
- Conserve natural resources
- Protects environment
