Edwards Flashcards
classification of HCS
–> aromatics and aliphatics–> alkenes, alkanes, alkynes
alicyclic
contain one or more rings
Alkanes - chemistry & physical
properties
• Alkanes are non-polar (electronegativities
of carbon and hydrogen are the same)
• Essentially insoluble in water
• Soluble in most organic solvents
(especially non-polar ones)
• Relatively low melting and boiling points
– Means low MW alkanes are gases or quite
volatile
• Generally less dense that water
Alkanes
• Are often present in complex mixtures, e.g., in petroleum distillate, kerosene, etc. • Many compounds in mixture may exist within a narrow boiling range • For petroleum fractions, you cannot consider individual compounds within these mixtures
Alkanes: general toxicology
• Generally: fairly innocuous
– There are some exceptions: hexane is worth
noting
• In humans and animals – acute toxicities
are fairly low
– Mildly irritating to mucous membranes
• Can cause problems in the atmosphere in
production of smog, hydroxyl radicals etc
Gaseous Alkanes C1 to C4
• All are relatively non-toxic, and are simple asphyxiants (i.e., exposure to high concentrations can cause suffocation) – Lowers amount of available O2 • Other possible problems: – Methane: Greenhouse gas – Low M.W. alkanes - highly flammable (can form explosive mixtures with air).
Tropospheric oxidation of methane
• Several natural and anthropogenic
sources of methane
– Anaerobic degradation of landfill
– Natural gas wells
• Methane is a greenhouse gas and absorbs
IR radiation
• Methane is oxidised by hydroxyl radicals
Sequence - one example
• CH4 + •OH ® •CH3 + H2O • •CH3 + O2 ® CH3OO• • CH3OO• + NO ® CH3O• + NO2 • CH3O• + O2 ® CH2O + HOO• • The formaldehyde can be further oxidised by •OH to ultimately give CO or CO2 • Subsequent reactions can be complex – Contributes to photochemical smog
Higher Molecular Weight Alkanes
• Above C4 - liquids of varying volatility
• Alkanes ≥ C13 have very low vapour pressures
• Liquid hydrocarbons are generally
combustible (their main problem)
– Also fat solvents: dermal irritants
• Eventually alkanes are waxes (solids) at
room temperature
– Solid high molecular weight alkanes virtually
non-toxic
Vapours of Alkanes
• Generally low toxicity • In high concentrations: – Asphyxiants – Narcotics (due to lipid solubility) • Can be achieved by deliberate inhalation (e.g., glue or petrol sniffing), or in enclosed spaces – Among other combustion products, car exhaust contains unburned hydrocarbons • TOXIC EFFECTS can include: – Disorientation, stupor, depression of central nervous system (CNS) leading to respiratory failure • In enclosed spaces – Fatalities have occurred cleaning storage tanks, and in railway carriages • EFFECTS ARE REVERSIBLE: emergency treatment - remove affected persons to clean air
Metabolism of linear alkanes
• Alkanes: absorbed from gastrointestinal tract and lungs • Also dermal absorption is possible but less significant – Removed by exhaling, and metabolism • Alkanes are lipophilic and can be deposited in fat • Longer chain alkanes cleared more slowly
Metabolism of linear alkanes: one fate
• One route: oxidation of terminal carbon to
alcohol by mixed function oxidase
• Further oxidation to carboxylic acid
• Degradation of chain by β-oxidation of the
carboxylic acid
– Same process as fatty acid metabolism
• Final products include propionyl or acetyl
Coenzyme A
– Or CO2
alkanes
• Contain only carbon and hydrogen
• Compounds are SATURATED
– All carbons have four single bonds, and are
sp3 hybridised
• Either aliphatic (open chain)
• Or alicyclic (contain one or more rings)
• Alkanes are non-polar (electronegativities
of carbon and hydrogen are the same)
• Essentially insoluble in water
• Soluble in most organic solvents
(especially non-polar ones)
• Relatively low melting and boiling points
– Means low MW alkanes are gases or quite
volatile
• Generally less dense that water
Are often present in complex mixtures,
e.g., in petroleum distillate, kerosene, etc.
• Many compounds in mixture may exist
within a narrow boiling range
• For petroleum fractions, you cannot
consider individual compounds within
these mixtures
Gaseous Alkanes C1 to C4
• All are relatively non-toxic, and are simple asphyxiants (i.e., exposure to high concentrations can cause suffocation) – Lowers amount of available O2 • Other possible problems: – Methane: Greenhouse gas – Low M.W. alkanes - highly flammable (can form explosive mixtures with air).
Tropospheric oxidation of methane
• Several natural and anthropogenic
sources of methane
– Anaerobic degradation of landfill
– Natural gas wells
• Methane is a greenhouse gas and absorbs
IR radiation
• Methane is oxidised by hydroxyl radicals
