Organic-Alkanes

Introduction

Simplest, least reactive homologous series

Often saturated hydrocarbons: Contain on C-C and C-H single bonds, maximum number of H atoms, only C and H.

Main use: Fuel

Classification of alkanes & nomenclature

Aliphatic alkanes(open-chain)

1. General Formula: C_nH_2n+2
2. Only contain sp³ hybridised carbon atoms
3. Meth, Eth, Prop, But, Pent, Hex, Hept, Oct, Non, Dec, …

Cycloalkanes(alicyclic)

1. Carbon atoms joined in a ring
2. General Formula: C_n­H_2n
3. Cycloprop, Cyclobut, Cyclopent, Cyclohex, …

Alkyl group

1. Partial structure left if hydrogen removed from alkane
2. Not independently stable
3. Substituent: Part of larger compounds
4. Represented by: R

Nomeclature

1. IUPAC rules

• Identify longest continuous carbon chain (parent name)
• Number carbon atoms in main chain beginning from end nearer first branch
• Identify and number substituents, cite in alphabetical order. Number prefix ignored.

Isomerism

Structural isomerism (chain isomerism)

Physical properties

Held together mainly by weak dispersion forces

Physical state:

1. At r.t.p., lower alkanes C₁-C­₄ are gases, C₅-C₁₇ are liquids, C­₁₈- are solids.

Boiling point:

1. Increases smoothly with increasing number of C atoms
2. Increases 30^oC-20^oC per additional CH₂ group
3. Size of electron cloud increases, stronger dispersion forces with increasing size
4. Branched-chain: lower boiling points, more compact, smaller surface area of contact, weaker dispersion forces than straight chain.

Melting point:

1. Increase with increasing relative molecular mass, not smooth
2. Intermolecular forces dependent on size of electron cloud and also how well packed the molecules are in solid state.
3. Branched-chain: higher melting points, more compact, more effective packing, stronger dispersion forces than straight chain.

Density:

1. Less dense than water, immiscible
2. Density increase slightly with increasing C atoms present, slightly smaller volume of liquid.

Solubility:

1. Non-polar, soluble in non-polar solvents (Example: Benzene, Ether)
2. Insoluble in polar solvents (Example: Water), unable to form favourable interactions with water molecules.

Viscosity: Fluid’s internal resistance to flow, may be thought of as measure of liquid friction

1. Increases with increase in relative molecular mass, as dispersion forces between the molecules increases.
2. Higher alkanes, higher viscosity, long molecules “tangle up” with one another.

Reactivity and Chemical Properties

• Little chemical reactivity
• Chemically inert to most polar substances
• Due to C-C and C-H bonds being strong and fairly non-polar
• No centres of electric charge
• No reactive sites (lone pair, polar bond, electron-deficient atom, expandable octet)
• React with oxygen and halogens

Combustion:

1. General Equation: C_xH_y + (x+y/4) O₂ -> xCO₂ + (y/2)H₂O
2. Highly exothermic
3. High activation energy
4. Burn only in gaseous state

Free Radical Substitution

1. React with halogens to form halogenoalkanes (solvents, refrigerants, anaesthetics)
2. Conditions: UV Light / Sunlight / Heat
3. Reagent: Cl₂(g) / Br₂(l)
4. Observations: Greenish-yellow chlorine fades, white fumes detected; Reddish-brown bromine fades, white fumes detected; Iodination is slow and reversible; Fluorination is dangerously exothermic due to low F-F bond energy and high C-F bond energy
5. Free radicals (unpaired electron, highly reactive, no electrical charge) as intermediates

Mechanism

Step 1: Initiation – Cl-Cl bond is homolytically broken to give chlorine free radicals (Cl^.) The energy required comes from the light absorbed or the heat supplied.

(C-H bonds in CH₄ are unaffected because the energy is insufficient to break them)

Cl-Cl -> 2Cl^.

Step 2: Propagation – Reactive chlorine radical collides with methane molecule and abstracts a hydrogen atom to produce HCl and a methyl radical (^.CH₃)

CH₄ + Cl^. -> ^.CH₃ + HCl —(a)

Methyl radical then reacts further with Cl_2­ in propagation step to give the product chloromethane and another chlorine radical.

^.CH₃ + Cl₂ -> CH₃Cl + Cl^. —(b)

The chlorine radical forms then reacts with another methane molecule as in step (a) and thus once the reaction has been initiated, it becomes a self-sustaining cycle of repeating steps (a) and (b), making the overall process a chain reaction.

Step 3: Termination

2 radicals collide and combine to form a stable product in this step.

When these happen, the reaction cycle is broken, and the chain reaction ends. However, the likelihood that 2 radicals collide is small as the concentration of radicals in the reaction mixture at any given moment is very small.

Cl^. + Cl^. -> Cl₂

Cl^. + ^.CH₃ -> CH₃Cl

^.CH₃ + ^.CH₃ -> CH₃CH₃

Problems with Free Radical Substitution

• Limited synthetic utility due to multiplicity of products formed
• In presence of excess halogen, further substitution may occur, resulting in multi-substituted halogenoalkanes
• In the case of alkanes with 3 or more carbon atoms, isomeric products are formed depending on which H atom is substituted.

Environmental Consequences Of Burning Fuel

Carbon monoxide and unburnt hydrocarbons

• Due to incomplete combustion caused by (a) short reaction time in the internal combustion engine, (b) poor maintenance of the engine and (c) a lack of oxygen supply in the engine.
• CO is dangerous

1. Binds irreversibly with haemoglobin in blood by forming a stable complex,
2. Reduces the capacity of haemoglobin to transport oxygen,
3. Causing drowsiness, headaches
4. Fatal above 2000 ppm
5. Odourless, giving no warning of presence

• Unburnt hydrocarbons cause little damage on their own, but can react with O_2, O₃ and NO_x to form photochemical smog

Oxides of Nitrogen

• Formed inside combustion chambers of motor vehicles
• Atmospheric N and O combine to form NO, further oxidised to NO₂
• NO_x irritate lungs, cause bronchitis and pneumonia and lower resistance to respiratory infections
• Often results in the formation of photochemical smog
• Photochemical smog has harmful effects on plants;
• May also result in chest pains and breathing difficulties in children

Nitrogen dissolves in water to give mixture of nitrous and nitric acids:

2NO₂(g) + H₂O(l) -> HNO₂(aq) + HNO₃(aq)

NO₂ catalyses the formation of SO₃ from SO₂, SO₃ dissolves in water to form H₂SO₄

2SO₂(g) + NO₂(g) -> 2SO₃(g) +NO(g)

NO(g) + ½ O₂(g) -> NO₂(g)

HNO₃ and H₂SO₄ are strongly acidic, producing acid rain

Acid rain renders farm land unsuitable for cultivation;

Causes corrosion of buildings and other man-made structures;

Adversely affects marine lives (due to lowered pH of water)

Greenhouse Effect

• Infrared radiation reflected from Earth into atmosphere will be absorbed by greenhouse gaseous molecules, keeping the surface warm
• Increased [greenhouse gases], greater proportion of radiation trapped in atmosphere, resulting in increase in global temperature

Catalytic Converter

Removes pollutant gases from the exhaust by oxidising or reducing them

Made of platinum or rhodium
3 main pollutants removed from exhaust gases

Converted to CO₂(g), N₂(g), H₂O(g)

2NO(g) + 2CO(g) -> N_2­(g) + 2CO₂(g)

C_xH_y(g) + (x+y/4) O₂(g) -> xCO₂(g) + (y/2)H₂O(g)

Work only with unleaded petrol as lead destroys their catalytic action

Lead Compounds

• Tetraethyl lead(IV), Pb(C₂H₅)₄ is anti-knocking agent

• Lead(IV)oxide, PbO₂ produced upon combustion, combines with free radicals to give smoother ignition
• Accumulation of solid lead(IV)oxide as deposits on the walls of the engine could cause serious damage to the engine
• 1,2-dibromoethane added to petrol to remove lead as lead(II)bromide, PbBr₂, volatile and easily disposed off through exhaust pipe to atmosphere.
• Prolonged exposure to lead in atmosphere causes brain damage, especially in children
• Lead is also enzyme inhibitor, causing nervous disorders and large doses lead to coma
• Leaded petrol phased out by legislation

Processing of Crude Oil

• Crude oil is a mixture of many hundred of hydrocarbons
• Also contains some compounds containing sulphur and Nitrogen
• Important source of aliphatic and aromatic hydrocarbons
• First stage of processing – fractional distillation

Cracking

• Only one-fifth of crude oil is petrol
• Used to convert more abundant fractions into petrol
• C-C bonds are broken in long hydrocarbons
• Provide extra petrol and is a source of alkenes

Thermal Cracking

• Involves heating the alkane mixture at moderate pressure in absence of air but presence of steam
• High temperature sufficient to cause C-C bonds to break
• Proceeds by free radical mechanism in which C-C break homolytically
• Very little rearrangement of chains

Catalytic Cracking

• Heat to 500^oC and pass under slight pressure over catalyst
• Catalyst made from aluminium and silicon oxides
• Involves formation of carbocations which can undergo internal rearrangement before forming final products
• Branched chain alkanes are useful components of high octane petrol

Reforming

• Suitability of petrol as fuel is measured by octane number
• Mixtures with characteristics of 2,2,4-trimethylpentane are most efficient, rated 100
• Mixtures with burning characteristics of heptanes rated 0
• High octane numbers are rich in branched alkanes and burn smoothly and efficiently
• 95-100 in use in Singapore
• Demand satisfied by reforming straight-chain alkanes
• No C atoms lost, but H atoms often lost
• Pass vapourised alkane mixture over platinum-coated aluminium oxide catalyst at 500^oC and moderately high pressure