alkanes

introduction

• hydrocarbons → organic compounds that contain only two types of elements, carbon and hydrogen

• alkanes form a homologous series of saturated hydrocarbons ⇒ no functional group

• all carbon atoms present in alkanes are sp³ hybridised (C-H bonds)

• general formula: C_nH_2n+2

cycloalkanes

• are cyclic saturated hydrocarbons

• a cyclic structure is formed when two terminal carbon atoms join together, and one hydrogen atom from each of these terminal carbon atoms are removed

• general formula: C_nH_2n

• are constitutional isomers of alkenes ⇒ same general formula with alkenes

nomenclature

straight chain alkanes

• chemical names are formed by taking the

  1. stem corresponding to the number of carbon atoms

  2. ending with the suffix ‘-ane’

• each successive member of the alkane family differs by a –CH₂ (methylene) group ⇒ true when descending through each homologous series

cycloalkanes

• ring structures are named with the word ‘cyclo’ attached to the front of the stem

branched chain alkanes

• for alkanes with 4 carbon atoms onwards, there are different ways in which the carbon atoms can be connected to each other ⇒ branched chain alkanes

• naming:

  1. name the stem by counting the longest unbranched chain of carbon atoms in the main carbon skeleton ⇒ parent chain of the alkane

  2. name the alkyl substituents (arranged in alphabetical order) as prefix to the name of the main carbon skeleton

  3. the position of each substituent on the parent chain is denoted by a number → each carbon atom on the parent chain is numbered, starting from one end of the chain

     • start from the end that indicates the positions of the substituent groups by a set of smallest numbers possible

     • the number denoting the position of substitution is stated in front of the prefix for that particular substituent → hyphens are used to separate numbers from words

  4. if there are multiple identical substituents, the numeric prefixes are added to the prefix → di-, tri-, tetra-

     • numeric prefixes do not affect the alphabetical order of the prefixes

     • hyphens are used to separate numbers from words, commas are used to separate numbers

• alkyl substituents

  • are alkane derivatives with one less hydrogen atom from the parent alkane

  • have their names end in ‘-yl’

  • general formula: $C_nH_2n+1

isomerism

constitutional isomerism

• alkanes with at least 4 C atoms exhibit chain isomerism

• chain isomerism is the only form of constitutional isomerism that alkanes can exhibit since they have no functional groups

enantiomerism

• longer-chain alkanes can exhibit chirality and enantiomerism

• the smallest alkane is C₇H₁₆, 3-methylhexane

cis-trans isomerism

• some cycloalkanes display cis-trans isomerism ⇒ ring structure restricts the free rotation of bonds in the ring

physical properties → boiling point

• at room temperature:

  • C₁ to C₄: alkanes are gases

  • C₅ to C₁₇: alkanes are liquids

  • > C₁₇: alkanes are solids

• alkanes (non-polar molecules) have relatively low boiling and melting point as boiling or melting involves overcoming weak instantaneous dipole-induced dipole interactions that exist between the molecules

• melting and boiling points increase with increasing Mr due to the larger electron cloud size, which results in greater polarisation of the electron cloud ⇒ stronger and more extensive instantaneous dipole – induced dipole attractions between alkane molecules. thus, more energy is needed to overcome these intermolecular forces of attraction for larger alkanes during melting and boiling

• boiling point of a branched isomer is lower than that of its straight chain isomer. the greater the extent of branching, the lower the boiling point. Increased branching gives the molecule a
  more spherical shape. this reduces the surface area of contact between molecules, leading to less extensive instantaneous dipole – induced dipole interactions between the molecules. thus, less energy required to overcome these intermolecular forces of attraction during boiling

physical properties → density

• density of alkanes increases with increasing Mr ⇒ due to stronger and more extensive instantaneous dipole – induced dipole interactions which hold the molecules more closely together

• all alkanes are less dense than water

physical properties → solubility

• alkanes being non-polar are soluble in non-polar solvents (e.g. CCl4) and are insoluble in polar solvents (e.g. water)

• liquid alkanes can act as non-polar solvents

reactivity of alkanes

• relatively unreactive

1. due to the high bond energies of C–H and C–C bonds, a large amount of energy is required to break these bonds

2. alkanes are unreactive towards polar reagents (e.g. strong acids and bases, oxidising agents and reducing agents), as the C–H and C–C bonds in alkanes are non-polar due to the similarity in electronegativities between C and H atoms

• can undergo combustion, and free-radical substitution reaction with halogens

combustion

• combustion of alkanes is highly exothermic ⇒ important fuel

C_xH_y + (x + y/4) O₂ → xCO₂ + y/2 H₂O

• with abundant/excess supply of oxygen, products are CO₂ and H₂O

• with limited supply of oxygen, products are C (soot) or CO and H₂O

free-radical substitution reaction with halogens

• reaction with chlorine or bromine does not occur in the dark → when exposed to uv light, substitution occurs, giving a mixture of products

general equation: C_nH_2n+2 + X₂ → C_nH_2n+1X + HX

• X is Cl or Br

• reaction takes place via the free-radical substitution mechanism, which involves 3 stages: initiation, propagation and termination

name of mechanism:

• free radical substitution

reagent and condition:

• limited Cl₂ (g), uv light and at room temperature

  • Br₂ (l) or Br₂ in CCl₄ can be used

• further substitution will give rise to a mixture of products

  • e.g.: free radical substitution of methane can give dichloromethane, trichloromethane and tetrachloromethane

• mono-substituted alkane predominates if excess alkane is used

  • e.g.: in order to form $CH_3Cl$ as the major product of the reaction, the reaction can be controlled by starting the reaction with a large excess of $CH_4$

relative amount of mono-substituted isomeric products in a free radical substitution reaction

• free radical substitution takes place randomly and a lack of control on the reaction is inevitable

  • coupled with the fact that the C–H bonds are of about the same energy, there is a high possibility of having a different hydrogen atom substituted such that isomeric mono-substituted products are obtained

  • assuming that all H-atoms are of the same reactivity and substitution is completely random

• in reality, the assumption does not hold true ⇒ not all H atoms are of the same reactivity

  • H atoms may have different reactivity since radicals of different stability are formed when H atoms in different structural environments are abstracted during the reaction

stability of radicals

• reason: the tertiary radical has the greatest number of electron-donating alkyl groups which will stabilise the radical to a greatest extent ⇒ most likely to be formed

• the more stable the radical, the faster it is formed → increases the proportion of the isomer formed from the more stable radical during the reaction

• both factors affect the final composition of isomeric products formed

  1. statistical factor → number of structurally different H atoms being substituted

  2. relative reactivity of structurally different H atoms → relative rate of formation of the radical leading to each isomeric product

thermodynamic considerations for free radical substitution reactions

• the trend in the calculated ΔH for the overall halogenation reaction corresponds to the trends observed for H–X and C–X bond energies ⇒ enthalpy changes become less
  exothermic (or more endothermic) from F to I

• order of reactivity: F₂ (most violent) >> Cl₂ > Br₂

  • fluorination of alkanes is highly exothermic, such that the reaction is too violent to be of practical use

• iodoalkanes are not produced as the first propagation step is highly endothermic

  • due primarily to the weak H–I and C–I bonds formed → propagation step 1 is highly endothermic and energy released from propagation step 2 is insufficient to compensate for step 1 ⇒ overall ΔH for iodination is endothermic and energetically unfavourable

reduction (hydrogenation) of alkenes → preparation of alkanes

general equation: C_nH_2n + H₂ → C_nH_2n+2

• reagent and condition:

  1. H₂ (g) with Pt or Pd catalyst

  2. H₂ (g) with Ni catalyst

motor vehicle pollution

• since air rather than pure oxygen is used, incomplete combustion occurs ⇒ unburnt hydrocarbons, carbon monoxide and oxides of nitrogen are produced

greenhouse gases

• includes gases such as carbon dioxide, methane, nitrous oxide and water vapour ⇒ absorb some of the infrared radiation, retain the energy and warm the earth’s surface (greenhouse effect)

• causes global warming and leads to drastic shifts in weather

carbon monoxide

• produced as a result of incomplete combustion in the car engine

• CO is odourless and highly toxic

  • combines with haemoglobin in blood more effectively than oxygen, preventing haemoglobin from carrying oxygen to body cells

  • may affect mental alertness and can lead to death if inhaled extensively

oxides of nitrogen

• under the high temperature in the car engine, oxides of nitrogen are formed:

  • O₂ + N₂ → 2NO

  • 2NO + O₂ → 2NO₂

• NO and NO₂ combine with water to form HNO₃ and HNO₂which contribute to acid rain formation ⇒ negative impact on soil, trees, buildings and aquatic lives

• NO₂ acts as a catalyst in the conversion of SO₂ to SO₃ which then dissolves in water to give dilute H₂SO₄ → SO₂ is the main contributor to acid rain

  • SO₂ (g) + NO₂ (g) → SO₃ (g) + NO (g)

  • NO (g) + 1⁄2 O₂ (g) → NO₂ (g)

• NO₂ also contributes to the formation of photochemical smog and in the formation of ground level ozone → ozone in smog is dangerous to health

unburnt hydrocarbons

• some hydrocarbons do not undergo combustion and end up in the exhaust gases.

• a mixture of unburnt hydrocarbons and nitrogen dioxide contributes to photochemical smog that is corrosive and irritating to the respiratory tract

lead (II) bromide

• formed from combustion of tetraethyllead(IV) (TEL), an additive to
  petrol

• TEL was being replaced by unleaded petrol starting in the US in the mid-1970s because of lead’s neurotoxicity and damaging effect on catalytic converters

solutions to the pollution problems

1. use unleaded petrol

   • catalytic converters cannot work with leaded petrol as lead poisons the catalyst → when a heterogenous catalyst is “poisoned”, the surface of the catalyst is no longer available
     to catalyse the substrate

2. use catalytic converters

   • catalytic converters are fitted onto car exhausts to remove pollutant gases before they reach the atmosphere

   • catalyst used: ceramic honeycomb coated with precious metals such as platinum, palladium and rhodium

   • in the converter,

     • 2NO + 2CO → N₂ + 2CO₂ (redox processes)

     • 2CO + O₂ → 2CO₂

     • C_xH_y + (x + y/4) O₂ → x CO₂ +y/2 H₂O

   • as a result, the exhaust gases from a car with a catalytic converter contain simply a mixture of nitrogen, carbon dioxide and water